WO2001091187A1 - A semiconductor device - Google Patents

A semiconductor device Download PDF

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Publication number
WO2001091187A1
WO2001091187A1 PCT/SE2001/001167 SE0101167W WO0191187A1 WO 2001091187 A1 WO2001091187 A1 WO 2001091187A1 SE 0101167 W SE0101167 W SE 0101167W WO 0191187 A1 WO0191187 A1 WO 0191187A1
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WO
WIPO (PCT)
Prior art keywords
layer
dopants
semiconductor
contact
doped
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PCT/SE2001/001167
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French (fr)
Inventor
Olof Hjortstam
Johan Hammersberg
Bo Breitholtz
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Abb Ab
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Publication date
Application filed by Abb Ab filed Critical Abb Ab
Priority to AU2001260938A priority Critical patent/AU2001260938A1/en
Publication of WO2001091187A1 publication Critical patent/WO2001091187A1/en

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/12—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
    • H01L29/16—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed including, apart from doping materials or other impurities, only elements of Group IV of the Periodic Table
    • H01L29/1602—Diamond
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/02—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having potential barriers; including integrated passive circuit elements having potential barriers
    • H01L27/04—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having potential barriers; including integrated passive circuit elements having potential barriers the substrate being a semiconductor body
    • H01L27/08—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having potential barriers; including integrated passive circuit elements having potential barriers the substrate being a semiconductor body including only semiconductor components of a single kind
    • H01L27/085—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having potential barriers; including integrated passive circuit elements having potential barriers the substrate being a semiconductor body including only semiconductor components of a single kind including field-effect components only
    • H01L27/095—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having potential barriers; including integrated passive circuit elements having potential barriers the substrate being a semiconductor body including only semiconductor components of a single kind including field-effect components only the components being Schottky barrier gate field-effect transistors
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/12—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
    • H01L29/16—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed including, apart from doping materials or other impurities, only elements of Group IV of the Periodic Table
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/12—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
    • H01L29/16—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed including, apart from doping materials or other impurities, only elements of Group IV of the Periodic Table
    • H01L29/167—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed including, apart from doping materials or other impurities, only elements of Group IV of the Periodic Table further characterised by the doping material
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/86—Types of semiconductor device ; Multistep manufacturing processes therefor controllable only by variation of the electric current supplied, or only the electric potential applied, to one or more of the electrodes carrying the current to be rectified, amplified, oscillated or switched
    • H01L29/861—Diodes
    • H01L29/872—Schottky diodes

