Optoelectronic pseudomorphic high-electron-mobility transistors

Abstract

We present experimental and theoretical results on in-situ photoconduction (PC) and photoemission (PE) studies on fully fabricated pseudomorphic high electron mobility transistors (PHEMTs). The measurements are performed on wafer and are non-destructive. The monochromatic light is chopped and lock-in amplifier techniques are used to obtain the PC and PE signals. The bandgap and optical transition energies in PHEMTs are smaller than the bandgap of the substrate and backside illumination is feasible. Optical absorption occurs by interband transitions, mainly from heavy hole sub-levels to electron sub-levels. In devices with rectangular quantum wells we observe the selection rule (Delta) n equals 0 where n represents the electron and hole quantum numbers. Consequently, the number of transitions are greatly reduced under these conditions and the lines can be identified easily. The photogenerated charge is amplified by the transistor since the photogenerated holes are stored in the channel area, modifying its threshold voltage. We experimentally observe that the photoconduction is proportional to the transconductance when the measurements are performed in the linear region of the transistor characteristics, i.e. at low drain voltage in agreement with theory. We evaluate the applicability of PHEMTs as photodetectors in optoelectronic integrated circuits.