US2852448A - Crystal rectifiers and method - Google Patents

Description

P 1958 'J. J. DYMON ErAL 2,852,448

CRYSTAL RECTIFIERS AND METHOD Filed Sept. 1, 19155 INVENTOR. 0 55ENH/LQD 58/422215 1 By 050% J fim/a/r NJHW United States CRYSTAL RECTIFIERS AND METHOD Application September 1, 1955, Serial No. 532,006

4 Claims. (CL 204-29) The present invention relates to the manufacture of electric current rectifying devices, and more particularly deals with the processing of rectifiers utilizing a single crystal of strontium titanate as the rectifier body, and to such rectifiers as improved articles of manufacture.

Structurally, a crystal rectifier includes a body or layer of semiconductive material to which are attached two metal contacts serving as electrodes. One of the contacts is essentially ohmic and is designated as the base electrode, while the other of the contacts allows for heavy current flow in the forward direction and has a high reverse resistance, and serves as a rectifying junction or barrier. Numerous materials are known to be suitable for the semiconducting layer, including germanium, silicon, sintered bodies of titanium dioxide, and sintered bodies of alkaline earth titanates and mixtures thereof. For a complete description of rectifiers incorporating a semiconductive body primarily of an alkaline earth titanate in a reduced state, reference may be made to copending applications Serial No. 341,726 filed March 11, 1953, now Patent No. 2,820,184, Serial No. 365,987 filed July 3, 1953, now Patent No. 2,806,189, Serial No. 366,038 filed July 3, 1953, Serial No. 399,456 filed December 21, 1953, now Patent No. 2,796,564, all of which applications are assigned to the assignee of the present invention.

' Broadly it is an object of the present invention to provide improved rectifying devices of the aforesaid type. Specifically it is within the contemplation of the present invention to provide useful rectifiers, both of the point contact and area contact types, incorporating reduced single crystals of strontium titanate.

As detailed in the aforesaid application, ceramic bodies of reduced titanates of the alkaline-earth metals, particularly those titanates of barium, strontium, calcium and mixtures of barium, strontium and calcium are suitable for incorporation into rectifiers after appropriate reduction, surface treatment and the application of electrodes and counter-electrodes. Such ceramic semiconductive bodies include a large number of small crystalline particles or grains which are compacted and sintered together and may be characterized as highly polycrystalline. The orientation of the crystallographic axes of these particles or grains are random; and between particles or grains there exist grain boundaries Within which occurs the transition from the crystal or lattice orientation one grain to the crystal or lattice orientation of the next adjacent grain. As a result of the random orientation of the crystallographic axes, it is impossible for the material within the grain boundaries to show the high degree of atomic order associated with a crystalline material; rather such sintered bodies may be expected to show a high degree of disorder on an atomic scale. The occurrence of grain boundaries in the rectifying barrier or surface presents regions of weakness, that is defective spots or localized areas of low resistance which are likely to cause rectifier failure.

atent In accordance with the present invention, it has been 2 found that single crystals of strontium titanate are useful as the semiconducting material in rectifying devices, both for point contact rectifiers which are exceptionally useful for communications applications in the high frequency and microwave frequency ranges and for area contact rectifiers which are useful for power applications in converting comparatively large amounts of alternating current power into direct current power. The meaning accorded the term single crystal, and equivalent terminology, as applied to the semiconductive bodies of the present invention, is a body of practically useful size in which the crystallographic axes throughout the body have the same orientation. Although the individual grains of a compacted and sintered ceramic body may also be considered as single crystals, their individual small size precludes their useful application as semiconductor bodies and as such do not fall within the above definition.

Although rectifying devices incorporating sintered and compacted ceramic bodies prepared according to the teachings of the aforementioned applications have been found to exhibit good rectifier properties, numerous important advantages are realized by using single crystals as the semiconductive material, as compared to rectifiers employing polycrystalline semiconductive bodies. Included among the advantages which may be attributed to devices incorporating single crystals of strontium titanate, without limitation, are the absence of grain boundaries and the expected regions of weakness which may occur in rectifying barriers or layers; the assurance of attaining greater homogeneity in the chemical composition of the single crystal devices and in the electrical properties of such devices; the facility for a higher degree of manufacturing control in that the measured electrical properties of a single crystal of semiconductive material yields a better indication of the characters determining rectifier performance; and the provision of reater mobility or velocity for the electrical current carriers resulting in a lower resistivity which makes available a wider range of compositions having practically useful values of resistivity for rectifiers yet capable of formation thereon of effec tive barrier or surface layers. The above factors, alone and in combination, make possible more efiicient rectifier manufacture, and assure better performance, reliability, and provide wider fields of application.