Definitions

  • the present invention relates to a semiconductor device com- prising a first semiconductor layer doped according to either a) n- type or b) p-type and a metal layer forming a Schottky-barrier contact thereto.
  • the present invention relates to all types of semi- conductor devices having a metal layer forming a contact to a doped semiconductor layer.
  • Such semiconductor devices may normally assume either a blocking state, in which the leakage current therethrough should be as low as possible, or a conducting state, in which the power losses generated through the cur- rent through the device should be as low as possible.
  • the features of the contacts of such a semiconductor device are essential for obtaining this object.
  • the present invention is in particular, but not exclusively, di- rected to semiconductor devices in which it is very important that such a contact has two active functions, namely a blocking function in a blocking state of the device and a low ohmic function in the conducting on-state of the device.
  • these two functions are solved separately with two types of contacts, namely a blocking Schottky-contact and a low resistance ohmic contact.
  • Diamond has some properties making it extremely interesting as a material in a device for high power applications, one of which is the very high breakdown field strength, which means that the number of devices to be connected in series for holding a voltage of a certain magnitude may be reduced considerably with respect to devices of other known materials involving important cost reduction even if such a device itself would be much more expensive than the prior art devices, which for the rest is not an evi- dent fact.
  • Other interesting properties of diamond is a very high thermal conductivity and high charge mobility.
  • said first layer is doped by dopants assuming such deep energy levels in the semiconductor material of said layer that the majority thereof will not be thermally activated at working temperature of the device, that the device comprises an irradiation source adapted to emit radiation of an energy being high enough for activating said dopants of the first layer for making the device conducting in a forward biased state thereof with a voltage of the same type as the doping of said first layer applied to said contact.”
  • Voltage of the same type as the doping of said first layer means that said contact will be connected to a potential being negative with respect to another contact of the device when said doping type of the first layer is n, and oppositely if the doping type of the first layer is p.
  • a Schottky-barrier contact in combination with such deep energy level doping makes it possible to obtain a blocking function of the contact thanks to the Schottky-barrier when the semiconductor material next thereto is not irradiated and a low ohmic function when the region of the semiconductor material next to the con- tact is irradiated.
  • the contact of such a semiconductor device is advantageous it may be improved with respect to the ability to withstand high electric fields in the forward blocking state of such a device.
  • the electric field at the contact of such a device has to be reduced considerably for being able to construct a component with a reasonable thickness able to hold voltages in the re- gion of several kV.
  • the electric field in question has to be reduced 100-1000 times at the contact with respect to the average electric field over the thickness of the device in order to design a device which can block approximately 50 kV over a thickness of 100 ⁇ m.
  • the object of the present invention is to provide a semiconductor device of the type defined in the introduction, which finds a solution to the problem of too high electric fields at said Schottky- barrier contact.
  • the electric field applied to the device in the forward block- ing state thereof may be concentrated to a region between this second layer and at least a further layer of the device not being said first layer, so that the electric field in said first layer next to said contact may be reduced. Accordingly, it will be possible to benefit from the superior characteristics of a device having a contact of this type with respect to low losses in the forward conducting state while still obtaining a blocking capability not being destroyed by a low blocking capability of the contact enabling the production of reasonably thin devices also for devices having to hold higher voltages in the forward blocking state thereof.
  • the dopants of the second layer are continuously activated at said working temperature by ensuring that the majority of the dopants of said second layer are thermally activated at said working temperature of the device.
  • the dopants of said second layer assume such deep energy levels in the semiconductor material of said layer that the majority thereof will not be thermally activated at working temperature of the device, and the device comprises an irradiation source adapted to substantially continuously subject said second layer to radiation of an energy being high enough for activating said dopants of the second layer.
  • This embodiment is particularly advantageous for semiconductor materials for which it is difficult to find candidates for dopants thermally activated at said working temperature, such as for example n-type dopants for diamond.
  • the dopants of said second layer have a lower acti- vation energy than the dopants of the first layer, and said irradiation source adapted to subject the second layer to irradiation is adapted to emit radiation of an energy being lower than the activation energy of the dopants of the first layer.
  • the radiation from the irradiation source for activating the dopants of the second layer may not influence the dopants of the first layer and by that the switching behaviour of the device.
  • Another way to achieve this is to arrange means for optically shielding said first layer from the radiation to which the second layer is subjected for activation of the dopants therein. In such a case it will even be possible to use dopants for said second layer located at a deeper energy level than the dopants of the first layer.
  • the device comprises an intrinsic layer laterally separating said first and second layers.
  • This is very advantageous, since this will reduce lateral internal fields otherwise formed at the junction of the first and second layer when depleted. Such internal fields may result in unacceptably high leakage currents between the first layer and the contact in the forward blocking state of the device for comparatively low operation voltages.
  • this is solved by the arrangement of the intrinsic layer, which means that the depleted charge of the different sides of the junction between the first and second layer will be separated by a large distance and thus the electric field will be reduced.
  • said second layer constitutes one of the doped layers of a pin-diode connected in anti-parallel with a charge carrier conducting path formed between two terminals of the device through said first layer when irradiated.
  • This structure is very advantageous, since it means on one hand that the electrical field will be concentrated to the depleted layer of the pin-diode and it will be low at the contact, so that possibly inherent high breakdown field strength of said semiconductor material may be fully utilised, and on the other an anti-parallel diode will be integrated in the same device as a switch, which results in a reduced number of separate components used in converter station applications, such as HVDC (High Voltage Direct Current) stations.
  • HVDC High Voltage Direct Current
  • the metal of the contact layers has a work function being for a) substantially higher than the electron affinity of the material of the semiconductor layer and for b) substantially lower than the sum of on one hand the band gap between the conduction band and the valence band and on the other the electron affinity of said semiconductor material.
  • the device comprises means promoting tunnelling of charge carriers through said Schottky-barrier when said first semiconductor layer is irradiated. Such means may for example be ion impact defects. This means will make it possible to be able to tunnel through a higher and wider barrier than oth- erwise improving the tunnel probability for a barrier of a certain width, i.e.
  • said semiconductor material is a wide band gap material, i.e. a material having an energy gap between the conduction band and the valence band exceeding 2 eV.
  • a wide band gap material i.e. a material having an energy gap between the conduction band and the valence band exceeding 2 eV.
