Control of threshold voltage in ZnO-based oxide thin film transistors

We investigated the feasibility of controlling the threshold voltage (Vth) by adjusting the thickness of the active layer (tactive) rather than by conventional chemical doping in indium-gallium-zinc oxide (IGZO) transistors with an inverted staggered structure. The value of Vth of the IGZO transistor was linearly modulated from −15.3±1.6to−0.1±0.21V by reducing tactive without any significant change in the field-effect mobility (μFE), subthreshold gate swing, or Ion∕off ratio. The free electron density extracted from the relationship between tactive and Vth was 1.9×1017cm−3, which was consistent with the value of 1.5×1017cm−3 obtained from the C-V measurement for the 30-nm-thick IGZO films. The slight increase in the μFE with increasing tactive, which was in contradiction with the behavior of the corresponding amorphous Si transistor, was explained by the anomalous behavior of the source/drain contact resistance.