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

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.