  • Such materials are particularly preferable when high voltages are to be handled by the device, since the breakdown field strength of such devices is high. Furthermore, it is easier to find energy levels for dopants being deep enough for not being activated at working temperature of the de- vice for such materials. Examples of these materials are SiC, diamond, GaN.
  • the invention also comprises a semiconductor device of the type defined in the introduction, which comprises two first semicon- ductor layers doped with dopants of different doping types connected in series between two terminals of the device and a said contact to each of the first layers and a second layer close to each first layer, said second layers are doped according to the opposite type to that of the respective first layer adjacent thereto for concentrating the electric field to a region between each second layer and at least a further layer of the device not being said first layer in a forward blocking state of the device reducing the electric field in the respective first layer next to the contact associated therewith.
  • a semiconductor device of the type defined in the introduction which comprises two first semicon- ductor layers doped with dopants of different doping types connected in series between two terminals of the device and a said contact to each of the first layers and a second layer close to each first layer, said second layers are doped according to the opposite type to that of the respective first layer adjacent thereto for concentrating the electric field to a region between each second layer and at least a further layer
  • this device may have a first layer with dopants on such energy levels that the majority thereof are thermally acti- vated at the working temperature of the device.
  • the invention also comprises the uses according to the appended use-claims, and the advantages thereof resides in the possibility to use materials as diamond while obtaining good contact functions and utilising the inherent properties of such materials, in particularly diamond and SiC.
  • Fig 1 is a schematic cross-section view of a type of semiconduc- tor device to which the present invention may be applied,
  • Fig 2 is an energy band diagram of a contact metal layer adjacent to a semiconductor layer doped with deep level dopants in the form of donors in a forward blocking state of a device ac- cording to the present invention
  • Fig 3 is a view corresponding to Fig 2 in a conducting state of the device
  • Fig 4 is a cross-section view of a device according to a first preferred embodiment of the invention in the forward blocking state
  • Fig 5 is a view corresponding to Fig 4 of that device in the forward conducting state
  • Fig 6 is a view corresponding to Fig 4 of that device in the reverse biased state
  • Fig 7 is a schematically illustration of the operation of the device according to Fig 4,
  • Fig 8 is a view corresponding to Fig 4 of a device according to a second preferred embodiment of the invention.
  • Fig 9 is a view illustrating the distribution of the electric field in the device according to Fig 8 close to one of the two active con- tacts thereof,
  • Fig 10 is a view corresponding to Fig 4 of a device according to a third preferred embodiment of the invention.
  • Fig 1 1 is a view corresponding to Fig 4 of a device according to a fourth preferred embodiment of the invention.
  • Fig 12 is a view corresponding to Fig 4 of a device according to a fifth preferred embodiment of the invention.
  • Fig 13 is a view corresponding to Fig 4 of a device according to a sixth preferred embodiment of the invention.
  • the device has two terminals 1 , 2 for connecting the device to an electric current path.
  • the device has also one first semiconductor layer 3 of diamond doped by dopants assuming such deep energy levels that the majority thereof will not be thermally activated at room temperature, and this means that the activation energy of said dopants should be higher than 0.3 eV.
  • the device comprises an- other first semiconductor layer 4 of diamond doped according to the same conditions as the first layer 3.
  • a further layer 5 of intrinsic diamond is arranged between the two first layers 3 and 4.
  • the layer 5 has typically a thickness of 3 to 200 ⁇ m, whereas the two doped layers 3, 4 have a thickness of 1 to 20 ⁇ m. It is within the scope of the present invention to dope the two layers 3, 4 with the dopants of the same conductivity type, n or p, but it is preferred to dope them with dopants of opposite conductivity type, and we therefor in the following assume that the layer 3 is doped with acceptors, whereas the layer 4 is doped with donors.
  • a metal contact 6, 7 connects the respective terminal to the diamond layer 3 and 4, respectively.
  • the material of the metal contact is chosen so that a Schottky-barrier of a preferable height and width is created at the interface between the contact and the respective first layer, which will be discussed below with reference to Fig 2 and 3 and is more in detail discussed in the Swedish patent application 00001 15-6.
  • the metal contact has preferably vents 8 for allowing penetration of light from a light source 9 through the metal contact and into the respective first layer 3, 4. It should be mentioned that characteristics of this device and the rest of the devices shown in the figures having nothing to do with the present invention, such as passivation layers have been omitted for the sake of clearity.
  • the light sources 9 are adapted to illuminate a region under the respective metal contact for activation of dopants occupying deep energy levels there from the opposite side of the device, so that the light source shown to the right in the figure will activate the acceptors in the layers 3 underneath the anodes through il- lumination through the intrinsic layer 5, whereas the light source shown to the left will illuminate and activate donors in the layers 4 underneath the cathodes of the device.
  • a voltage source 30 and a load 31 for the circuit, to which the device is connected, are also schematically indicated in this figure.
  • the acceptors in the layer 3 occupy levels at a considerable distance to the valence band of the diamond lattice, whereas the dopants of the first layer 4, which are assumed to be donors, occupy levels lo- cated at a substantial distance below the conduction band of the diamond lattice.
  • Said energy distance is preferably above 0.3 eV, so that practically none of these dopants are thermally activated at room temperature.
  • the first layers 3 and 4 will without said illumination have practically no free charge carriers for transport between the anode and the cathode and they will act as intrinsic layers so that the switch should be able to block very high voltages supplied thereacross in any direction as long as no charge carriers are injected at the contacts, and the pres- ent invention aims at taking care of that issue.
  • the layers 3 and 4 are illuminated by light having an energy exceeding the activation energy of the dopants, these will be activated.
  • the activation energy of said dopants is lower than 2.5 eV a standard high power semiconductor laser may be used as said irradiation source 9, since it has only to deliver light with an energy above said activation energy, which for instance for donors of N is 1 .7 eV, which should be compared with the energy needed to lift electrons from the valence band to the con- duction band in diamond (5.4 eV).
  • Fig 2 illustrates the choice of a metal for the contact 6 creating a Schottky-barrier between the contact and the first layer 4.
  • the lower limit of the conduction band and the upper limit of the va- lence band are referenced by 26 and 27, respectively. It is schematically illustrated how defects 10 are introduced in the first layer close to the metal contact for promoting tunnelling of charge carriers through the Schottky-barrier in the forward conducting state illustrated in Fig 3, in which the donors 1 1 are acti- vated.
  • the donors 1 1 are not activated, but also in this state a considerable leakage current, which would be too high in case of utilising the blocking capability of the other layers of the device, would result if not particular measures are taken, and such measures form the basis of the present invention.
  • the concentration of dopants on deep levels in the region of the layer 4 closest to the contact is above 10 15 cm “3 , preferably above 10 17 cm "3 to achieve good absorption. The same is valid for the corresponding region of the layer 3.
  • FIG 4 A device according to a first preferred embodiment of the inven- tion for addressing this problem is illustrated in Fig 4.
  • the parts having correspondence in Fig 1 are provided with the same reference numerals.
  • a second layer 12, 13 doped according to the opposite type to that of the first layer 4 and 3, respectively, are located laterally close to the respective first layer.
