Analogy of Photogating to Voltage-Gating in Zinc-Tin Oxide Thin-Film Transistor: Efficiency and Current Saturation Mechanism

In contrast to the conventional voltage-gated channels of thin-film transistors (TFTs), the photo-induced gating effect provides a promising approach to control the carrier concentration in the channels and benefits the advanced application in a remote operation mode. In this work, the zinc-tin oxide TFT (ZTO TFT) is revealed the feasibility of being photo-gated at ${V}_{\text {G}} =0$ V to obtain a series of output ( ${I}_{\text {D}}$ – ${V}_{\text {D}}{)}$ characteristics. The current saturation in ${I}_{\text {D}}$ – ${V}_{\text {D}}$ curves of the photo-gated ZTO TFT confirms the photo-induced pinch-off region in the ZTO channel, suggesting the presence of positive pseudogate voltage at the channel under lighting. The photo-gated output characteristic at a given ${V}_{\text {D}}$ is converted to the log ${I}_{\text {D}}$ verse optical power density ( $\rho {)}$ curve, showing a rapid switching of ${I}_{\text {D}}$ by 405-nm light illumination with the reciprocal log ( ${I}_{\text {D}}{)}$ – $\rho $ slope of 0.04 (mW/cm $^{{2}}{)}$ /decade. Parallel output characteristics triggered electrically with various gate voltages are also performed to obtain an equivalent drain current. The correlation of ${V}_{\text {G}}$ versus $\rho $ for equivalent drain current reveals that photogating efficiency is 7.93 V/(mW/cm $^{{2}}{)}$ . The photogating mechanism and efficiency are discussed based on the photoionization of the neutral oxygen vacancies to positively charged oxygen vacancies, which are responsible for photo-induced gate voltage (VG,ph).