The Effect of Junction Shape and Surface Recombination on Transistor Current Gain

An experimental and theoretical study is presented which shows that the current gain of an alloy transistor is greatly affected by the geometry of emitter and collector junctions and by surface treatment of the base germanium, but is hardly affected at all by bulk recombination (lifetime) in the base. The current gain is computed for specific three-dimensional geometries by an electric analog method which assumes that surface recombination is the major factor in minority-carrier loss. By this method, a new way of measuring surface-recombination velocity, s, from simple measurements on transistors has been devised. The value of s is obtained directly from a suitable calibration curve, and thus may be useful as a quality control on surface condition. The transit-time path-length dispersion of minority carriers in a transistor structure with nonparallel junctions has been computed. The results show that the effect is significant only above 1 mc/second in a typical structure.