The role of substrate in the photoresponse of graphene transistors

Recent experiments on graphene phototransistors, composed of a graphene channel back-gated by an undoped 6H-SiC, have demonstrated impressive photo-responsivity (> 1A/W). Despite several experimental reports and significant potential for commercial application, the theoretical basis of this high sensitivity is not fully understood. Here we use detailed computational modeling to demonstrate that the extraordinary photosensitivity arises from the electrostatic doping of graphene due to surface accumulation of photo-generated carriers in the substrate. We find that further optimization of substrate mobility, lifetime, and, bandgap may substantially improve photosensitivity as well as the gain-bandwidth of photo-detection.