Photodiode tutorial

Photodiode – Characteristic, Symbol, Definition



Photodiode – a type of semiconductor diode that serves as a photo detector. Its electrical properties are dependent on the light radiation that is illuminating it. It is easy to recognize this component among other semiconductor diodes due to its characteristic housing with transparent window. Internal photodiode component construction is based on the p-n junction or p-i-n structure (p-i-n-with an intrinsic or undoped layer between two p-n doped semiconductor crystals).

photodiode symbol

Fig. 1. Photodiode Symbol

Photodiode – Principle of operation

Energy is delivered to the photodiode junction while it is being illuminated by photons (light) – injection of minority carriers takes place, wherein the pairs of hole and electron are generated. It causes the generation of current, which is proportional to the light flux that  “is put” on the photodiode (photoelectric effect). Total current that flows through the junction is the sum of two components: the saturation current (dark current) and current dependent from illumination intensity. The free electrons are attracted by the positive charged particles on the n-type area border, while the holes penetrate into the p-type area. 

Photodiode – Modes of operation

  • Photovoltaic mode (no-bias mode) – Photodiode acts as a source of electric current. While its junction is being illuminated by photons, it then creates EMF  (electromotive force), which means electric current. This phenomenon is called photovoltaic phenomenon, which is the basis for solar cells. We can say, that a typical solar cell is just a much bigger photodiode,
  • Photoconductive mode (reverse-bias mode) – Very often, reverse voltage is applied to a photodiode (reverse-bias of the cathode with respect to the anode) to widen the depletion layer, which decreases the junction’s capacitance. (If you are confused what I am talking about, check this article about Semiconductor Diode’s internal construction.)In case of absence of the component illumination, so called dark current flows, which has low values. In addition, reverse-bias also has influence on the dark current. After illuminating, the number of minority carriers raise, which causes increase of the reverse current.
photodiode characteristic

Fig. 2. Current-voltage characteristics of photodiode

When observing current-voltage characteristics of photodiode, take a look at the waveforms in different quadrants of coordinate system. The third quarter characteristic shows the usage of photodiode as a photodetector (light sensor). It corresponds to the current saturation of the p-n junction. However, in the fourth quarter of characteristic, photodiode 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 for this purpose.

Photodiode – Applications

Photodiodes are used in many industrial areas and devices, which include:

  • industrial automation,
  • optical switching devices,
  • measuring systems of electrical quantities,
  • measuring systems of non-electrical quantities,
  • photometry,
  • remote control systems,
  • high-speed Analog/Digital (A/D) converters.

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