Photodiode tutorial

Photodiode – Characteristic, symbol, definition




Photodiode – a semiconductor diode that serves as a photo detector. Its electrical characteristics are dependent on the light radiation put on it. It is placed in a characteristic casing with transparent window. Photodiodes are made as elements with pn or pin junctions with an undoped layer.


photodiode symbol

Photodiode Symbol

Photodiode operating principles:

Energy is delivered to the junction by illuminating photodiode (injection of minority carriers) wherein the pairs of hole – electron are generated. It causes the generation of current proportional to the light flux that incident on the diode. Total current that flows through the junction is the sum of two components: the saturation current (dark current) and current dependent from light intensity. The free electrons are attracted by the positive space charge on the n-type area border, while the holes penetrate into the type p area. It is required for junction to be reversed biased from an external source.

Photodiode operating modes:

  • no bias mode – photodiode acts as a source of electric current, while junction is being illuminated it creates EMF  (electromotive force) that generates electric current. This phenomenon is called photovoltaic phenomenon,
  • reversed bias mode – barrier voltage is applied to a diode, it acts as resistor that resistance depends on light intensity that enters “inside the component” through the transparent window. In case of absence of light, so called small, dark current flows. After illuminating, the number of minority loads increase causing increasing the reverse currentphotodiode characteristicCurrent-voltage characteristics of photodiodes


When observing current-voltage characteristics of photodiode, we can notice the waveforms in different quadrants of coordinate system. The third quarter characteristics show the usage of photodiode as a photo detector (light sensor). It corresponds to the current saturation of pn junction. However, in the fourth quarter of characteristics, the diode works as a light radiation converter – solar battery. If current doesn’t flow through illuminated diode, the amount of voltage that arises on the terminals is called photovoltaic voltage. Solar batteries are commonly used in electrical equipment power supply for example sanitation and battery charging when weather conditions or light are favourable.

Practical applications of photodiode:

  • in industrial automation,
  • in optical switching devices,
  • in metering systems of electrical quantities,
  • in metering systems of not electrical quantities,
  • in photometry,
  • in remote control systems,
  • in high-speed A/C converters.




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