Model for the above-threshold characteristics and threshold voltage in polycrystalline silicon transistors

A new theory of polycrystalline silicon thin-film transistors is proposed, based on a continuous trap state density. Three different regimes are predicted: subthreshold, transitional, and crystallinelike. Two characteristic voltages are identified: the threshold voltage, corresponding to the condition of equal trapped and free charge concentration at the oxide/semiconductor interface and the on-voltage, corresponding to the condition of equal trapped and free charge in the whole space-charge region. In the case of an exponential distribution of gap states, approximated analytical expressions can be deduced and a simple accurate fitting procedure is presented. A very good agreement with the experiment is obtained, confirming the importance of taking into account the detailed energy dependence of the trap distribution.