Effective mobility of polycrystalline semiconductors

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Abstract
Theoretical investigations are presented to explain the behaviour of effective mobility (μ*) as a function of doping density, valid for all grain sizes, in polycrystalline silicon, by considering the dynamics of capture and release of free carriers at the grain boundary trapping sites, the finite thicknesses of the grain boundaries and the contributions of the bulk and depletion regions towards the electrical conductivity. An empirical formula is used for the value of grain boundary trapping states density (N T) as a function of grain size. Computations show that (a) μ* has a minimum for a particular doping concentration for a given grain size D, and it shifts towards lower doping concentration with increasing D; and (b) the polycrystalline behaviour approaches the single-crystal behaviour for D ≥: 3000μm. The predicted results reasonably represent the experimentally observed variations of effective resistivity and effective mobility of poly-semiconductors as a function of doping density for all grain sizes.