  • the two layers 12, 13 are doped with dopants being thermally activated at working temperature of the device. Accordingly, the layer 12 is a p-type layer and the layer 13 is an n- type layer.
  • a metal contact 33, 34 contacts the respective terminal to the diamond layer 12 and 13, respectively.
  • the forward conducting state of the device is illustrated in Fig 5.
  • the light sources 9 do then irradiate not only the regions of the first layers 3, 4 closest to the respective contact for activating them, but they do also irradiate the entire first layers 3 and 4 for activating the deep energy level dopants therein. This means that the switch formed by the structure of the layers 4, 5 and 3 will form a low resistance current path 28 between the two contacts 6. (Similar to a forward biased pin-diode).
  • the device has a function being identical to an IGBT 15 and a diode 16 connected in anti-parallel therewith as illustrated in Fig 7, so that it may be used in a current valve of a converter, possibly connected in series with a plurality of such devices for being able to jointly handling very high voltages and being switched simultaneously for function as one single switch. This could involve considerable cost savings for such a valve.
  • Fig 8 illustrates a device differing from that according to Fig 4 by the introduction of an intrinsic layer 17 laterally separating the respective first and second layers. It has namely been found that if the first layer 3 and/or the first layer 4 is strongly doped a large internal field will be formed as these regions are depleted, so that the leakage current through the contact will then be unacceptably high. However, this is addressed by introducing said intrinsic layer 17 through which the depleted charge of the different sides of the junction between the second layer and the first layer associated therewith will be separated by a larger distance and thus the electric field will be reduced.
  • the electric field E close to the contact 6 at the first layer 4 ver- sus the distance d away from this contact 6 is illustrated in Fig 9. It is shown how the electric field is kept down at the interface between the contact 6 and the layer 4 thanks to the arrangement of the intrinsic layer 17. The entire potential difference between the layer 12 and the layer 4 had otherwise to be taken over a short distance at the junction between these layers resulting in a high local electric field. The same type of reduction of the field in the depletion region can be achieved by a graded doping concentration profile.
  • Fig 10 illustrates a device according to a third preferred embodi- ment of the invention, which has a thyristor-like structure formed by a pin-structure 18 vertically separating the two first semiconductor layers 4, 3 with the p-type layer 19 of the pin-structure next to the first layer 4 with dopants of n-type and the n-type layer 20 next to the first layer 3 with the dopants of p-type.
  • An intrinsic layer 21 separates the layers 19 and 20.
  • the shorts 22, 23 so created will guaran- tee a region free from external field for the first layers 4 and 3 when they are not irradiated and the device is forward biased as shown in Fig 10.
  • this device When this device is irradiated it will work as a switched on thyristor. Possibly a switch-on UV-pulse has to be applied during or after the irradiation has been ramped up for triggering the thyristor.
  • the shorts 22, 23 of the layers 19 and 20 to the contacts could be weakly doped so that under irradiation the excess of charge carriers of opposite sign will effectively quench the conducting of the short circuiting channels between the respective layers 19, 20 and the contacts.
  • a fast switch-off due to high injection of minority charge carriers into the respective first layers 4, 3.
  • the switch-off time may here be as short as 10-100 ns.
  • Fig 1 1 schematically illustrates a device according to a fourth preferred embodiment of the invention differing from the device according to Fig 4 only by the fact that one of the second layers 24, in this case the one doped with donors, is doped with dopants assuming such deep energy levels in the semiconductor material of that layer that the majority thereof will not be ther- mally activated at working temperature of the device.
  • This device comprises an irradiation source 25 schematically indicated adapted to substantially continuously subject said second layer 24 to radiation of an energy being high enough for activating the dopants of that layer, so that the same function as for the device according to Fig 4 will be obtained for this device.
  • the dopants of the layer 24 has to have a lower activation energy than the dopants of the first layer 3 next thereto and the energy of the radiation from the light source 25 has to be lower than the activa- tion energy of the dopants of the first layer 3 or means for optically shielding the first layer 3 from the radiation to which the second layer 24 is subjected have to be arranged for avoiding that the radiation from the irradiation source 25 disturbs the function of the device.
  • This embodiment is particularly advanta- geous for and applicable to semiconductor materials being difficult to dope with dopants being thermally activated at normal working temperatures of a device of this type. This is for example true for diamond, for which there is today no established thermally achieved donors available that can be incorporated during diamond growth.
  • the layer 24 It is then preferred to dope the layer 24 with phosphorus (P) having an activation energy of 0.6 eV, whereas the doping of the n-type first layer 4 is realised through nitrogen (N) having an activation energy of 1 .7 eV. It would of course be possible to have the same dopants for the layers 4 and 24, but a shielding described above has then to be introduced. It would of course also be possible to have the p-type second layers doped with dopants on deep energy levels, but dopants being thermally activated at working temperatures of the device would be preferable for the second layers for simplifying the operation of the de- vice.
  • Fig 12 schematically illustrates a device according to a fifth preferred embodiment of the invention differing from the device according to Fig 4 only by the fact that holes 32 have been etched into the semiconductor material at the locations for said second layers 12, 13, which have then been formed by implanting dopants into the walls of the holes. This enables a location of the second layers deeper into the material, so that the field will be easier reduced close to the contacts 6, 7.
  • Metal contacts 35, 36 filling out the holes contact the second layers 12 and 13, respectively.
  • Fig 13 schematically illustrates a device according to a sixth preferred embodiment of the invention differing from the device according to Fig 4 only by the fact that said second layers 12, 13 here are formed by another semiconductor material than the first layer for forming a heterostructure. They may for example be of Si or SiC appropiately doped in the case of diamond as the semiconductor material for the rest of the device.
  • the second layers may be formed by carrying out an etching as for forming the holes 32 of the embodiment according to Fig 12 and then epi- taxially growing said second layers. Also this embodiment enables a location of the second layers deeper into the material, so that the field will be easier reduced close to the contacts 6, 7.
  • Metal contacts 37, 38 contact the second layers 12 and 13, respectively.
  • semiconductor materials as used in the claims is to be interpreted broadly and also include an insulator as diamond being doped with deep level dopants, which may be activated by irra- diation. It is also pointed out that irradiation does not necessarily have to be illumination, but it is also conceivable to use for instance beams of electrons. Furthermore “metal” is in this disclosure defined as covering all materials being electric conducting. This has to be considered when interpreting the claims.
  • the semiconductor device according to the present invention may also have other combinations of layers than those described above, and the invention is mainly directed to the layers close to a Schottky-barrier contact of any type of semiconductor device having the features of appended claim 1.
  • Continuous activated with respect to the dopants of the second layer as defined in the claims does only mean that the dopants are continuously activated when the device is in operation, but the activation may well cease when the device is taken out of operation, as will be the case for a device according to Fig 1 1 , for which the additional irradiation source will be switched off and the dopants of the second layer will then not be activated when the device is not in operation.

