Photodiode Vs. Phototransistors: Applications, Working Principle and Uses

a key difference between the photodiode and the phototransistor is that the photodiode uses a PN-junction diode which converts the light energy into an electric current, while the phototransistor uses the ordinary transistor (NPN transistor) for the conversion of light into current.

What is Photodiode? (Definition)

a photodiode is a 2-terminal semiconductor device that produces current when illuminated. A photodiode works on the principle of the photoelectric effect, thereby converting the light energy falling on the surface of the material into electrical energy. The greater the intensity of the incoming radiation, the greater the current through the device.symbol ans construction of photodiode

working principle:

  • A device that converts light into electrical energy and belongs to a family of semiconductors called photodiodes.
  • It consists of a PN junction as a normal diode.
  • Electric current is produced by this device when photons of light collide with the body.
  • There is an optical filter for light collection, lenses are placed on it.

What is Phototransistor? (Definition)

A phototransistor is a device almost similar to a regular junction transistor. But the only variation is that the base collection zone is a large area. In a phototransistor, no fundamental current is provided as the input, but instead, the light energy is used to trigger the transistor. This transmitted light energy generates an electric current in the device due to the photoelectric effect.working structure of phototransistor

working principle:

  • The transistor that detects the absorbed light and allows the current to flow between the emitter and the collector is called a phototransistor.
  • It has 3 main parts along with the base zone adjusted to detect light.
  • It has a structure like a transistor with the difference that it works with light.

Difference Between Photodiode Vs. Phototransistor in tabular form:

Photodiode Phototransistor
Photodiode consist of normal p-n junction housed in a small enclosure which a transparent window through which light can fall inside. Phototransistors can resemble photodiodes giving current amplification due to transistor action. Few of these devices are molded in transparent cases with convex. This convex acts as a lens that focuses light on the transistor. As a result of this extra minority, carriers are liberated at the reverse-biased CB Junction (collector or base).
A photodiode is operated in reverse bias in which leakage current increases in proportion to the amount of light falling on the junction. This is the result of the result which breaks the bonds in the crystal lattice of the semiconductor producing and electrons and holes. This effect is similar to the photovoltaic cells. This generated leakage current is later amplified. If used in this way, a connection to the base terminal is not needed. Many phototransistors do not have a base lead.
Photodiodes are used as fast counters and used in light meters to measure light energy. A phototransistor is about one hundred times more sensitive than a photodiode.

Advantages and Disadvantages of Photodiode and Phototransistor:

Advantages and Disadvantages of  Photodiode and Phototransistor
Advantages Disadvantages
Photodiode Linear response, Very fast, Small size. Photodiodes can be used in multisensory applications (e.g. tape reader) A small change in characteristics, low-level output.
Phototransistor Phototransistors can directly interface to small loads reasonably fast Nonlinear,


Photodiode Vs. Phototransistors both work on the principle of the inner photoelectric effect. a photodiode uses an ordinary PN junction diode that has two terminals namely cathode and anode. And in phototransistors, the normal transistor is used. The only difference b/w the Photodiode Vs. Phototransistors is that the phototransistor does not have a base terminal. The base side of the phototransistor captures the light from the source.

Basic information about photodiode and phototransistor:

Operational speed High Low
Sensitivity Low High
Noise interference Less immune More immune
Output response Fast Slow
High-frequency response Provides better results. Provides poor results.

Applications of photodiode:

main applications of a photodiode in our daily life are listed below:

  • Photodiodes with the help of opt couplers provide electric isolation. When two isolated circuits are illuminated by light, optocouplers are used to couple the circuit optically. Optocouplers are faster compared to conventional devices.
  • Photodiodes are used in safety electronics such as fire and smoke detectors.
  • Photodiodes are used in simple day-to-day applications. The reason for their prominent use is their linear response of photodiode to light illumination.
  • They are used in numerous medical applications. Photodiodes are used in instruments that analyze samples, detectors for computed tomography and also used in blood gas monitors.
  • Photodiodes are used in solar cell panels.
  • Photodiodes are used in logic circuits.
  • Photodiodes are used in the detection circuits.
  • Photodiodes are used in character recognition circuits.
  • Photodiodes are used for the exact measurement of the intensity of light in science and industry.
  • Photodiodes are faster and complex than normal PN-junction diode and hence are frequently used for lighting regulation and optical communication.

Applications of phototransistor:

phototransistors are used in many applications:

  • Opto-isolators: phototransistor is used as the light sensor, the light emitter being relatively close, but at a different potential. The physical gap between the light emitter and detector provides a considerable degree of electrical isolation.
  •  Position sensing: the optoisolator can be used to detect the position of a moving element, often the moving element has a light or interrupts a beam of light that the phototransistor detects.
    • Security systems: Phototransistors can be used in many ways in security systems, often detecting whether a beam of light is present or has been broken by an intruder.
  •  Coin counters: phototransistor can be used in coin and other counting applications. A beam of light is interrupted each time a coin or other item passes a given point. The number of times the beam is interrupted equals the number of coins or objects to be counted.

Stay tuned to whatmaster to know more about the modern-day uses of photodiodes and phototransistors with interesting information and fall in love with learning!!

Physics Differences : 

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Back to top button