QWIP

QWIP stands for quantum well infrared photodetector, a type of infrared detector that relies on quantum wells to detect infrared radiation. These devices use a stack of thin semiconductor wells and barriers grown in a heterostructure, commonly GaAs/AlGaAs or related material systems, to create discrete electronic subbands within the conduction band. When infrared photons promote electrons between these subbands, a photocurrent is generated and can be measured as a detector signal.

The operating principle is based on intersubband transitions. In a quantum well, electrons occupy quantized energy levels. Absorption of a photon can cause a vertical transition between subbands within the same conduction band. This mechanism is selective for photon polarization with an electric field component perpendicular to the well layers (TM polarization) and tends to be narrow in spectral response unless engineered with different well depths and widths. As a result, QWIPs are typically designed to detect mid- to long-wavelength infrared radiation.

A typical QWIP device is structured as a stack of quantum wells integrated into a detector diode, often a p-i-n or Schottky configuration. The spectral response, and thus the wavelength range, is controlled by the well width, barrier height, and material system. QWIPs are compatible with focal plane arrays and standard semiconductor processing, enabling the production of infrared cameras for military, space, and scientific applications.

Advantages include mature fabrication and uniformity across large arrays. Limitations include the need for cryogenic cooling to reduce dark current and noise, polarization sensitivity, and sometimes limited room-temperature performance. QWIPs have contributed to infrared imaging, spectroscopy, and sensing across various fields.