Introduction of Photodiode

A photodiode is a special type of PN junction diode in the light energy are converted into an electric current or that generate the eclectic current when light exposed known as photodiode, it has also called photo-detector or photo-sensor and light detector. It is design to operate in reverse bias region. The material uses for photodiode construction are Germanium Silicon Indium and gallium arsenide.

What is a Photodiode?

A photodiode is a semiconductor device that converts the light energy to electrical energy (voltage or current). The other name of photodiode is photo-detector, photo-sensor, or light detector. It is operate in reverse bias region mans the positive terminal of the supply is connected to the N terminal and the negative side of the batter is connected to the P terminal. It is highly sensitive diode to light so when light or photons falls on the it. It convert the light to electrical energy. The solar cells are the best example. Because it converts the light energy to electrical energy. The solar cells only work on bright light.

The few photodiodes are look like the Light Emitting Diode (LED) because It has two terminals smaller terminal acts as cathode and longer terminal acts as anode as shown below.


Photodiode symbol

The symbol of photo diode is similar to the Light Emitting Diode (LED) but the inwards arrow is the opposed to outwards in the LED. It has two terminal anode and cathode. The symbol of photodiode is shown below.



The construction of photodiode is similar to the PN junction diode because it has two layers of P-type and N-type semiconductor, the P-type semiconductor are form to add pentavalant impurity and N-type semiconductor form by the addition  of trivalent impurity. The contacts of two layers are made up of metals to form two terminal cathode and anode. The PIN diode (p-type, intrinsic and n-type) structure is mostly use in photodiode in place of PN junction diode because the PIN diode has fast response time, PIN are mostly use in high speed applications.


The diode is dividing in two parts one is active surface and other is non-active surface. The non active region are SiO(Silicon-di Oxide) and the active surface is coated with anti-reflection material so that the light energy is not lost and the maximum of it can be converted into current. The lights are fallen in active surface area and non active surface the light ray do not strike. The range of dimensions has in order of 2.5 mm. it is noted that a current through the diode is in micro-ampere and is measured through an ammeter.


In photodiode, a small reverse current flows through the diode that is called dark current. The dark current followed by minority carrier, and also flow when no radiation expose on it.

Generally, when the light fall on the photodiode the covalent bond are ionized, due to this hole and electron pairs produce. The holes and elections are the cause of photocurrent. These pair is form when the energy of photons is more than the 1.1eV hits the diode. When the high energy of photons is entered into the depletion region of diode, it breaks the covalent bond and releases the elections from the atom. When the electron are eject that produce the hole and flow of current.


As we know, an electron is negative charge and holes are positive charge. The electric field is created by depletion energy and this electric field moves the electron hole pairs away from the junction. Hence the movement of holes to anode and elections moves to cathode, this movement produce photocurrent.

The photon energy is directly proportional to the photon absorption intensity. The sum of the absence of light and the photocurrent is total current through the photodiode. So the sensitivity of the device is depends on the absent current.

Modes of operation 

The operation of photodiode is in three mode and they are given below:-

  • Photoconductive Mode
  • Photovoltaic Mode
  • Avalanche Diode Mode

Let us discuss of these three modes in brief:

Photoconductive Mode

In photoconductive mode, photodiode will act in reverse bias condition, the cathode is connected to the positive terminal of the batter and the anode terminal is connected to the negative terminal of the batter. When we increase reverse voltage. Due to this increase voltage the width of the depletion layer will also increase. The increment of depletion region will reduce the junction capacitance and response time. In this mode of operation is fast but produce electronic noise.


Photovoltaic Mode

The overview of photovoltaic mode is called zero bias modes when the photodiode is operate in ultra level light and low frequency application. So this mode is used. The voltage is produce when photodiode is irradiated by a flash of light. And the voltage will be very small range and non liner characteristic. When it is connected with OP-Amp in photovoltaic mode, here will be a very less variation with temperature.


Avalanche Diode Mode

The avalanche diode mode, the diode operates in high reverse bias voltage. It allows producing to electron hole pair and it also produces internal gain within the photodiode. Hence the internal gain increases the device response.

