What is Photodiode and How it works

A photodiode is a type of diode that converts light energy into electrical energy. This is essentially a light sensor that produces a current when light hits it. Also known as a photodetector, light sensor, or light sensor. The phenomenon of converting light energy into electrical energy is called the photovoltaic effect. A solar cell or solar panel consists of a series of photodiodes (also called photovoltaic cells) that convert solar energy into electricity.


Features of Photodiode

  • The photodiode should always be reverse-biased.
  • The applied reverse bias voltage should be low.
  • Produces low noise
  • Low-temperature sensitivity
  • High gain

Symbol of Photodiode


Construction of Photodiode

A photodiode consists of two layers, a P-type semiconductor, and an N-type semiconductor. Here, the P-type material is formed by diffusion of a lightly doped P-type substrate. Thus, the diffusion process forms a layer of P+ ions. Then, an N-type epitaxial layer is grown on the N-type substrate. A P+ diffusion layer is formed on the heavily doped N-type epitaxial layer. The contacts are metallic and form a two-terminal cathode and anode.

The front area of ​​the diode is divided into two types: active surface and non-active surface. The non-active surface is composed of SiO2 (silicon dioxide) and the active surface is coated with an anti-reflective material. The active area is so-called because the rays hit it.

Rays do not hit inactive surfaces. The active layer is coated with an anti-reflection material to ensure that light energy is not lost and converted to electricity as much as possible. The overall dimensions of the unit are about 2.5 mm.


VI Characteristics of Photodiode

The VI characteristic shows the relationship between voltage and current for a device. The horizontal or x-axis represents voltage and the vertical or y-axis represents the current through the device. Below is the characteristic curve of the photodiode.


We know that photodiodes operate in reverse bias, so the graph is between reverse voltage and reverse current. Reverse voltage is plotted on the negative x-axis and reverse current in microamperes is plotted on the negative y-axis.

The reverse current does not change as the reverse voltage changes. However, the reverse current increases with increasing incident light intensity. When the light intensity is zero, there is no visible reverse current except for a very small constant current called dark current. With increasing light intensity, the reverse current increases linearly.

Working of Photodiode

The principle of operation of a photodiode is that when a photon of sufficient energy hits the diode, a pair of holes is created. This mechanism is also known as the internal photoelectric effect. When absorption occurs at the depletion region junction, the charge carriers are removed from the junction by the built-in electric field in the depletion region.

Therefore, holes in this region move toward the anode and electrons toward the cathode, generating a photocurrent. The total current through the diode is the sum of the light deficit and the photocurrent. Therefore, the missing current should be reduced to maximize the sensitivity of the device.


Types of Photodiode

  • PN junction photodiode
  • PIN photodiode
  • Avalanche photodiode

Modes of Operation

  • Photovoltaic mode
  • Photoconductive mode
  • Avalanche diode mode

Photovoltaic mode

It is also called zero-bias mode, this mode produces a voltage across the lit photodiode. As a result, the dynamic range is very small and non-linearity of the formed voltage is required.

Photoconductive mode

Photodiodes used in this photoconductive mode are typically reverse-biased. Applying a reverse voltage widens the depletion layer and reduces the response time and junction capacitance. This mode is too fast and produces electronic noise

Avalanche diode mode

Avalanche diodes operate under high reverse bias conditions, allowing for increased avalanche breakdown of each photo-generated electron-hole pair. The result is an internal amplification of the photodiode that slowly increases the response of the device.

S. NoAdvantages of PhotodiodeDisadvantages of Photodiode
1Works on light energy and low voltageThe active area is relatively small
2Enables high-speed communication over fiber optic cablesLess sensitive than other components
3Low dark current compared to phototransistorsAn external power source is required to supply sufficient current to the load
4High quantum efficiencyAmplification is required when operating in low light
5It has very low resistanceThermal noise greatly affects its performance
6Robust and long service lifePoor temperature stability

Applications of Photodiode

  • Digital and Logic circuits
  • Solar cell panels
  • Detection circuits
  • Lighting regulation
  • Optical communication
  • Compact disc players
  • Space applications

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