One widely used photodetector is the pin photodetector (pin-PD), which has a depleted absorption layer of intrinsic semiconductor (i-layer) between positively and negatively doped electrode layers (p- and n-type semiconductor layers). In a pin-PD, two types of charge carriers—both negative electrons and positive holes—are photogenerated and contribute to the external induced current. Because holes have a much lower velocity than electrons (1/10 the speed), the device’s response speed is dominated by hole transport. Therefore, we developed the UTC-PD, which can achieve ultrahigh-speed operation, by removing this limiting factor for the pin-PD. As illustrated in Fig. 1, carriers are generated in the p-type absorption layers and only electrons are injected into the depleted carrier collection layer, whose thickness is designed independently of the absorption layer. Since the holes are majority carriers in the p-type absorption layer, their transport is the collective motion with a high response time determined by the dielectric relaxation time. Eventually, only electrons behave as active carriers in the UTC-PD; thus, UTC-PDs can achieve ultrahigh-speed and high-current operation without the space-charge-effect induced by hole accumulation.