Connecting a Photodiode in an External Circuit

The photodiode is operating in reverse bias in a circuit. The cathode is connected to the positive of the supply voltage and anode to ground. When illuminated by light, current will flow from the cathode to anode.


The amount of current flow form anode to cathode is very small and this current value is insufficient to drive an electronic circuit. When we use the photodiode in external circuit we need a battery source help to increase the current in a circuit.

V-I Characteristics of Photodiode

It is operated in reverse bias region below the figure shows the V-I characteristics of photodiode. The X-axis represents the reverse voltage and the Y-axis is reverse current in microampere and the Reverse current does not depend on reverse voltage both are independent with each other. Reverse current will be almost zero when no light illumination. The dark current is the minimum amount of current present. Once when the light illumination increases the photodiode will start conduct and produce a reverse current and the reverse current increases linearly.


Types of Photodiode

The types of photodiodes are classified on the working and construction. The working of different types of photodiodes is slightly different.

Applications of Photodiode

  • It is use in linear response of photodiode to a light illumination. When light fall a current will produce and this produce current will be measure by galvanometer connected to the circuit.
  • It is use in electric isolation with help of optocouplers. When two isolated circuits are illuminated by light, the photodiode is use as an optocouplers to couple the circuit optically. Compared with a conventional device, the optocouplers are high speed.
  • It is use in safety electronics device like fire, TV units and smoke detectors.
  • It is use in cameras act as photo sensors.
  • It is used in scintillators charge-coupled devices, photomultiplier tubes and photoconductors.
  • It is widely used medical applications like analyze samples, detectors for computed tomography and blood gas monitors.
  • It is much faster and more complex than normal PN junction diode so it is use in lighting regulation and optical communication

Advantages of photodiode

The advantages of photodiode are following which are given below.

  • It has Less resistance and low noise
  • Its operation is high Quick and high
  • Long life span
  • Fastest photodetector
  • Spectral response and Frequency response are good
  • Solid and low-weight
  • It is extremely responsive to the light
  • It has small Dark current
  • High quantum efficiency

Disadvantages of photodiode

The disadvantages of photodiode are following which are given below.

  • It has poor Temperature stability
  • It produce small current, they are not enough to drive the electronic circuit.
  • It has small active area
  • Photodiode has high response time
  • It has less sensitivity
  • It uses offset voltage

Which is better Photodiode or Phototransistor?

The working of photodiode and phototransistor is same because both convert the light energy to electrical energy. However the phototransistor is more responsive as compare to photodiode. The comparison of photodiode and phototransistor is give below:

Photodiode Phototransistor
It converts the energy from light to electrical current is known as a photodiode. It is used to change the energy of light into an electrical current using the transistor.
It produce both current and voltage It only produce current
The response time is speed The response time is slow
It is less responsive as compared with a phototransistor It is responsive and generates a huge o/p current.
It works in both the biasing conditions It works in forward biasing only.
It is used in a light meter, solar power plant, etc It is used to detect the light



The photo diode is a light sensitive electronics component. It is made by semiconductor and the photo diode convert the light energy to electrical energy. The current produce by photodiode is very small. It is mostly use in solar panel.

Below the table shows a simplified chart comparing three different photodiodes.

Parameters P-N PIN APD
LOW LIGHT Poor Good Best
COST Best Good Poor
LOW NOISE Good Best Poor


Frequently ask question

  1. What is photodiode explain its working principle?

The photo diode is convert the light energy to electrical energy

Working: In photo diode, a small reverse current flows through the diode that is called dark current. The dark current followed by minority carrier, and also flow when no radiation expose on it. Generally, when the light fall on the photo diode the covalent bond are ionized, due to this hole and electron pairs produce. The holes and elections are the cause of photocurrent. These pair is form when the energy of photons is more than the 1.1eV hits the diode.

  1. For what purpose photodiode is used?

The purpose of photo diode is use to convert the light energy to electrical energy, optocoupler, communication and consumer electronic

  1. How is photodiode different from LED?

The difference of LED is emitting the light but photo diode absorb the light. And the LED convert the electrical energy to light energy and photo diode is just opposite of the LED.

Also read: Zener Diode, The Diode, Half Wave Rectifier, Schottky Diode,

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