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Abstract

A semiconductor device comprises a first doped semiconductor layer (3, 4) and a metal layer (6) forming a Schottky-barrier contact thereto. The first layer is doped by dopants assuming such deep energy levels in the semiconductor material of said layer that the majority thereof will not be thermally activated at the working temperature of the device. A second layer located close to the first layer is doped with dopants of opposite type to that of the first layer and is continuously activated at said working temperature for concentrating electric field to a region between the second layer (12, 13) and a further layer (5) of the device not being said first layer in a forward blocking state of the device while reducing the electric field in said first layer next to said contact.

Description

A semiconductor device
FIELD OF THE INVENTION AND PRIOR ART
The present invention relates to a semiconductor device com- prising a first semiconductor layer doped according to either a) n- type or b) p-type and a metal layer forming a Schottky-barrier contact thereto.
Accordingly, the present invention relates to all types of semi- conductor devices having a metal layer forming a contact to a doped semiconductor layer. Such semiconductor devices may normally assume either a blocking state, in which the leakage current therethrough should be as low as possible, or a conducting state, in which the power losses generated through the cur- rent through the device should be as low as possible. The features of the contacts of such a semiconductor device are essential for obtaining this object.
The present invention is in particular, but not exclusively, di- rected to semiconductor devices in which it is very important that such a contact has two active functions, namely a blocking function in a blocking state of the device and a low ohmic function in the conducting on-state of the device. In normal semiconductor technologies these two functions are solved separately with two types of contacts, namely a blocking Schottky-contact and a low resistance ohmic contact.
However, when we consider a light activated switch device in for example diamond (as for example in the Swedish patent applica- tion 9903149-4 discussed below), it would be of particularly interest to be able to improve contacts for these types of materials, so that the other excellent properties of especially diamond could be used for producing semiconductor devices. Some of the advantages of using such material for a semiconductor device will therefor be briefly discussed here, but it is emphasised that the present invention is directed to all types of semiconductor mate- rials.
Diamond has some properties making it extremely interesting as a material in a device for high power applications, one of which is the very high breakdown field strength, which means that the number of devices to be connected in series for holding a voltage of a certain magnitude may be reduced considerably with respect to devices of other known materials involving important cost reduction even if such a device itself would be much more expensive than the prior art devices, which for the rest is not an evi- dent fact. Other interesting properties of diamond is a very high thermal conductivity and high charge mobility.
Besides the desire to provide a contact having low forward losses in the on-state of the device, it would also be interesting to combine a good blocking function and a low ohmic function in the same contact.
A semiconductor device addressing these problems have recently been suggested by the applicant in the Swedish patent applica- tion 00001 15-6, but that application is still unpublished, so that the following features of the device according to the present invention in common with the device according to that application are placed in the characterising part of the appended independent claim 1 :
"Said first layer is doped by dopants assuming such deep energy levels in the semiconductor material of said layer that the majority thereof will not be thermally activated at working temperature of the device, that the device comprises an irradiation source adapted to emit radiation of an energy being high enough for activating said dopants of the first layer for making the device conducting in a forward biased state thereof with a voltage of the same type as the doping of said first layer applied to said contact."
"Voltage of the same type as the doping of said first layer" means that said contact will be connected to a potential being negative with respect to another contact of the device when said doping type of the first layer is n, and oppositely if the doping type of the first layer is p.
A Schottky-barrier contact in combination with such deep energy level doping makes it possible to obtain a blocking function of the contact thanks to the Schottky-barrier when the semiconductor material next thereto is not irradiated and a low ohmic function when the region of the semiconductor material next to the con- tact is irradiated.
It is here also to be mentioned that this way of doping, which is particularly interesting in the case of diamond as a semiconductor layer in a semiconductor device, since there are no shallow dopants for diamond, i.e. with dopants being thermally activated at the operation temperature of the device in question, so that some devices of interest for other materials may not be envisaged for diamond, has been disclosed in the unpublished Swedish patent application No. 9903149-4 of the applicant for the doping of the active layers of a semiconductor device. This method of deep energy level doping of diamond combined with activation by irradiation makes it possible to benefit from the superior properties of diamond in semiconductor devices of this type. Accordingly, the active layers in question may hold a very high voltage when not irradiated thanks to the high breakdown field strength of diamond, but conduct a high current with a low on-state voltage and thereby low losses when irradiated.
However, although the contact of such a semiconductor device is advantageous it may be improved with respect to the ability to withstand high electric fields in the forward blocking state of such a device. For the different types of contacts disclosed in said un- published Swedish patent application 0000115-6, it has been estimated that the electric field at the contact of such a device has to be reduced considerably for being able to construct a component with a reasonable thickness able to hold voltages in the re- gion of several kV. It may as an example be mentioned that in the case of diamond as said semiconductor material the electric field in question has to be reduced 100-1000 times at the contact with respect to the average electric field over the thickness of the device in order to design a device which can block approximately 50 kV over a thickness of 100 μm.
SUMMERY OF THE INVENTION
The object of the present invention is to provide a semiconductor device of the type defined in the introduction, which finds a solution to the problem of too high electric fields at said Schottky- barrier contact.
This object is according to the invention obtained by providing a device according to the appended independent claim 1.
By arranging such a second layer doped with dopants continuously activated at said working temperature close to said first layer the electric field applied to the device in the forward block- ing state thereof may be concentrated to a region between this second layer and at least a further layer of the device not being said first layer, so that the electric field in said first layer next to said contact may be reduced. Accordingly, it will be possible to benefit from the superior characteristics of a device having a contact of this type with respect to low losses in the forward conducting state while still obtaining a blocking capability not being destroyed by a low blocking capability of the contact enabling the production of reasonably thin devices also for devices having to hold higher voltages in the forward blocking state thereof.
According to a preferred embodiment of the invention it is achieved that the dopants of the second layer are continuously activated at said working temperature by ensuring that the majority of the dopants of said second layer are thermally activated at said working temperature of the device.
According to another preferred embodiment of the invention the dopants of said second layer assume such deep energy levels in the semiconductor material of said layer that the majority thereof will not be thermally activated at working temperature of the device, and the device comprises an irradiation source adapted to substantially continuously subject said second layer to radiation of an energy being high enough for activating said dopants of the second layer. This embodiment is particularly advantageous for semiconductor materials for which it is difficult to find candidates for dopants thermally activated at said working temperature, such as for example n-type dopants for diamond.
According to another preferred embodiment of the invention, which constitutes a further development of the embodiment last mentioned, the dopants of said second layer have a lower acti- vation energy than the dopants of the first layer, and said irradiation source adapted to subject the second layer to irradiation is adapted to emit radiation of an energy being lower than the activation energy of the dopants of the first layer. This means that the radiation from the irradiation source for activating the dopants of the second layer may not influence the dopants of the first layer and by that the switching behaviour of the device. Another way to achieve this is to arrange means for optically shielding said first layer from the radiation to which the second layer is subjected for activation of the dopants therein. In such a case it will even be possible to use dopants for said second layer located at a deeper energy level than the dopants of the first layer.
According to another preferred embodiment of the invention the device comprises an intrinsic layer laterally separating said first and second layers. This is very advantageous, since this will reduce lateral internal fields otherwise formed at the junction of the first and second layer when depleted. Such internal fields may result in unacceptably high leakage currents between the first layer and the contact in the forward blocking state of the device for comparatively low operation voltages. However, this is solved by the arrangement of the intrinsic layer, which means that the depleted charge of the different sides of the junction between the first and second layer will be separated by a large distance and thus the electric field will be reduced.
