Patent GB-817905-A (GB)
Title: Improvements in or relating to electric circuit controlling devices utilizing semiconductor bodies
IPC: H01L29/00
Filed: 1956-07-10
Abstract: 817,905. Semi-conductor devices. WESTERN ELECTRIC CO. Inc. July 10, 1956 [July 13, 1955], No. 21332/56. Class 37. In an avalanche multiplication diode comprising a wafer of semi-conductor of one conductivity type with a pair of regions of the opposite conductivity type formed in its opposite faces, one of the regions, to be used as the emitter, is of greater lateral extent than the other. Alternatively there may be a resistivity gradient in the wafer adjacent the emitter region parallel to the emitter junction. As a further alternative the said region is connected by a low-resistance connection to a zone of the same conductivity type as the body but of higher conductivity on the same face of the body as the emitter region but spaced therefrom. In one embodiment (Fig. 1) the electrodes are zones of P-type material on an N-type base body. In operation the external bias applied is such as to bias the large area junction 15 as an emitter and the junction 16 as a collector. When the bias is small the emitter emits minority carriers more or less uniformly throughout its area, and under these conditions the average α is low since a large fraction of the emitter current has to flow from the edge of the emitter to the collector, suffering appreciable losses by recombination on the way. However, when the bias is large, e.g. sufficient to produce avalanche breakdown in the N-type Ge there is an appreciable potential gradient in the base region due to the increased emitter current, resulting in the reduction of the forward bias at the edges of emitter compared with that at the centre. Under these conditions the bulk of the emitter current is injected from the centre of the emitter for which α is high thereby giving a rise in the average value of a for the whole emitter. The variation of a may be further increased by making the edges of the emitter much less efficient than the centre by making the parts 17 of the base region adjacent the edges of the emitter of lower resistivity. The lower resistivity region may be confined to one face of the wafer as shown or may extend through the wafer. The variation of α may also be increased by using an annular emitter zone coaxial with the collector (Fig. 4). In yet another arrangement (Fig. 5) a low resistance connection is made between the emitter zone and a layer of N + material spaced therefrom on the same face of the wafer. For low current most of the current flows through the N + zone to give a low overall a whereas at high current the bulk of the current flows through the now low-impedance emitter junction thereby increasing α. A typical device is made as follows: indium discs are punched from a sheet, etched, formed into spheres by heating to 300‹ C. in dry hydrogen, and the larger sphere mounted in a graphite jig and pressed against the wafer while heated to 450‹ C. in dry hydrogen for 5 minutes. The smaller sphere is then fused to the opposite face by heating to 500‹ C. in a similar process. The device is then etched, first in a mix of nitric and hydrofluoric acids, then in hydrochloric acid, and finally in concentrated nitric acid. After successive rinsings in water and alcohol nickel wires are soldered to the indium masses by a indium solder. Low resistivity may be produced at the edges of the emitter in the base zone by diffusion of donor impurities, e.g. from a surface film deposited from the vapour phase through a mask. Si, Ge-Si alloys or A III B v  compounds may be used in place of Ge and an NPN structure instead of the PNP structures described though the former is preferred on account of the superior avalanche multiplication in N-type material. Specification 790,387 is referred to.