“…If we assume a simple velocity saturation model for both carriers , the output current saturation starts to occur when the current density reaches a critical current density (J c ) at a critical electric filed (Tebbenham et al, 1975, Hill et al, 1987. Because of the larger saturation velocity and smaller critical electric field for electrons, J c can be much larger for the UTC-PD than that for a pin-PD.…”