Nonsaturating Drain Current Characteristic in Short-Channel Amorphous-Silicon Thin-Film Transistors

Nonsaturating drain current characteristics are analyzed in terms of the channel length modulation (CLM) and the self-heating effect. According to this analysis, the nonsaturating drain current arises if the effective channel length is sufficiently reduced such that the CLM effect leads to a superlinear increase of the drain current beyond saturation. The extracted CLM parameter was around ¿´ = 1/15 ¿m/V for the samples investigated, and a nonsaturating characteristic was observed in hydrogenated-amorphous-silicon (a-Si:H) thin-film transistors (TFTs) with a channel length of 10 ¿m or less. Furthermore, in a bias-temperature-stressed short-channel a-Si:H TFT, which has a laterally nonuniform threshold voltage, the experimental data showed a pronounced nonsaturating current in the reverse output characteristics and a much lower and flatter characteristic in the forwardId-Vds data. The nearly flat forward saturation characteristic is discussed in terms of the cancellation of the CLM effect by the effect from the rising threshold voltage at the pinchoff point as the drain bias increases. The pronounced nonsaturating reverse characteristic is explained in terms of the compounding effects of the rising CLM current and the rising current due to the falling threshold voltage of the pinchoff point. We also discuss a split-channel design to suppress the nonsaturating drain currents in a-Si:H TFTs.