According to another preferred embodiment of the invention said second layer constitutes one of the doped layers of a pin-diode connected in anti-parallel with a charge carrier conducting path formed between two terminals of the device through said first layer when irradiated. This structure is very advantageous, since it means on one hand that the electrical field will be concentrated to the depleted layer of the pin-diode and it will be low at the contact, so that possibly inherent high breakdown field strength of said semiconductor material may be fully utilised, and on the other an anti-parallel diode will be integrated in the same device as a switch, which results in a reduced number of separate components used in converter station applications, such as HVDC (High Voltage Direct Current) stations.
According to another preferred embodiment of the invention the metal of the contact layers has a work function being for a) substantially higher than the electron affinity of the material of the semiconductor layer and for b) substantially lower than the sum of on one hand the band gap between the conduction band and the valence band and on the other the electron affinity of said semiconductor material. The device comprises means promoting tunnelling of charge carriers through said Schottky-barrier when said first semiconductor layer is irradiated. Such means may for example be ion impact defects. This means will make it possible to be able to tunnel through a higher and wider barrier than oth- erwise improving the tunnel probability for a barrier of a certain width, i.e. a certain intensity of the irradiation source, or reducing the required intensity of the irradiation source while maintaining the tunnel probability. Such a contact will have attractively low conduction losses, but it is very sensitive to high electric fields, which however is taken care of by the reduction of the electric field in said first layer next to said contact in accordance with the present invention.
According to another preferred embodiment of the invention said semiconductor material is a wide band gap material, i.e. a material having an energy gap between the conduction band and the valence band exceeding 2 eV. Such materials are particularly preferable when high voltages are to be handled by the device, since the breakdown field strength of such devices is high. Furthermore, it is easier to find energy levels for dopants being deep enough for not being activated at working temperature of the de- vice for such materials. Examples of these materials are SiC, diamond, GaN.
The invention also comprises a semiconductor device of the type defined in the introduction, which comprises two first semicon- ductor layers doped with dopants of different doping types connected in series between two terminals of the device and a said contact to each of the first layers and a second layer close to each first layer, said second layers are doped according to the opposite type to that of the respective first layer adjacent thereto for concentrating the electric field to a region between each second layer and at least a further layer of the device not being said first layer in a forward blocking state of the device reducing the electric field in the respective first layer next to the contact associated therewith. The advantages of the field reduction close to the contacts are in such a device the same as for the device of the type according to appended claim 1 with a special contact formed through the particular way of doping said first layer there. Accordingly, this device may have a first layer with dopants on such energy levels that the majority thereof are thermally acti- vated at the working temperature of the device. The invention also comprises the uses according to the appended use-claims, and the advantages thereof resides in the possibility to use materials as diamond while obtaining good contact functions and utilising the inherent properties of such materials, in particularly diamond and SiC.
Further advantages as well as advantageous features of the invention will appear from the following description and the other dependent claims.
BRIEF DESCRIPTION OF THE DRAWINGS
With reference to the appended drawings, below follows a specific description of preferred embodiments of the invention cited as examples.
In the drawings:
Fig 1 is a schematic cross-section view of a type of semiconduc- tor device to which the present invention may be applied,
Fig 2 is an energy band diagram of a contact metal layer adjacent to a semiconductor layer doped with deep level dopants in the form of donors in a forward blocking state of a device ac- cording to the present invention,
Fig 3 is a view corresponding to Fig 2 in a conducting state of the device,
Fig 4 is a cross-section view of a device according to a first preferred embodiment of the invention in the forward blocking state,
Fig 5 is a view corresponding to Fig 4 of that device in the forward conducting state,
Fig 6 is a view corresponding to Fig 4 of that device in the reverse biased state, Fig 7 is a schematically illustration of the operation of the device according to Fig 4,
Fig 8 is a view corresponding to Fig 4 of a device according to a second preferred embodiment of the invention,
Fig 9 is a view illustrating the distribution of the electric field in the device according to Fig 8 close to one of the two active con- tacts thereof,
Fig 10 is a view corresponding to Fig 4 of a device according to a third preferred embodiment of the invention,
Fig 1 1 is a view corresponding to Fig 4 of a device according to a fourth preferred embodiment of the invention,
Fig 12 is a view corresponding to Fig 4 of a device according to a fifth preferred embodiment of the invention, and
Fig 13 is a view corresponding to Fig 4 of a device according to a sixth preferred embodiment of the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
A device of the type to which the present invention is particularly applicable will now be described with reference to Fig 1 , but reference is in this connection also made to the Swedish patent ap- plication number 9903149-4, which more thoroughly describes the construction and function of that device. The device has two terminals 1 , 2 for connecting the device to an electric current path. The device has also one first semiconductor layer 3 of diamond doped by dopants assuming such deep energy levels that the majority thereof will not be thermally activated at room temperature, and this means that the activation energy of said dopants should be higher than 0.3 eV. The device comprises an- other first semiconductor layer 4 of diamond doped according to the same conditions as the first layer 3. A further layer 5 of intrinsic diamond is arranged between the two first layers 3 and 4. The layer 5 has typically a thickness of 3 to 200 μm, whereas the two doped layers 3, 4 have a thickness of 1 to 20 μm. It is within the scope of the present invention to dope the two layers 3, 4 with the dopants of the same conductivity type, n or p, but it is preferred to dope them with dopants of opposite conductivity type, and we therefor in the following assume that the layer 3 is doped with acceptors, whereas the layer 4 is doped with donors. A metal contact 6, 7 connects the respective terminal to the diamond layer 3 and 4, respectively. The material of the metal contact is chosen so that a Schottky-barrier of a preferable height and width is created at the interface between the contact and the respective first layer, which will be discussed below with reference to Fig 2 and 3 and is more in detail discussed in the Swedish patent application 00001 15-6. The metal contact has preferably vents 8 for allowing penetration of light from a light source 9 through the metal contact and into the respective first layer 3, 4. It should be mentioned that characteristics of this device and the rest of the devices shown in the figures having nothing to do with the present invention, such as passivation layers have been omitted for the sake of clearity.
The light sources 9 are adapted to illuminate a region under the respective metal contact for activation of dopants occupying deep energy levels there from the opposite side of the device, so that the light source shown to the right in the figure will activate the acceptors in the layers 3 underneath the anodes through il- lumination through the intrinsic layer 5, whereas the light source shown to the left will illuminate and activate donors in the layers 4 underneath the cathodes of the device. A voltage source 30 and a load 31 for the circuit, to which the device is connected, are also schematically indicated in this figure. The acceptors in the layer 3 occupy levels at a considerable distance to the valence band of the diamond lattice, whereas the dopants of the first layer 4, which are assumed to be donors, occupy levels lo- cated at a substantial distance below the conduction band of the diamond lattice. Said energy distance is preferably above 0.3 eV, so that practically none of these dopants are thermally activated at room temperature. This means that the first layers 3 and 4 will without said illumination have practically no free charge carriers for transport between the anode and the cathode and they will act as intrinsic layers so that the switch should be able to block very high voltages supplied thereacross in any direction as long as no charge carriers are injected at the contacts, and the pres- ent invention aims at taking care of that issue.
When the layers 3 and 4 are illuminated by light having an energy exceeding the activation energy of the dopants, these will be activated. When the activation energy of said dopants is lower than 2.5 eV a standard high power semiconductor laser may be used as said irradiation source 9, since it has only to deliver light with an energy above said activation energy, which for instance for donors of N is 1 .7 eV, which should be compared with the energy needed to lift electrons from the valence band to the con- duction band in diamond (5.4 eV).
Fig 2 illustrates the choice of a metal for the contact 6 creating a Schottky-barrier between the contact and the first layer 4. The lower limit of the conduction band and the upper limit of the va- lence band are referenced by 26 and 27, respectively. It is schematically illustrated how defects 10 are introduced in the first layer close to the metal contact for promoting tunnelling of charge carriers through the Schottky-barrier in the forward conducting state illustrated in Fig 3, in which the donors 1 1 are acti- vated. In the forward blocking state illustrated in Fig 2 the donors 1 1 are not activated, but also in this state a considerable leakage current, which would be too high in case of utilising the blocking capability of the other layers of the device, would result if not particular measures are taken, and such measures form the basis of the present invention. The concentration of dopants on deep levels in the region of the layer 4 closest to the contact is above 1015 cm"3, preferably above 1017 cm"3 to achieve good absorption. The same is valid for the corresponding region of the layer 3.
