Nonlinear photocurrent-intensity behavior of amorphous InZnO thin film transistors

The photocurrent (I_PH) of amorphous InZnO thin film transistors in the off-state is investigated as a function of incident optical power (P). The results show that I_PH exhibits a nonlinear dependence on P. Additionally, the dependence of I_PH on P exhibits a strong photon energy (hυ)-dependent feature. When P is relatively low, I_PH is shown to be proportional to P^γ, where γ is greater than 1. The γ > 1 behavior may be ascribed to the source-barrier-lowering effect due to the accumulation of photo-induced positive charges at the source side. When P is relatively high, while I_PH remains proportional to P^γ under the incident light with hυ larger than the optical bandgap (E_g) of a-IZO, it turns to increase at an exponential rate with P if hυ of the incident light is smaller than the E_g. The exponential increase in I_PH is attributed to the source-barrier-thinning effect, which leads to a significantly enhanced tunneling current.