The Double-Surface Transistor

In a series of Letters to the Editor [1] appearing in a recent issue of this journal, there are described the physical construction and proposed theory of operation of a solid state semiconductor triode. This device, which is now called the type A transistor, comprises a block of high back-voltage germanium on one of the faces of which are two contacts, side by side with each other and close together. These contacts are called the emitter and collector, respectively. A large-area contact to the opposite face of the semiconductor block is called the base contact. The present communication describes another semiconductor triode, the double-surface transistor, in which the emitter and collector contacts bear on the two opposite faces of a thin wedge or slab of semiconductor. This slab is prepared from an ingot of high back-voltage germanium of N-type [2]. After being ground approximately to the desired shape, the slab is etched and provided with a suitable large-area base contact. For good gain characteristics it is advantageous that the thickness of the slab be no greater than about 0.01 cm at the place where the contacts bear upon it. These contacts should be within about the same distance of coming exactly opposite each other on the two faces of the slab. Tungsten, copper, and phosphor bronze have been used successfully as contact materials. This device, together with its electrical connections for use as a grounded-base amplifier, is illustrated schematically in Fig. 1. In operation a comparatively large dc reverse bias ( 50 to 100 V) is applied to the collector, while a comparatively small dc forward bias (a few tenths of a volt) is applied to the emitter. Because of positive feedback effects in the base contact and semiconductor body, the emitter bias voltage-to-base may in some cases be zero or even negative. The static characteristics of a double-surface transistor are presented in Fig. 2. Families of collector voltage versus collector current curves are given, with constant emitter current as parameter for the solid lines, and with constant emitter voltage as parameter for the dashed lines. Such a plot allows one to make judicious choice of dc operating point. It furnishes in addition complete information from which can be obtained, almost by inspection, the dynamic