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

We investigated the feasibility of controlling the threshold voltage $\left(V_{\mathrm{th}}\right)$ by adjusting the thickness of the active layer $\left(t_{\text{active}}\right)$ rather than by conventional chemical doping in indium-gallium-zinc oxide (IGZO) transistors with an inverted staggered structure. The value of $V_{\mathrm{th}}$ of the IGZO transistor was linearly modulated from $-15.3\pm 1.6\text{to}-0.1\pm 0.21\mathrm{V}$ by reducing $t_{\text{active}}$ without any significant change in the field-effect mobility $\left({\mu }_{\mathrm{FE}}\right)$ , subthreshold gate swing, or $I_{\text{on}∕\text{off}}$ ratio. The free electron density extracted from the relationship between $t_{\text{active}}$ and $V_{\mathrm{th}}$ was $1.9\times {10}^{17}{\mathrm{cm}}^{-3}$ , which was consistent with the value of $1.5\times {10}^{17}{\mathrm{cm}}^{-3}$ obtained from the $C\text{-}V$ measurement for the $30\text{-}\mathrm{nm}$ -thick IGZO films. The slight increase in the ${\mu }_{\mathrm{FE}}$ with increasing $t_{\text{active}}$ , 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.