GB1568526A

GB1568526A - Reverse switching rectifier device

Info

Publication number: GB1568526A
Application number: GB1099/77A
Authority: GB
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1976-01-22
Filing date: 1977-01-12
Publication date: 1980-05-29
Also published as: FR2339256A1; IN144812B; BE850349A; CA1070434A; DE2701991A1

Description

(54) REVERSE SWITCHING RECTIFIER DEVICE (71) We, WESTINGHOUSE ELEC TRIC CORPORATION of Westinghouse Building, Gateway Center, Pittsburgh, Pennsylvania, United States of America, a company organised and existing under the laws of the Commonwealth of Pennsylvania, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates generally to semiconductor switching devices and more particularly to two terminal silicon thyristor devices or reverse switching rectifier (RSR) devices.

Prior art RSR device 100 has a general structural configuration shown in Figure 1 wherein a body of semi-conductor material in the form of a wafer 110 is doped to provide four alternate semiconductivity zones. An anode-emitter zone 112 of P-type semiconductivity extends from one major surface 111 of the wafer 110 into the semiconductor material to meet a middle zone 114 of N-type semiconductivity. PN junction 113 is formed at the interface of zones 112 and 114. Similarly, P-type base zone 116 forms PN junction 115 with zone 114. Base zone 116 extends from PN junction 115 to cathode-emitter zone 118 of N-type semiconductivity where PN junction 117 is formed. In addition, zone 116 typically extends past zone 118 to the outer portion of major surface 119.

A supporting anode electrode 120 is affixed to major surface 111 to provide good electrical and thermal contact to zone 112 as well as to provide mechanical support for the wafer 110. Typical examples of metals used for the electrode 120 are molybdenum and tungsten, which are preferred for their favourable expansion properties.

Typically, a shorted emitter construction is used whereby a cathode electrode 122 is affixed to major surface 119 contacting the cathode-emitter zone 118 and a peripheral portion of the base zone 116 surrounding zone 118. The electrode 122 may be provided, for example, by aluminium deposition in a known manner.

The wafer 110 has a beveled edge 125 produced in a known manner in order to optimize electrical characteristics. Disposed on the beveled edge 125 is an insulating and protective coating 126. The coating composition and manner of application is known in the art, a high temperature curing silicone varnish being an example of a suitable coating material.

RSR device 100 of the prior art shown in Figure 1 operates as an electrical current switch. Briefly described, RSR device 100 blocks voltage in both directions unless the device 100 is "turned on" or "fired" in which case it carries current in the forward direction as indicated by the arrow 127.

When RSR device 100 is forward biased as indicated by the polarity marks + and -, it may be turned on by impressing a forward voltage pulse across electrodes 120 and 122, which pulse has a sufficiently high DV/DT to cause the device 100 to turn on. Typically device 100 will turn on when pulsed with a voltage of greater than 5000 volts per microsecond.

It has been found that device structures of the prior art, as shown in Figure 1, do not turn on uniformly along PN junction 115, rather such prior art devices initially turn on in a relatively small region located under an edge of the cathode-emitter zone 118 causing hot spotting and failure of the device.

An example of a typical failure mode is illustrated in Figure 1 in which emission of electrons from zone 118 into zone 116 more readily occurs in the dashed region 128 causing initial conduction of current through PN junction 115 to pass through the rela tively small area of region 128 as illustrated by path X. The very high current density along path X causes localized heating which permanently destroys the blocking capability of Pn junction 115. Uniform firing of device 100 is illustrated by paths Y and Z which may or may not occur with device 100 due to microvariations in the resistivity of standard silicon bar stock.

It is the principal object of the present invention to provide an RSR device structure which assures uniform firing in spite of microvariations in resistivity.

The invention resides broadly in a semiconductor switching device comprising: a base layer of a first conductivity type disposed on a layer of a second conductivity type; a triggering-emitter region and at least one cathode-emitter region penetrating into the surface of the base layer opposite the second conductivity layer; the triggeringemitter penetrating deeper into the base than the or each cathode-emitter, such that, the thickness of the base between said triggering emitter and said second conductivity layer is less than the thickness of the base between said or each cathode-emitter and said second conductivity layer; and a gate electrode bridging the PN junction interfacing said triggering-emitter and said base layer on a side of said triggeringemitter facing said cathode-emitter.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a vertical cross-sectional view of a device of the prior art; Figure 2 is a vertical cross-sectional view of one embodiment of the present invention; Figure 3 is a vertical cross-sectional view of a second embodiment of the present invention; and, Figure 4 is a vertical cross-sectional view of a presently preferred embodiment of the present invention.

Figures 2, 3 and 4 illustrate various embodiments of the present invention which are similar in many respects to prior art device 100 of Figure 1, similar parts being designated by similar numerals. Since a discussion of the similar parts is given above, the balance of the specification will be devoted to the structural differences between the present invention and prior art device 100.

