Current gain in silicon near infrared optical sensor containing multiple low–high junctions

Abstract Properties of multiple junctions between lightly and heavily doped silicon of the same conductivity type, namely n–n + , or low–high (L–H) junctions, are described. Structures containing multiple L–H junctions were fabricated on the surface of a lightly doped silicon epitaxial layer by phosphorus diffusion through 4 × 10 −6  cm 2 square “windows” etched in the silicon dioxide layer. A heavily tin-doped indium oxide film (ITO) deposited on the SiO 2 surface is used as a conducting and transparent metallic gate connecting in parallel the formed L–H junctions and the ITO-SiO 2 -Si capacitors surrounding these L–H junctions. The structures are packaged in a special black resin to obtain a selective response in the near infrared spectral range. The irradiated sensors present a transistor action that produces a considerable internal photocurrent gain. The nature of this gain is an additional electron injection over the potential barriers at the L–H junctions caused by the accumulation of the photo generated minority carriers in the potential well near the boundary of the L–H junctions. Based on the diffusion-drift equations coupled with the Poisson equation, a theoretical model for the injection and transport of charge carriers has been developed to clarify the origin of the internal gain under irradiation.