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Theory of conductivity modulation in semiconductors Nishi, Ronald Yutaka

Abstract

The theory of conductivity modulation in semiconductors and the conditions under which negative resistance can be obtained are investigated. The ambipolar transport equation is derived for one-dimensional flow in a two-terminal homogeneous semiconductor with no trapping and no temperature gradients. Charge neutrality is assumed in the majority of the models studied. A phenomenological model is considered first to show how conductivity modulation can lead to negative resistance. Since the general problem of carrier transport with drift and diffusion currents is difficult, the models investigated are mainly concerned with either drift or diffusion as the dominant transport mechanism. For a unipolar space-charge limited drift model, negative resistance in frequency bands is found. For bipolar, neutral drift models, negative resistance is found under special conditions for the case of no recombination and for recombination with a carrier lifetime increasing with carrier density. For recombination with a constant lifetime, the bipolar drift model gives current-voltage characteristics of the form: J α V at low injection levels, J α V² at high injection levels, and J α V³ at very high injection levels. Space charge is important in the cube law case. Models ignoring diffusion are shown to be valid only for extrinsic semiconductors. Bipolar neutral flow with equal carrier densities leads to diffusion solutions of the ambipolar equation. This case applies to extrinsic material at high injection levels as well as to intrinsic material and is found to exhibit negative resistance under certain conditions. The most favourable situation is the case where the carrier lifetime increases with carrier density. The dependence of the lifetime with carrier density determines the possibility of defining completely a negative resistance model. It is found that the properties of the contacts are important in attaining negative resistance. Contacts and their properties are briefly discussed in relation to the carrier density boundary conditions. Avalanche injection and its relation to the conductivity modulation problem is considered. Several outstanding problems, both theoretical and experimental, arising from this thesis are outlined in the last chapter.

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