Physical models for amorphous-silicon thin-film transistors and their implementation in a circuit simulation program

A semianalytic theory to describe both the current-voltage and capacitance-voltage characteristics of amorphous silicon thin-film transistors on the basis of their physics of operation is presented. In this model, the drain current is directly related to the electron concentration at the source side of the channel. This enables one to describe the various regimes of operation of these devices (i.e. subthreshold or above threshold) using only one equation. The output conductance of these devices in saturation is also considered, and it is shown that the finite output impedance is a consequence of the drain voltage modulating the effective channel length by creating a space-charge limited current region of variable length near the drain. The results of this model are in good agreement both with experimental data and the results of comprehensive two-dimensional simulations. These device models have been successfully incorporated into a SPICE circuit simulation program