Photodiode Working Principle, Characteristics and Applications

A photodiode is a PN-junction diode that consumes light energy to produce electric current. Sometimes it is also called as photo-detector, a light detector, and photo-sensor. These diodes are particularly designed to work in reverse bias condition, it means that the P-side of the photodiode is associated with the negative terminal of the battery and n-side is connected to the positive terminal of the battery. This diode is very complex to light so when light falls on the diode it easily changes light into electric current. The solar cell is also branded as large area photodiode because it converts solar energy into electric energy. Though, solar cell works only in bright light.

What is a Photodiode?

A photodiode is one type of light detector, used to convert the light into current or voltage based on the mode of operation of the device. It comprises of optical filters, built-in lenses and also surface areas. These diodes have a slow response time when the surface area of the photodiode increases. Photodiodes are alike to regular semiconductor diodes, but that they may be either visible to let light reach the delicate part of the device. Several diodes intended for use exactly as a photodiode will also use a PIN junction somewhat than the usual PN junction.

Photodiode Symbol
Photodiode Symbol

Some photodiodes will look like a light emitting diode. They have two terminals coming from the end. The smaller end of the diode is the cathode terminal, while the longer end of the diode is the anode terminal. See the following schematic diagram for the anode and cathode side. Under forward bias condition, conventional current will flow from the anode to the cathode, following the arrow in the diode symbol. Photocurrent flows in the reverse direction.


Types of Photodiode

Although there are numerous types of photodiode available in the market and they all works on the same basic principles, though some are improved by other effects. The working of different types of photodiodes work in a slightly different way, but the basic operation of these diodes remains the same. The types of the photodiodes can be classified based on its construction and functions as follows.

These diodes are widely used in the applications where the detection of the presence of light, color, position, intensity is required. The main features of these diodes include the following.

  • The linearity of the diode is good with respect to incident light
  • Noise is low.
  • The response is wide spectral
  • Rugged mechanically
  • Light weight and compact
  • Long life

The required materials to make a photodiode and the range of electromagnetic spectrum wavelength range includes the following


  • For silicon material, the electromagnetic spectrum wavelength range will be (190-1100) nm
  • For Germanium material, the electromagnetic spectrum wavelength range will be (400-1700) nm
  • For Indium gallium arsenide material, the electromagnetic spectrum wavelength range will be (800-2600) nm
  • For Lead (II) sulfide material, the electromagnetic spectrum wavelength range will be <1000-3500) nm
  • For Mercury, cadmium Telluride material, the electromagnetic spectrum wavelength range will be (400-14000) nm

Because of their better band gap, Si-based photodiodes produce lower noise than Ge-based photodiodes.

Working of Photodiode

The working principle of a photodiode is, when a photon of ample energy strikes the diode, it makes a couple of an electron-hole. This mechanism is also called as the inner photoelectric effect. If the absorption arises in the depletion region junction, then the carriers are removed from the junction by the inbuilt electric field of the depletion region. Therefore, holes in the region move toward the anode, and electrons move toward the cathode, and a photocurrent will be generated. The entire current through the diode is the sum of the absence of light and the photocurrent. So the absent current must be reduced to maximize the sensitivity of the device.

PN Junction Diode
PN Junction Diode

Modes of Operation

The operating modes of the photodiode include three modes, namely Photovoltaic mode, Photoconductive mode and avalanche diode mode

Photovoltaic Mode: This mode is also known as zero bias mode, in which a voltage is produced by the lightened photodiode. It gives a very small dynamic range & non-linear necessity of the voltage formed.

Photoconductive Mode: The photodiode used in this photoconductive mode is more usually reverse biased. The reverse voltage application will increase the depletion layer’s width, which in turn decreases the response time & the junction capacitance. This mode is too fast and displays electronic noise

Avalanche Diode Mode: Avalanche diodes operate in a high reverse bias condition, which permits multiplication of an avalanche breakdown to each photo-produced electron-hole pair. This outcome in an internal gain in the photodiode, which slowly increases the device response.

Applications of Photodiode

  • The applications of photodiodes involve in similar applications of photodetectors like charge-coupled devices, photoconductors, and photomultiplier tubes.
  • These diodes are used in consumer electronics devices like smoke detectors, compact disc players, and televisions and remote controls in VCRs.
  • In other consumer devices like clock radios, camera light meters, and street lights, photoconductors are more frequently used rather than photodiodes.
  • Photodiodes are frequently used for exact measurement of the intensity of light in science & industry. Generally, they have an enhanced, more linear response than photoconductors.
  • Photodiodes are also widely used in numerous medical applications like instruments to analyze samples, detectors for computed tomography and also used in blood gas monitors.
  • These diodes are much faster & more complex than normal PN junction diodes and hence are frequently used for lighting regulation and in optical communications.

V-I Characteristics of Photodiode

A photodiode continually operates in a reverse bias mode. The characteristics of the photodiode are shown clearly in the following figure, that the photocurrent is nearly independent of reverse bias voltage which is applied. For zero luminance, the photocurrent is almost zero excluding for small dark current. It is of the order of nano amperes. As optical power rises the photo current also rises linearly. The max photocurrent is incomplete by the power dissipation of the photo diode.

V-I Characteristics
V-I Characteristics

Thus, this is all about photodiode working principle, characteristics, and its applications. We hope that you have got a better understanding of this concept. Furthermore, any queries regarding this concept or to implement electrical and electronic projects for Engineering Students. Please give your valuable suggestions by commenting in the comment section below. Here is a question for you, what is the function of photodiode?

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  1. how the respomnse time of photodiode is slow due to increase in intrinci region

  2. Jaden Massey says:

    Can a photodiode be used to power a LED light? If so, then how.

    Thank you in advance

    1. what about the photovoltaic mode of operation of the photodiode???

      1. Tarun Agarwal says:

        Hi Alex
        The photovoltaic mode is also known as zero bias mode, in which a voltage is generated by the lightened photodiode. It gives a very small dynamic range as well as the non-linear necessity of the voltage formed

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