Graded gap inversion layer photodiode array

Abstract

A HgCdTe photodiode array for detecting mid-wavelength infrared radiation has a laminated structure consisting of a substrate, a heavily doped wide bandgap buffer layer, and a more lightly doped narrow bandgap base layer. Two sets of a orthogonally disposed U-shaped grooves are etched completely through the base layer and partially through the buffer layer, thereby forming a plurality of mesa-shaped structures. Overlying the portion of base layer contained within each mesa is a capping layer of opposite conductivity. The junction of the base and capping layer within each mesa forms a photodiode. In contact with each capping layer is a metalization area for connection of the underlying diode to a readout device. In contact with the buffer layer is another metalization layer for making a common electrical connection to the array of photodiodes. Overlying the mesa surfaces is a layer of passivation which contains a fixed positive charge. The charge creates an inversion layer within the surface of the base layer exposed along the mesa walls, thereby enlarging the p-n junction of each photodiode. Radiation impinging on a lower surface of the transparent substrate passes through the substrate and into the buffer layer, which is also transparent. Transmitted through the buffer layer, the radiation is absorbed within that portion of the base layer contained within each mesa, resulting in the generation of diode currents.