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An optical detector is a semiconductor device that is most often used to capture signals in an optical communication system. First let's go over some physics. While light is often called an electromagnetic wave, it also has particle-like behavior (see wave/particle duality). Light comes in discrete packets called photons that have energies hc/λ, where λ is the light's wavelength and h is Planck's constant. Photons with energies smaller than the bandgap of a semiconductor cannot easily cause electrons to transition from the valence band to the conduction band because the law of conservation of energy must be satisfied. It is reasonable for us to assume that only photons with energies greater than the bandgap are absorbed. The cutoff wavelength is given by

λ(μm) = 1.24/Eg,

where Eg is the bandgap in units of eV.

The simplest optical detector is the photoconductive detector. Since the absorption of photons creates electron-hole pairs, the conductivity of a semiconductor is modulated by incident light. For good sensitivity, undoped semiconductors are used. If enhanced sensitivity is needed, the device can be operated a lower temperature, at which there are fewer thermally-generated electron-hole pairs. The photoconductive detector is simple but it is not popular because of slow response time and poor noise performance.

The structure of the p-n junction photodetector is the same as that of a p-n diode. The idea behind the p-n junction photodetector is that light incident on the depletion region of the device will create electron-hole pairs that are quickly swept from the depletion region because of the high electric field. Therefore the bandwidth of the p-n junction detector can be very high. The p-n junction is reverse-biased to increase the size of the depletion region (allowing more photons to be collected) and to decrease the speed-limiting capacitance of the depletion region.

The most popular photodetector is the pin photodiode. The structure is the same as that of a p-n junction but with an intrinsic, undoped region between the p-type and n-type regions. This structure effectively increases the photon collection area and decreases parasitic capacitance. A typical germanium pin photodiode has a spectral range of 0.5-1.8μm, a responsivity of about 1A of current per 1W of incident light, and a bandwidth of 4GHz.

Another type of photodetector based on the p-n junction is the avalanche photodiode. This detector works the same way as the previous photodiodes, but is operated with a depletion-region electric field high enough to cause impact ionization. Impact ionization greatly increases the sensitivity of photodiodes since it generates more current per incident photon. However, impact ionization is a noisy process that can deteriorate the detector's signal-to-noise ratio.

Interestingly, the photodetection process can be used as an energy source as well as a signal-detection mechanism. Semiconductor devices that use photodetection to generate energy are called solar cells.

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