QWIPs

QWIPs, or quantum well infrared photodetectors, are semiconductor devices that detect infrared light by exploiting intersubband transitions within quantum wells. In these structures, electrons are confined in thin layers (quantum wells) and occupy discrete energy subbands in the conduction band. When infrared photons matching the energy difference between subbands are absorbed, electrons are promoted between subbands, producing a detectable electrical signal. The absorption is strongly polarization dependent and is most efficient for TM-polarized light; absorption at normal incidence is typically weak and requires optical coupling structures such as gratings or waveguides.

The active region of a QWIP typically consists of many quantum wells and barriers grown in a single crystal, often in the intrinsic region of a p-i-n photodiode or in a Schottky- or metal-insulator-metal-type detector. The well/barrier materials are usually chosen from III-V semiconductors, with GaAs/AlGaAs being the most common system for mid- to long-wavelength infrared detection. Other material systems, such as InGaAs/InAlAs for different wavelength ranges, have also been explored. The spectral response is tuned by adjusting well width, barrier height, and the number of wells, enabling detection across a range from about 3 to 25 micrometers, depending on material choices and design.

QWIPs generally require cooling to reduce dark current, with typical operation at cryogenic temperatures or with thermoelectric coolers. They offer mature fabrication compatible with standard semiconductor processing and can be implemented in focal plane arrays for imaging. Limitations include polarization- and angle-sensitive absorption, narrow spectral bandwidth for a given structure, and the need for efficient optical coupling to normal-incidence light. Applications include infrared imaging, spectroscopy, and environmental sensing.