A device according to a first preferred embodiment of the inven- tion for addressing this problem is illustrated in Fig 4. The parts having correspondence in Fig 1 are provided with the same reference numerals. It is illustrated how a second layer 12, 13 doped according to the opposite type to that of the first layer 4 and 3, respectively, are located laterally close to the respective first layer. The two layers 12, 13 are doped with dopants being thermally activated at working temperature of the device. Accordingly, the layer 12 is a p-type layer and the layer 13 is an n- type layer. A metal contact 33, 34 contacts the respective terminal to the diamond layer 12 and 13, respectively. These layers are separated by the intrinsic layer 5, which means that a pin-diode, which will be reverse-biased when the switch is forward-biased, will be formed in this way. This means that the major part of the electric field, as illustrated by the arrows 14, at the forward blocking state of the device, will be distributed in the intrinsic re- gion between the two thermally activated layers 12 and 13. Accordingly, the field will be reduced at the contacts 6. It is namely no problem to design a device of this type, which can block approximately 50 kV over a 100 μm thick layer of diamond, but a contact of the type illustrated in Fig 2 and 3 having the low con- duction losses aimed at may only take an electric field being at the most 1 /100-1 /1 000 of the average electric field over such a layer. Otherwise expressed, the device would only be able to block a voltage of less than 100 V if the thickness were 100 μm and the electric field were homogeneously distributed. The leak- age current through the contact will otherwise be unacceptably high. However, this is taken care of thanks to the field concentration in the intrinsic region reducing the electrical field in the layers 4 and 3 next to the contact 6.
The forward conducting state of the device is illustrated in Fig 5. The light sources 9 do then irradiate not only the regions of the first layers 3, 4 closest to the respective contact for activating them, but they do also irradiate the entire first layers 3 and 4 for activating the deep energy level dopants therein. This means that the switch formed by the structure of the layers 4, 5 and 3 will form a low resistance current path 28 between the two contacts 6. (Similar to a forward biased pin-diode).
It is illustrated in Fig 6 how the pin-diode 12, 5, 13 of this device will be conducting (29) in the reverse-biased state of the switch 4, 5, 3. Accordingly, the device has a function being identical to an IGBT 15 and a diode 16 connected in anti-parallel therewith as illustrated in Fig 7, so that it may be used in a current valve of a converter, possibly connected in series with a plurality of such devices for being able to jointly handling very high voltages and being switched simultaneously for function as one single switch. This could involve considerable cost savings for such a valve.
Fig 8 illustrates a device differing from that according to Fig 4 by the introduction of an intrinsic layer 17 laterally separating the respective first and second layers. It has namely been found that if the first layer 3 and/or the first layer 4 is strongly doped a large internal field will be formed as these regions are depleted, so that the leakage current through the contact will then be unacceptably high. However, this is addressed by introducing said intrinsic layer 17 through which the depleted charge of the different sides of the junction between the second layer and the first layer associated therewith will be separated by a larger distance and thus the electric field will be reduced.
The electric field E close to the contact 6 at the first layer 4 ver- sus the distance d away from this contact 6 is illustrated in Fig 9. It is shown how the electric field is kept down at the interface between the contact 6 and the layer 4 thanks to the arrangement of the intrinsic layer 17. The entire potential difference between the layer 12 and the layer 4 had otherwise to be taken over a short distance at the junction between these layers resulting in a high local electric field. The same type of reduction of the field in the depletion region can be achieved by a graded doping concentration profile.
Fig 10 illustrates a device according to a third preferred embodi- ment of the invention, which has a thyristor-like structure formed by a pin-structure 18 vertically separating the two first semiconductor layers 4, 3 with the p-type layer 19 of the pin-structure next to the first layer 4 with dopants of n-type and the n-type layer 20 next to the first layer 3 with the dopants of p-type. An intrinsic layer 21 separates the layers 19 and 20. Furthermore, the layers 19 and 20, which have their dopants substantially continuously activated, preferably by being thermally activated, also laterally surround the respective first layer and extend to the metal contact 6 there. The shorts 22, 23 so created will guaran- tee a region free from external field for the first layers 4 and 3 when they are not irradiated and the device is forward biased as shown in Fig 10. When this device is irradiated it will work as a switched on thyristor. Possibly a switch-on UV-pulse has to be applied during or after the irradiation has been ramped up for triggering the thyristor. The shorts 22, 23 of the layers 19 and 20 to the contacts could be weakly doped so that under irradiation the excess of charge carriers of opposite sign will effectively quench the conducting of the short circuiting channels between the respective layers 19, 20 and the contacts. When irradiation is terminated there will be a fast switch-off due to high injection of minority charge carriers into the respective first layers 4, 3. This means that it may preferably be used where a fast switch-off (turn-off) is desired, for instance as a switch for Pulse Width Modulation (PWM) in converters. The switch-off time may here be as short as 10-100 ns.
Fig 1 1 schematically illustrates a device according to a fourth preferred embodiment of the invention differing from the device according to Fig 4 only by the fact that one of the second layers 24, in this case the one doped with donors, is doped with dopants assuming such deep energy levels in the semiconductor material of that layer that the majority thereof will not be ther- mally activated at working temperature of the device. This device comprises an irradiation source 25 schematically indicated adapted to substantially continuously subject said second layer 24 to radiation of an energy being high enough for activating the dopants of that layer, so that the same function as for the device according to Fig 4 will be obtained for this device. The dopants of the layer 24 has to have a lower activation energy than the dopants of the first layer 3 next thereto and the energy of the radiation from the light source 25 has to be lower than the activa- tion energy of the dopants of the first layer 3 or means for optically shielding the first layer 3 from the radiation to which the second layer 24 is subjected have to be arranged for avoiding that the radiation from the irradiation source 25 disturbs the function of the device. This embodiment is particularly advanta- geous for and applicable to semiconductor materials being difficult to dope with dopants being thermally activated at normal working temperatures of a device of this type. This is for example true for diamond, for which there is today no established thermally achieved donors available that can be incorporated during diamond growth. It is then preferred to dope the layer 24 with phosphorus (P) having an activation energy of 0.6 eV, whereas the doping of the n-type first layer 4 is realised through nitrogen (N) having an activation energy of 1 .7 eV. It would of course be possible to have the same dopants for the layers 4 and 24, but a shielding described above has then to be introduced. It would of course also be possible to have the p-type second layers doped with dopants on deep energy levels, but dopants being thermally activated at working temperatures of the device would be preferable for the second layers for simplifying the operation of the de- vice.
Fig 12 schematically illustrates a device according to a fifth preferred embodiment of the invention differing from the device according to Fig 4 only by the fact that holes 32 have been etched into the semiconductor material at the locations for said second layers 12, 13, which have then been formed by implanting dopants into the walls of the holes. This enables a location of the second layers deeper into the material, so that the field will be easier reduced close to the contacts 6, 7. Metal contacts 35, 36 filling out the holes contact the second layers 12 and 13, respectively.
Fig 13 schematically illustrates a device according to a sixth preferred embodiment of the invention differing from the device according to Fig 4 only by the fact that said second layers 12, 13 here are formed by another semiconductor material than the first layer for forming a heterostructure. They may for example be of Si or SiC appropiately doped in the case of diamond as the semiconductor material for the rest of the device. The second layers may be formed by carrying out an etching as for forming the holes 32 of the embodiment according to Fig 12 and then epi- taxially growing said second layers. Also this embodiment enables a location of the second layers deeper into the material, so that the field will be easier reduced close to the contacts 6, 7. Metal contacts 37, 38 contact the second layers 12 and 13, respectively.
The invention is of course not in any way restricted to the preferred embodiments described above, but many possibilities to modifications thereof would be apparent to a man with ordinary skill in the art without departing from the basic idea of the inven- tion as defined in the appended claims. All definitions of materials in this disclosure of course also include inevitable impurities except for intentional doping, so that the layers provided with deep level dopants may also contain a small concentration of thermally activated dopants unintentionally built into the structure during the processing thereof.
"Semiconductor materials" as used in the claims is to be interpreted broadly and also include an insulator as diamond being doped with deep level dopants, which may be activated by irra- diation. It is also pointed out that irradiation does not necessarily have to be illumination, but it is also conceivable to use for instance beams of electrons. Furthermore "metal" is in this disclosure defined as covering all materials being electric conducting. This has to be considered when interpreting the claims.
It is also stressed that the semiconductor device according to the present invention may also have other combinations of layers than those described above, and the invention is mainly directed to the layers close to a Schottky-barrier contact of any type of semiconductor device having the features of appended claim 1.
"Continuously activated" with respect to the dopants of the second layer as defined in the claims does only mean that the dopants are continuously activated when the device is in operation, but the activation may well cease when the device is taken out of operation, as will be the case for a device according to Fig 1 1 , for which the additional irradiation source will be switched off and the dopants of the second layer will then not be activated when the device is not in operation.

