In this paper, using finite difference method, the effect of adding a p-layer at the back of a metal-semiconductor-metal (MSM) photodetector (PD) on the spatial electric charge distribution and the transient response of the device is numerically studied. To this aim, the fundamental equations of the semiconductor device, i.e., two current continuity time-dependent equations have been considered coupled with Poisson's equation. The I-V curve of the MSM photodetector is obtained as the main characteristics of each semiconductor device. Moreover, the variations of electrostatic potential, electron and hole concentrations are determined in the MSM photodetector with a p-layer at the back of the active layer. It is observed that the peak transient response of an MSM device is improved by back-gating the device as more electrons are injected to the semiconductor layer and the slower charge carriers (the holes) to be removed from the top circuit. Manuscript profile