The above brief description, as well as other objects, features, and advantages of the present invention will be best appreciated by reference to the following detailed description of several presently preferred but illustrative embodiments, when taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a perspective view, partly in section, showing an area contact or junction rectifier embodying features of the present invention;

Fig. 2 is a perspective view, partly in section, of the rectifier illustrated in Fig. 1 with the addition of a counter electrode;

Fig. 3 is a perspective view, partly in section, of a point contact type of rectifier embodying features of the present invention; and,

Fig. 4 is a curve showing the relationship between current and voltage during the formation of rectifying surfaces or junctions in the processing of area rectifiers in accordance with aspects of the present invention.

In accordance with the present invention, it has been found that strongly reduced single crystals of strontium titanate exhibit both point contact and area rectification; and when appropriate electrodes are formed in contact with a single crystalline strontium titanate body, usually by techniques well known per se, such rectifiers exhibit useful properties both for the detection of radio frequency energy and for the conversion of power.

Referring now specifically to Fig. 1, there is shown an area contact or junction rectifier which incorporates a single crystal of strontium titanate which has a thermally and anodically conditioned surface 10a, a lead or manganese dioxide coating 12 formed in electrical contact with and coextensive with the conditioned surface 101:, a further conductive coating 14 coextensive with the coating 12 and a base electrode 18'coextensive with the surface of the single crystalline body 10 remote from the conditioned surface 10a. Optionally, and as shown in Fig. 2, a further layer 16 may be superimposed upon the layer 12 to provide a counter electrode; the further layer 16 may be made of a soft metal, such as lead or cadmium.

A typical method for the preparation of single crystal area rectifiers will now be described in detail, it being expressly understood that the method is subject to a latitude of variation as detailed in the mentioned copending applications, except for the preparation of the single crystalline body 10. Rectifier bodies can be prepared from a single crystalline boule of either the clear and unreduced strontium titanate or the darker reduced strontium titanate. The unreduced material is fired in a dry reducing atmosphere to impart the desired semiconductive properties. Reduction may take place at a temperature of between 1000 C. and 1400 C. for a time of between and minutes and in an atmosphere of between 10 to 100% purified hydrogen and the balance a purified inert gas, such as helium, with a total rate of flow between 500 to 2000 cubic centimeters per minute. It is preferred to have all of the rectifier bodies cut from single crystals subjected to the above controlled reduction. Accordingly if the bodies or plates 10 are cut out from the dark reduced boule, it is preferred to oxidize the reduced boule and then to re-reduce under the controlled conditions.

Following the reduction of the single-crystalline bodies 10, the several electrodes 12, 14, 16, and 18 are applied to the body, after suitable treatment to provide the rectifying surface or barrier 10a. The surface treatment may consist of sand blasting the surface 1011 upon which the rectifying barrier is formed. Following the sand blasting, which is optional, the surface 10a may be thermally oxidized or conditioned. This is accomplished by firing the single crystals in an atmosphere furnace. Since only the one surface is to be formed with the rectifying barrier, it is preferable to place the units in the atmosphere furnace in a manner to expose only the surface 10a to the atmosphere. For best results, the side of the body removed from the surface 10a may be protected during the thermal firing by any suitable protective coating so that the surface Which is to make electrical connection to the base electrode 18 is subjected to a minimum degree of oxidation.

Following the thermal oxidation, the surface 10a is electrolytically conditioned. Such electrolytic conditioning and treatment involves establishing the body 10 as the anode in a plating bath and operating the bath at a current density sufiicient to anodically oxidize the surface iiia. The parameters for anodic conditioning will be best appreciated by considering the current voltage relationship illustrated in Fig. 4. As the current is slowly increased the voltage rises in a linear relationship. At the current value I a sharp and marked rise in voltage occurs. For this polarizing current, the single crystalline body is anodically oxidized or conditioned. Although it is not necessary that metallic salts be present in the plating bath to achieve such anodic conditioning, it has been found that after conditioning or polarization the surface 10a will readily accept the required layer of manganese or lead dioxide making up the coating or layer 12. For example, plating may be achieved following polarization by initially conditioning the semiconductive body in a lead nitrate solution. It has been found that the value of the current of polarization I varies widely. Therefore during both the conditioning and plating, voltage control is more appropriate for practicing the process. Following the polarization or conditioning, the voltage is cut back by an amount sutficient to operate at a fixed voltage V and a corresponding current I appropriate for plating from the bath containing the lead salts. A lead dioxide plated layer of a few molecules thickness is sufiicient for the layer 12.

