Photoconductive Gain in Barrier Heterostructure Infrared Detectors

Infrared (IR) detector technologies with the ability to operate near room temperature are important for many applications including chemical identification, surveillance, defense and medical diagnostics. Reducing the need for cryogenics in a detector system can reduce cost, weight and power consumption; simplify the detection system design and allow for widespread usage. In recent years, infrared (IR) detectors based on uni-polar barrier designs have gained interest for their ability to lower dark current and increase a detector's operating temperature. Our group is currently investigating detectors based on the InAs/GaSb strain layer superlattice (SLS) material system that utilize barrier heterostructure engineering. Examples of such engineering designs include pBp, nBn, PbIbN, CBIRD, etc. For this paper I will focus on LW (long wave) pBp structures. Like the built-in barrier in a p-n junction, the heterojunction barrier blocks the majority carriers allowing free movement of photogenerated minority carriers. However, the barrier in a pBp detector, in contrast with a p-n junction depletion layer, does not significantly contribute to generation-recombination (G-R) current due to the lack of a depletion region across the narrow band gap absorber material. Thus such detectors potentially work like a regular photodiode but with significantly reduced dark current from G-R mechanisms. The mechanism of photoconductive (PC) gain has not been fully characterized in such device architectures and in many recent studies has been assumed to be unity. However, studies conducted with similar device structures have shown the presence of PC gain. In this report we will measure and analyze the impact of PC gain in detectors utilizing single unipolar barriers such as the case of pBp detectors.