Quantum well infrared photodetector

A Quantum Well Infrared Photodetector (QWIP) is an infrared photodetector, which uses electronic intersubband transitions in quantum wells to absorb photons. The basic elements of a QWIP are quantum wells, which are separated by barriers. The quantum wells are designed to have one confined state inside the well and a first excited state which aligns with the top of the barrier. The wells are n-doped such that the ground state is filled with electrons. The barriers are wide enough to prevent quantum tunneling between the quantum wells. Typical QWIPs consists of 20 to 50 quantum wells. When a bias voltage is applied to the QWIP, the entire conduction band is tilted. Without light the electrons in the quantum wells just sit in the ground state. When the QWIP is illuminated with light of the same or higher energy as the intersubband transition energy, an electron is excited.

Once the electron is in an excited state, it can escape into the continuum and be measured as photocurrent. To externally measure a photocurrent the electrons need to be extracted by applying an electric field to the quantum wells. The efficiency of this absorption and extraction process depends on several parameters.

Photocurrent

Assuming that the detector is illuminated with a photon flux ${\displaystyle \phi }$ (number of photons per unit time), the photocurrent ${\displaystyle I_{ph}}$ is

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