Degradation on the Current Saturation of Output Characteristics in Amorphous InGaZnO Thin-Film Transistors

Degradation on the current saturation of the output characteristics in amorphous indiumgallium-zinc-oxide (a-IGZO) thin-film transistors with the bottom-gate structure is investigated. As the drain-tosource voltage (V-DS) increases at a fixed gate-to-source voltage (V-GS), the current from drain to source (I-DS) becomes nonsaturated and increases due to the mobility enhancement and I-DS also decreases again due to the electron trapping into the gate insulator and/or an interface, which occurs mainly at the channel edge near a drain. Analysis is validated by using the pulsed I-DS-V-DS measurement. Nonsaturated current is attributed to the Joule heating-assisted mobility enhancement and the thermally activated electron trapping, which is the reason why the nonsaturation becomes more prominent as the IGZO active thin film becomes thinner. Furthermore, it is found that the rate of the increased I-DS per increasing V-DS gets higher as either the channel width (W) increases or the channel length (L) decreases; whereas, the rate of the decreased I-DS per increasing V-DS becomes higher as the device size, i.e., W x L, increases. The former is well correlated with Joule heating while the latter with the self-heating-assisted electron trapping due to a total heat accumulated in an active layer.