As pointed out in application Serial No. 365,987, rectification properties can be improved by applying a further metallic oxide coating 14 to the plated layer 12. The layer i4 is preferably applied in the form of a paste which is prepared by intimately mixing the metal oxide powder, for example lead dioxide or manganese dioxide, in a binder, such as silicone cement. For further details reference may be made to the mentioned copending application.

Finally the counter electrode 16 and the base electrode are applied, for example by provision of solder, cadmium or similar metals.

For the point-contact rectifier illustrated in Fig. 3, the processing is essentially the same as for the area contact rectifier except that the area layers 12, 14, 16 are replaced by a point 16' of a conducting material, such as lead, tin, tungsten or lead dioxide.

Among the advantages which may be attributed to rectifiers embodying a semiconductive body of strontium titanate are the lower cost of materials as compared to present devices incorporating germanium or silicon and the assurance of greater temperature stability due to the inherent properties of the strontium titanate.

Experimental results indicate that single crystals of strontium titanate show better rectification properties than polycrystalline materials prepared by ceramic procedures. Grain boundaries may be expected to be regions of weakness in a recetifying barrier layer; such grain boundaries are absent in single crystals of strontium titanate. Further, it is possible to form barrier layers of more uniform properties due to the greater homogeneity of the chemical composition and electrical properties of single crystals. The measured electrical properties of single crystals of strontium titanate give av better reflection of the factors determining performance as a rectifier. This tends to make a higher degree of manufacturing control possible. Still further the mobility or velocity of the electron current carriers is greater for single crystals. As a result such single crystals have lower resistivity than polycrystalline materials of the same composition. Thus there is made available a wider range of compositions having practically useful values of resistivity and on which at the same time sufiiciently effective barrier layers may be formed. Still further if any significant difierence exists between the rectifying capabilities of various crystal faces of a strontium titanate body such differences may be applied to advantage by the correct orientation of the single crystal. As compared to polycrystalline bodies of the same material, the rectifying surfaces consist of a random mixture of various crystal faces.

Numerous other advantages of the present invention will occur to those skilled in the art and in some instances some features of the invention can be used without a corresponding use of other features. Accordingly the appended claims should be construed broadly and as is consistent with the disclosure.

What we claim is:

l. A rectifier comprising a single crystalline body of reduced strontium titanate having a thermally oxidized and anodically conditioned surface, and a metallic oxide layer formed on said surface.

2. In the manufacture of a titanate rectifier, the steps including preparing a single-crystalline body of reduced strontium titanate, introducing a surface of said body into an electrolytic bath containing a metallic salt which plates out as the metal dioxide, operating said bath at a current density selected to anodically oxidize said surface of said body and to condition said surface to receive said metal dioxide, and operating said bath at a reduced current density selected to plate a layer of said metal dioxide onto said surface.

3. In the manufacture of a rectifier, the steps of preparing a single-crystalline body of a reduced strontium titanate, mechanically roughening and thermally oxidizing a surface of said body, anodically oxidizing said surface of said body to condition said surface to receive a metallic oxide layer, electrolytically depositing said metallic oxide layer on said conditioned surface, and

coating the electrolytically deposited metallic layer with a further layer of said metallic oxide.

4. In the manufacture of a rectifier, the steps including preparing a rectifier body from a single crystalline boule of strontium titanate, firing said body in a reducing atmosphere at a temperature in the range of 1000' C. to 1400 C. to render said body semiconductive, thermally oxidizing said surface of said body, anodically oxidizing said surface of said body, and electrolytically depositing a metallic oxide layer onto said surface.

References Cited in the file of this patent UNITED STATES PATENTS 2,633,543 Howatt Mar. 31, 1953 2,711,496 Ruben June 21, 1955 2,711,497 Ruben June 21, 1955 2,760,126 Kopelman Aug. 21, 1956 FOREIGN PATENTS 497,474 Germany May 8, 1930

Claims (1)

1. A RECTIFIER COMPRISING A SINGLE CRYSTALLINE BODY OF REDUCED STRONTIUM TITANATE HAVING A THERMALLY OXIDIZED AND ANODICALLY CONDITIONED SURFACE, AND A METALLIC OXIDE LAYER FORMED ON SAID SURFACE.

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