A device 200 of the present invention is illustrated in Figure 2. A comparison of Figures 1 and 2 discloses differences in the structure of the emitter zones and associated electrodes. Prior art device 100 has a single N-type emitter zone 118 which is contacted by a single electrode 122, whereas device 200 has a segmented emitter and associated electrodes. Specifically, device 200 of the present invention has a ring-shaped cathode-emitter zone 230 and a circular triggering-emitter zone 232 disposed within but spaced apart from the ring of zone 230.

A ring-shaped cathode electrode 240 is affixed to major surface 219 of the wafer 210 contacting the cathode-emitter zone 230.

Although not essential to the operation of device 200, cathode-emitter electrode 240 preferably contacts a peripheral portion of base zone 216 thereby bridging PN junction 231 as shown. A circular triggering or gate electrode 242 is affixed to major surface 219 contacting triggering-emitter zone 232. It is essential to the operation of device 200 that gate electrode 242 be in contact with base zone 216 thereby bridging PN junction 233 which interfaces zones 216 and 232.

The internal operation of device 200 will now be described insofar as it differs from the internal operation of prior art device 100, external circuitry being the same. With a forward voltage impressed on device 200, as indicated by polarity marks + and -, firing will occur when the device 200 is pulsed in the same manner as prior art device 100. However, the firing of device 200 occurs in a uniform manner, since emission of electrons initially occurs in the area of base zone 216 between zone 232 and zone 214. This condition is achieved since base zone 214 has a thickness separating cathode-emitter zone 230 from middle zone 214 which exceeds the thickness of the portion of base zone 216 separating triggering-emitter zone 232 from middle zone 214.

Specifically, base zone 214 has a thickness designated by T1 separating zones 230 and 214. and a thickness designated by T2 separating zones 232 and 214, as shown in Figure 2. Typically, T, is in the range of 25 to 45 microns with about 30 microns being preferred, while T2 is in the range of 15 to 35 microns with about 20 microns being preferred.

Those skilled in the art will appreciate that a greater degree of emission will occur from zone 232 than from zone 230 due to the relative proximity of zone 232 to zone 214.

With the occurrence of emission from triggering-emitter zone 232, a current designated by path A flows from zone 232 through gate electrode 242 into base zone 216 in the direction of the negative potential of the cathode electrode 240. The current of path A flows radially outwards in a uniform fashion, thus causing uniform initial firing of cathode-emitter zone 230 to occur in a circular area along the edge of zone 230 facing gate electrode 242, as illustrated by path B. The firing spreads uniformly and rapidly outwards from path B to paths C and D, as schematically illustrated in Figure 2, until the device 200 is in full conduction.

The structure of the present invention is not limited to the embodiment of Figure 2, rather the invention may be practiced by other structures within the scope of the claimed subject matter, as for example, those structures of Figures 3 and 4.

Figure 3 depicts a device 300 of the present invention similar to device 200 of Figure 2 except with the emitter segments and associated electrodes transposed. Device 300 has a centrally-located, circularshaped cathode-emitter zone 330, which is surrounded by a ring-shaped triggeringemitter zone 332. A circular-shaped cathode electrode 340 is affixed to major surface 319 contacting zone 330. A ring-shaped gate electrode 342 is affixed to major surface 319 contacting zone 332 and bridging PN junction 333 thereby contacting a portion of base zone 316. The portion of base zone 316 contacted by gate electrode 342 preferably includes about 5 mils of base zone 316 as measured from the edge of zone 332 in the direction of zone 330.

The operation of device 300 is fully analogous to the operation of device 200 with the exception that firing is initiated at the outer edge of cathode-emitter zone 330 as schematically illustrated by path B. The firing spreads uniformly and rapidly inwards from path B to paths C and D, until the device 300 is in full conduction.

Figure 4 depicts a presently preferred device embodiment 400 similar to the embodiments of Figures 2 and 3 except that firing is caused to spread both inwards and outwards. Device 400 has a ring-shaped triggering-emitter zone 432, a circular cathodeemitter zone 430 disposed within zone 432, and a ring-shaped cathode-emitter zone 434 surrounding zone 432. A circular-shaped cathode electrode 440 is affixed to major surface 419 contacting zone 430. A ringshaped gate electrode 442 is affixed to major surface 419 contacting zone 432 and bridging PN junction 433 thereby contacting a portion of base zone 416, which portion includes at least 2 mils of base zone 416 on all sides of zone 432, 5 mils being preferred.

A ring-shaped cathode electrode 444 is affixed to major surface 419 contacting zone 434, as shown. Electrodes 440 and 444 are electrically connected in common by external circuit means as schematically illustrated in Figure 4. Interfacing P-type base zone 416 with N-type emitter zones 430, 432 and 434 are PN junctions 431, 433 and 435, respectively.

The operation of device 400 is fully analogous to the operation of devices 200 and 300 with the exception that firing is initiated at the outer edges of zone 430 and the inner edges of zone 434 as illustrated schematically by the paths B.

