A semiconductor photodetector for photon detection without the use of avalanche multiplication, and capable of operating at low bias voltage and without excess noise. In one embodiment, the photodetector comprises a plurality of InP/AlInGaAs/AlGaAsSb layers, capable of spatially separating the electron and the hole of an photo-generated electron-hole pair in one layer, transporting one of the electron and the hole of the photo-generated electron-hole pair into another layer, focalizing it into a desired volume and trapping it therein, the desired volume having a dimension in a scale of nanometers to reduce its capacitance and increase the change of potential for a trapped carrier, and a nano-injector, capable of injecting carriers into the plurality of InP/AlInGaAs/AlGaAsSb layers, where the carrier transit time in the nano-injector is much shorter than the carrier recombination time therein, thereby causing a very large carrier recycling effect.