page_title: Quantum well infrared photodetector
section_title: Function
hierarchical_section_title: Quantum well infrared photodetector / Function
caption_attribution_description: English: Photoconductive gain in a quantum well infrared photodetector. To balance the loss of electrons from the quantum well, electrons are injected from the top emitter contact. Since the capture probability is smaller than one, extra electrons need to be injected and the total photocurrent can become larger than the photoemission current.
context_page_description: A Quantum Well Infrared Photodetector is an infrared photodetector, which uses electronic intersubband transitions in quantum wells to absorb photons. In order to be used for infrared detection, the parameters of the quantum wells in the quantum well infrared photodetector are adjusted so that the energy difference between its first and second quantized states match the incoming infrared photon energy. QWIPs are typically made of gallium arsenide, a material commonly found in smartphones and high-speed communications equipment. Depending on the material and the design of the quantum wells, the energy levels of the QWIP can be tailored to absorb radiation in the infrared region from 3 to 20 µm.
  QWIPs are one of the simplest quantum mechanical device structures that can detect mid-wavelength and long-wavelength infrared radiation. They are known for their stability, high pixel-to-pixel uniformity, and high-pixel operability.
context_section_description: Infrared detectors generally work by detecting the radiation emitted by an object, and the intensity of the radiation is determined by factors such as the object’s temperature, distance, and size. Unlike most infrared photodetectors, QWIPs are independent of the band gap of the detecting material, because they are based on the optical transition within a single energy band. As a result, it can be used to detect objects with much lower energy radiation than what was previously possible.
  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  (number of photons per unit time), the photocurrent  is
  where  is the elementary charge,  is the absorption efficiency and  is the photoconductive gain.  and  are the probabilities for a photon to add an electron to the photocurrent, also called quantum efficiency.  is the probability of a photon exciting an electron, and  depends on the electronic transport properties.
  Photoconductive gain
  The photoconductive gain  is the probability that an excited electron contributes to the photocurrent—or more generally, the number of electrons in the external circuit, divided by the number of quantum well electrons that absorb a photon. Although it might be counterintuitive at first, it is possible for  to be larger than one. Whenever an electron is excited and extracted as photocurrent, an extra electron is injected from the opposite (emitter) contact to balance the loss of electrons from the quantum well. In general the capture probability , so an injected electron might sometimes pass over the quantum well and into the opposite contact. In that case, yet another electron is injected from the emitter contact to balance the charge, and again heads towards the well where it might or might not get captured, and so on, until eventually an electron is captured in the well. In this way,  can become larger than one.
  The exact value of  is determined by the ratio of capture probability  and escape probability .
  where  is the number of quantum wells. The number of quantum wells appears only in the denominator, as it increases the capture probability , but not the escape probability .
caption: Photoconductive gain in a quantum well infrared photodetector. To balance the loss of electrons from the quantum well, electrons are injected from the top emitter contact. Since the capture probability is smaller than one, extra electrons need to be injected and the total photocurrent can become larger than the photoemission current.
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