Separate absorption and detection diode for vlwir

Abstract

A photodiode for detection of preferably very long wavelength infrared radiation wherein low energy photons are absorbed in one region and detected in another. In one example embodiment, an absorbing P region is abutted with an N region of lower doping such that the depletion region is substantially (preferably completely) confined to the N region. The N region is also chosen with a larger bandgap than the P region, with compositional grading of a region of the N region near the P region. This compositional grading mitigates the barrier between the respective bandgaps. Under reverse bias, the barrier is substantially reduced or disappears, allowing charge carriers to move from the absorbing P region into the N region (and beyond) where they are detected. The N region bandgap is chosen to be large enough that the dark current is limited by thermal generation from the field-free p-type absorbing volume, and also large enough to eliminate tunnel currents in the wide bandgap region of the diode.