Enhanced Sub-Band Gap Photosensitivity by an Asymmetric Source-Drain Electrode Low Operating Voltage Oxide Transistor

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, sub-bandgap photosensitivity of a photodetector required a suitable material interface engineering.<b> </b>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 TFT allows a 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 asymmetric TFT is 10² times greater than symmetric TFT. More interestingly, the subthreshold swing (SS) of this asymmetric S-D electrode TFT (210 mV/decade), was reduced more than four times than that of the symmetric electrode (975 mV/decade) device. The LiInSnO₄/ZnO interfaces states which has been identified in the UV-Vis absorption of LiInSnO₄/ZnO thin film is capable to generate 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, 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