It will be apparent to those skilled in the art that a variety of structures are contemplated by the present invention and are within the scope of the appended claims, device structures 200, 300 and 400 representing several preferred embodiments. For example, more complex arrangements of the N-type emitter zones, such as the so-called "interdigitated" arrangements known in the thyristor art, may be used. It will be further apparent that a complementary device may be produced by interchanging the P and N regions of the abovedescribed device embodiments.

WHAT WE CLAIM IS: 1. A semiconductor switching device comprising: a base layer of a first conductivity type disposed on a layer of a second conductivity type; a triggering-emitter region and at least one cathode-emitter region penetrating into the surface of the base layer opposite the second conductivity layer; the triggering-emitter penetrating deeper into the base than the or each cathode-emitter, such that, the thickness of the base between said triggering emitter and said second conductivity layer is less than the thickness of the base between said or each cathodeemitter and said second conductivity layer; and a gate electrode bridging the PN junction interfacing said triggering-emitter and said base layer on a side of said triggeringemitter facing said cathode-emitter.

2. A device according to claim 1 wherein the thickness of said base zone between said cathode-emitter and said second conductivity layer lies in a range from 25 to 45 microns, and the thickness of said base zone between said triggering-emitter and said second conductivity layer is in a range from 15 to 35 microns.

3. A device according to claim 2 wherein the thickness between the second layer and said or each cathode emitter is substantially 30 microns and the thickness between the second layer and the triggering-emitter is substantially 20 microns.

4. A device according to any of the preceding claims wherein said gate electrode extends at least 2 mils away from said PN junction.

5. A device according to any of claims 1-3 wherein said gate electrode extends at least 5 mils away from said PN junction.

6. A semiconductor switching device substantially as hereinbefore described with reference to Figures 2-4 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. The structure of the present invention is not limited to the embodiment of Figure 2, rather the invention may be practiced by other structures within the scope of the claimed subject matter, as for example, those structures of Figures 3 and 4. Figure 3 depicts a device 300 of the present invention similar to device 200 of Figure 2 except with the emitter segments and associated electrodes transposed. Device 300 has a centrally-located, circularshaped cathode-emitter zone 330, which is surrounded by a ring-shaped triggeringemitter zone 332. A circular-shaped cathode electrode 340 is affixed to major surface 319 contacting zone 330. A ring-shaped gate electrode 342 is affixed to major surface 319 contacting zone 332 and bridging PN junction 333 thereby contacting a portion of base zone 316. The portion of base zone 316 contacted by gate electrode 342 preferably includes about 5 mils of base zone 316 as measured from the edge of zone 332 in the direction of zone 330. The operation of device 300 is fully analogous to the operation of device 200 with the exception that firing is initiated at the outer edge of cathode-emitter zone 330 as schematically illustrated by path B. The firing spreads uniformly and rapidly inwards from path B to paths C and D, until the device 300 is in full conduction. Figure 4 depicts a presently preferred device embodiment 400 similar to the embodiments of Figures 2 and 3 except that firing is caused to spread both inwards and outwards. Device 400 has a ring-shaped triggering-emitter zone 432, a circular cathodeemitter zone 430 disposed within zone 432, and a ring-shaped cathode-emitter zone 434 surrounding zone 432. A circular-shaped cathode electrode 440 is affixed to major surface 419 contacting zone 430. A ringshaped gate electrode 442 is affixed to major surface 419 contacting zone 432 and bridging PN junction 433 thereby contacting a portion of base zone 416, which portion includes at least 2 mils of base zone 416 on all sides of zone 432, 5 mils being preferred. A ring-shaped cathode electrode 444 is affixed to major surface 419 contacting zone 434, as shown. Electrodes 440 and 444 are electrically connected in common by external circuit means as schematically illustrated in Figure 4. Interfacing P-type base zone 416 with N-type emitter zones 430, 432 and 434 are PN junctions 431, 433 and 435, respectively. The operation of device 400 is fully analogous to the operation of devices 200 and 300 with the exception that firing is initiated at the outer edges of zone 430 and the inner edges of zone 434 as illustrated schematically by the paths B. It will be apparent to those skilled in the art that a variety of structures are contemplated by the present invention and are within the scope of the appended claims, device structures 200, 300 and 400 representing several preferred embodiments. For example, more complex arrangements of the N-type emitter zones, such as the so-called "interdigitated" arrangements known in the thyristor art, may be used. It will be further apparent that a complementary device may be produced by interchanging the P and N regions of the abovedescribed device embodiments. WHAT WE CLAIM IS:

1. A semiconductor switching device comprising: a base layer of a first conductivity type disposed on a layer of a second conductivity type; a triggering-emitter region and at least one cathode-emitter region penetrating into the surface of the base layer opposite the second conductivity layer; the triggering-emitter penetrating deeper into the base than the or each cathode-emitter, such that, the thickness of the base between said triggering emitter and said second conductivity layer is less than the thickness of the base between said or each cathodeemitter and said second conductivity layer; and a gate electrode bridging the PN junction interfacing said triggering-emitter and said base layer on a side of said triggeringemitter facing said cathode-emitter.