Enhanced sub-band gap photosensitivity by an asymmetric source-drain electrode low operating voltage oxide transistor

Abstract

The electrical characteristics of a thin film transistor (TFT) can be tuned by using an asymmetric work function source-drain (S-D) electrode. However, to realize the effect of this asymmetric S-D electrode, a low operating voltage TFT is required. On the other hand, the sub-bandgap photosensitivity of a photodetector requires suitable material interface engineering. In this work, an asymmetric S-D electrode has been used to enhance the photosensitivity of a solution processed low voltage driven metal oxide TFT. An ion-conducting LiInSnO4 thin film has been used as the gate dielectric of this TFT that limits the operating-voltage of this TFT within 2 V whereas ZnO has been used as the channel semiconductor. This asymmetric S-D electrode of the TFT allows selective carrier (electron or hole) injection and collection from the channel. As a consequence, the on/off ratio and photosensitivity of the device improve significantly. The on/off ratio of the asymmetric TFT is 102 times greater than that of the symmetric TFT. More interestingly, the subthreshold swing (SS) of this asymmetric S-D electrode TFT (210 mV per decade) has been reduced more than four times than that of the symmetric electrode (975 mV per decade) device. The LiInSnO4/ZnO interface states which have been identified in the UV-Vis absorption of the LiInSnO4/ZnO thin film are capable of generating sub-band gap photocurrent in the devices. As a consequence, this ZnO based phototransistor can detect light efficiently ranging from 400 to 800 nm. Overall, the photosensitivity of this asymmetric S-D electrode TFT has been enhanced by ∼405 and ∼377 times under red and blue illumination, respectively, with respect to the symmetric S-D electrode TFT whereas the detectivity of the device increases by ∼10 and ∼4 times. © 2023 The Royal Society of Chemistry.