Variation of lifetime with injection level in germanium

In most germanium transistors, even those operating at high injection levels, surface and volume recombination losses may have a substantial effect on beta. It is of interest therefore, to determine how lifetime varies with injection level in order to estimate how much of the beta fall-off is due to increasing surface recombination. Uniform and known injection levels may be produced in small bars of germanium by illumination with unmodulated light. By the use of material having a bulk diffusion length large compared to the thickness of the bars, surface lifetime may be measured by super-imposing on the DC light a small modulated radiation. It is found that recombination is increasing at a considerable rate, even at the highest injection levels obtainable, which are comparable to the levels present in power transistors. The amount of beta fall-off observed in power transistors will be, therefore, influenced by the nature of the surface as well as by the emitter efficiency of the transistors.