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At very high operating levels the density of carriers injected into the body of a semiconductor junction device is comparable with the carrier density in the emitter regions of the device. The effect of these high densities on the lifetime and mobility of carriers is considered, and new equations are derived relating the carrier densities on either side of a forward biased junction. These equations are applied to derive the forward characteristics of two diode types, and to consider the dependence of emitter efficiency on current density for alloy junction transistors. For a PIN diode, over a considerable current range, the forward current varies approximately as exp (Â¿qV/kT), where Â¿ Â¿ Â¿ Â¿ 1; Â¿ = 1 for very thin diodes, and Â¿ = Â¿ for thick diodes. For PIR diodes, the forward current varies as exp (qV/2kT). Both types show additional voltage drop at high currents. Transistor emitter efficiency decreases with increasing current, but saturates to a finite value at high currents. These predictions are in accord with experiment and suggest design considerations for optimum performance. A brief discussion is also given of the usefulness of these new results in device applications.