Photosensitivity enhancement in hydrogenated amorphous silicon thin-film phototransistors with gate underlap

Conventional ɑ-Si:H phototransistors exhibit poor photosensitivity due to low photo-conversion efficiency. To overcome this intrinsic limit, we introduce gate underlap in ɑ-Si:H phototransistors and demonstrate photosensitivity enhancement. We show that photocurrent can be significantly larger than dark current by 4 orders of magnitude, using 1-μm gate underlap at incident optical power density (Pinc) of 3.2 W/cm2. Our 1-μm gate-underlap phototransistor exhibits higher photosensitivity than the device without gate underlap by 64 times with Pinc = 0.2 W/cm2 and a wavelength of 785 nm. Our gate-underlapped phototransistors also show excellent time-resolved photoswitching behaviors, demonstrating the great potential for highly sensitive photodetectors.