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(foh-toh-dÿ -ohd) A semiconductor device that produces an electric current when illuminated. Infrared-sensitive photodiodes are used, for example, in photometry.



a semiconductor diode characterized by unidirectional photoconductivity when exposed to optical radiation. A photodiode is a semiconductor crystal that usually has p-n junction with two metal terminals—one from the p-region and the other from the n-region—and is enclosed in a metal or plastic housing. Photodiodes are made of such materials as Ge, Si, GaAs, or HgCdTe.

A photodiode may operate in either of two modes. In the first mode, the external circuit contains a DC source, thus producing a reverse bias across the p-n junction (see Figure 1); in the second mode, when no DC source is present, a photodiode functions as a valve. When operating in the first mode, a photodiode is used as a photoconductive cell to control the electric current in a circuit in accordance with the variation in the intensity of the incident radiation. The minority carriers produced upon exposure to radiation diffuse through the p-n junction and weaken the junction’s electric field. Over a wide range, the photoelectric current is linearly related to the intensity of the incident radiation and is practically independent of the bias voltage. As a valve, a photodiode—like a semiconductor photocell—is used to generate a photoelectromotive force.

Figure 1. Schematic diagram of a photodiode and its circuitry when operating with reverse bias: (1) semiconductor crystal, (2) contacts, (3) terminals; (F) electromagnetic radiation flux, (n) semiconductor donor region, (p) semiconductor acceptor region, (E) DC source, (RL) load resistance

The main parameters of a photodiode are as follows: (1) the threshold sensitivity—that is, the minimum signal detected by a photodiode per unit operating-frequency band—which may be as low as 10–14 watt/hertz½ (W/Hz½); (2) the noise level, which does not exceed 10–9 ampere (A); (3) the spectral sensitivity region, which lies in the range 0.3–15 micrometers; (4) the spectral sensitivity—that is, the ratio of the photoelectric current to the flux of incident monochromatic radiation with a known wavelength-—which is 0.5–1 A/W; and (5) the rise time—that is, the time required to reach a rated value of a photoelectric current—which is of the order of 10–7–10–8 sec.

In an avalanche photodiode, which is a p-n variety, the sensitivity is increased by using the avalanche effect in the p-n junction; the effect is based on the impact ionization of atoms in the junction region by photoelectrons. The avalanche effect multiplies the charge carriers by a factor of 102–104.

Photodiodes with a p-i-n structure are also used. Such photo-diodes have characteristics similar to those of p-n photodiodes, but their rise time is substantially smaller—as low as 10–10 sec.

Photodiodes are used, for example, in automated devices, laser technology, computer technology, and measurement technology.


Trishenkov, M. A., and A. I. Frimer. “Fotoelektricheskie poluprovodnikovye pribory s p-n perekhodami.” In the collection Poluprovodnikovye pribory i ikh primenenie. Moscow, 1971.
Riabov, S. G., G. N. Toropkin, and I. F. Usol’tsev. Pribory kvantovoi elektroniki. Moscow, 1976.



A semiconductor diode in which the reverse current varies with illumination; examples include the alloy-junction photocell and the grown-junction photocell. Also known as photoconductor diode.


A light sensor (photodetector) that allows current to flow in one direction from one side to the other when it absorbs photons (light). The more light, the more current. Used to detect light in camera sensors, optical fibers and other light-sensitive applications, a photodiode is the opposite of a light emitting diode (see LED). Photodiodes detect light and let electricity flow; LEDs receive electricity and emit light.

Solar Cells Are Photodiodes
Solar cells are photodiodes that are chemically treated (doped) differently than the photodiode used as a switch or relay. When solar cells are struck by light, their silicon material is excited to a state where a small electrical current is generated. Many arrays of solar cell photodiodes are required to power a house. See photocell and phototransistor.
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