Temperature Dependent Transient Leakage Currents in Amorphous Silicon Thin Film Transistors

Abstract

AbstractThe transient response of a-Si:H thin film transistors (TFT) provides information about gap states in the channel and about the TFT conduction mechanisms. We present results of a detailed parametric study of transient leakage currents in TFTs, extending from 10-3 sec to >100 sec and at temperatures up to 400K. The form of the transient decay depends most notably on the off-state gate voltage, which determines the leakage current mechanism and magnitude. The measurements use chains of many TFTs, such that we can measure dc on/off ratios of >1011.In the sub-threshold region (e.g. Vg~-2V) some samples exhibit non-monotonic decay of the drain current, with a minimum followed by a slow increase to a steady state. Typically this effect is barely visible at room temperature due to the extremely low values of the dc current. Higher temperatures enhance the phenomenon, which has activation energy of about 1 eV for the current magnitude and time constant. We attribute the effect to back-channel electron conduction. This explanation is consistent with the form of the TFT transfer characteristics, and the weak drain-source voltage dependence excludes a contact injection mechanism.At more negative gate off-state voltages, some samples exhibit increasing leakage currents, indicative of hole conduction. These samples feature a monotonically decreasing transient decay with a pronounced drain-source voltage dependence, consistent with contact injection being enhanced by a hole-enriched channel in the high field drain region.Effects of on-state gate pulse width and amplitude are also discussed, as well as the effect of high temperature stress on the TFT transients.