Pin photodiode with improved frequency response and saturation output

Abstract

A pin photodiode having a structure capable improving the frequency response and the saturation output while maintaining the effective internal quantum efficiency and CR time constant. A pin photodiode is formed by: a first semiconductor layer in a first conduction type; a second semiconductor layer in a second conduction type; a third semiconductor layer sandwiched between the first and second semiconductor layers, having a doping concentration lower than those of the first and second semiconductor layers; a fourth semiconductor layer in the first conduction type, provided at one side of the first semiconductor layer opposite to a side at which the third semiconductor layer is provided; and a cathode electrode and an anode electrode connected directly or indirectly to the second semiconductor layer and the fourth semiconductor layer, respectively. The first semiconductor layer has a bandgap energy by which a charge neutrality condition is maintained in at least a part of the first semiconductor layer and the first semiconductor layer is made to function as a light absorption layer, while the second and third semiconductor layers have bandgap energies by which the second and third semiconductor layers are made not to function as a light absorption layer, and the fourth semiconductor layer has a bandgap energy greater than that of the first semiconductor layer.