Quasidrift limited field-effect current in polycrystalline silicon: Wide grain boundary approximation

The current flowing along polycrystalline silicon surfaces, as a function of the field applied normal to the surface, is developed for grains of finite size with finite grain boundaries which may contain amorphous material. The work is relevant to the operation of thin film transistors and expressions are developed for the dependence of drift channel current and field effect mobility on gate and drain voltage and, particularly, grain size. Comparison is made with real device characteristics. The results are consistent with a trapping level at the conduction band edge. In electrical terms, such a condition is indistinguishable from no trapping but with a proportionately lower mobility. The assumption of finite grain boundaries, with effective concentrations of traps, is demonstrated to provide potentially very useful analytical results that further the understanding of field effect in all polycrystalline materials.