Transient Double Injection in Trap-Free Semiconductors

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Abstract
When a step voltage is applied to injecting contacts on the ends of a long trap‐free semiconductor, such as p‐type germanium, an electron‐hole plasma forms at the minority‐carrier injecting end and propagates down the length of the bar. When the recombination time (τ) is much longer than the small‐pulse transit time (t0), the leading edge of the propagating plasma arrives at the far end at ta=(⅚)t0. The arrival is marked by a cusp in the time derivative of the current, which can be used to measure the minority‐carrier mobility. After ta, the current acquires an exponential form, with a time constant equal to the recombination time. Approximate analytic solutions and numerical results indicate that the important recombination effects are small (t0 greater than about 2. Early‐transient diffusion effects are small when (applied voltage/thermal voltage)1/2 is greater than about 40. All the prominent features can be described in terms of simple physical concepts, and the theoretical predictions are verified by experiment.