Alternating gradient photodetector

Abstract

A far infrared (FIR) range responsive photodetector. There is a substrate (28) of degenerate germanium. A a plurality of alternating impurity-band (32) and high resistivity (30) layers of germanium are disposed on the substrate (28). The impurity-band layers (32) have a doping concentration therein sufficiently high to include donor bands which can release electrons upon impingement by FIR photons of energy hv greater than an energy gap .epsilon.. The high resistivity layers (30) have a doping concentration therein sufficiently low as to not include conducting donor bands and are depleted of electrons. Metal contacts (36, 38) are provided for applying an electrical field across the substrate (28) and the plurality of layers (30, 32). In the preferred embodiment as shown, the substrate (28) is degenerate n-type (n.sup.++) germanium; the impurity-band layers (32) are n.sup.+ layers of germanium doped to approximately the low 10.sup.16 cm.sup.-3 range; and, the high resistivity layers (30) are n.sup.- layers of germanium doped to a maximum of approximately 10.sup.15 cm.sup.-3. Additionally, the impurity-band layers (32) have a thickness less than a conduction-electron diffusion length in germanium and likely to bein the range of 0.1-1.0 micron, the plurality of impurity-bands (33) is of a number such that the flux of FIR photons (20) passing therethrough will be substantially totally absorbed therein, the thickness of the high resistivity layers (30) is such compared to the voltage applied that the voltage drop in each the high resistivity layers (30) controls the occurance of impact ionization in the impurity-band layers (32) to a desired level.