Claims

CLAIMS:
1. A semiconductor device comprising a first semiconductor layer (3, 4) doped according to either a) n-type or b) p-type and a metal layer (6) forming a Schottky-barrier contact thereto, characterized in that said first layer is doped by dopants assuming such deep energy levels in the semiconductor material of said layer that the majority thereof will not be thermally activated at working temperature of the device, that the device comprises an irradiation source (9) adapted to emit radiation of an energy being high enough for activating said dopants of the first layer for making the device conducting in a forward biased state thereof with a voltage of the same type as the doping of said first layer (3, 4) applied to said contact, and that the device further comprises at least a second layer (12, 13, 24) doped according to the opposite type to that of said first layer with dopants continuously activated at said working temperature and located close to said first layer for concentrating the electrical field to a region be- tween this second layer and at least a further layer of the device not being said first layer in a forward blocking state of the device corresponding to said forward biased state without said irradiation, reducing the electrical field in said first layer next to said contact.
2. A device according to claim 1 , characterized in that the majority of the dopants of said second layer (12, 13) are thermally activated at said working temperature of the device.
3. A device according to claim 1 , characterized in that the dopants of said second layer (24) assume such deep energy levels in the semiconductor material of said layer that the majority thereof will not be thermally activated at working temperature of the device, and that the device comprises an irradiation source (25) adapted to substantially continuously subject said second layer to radiation of an energy being high enough for activating said dopants of the second layer.
4. A device according to claim 3, characterized in that the dopants of said second layer (24) have a lower activation energy than the dopants of the first layer (3, 4), and that said ir- radiation source (25) adapted . to subject the second layer to radiation is adapted to emit radiation of an energy being lower than the activation energy of the dopants of the first layer.
5. A device according to claim 3, characterized in that it com- prises means for optically shielding said first layer (3, 4) from the radiation to which the second layer (24) is subjected for activation of the dopants therein.
6. A semiconductor device comprising a first semiconductor layer (3, 4) doped according to either a) n-type or b) p-type and a metal layer (6) forming a Schottky-barrier contact thereto, characterized in that it comprises two first semiconductor layers (3, 4) doped with dopants of different doping types connected in series between two terminals of the device and a said contact (6, 7) to each of the first layers and a second layer close to each first layer, that said second layers (12, 13, 24) are doped according to the opposite type to that of the respective first layer adjacent thereto for concentrating the electric field to a region between each second layer and at least a further layer of the device not being said first layer in a forward blocking state of the device reducing the electric field in the respective first layer next to the contact associated therewith.
7. A device according to any of the preceding claims, characterized in that said second layer (12, 13) is formed by an implant of dopants into walls of a hole (32) etched into said semiconductor material.
8. A device according to any of claims 1 -6, characterized in that said second layer (12, 13) is made of another semiconductor material than the first layer for forming a heterostructure.
9. A device according to any of the preceding claims, characterized in that it comprises an intrinsic layer (17) laterally separating said first and second layers.
10. A device according to any of claims 1 -8, characterized in that it comprises a graded or compensation doped region sepe- rating said first and second layers.
1 1 . A device according to any of claims 1 -5 and 7-10, characterized in that it comprises two first semiconductor layers (3, 4) doped with dopants on deep energy levels of different doping types connected in series between two terminals of the device and a said contact (6) to each of the first layers and a said second layer close to each first layer.
12. A device according to claim 6 or 1 1 , characterized in that it comprises a third layer (5) of a substantially intrinsic semiconductor material arranged between said two first semiconductor layers.
13. A device according to any of the preceding claims, characterized in that said second layer (12, 13, 24) constitutes one of the doped layers of a pin-diode connected in anti-parallel with a charge carrier conducting path formed between two terminals of the device through said first layer when irradiated.
14. A device according to claim 1 1 , characterized in that it com- prises a pin-structure (18) vertically separating said two first semiconductor layers with the p-type layer (19) of the pin- structure next to the first layer (4) with dopants of n-type and the n-type layer (20) next to the first layer (3) with dopants of p-type for forming a thyristor-like structure.
15. A device according to claim 14, characterized in that the second layer (19, 20) arranged next to the respective first layer also laterally surrounds the latter and extends to said metal contact (6).
16. A device according to any of the preceding claims, charac- terized in that the metal of the contact layer (6) has a work function being for a) substantially higher than the electron affinity of the material of the semiconductor layer and for b) substantially lower than the sum of on one hand the band gap between the conduction band and the valence band and on the other the electron affinity of said semiconductor material.
17. A device according to claim 16, characterized in that substantially higher and substantially lower means a difference of at least 0.3 eV.
18. A device according to claim 16 or 17 depending on claim 1 , characterized in that it comprises means promoting tunnelling of charge carriers through said Schottky-barrier when said first semiconductor layer is irradiated.
19. A device according to claim 18, characterized in that said means is formed by defects (10) introduced close to the contact layer.
20. A device according to claim 19, characterized in that it comprises a third thin layer arranged between said semiconductor layer and the contact layer and being of a material introducing said defects for promoting tunnelling of charge carriers between the contact layer (6) and said first semiconductor layer (3, 4).
21 . A device according to claim 19, characterized in that said defects (10) are formed by radiation damage in a thin region of the semiconductor layer next to the contact layer (6).
22. A device according to any of the preceding claims, characterized in that said semiconductor material is a wide band gap material, i.e. a material having an energy gap between the conduction band and the valence band exceeding 2 eV.
23. A device according to any of the preceding claims, charac- terized in that said semiconductor material is diamond.
24. A device according to any of claims 1 -22, characterized in that said semiconductor material is SiC.
25. A device according to claim 4, 1 1 and 23, characterized in that said second layer (24) is doped by phosphorus (P) and the n-type first layer (4) at the other contact is doped by nitrogen (N).
26. A device according to claim 1 , characterized in that said irradiation source (9, 25) is a light source.
27. A device according to any of the preceding claims, characterized in that at least the region of said first semiconductor layer closest to the contact layer is highly doped with dopants on deep energy levels.
28. A device according to claims 22 and 26, characterized in that the concentration of dopants on deep energy levels in said region of the first semiconductor layer is above 1015cm"3, preferably above 1017cm"3.
29. A device according to any of the preceding claims, characterized in that it is adapted to hold voltages above 1 kV, pref- erably above 10 kV, when said first semiconductor layer is not irradiated by said irradiation source.
30. A use of a device according to any of the preceding claims as a switch in a converter.
31 . A use of a device according to any of the claims 1 -29 as a switch for switching high powers and/or high voltages and/or high currents.
32. A use of a device according to any of the claims 1 -29 as a switch in a high voltage converter operating at a voltage in the region of 10 kV-500 kV.
33. A use of a device according to any of the claims 1 -29 as a switch in a current valve of a converter.
34. A use of a device according to any of the claims 1 -29 as a switch in a current valve of a converter, in which a plurality of said devices are connected in series for together holding the voltage across said current valve in the blocking state thereof.
35. A use of a device according to any of the claims 1 -29 as a switch for switching high powers and/or high voltages and/or high currents in an arrangement for protection of an equipment for electric power applications.
PCT/SE2001/001167 2000-05-22 2001-05-22 A semiconductor device WO2001091187A1 (en)

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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2006027669A1 (en) * 2004-09-10 2006-03-16 Element Six Limited Switching device
CN102790077A (en) * 2012-08-24 2012-11-21 电子科技大学 Insulated gate bipolar transistor
TWI594443B (en) * 2016-08-29 2017-08-01 雋佾科技有限公司 Shottcky diode structure and method for forming the same
CN107924843A (en) * 2015-06-09 2018-04-17 Abb瑞士股份有限公司 Method for manufacturing the edge termination for silicon carbide power semiconductor devices

Citations (4)

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Publication number Priority date Publication date Assignee Title
US4783688A (en) * 1981-12-02 1988-11-08 U.S. Philips Corporation Schottky barrier field effect transistors
US5158909A (en) * 1987-12-04 1992-10-27 Sanken Electric Co., Ltd. Method of fabricating a high voltage, high speed Schottky semiconductor device
US5166760A (en) * 1990-02-28 1992-11-24 Hitachi, Ltd. Semiconductor Schottky barrier device with pn junctions
US5914500A (en) * 1997-01-21 1999-06-22 Abb Research Ltd. Junction termination for SiC Schottky diode

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4783688A (en) * 1981-12-02 1988-11-08 U.S. Philips Corporation Schottky barrier field effect transistors
US5158909A (en) * 1987-12-04 1992-10-27 Sanken Electric Co., Ltd. Method of fabricating a high voltage, high speed Schottky semiconductor device
US5166760A (en) * 1990-02-28 1992-11-24 Hitachi, Ltd. Semiconductor Schottky barrier device with pn junctions
US5914500A (en) * 1997-01-21 1999-06-22 Abb Research Ltd. Junction termination for SiC Schottky diode

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2006027669A1 (en) * 2004-09-10 2006-03-16 Element Six Limited Switching device
US8053783B2 (en) 2004-09-10 2011-11-08 Element Six Limited Switching device
CN102790077A (en) * 2012-08-24 2012-11-21 电子科技大学 Insulated gate bipolar transistor
CN102790077B (en) * 2012-08-24 2014-12-10 电子科技大学 Insulated gate bipolar transistor
CN107924843A (en) * 2015-06-09 2018-04-17 Abb瑞士股份有限公司 Method for manufacturing the edge termination for silicon carbide power semiconductor devices
TWI594443B (en) * 2016-08-29 2017-08-01 雋佾科技有限公司 Shottcky diode structure and method for forming the same

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AU2001260938A1 (en) 2001-12-03

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