Influence of wafer thickness and carrier recombination on the cutoff frequency of alloy-junction transistors

The influence of wafer thickness and carrier recombination on the -cutoff frequency of the “intrinsic” transistor is studied. First, M ' solution of the diffusion of minority carriers in a symmetrical junction transistor is used to find the d.c. transport factor. This is then converted to the a.c. case and finally an implicit equation is found for the cutoff frequency as a function of wafer thickness θ and surface recombination velocity S. It is shown that, for S < 3000 cm/sec, the influence of surface recombination is negligible.

At very narrow basewidths the current density is very high at the emitter edge and this decreases the diffusion length. More important seems to be the fact that the carriers' path (from slanted edge of emitter to slanted edge of collector) is longer than the geometrical basewidth. A reduction in wafer thickness decreases this effect and thus increases the cutoff frequency, in spite of increasing the current density at the emitter edge. The most important equations have been represented graphically for the particular case of the STC TK20 transistor.