Photodiodes
Photodiodes are a two-electrode, radiation-sensitive junction formed in a semiconductor material in which the reverse current varies with illumination. Photodiodes are used for the detection of optical power and for the conversion of optical power to electrical power. Photodiodes can be PN, PIN, or avalanche. PN photodiodes feature a two-electrode, radiation-sensitive PN junction formed in a semiconductor material in which the reverse current varies with illumination. PIN photodiodes are diodes with a large intrinsic region sandwiched between P-doped and N-doped semiconducting regions. Photons absorbed in this region create electron-hole pairs that are then separated by an electric field, thus generating an electric current in a load circuit. Avalanche photodiodes are devices that utilize avalanche multiplication of photocurrent by means of hole-electrons created by absorbed photons. When the device’s reverse-bias voltage nears breakdown level, the hole-electron pairs collide with ions to create additional hole-electron pairs, thus achieving a signal gain.
The photodiode spectral response can be measured in X-ray, UV, visible, or IR. X-ray photodiodes are optimized for X-ray, gamma ray, and beta radiation detection. UV enhanced photodiodes are optimized for the UV and blue spectral regions, which requires special fabrication processes. Visible photodiodes operate in the visible range without enhancement for operation in UV or IR. IR enhanced photodiodes are optimized for the near IR spectral region, which requires special fabrication processes. The spectral response range of incident light the photodiode detects is also called wavelength range. Important active area specifications to consider include active area diameter or length and active area height. If the active area is circle shaped, specify the diameter, otherwise specify the length of the active area. The active area height is applicable to photodiodes that are not circular. Photodiode arrays are packaged as multiples. Photodiode arrays will contain a certain number of elements (Photodiodes). Some photodiodes can be a position sensitive detector. Important photodiode performance specifications to consider include sensitivity, rise time, quantum efficiency, and operating temperature. Sensitivity is a measure of the effectiveness of a detector in producing an electrical signal at the peak sensitivity wavelength. Rise time is the time necessary for a detector’s output to go from 10% to 90% of its final value. A photodiode’s capability to convert light energy to electrical energy, expressed as a percentage, is its quantum efficiency.
Photodiodes can have a noise factor. This is measured as the dark current and noise equivalent power (NEP). Dark current is the current associated with a detector during operation in the dark with an applied reverse bias. Increased temperature and reverse bias will result in increased dark current. Also, larger active areas will generally have a higher dark current. Noise equivalent power is the power of incident light, at a specific wavelength, required to produce a signal on the detector that is equal to the noise. Common materials of construction for photodiodes include silicon, indium gallium arsenide, germanium, gallium nitride, and silicon carbide.