Non-linear source/drain effects in amorphous-silicon thin-film transistors

The authors discuss the effect of space-charge-limited conduction through the a-Si layer on n-channel staggered TFT (thin-film transistor) current-voltage characteristics by introducing a new model that incorporates this effect. Current density through the a-Si film can be described by J= beta phi /sup 4/, where phi is the potential from the channel to the source (or drain) contact. Field-effect mobility in an a-Si TFT increases with gate voltage because a greater fraction of the gate-induced electrons is in extended tail states as the Fermi level approaches the conduction band edge. Current flow at the ends of the channel is roughly analogous to the transmission line effect in shallow MOSFET source-drain diffusions, but the effect in TFTs is highly nonlinear because of the space-charge-limited current behavior. Experimental current-voltage characteristics measured on a variety of staggered TFTs closely match the simulated curves over a wide range of terminal voltages. Differential channel-conductance measurements of a-Si TFTs at a fixed gate voltage reveal a peak at low drain voltage with a sharp drop on the lower side. This behavior cannot be explained by models using linear series impedance, but it is accurately duplicated by the new model.