Light Sensors or Photosensors

Semiconductor-based light sensors as well as light sources are needed in optoelectronics. A light sensor is also known as a photodetector or photosensor. It is a device that is sensitive to light. Some photosensors can serve as energy sources (cells) as well.

A photosensor may be an integral component of an optoisolator or other optically coupled system. In particular, a commercial optical coupler typically has an LED light source and a photosensor in the same package, with leads for connecting it to other circuits, together with power leads.

By definition, the purpose of a photodetector or photosensor is to sense visible light. But there are many applications where sensing of adjoining bands of the electromagnetic spectrum, namely infrared.


A photodiode is a pn junction of semiconductor material that produces electron–hole pairs in response to light. The symbol for a photodiode is shown in Figure.

Symbol of photodiode, photodiode symbol

Two types of photodiodes are available.

  1. A photovoltaic diode generates a sufficient potential at its junction in response to light (photons) falling on it. Hence an external bias source is not necessary for a photovoltaic diode.
  2. A photoconductive diode undergoes a resistance change at its junction in response to photons. This type of photodiode is operated in reverse-biased form.


  • Semiconductor materials such as silicon, germanium, cadmium sulfide, and cadmium selenide are commonly used in photodiodes.
  • The photodiode is connected in reverse bias mode that is the p-lead of the diode is connected to the negative terminal of supply and n-lead is connected to the positive terminal of supply.
working of photodiode, Light sensors,
  • The electrons of the semiconductor material start moving when the photodiode absorbs the light energy.
  • The breakdown condition may occur at about 10 V and the corresponding current will be nearly proportional to the intensity of light falling on the photodiode. Hence this current can be used as a measure of the light intensity.
  • The sensitivity of a photodiode is rather low due to the reverse-bias operation because the output current level is usually low (fraction of mA). The response speed of a photodiode is high.

Advantages of Photodiode

  • They possess a good frequency response compared to photoconductors.
  • They produce a spectral response.
  • They have good linearity.
  • The problem of noise is low.

Disadvantages of Photodiode

  • The active area of the photodiode is very small.
  • Requirement of bias voltage to maintain the linearity over a wide range.
  • The dark current increases rapidly with an increase in temperature.
  • If the illumination level is low then a very low voltage is available at the output which needs amplification.

Applications of Photodiode

  • Photodiodes are widely used in high-quality light meters and demodulations.
  • Since the response time of photodiodes is very fast, they are used in film soundtrack recorders.
  • They are used in optical communication systems to detect the modulated light.
  • It is used in barcode scanner and character recognition.
  • It is used in the obstacle detection system.
  • It can be used in printers as page presence and page counter
  • It is used in proximity detection and oximeters.
  • It is also used in optical encoders and decoders.


Any semiconductor photosensor with amplification circuitry built into the same package (chip) is popularly called a phototransistor. Hence a photodiode with an amplifier circuit in a single unit might be called a phototransistor.


  • Strictly, a phototransistor is manufactured in the form of a conventional bipolar junction transistor with base (B), collector (C) and emitter (E) leads.
  • The semiconductor material is enclosed in an opaque container in which the light easily reaches to the photosensitive element.
  •       The symbol for a phototransistor is shown in Figure. 

phototransistor symbol

This is an npn transistor. The base is the central (p) region of the transistor element. The collector and the emitter are the two end regions (n) of the element.


Under operating conditions of the phototransistor the collector–base junction is reverse biased (i.e., a positive lead of the circuit is connected to the collector and a negative lead of the circuit is connected to the base of an npn transistor).

Alternatively, a phototransistor may be connected as a two-terminal device with its base terminal floated and the collector terminal properly biased (positive for an npn transistor).

working of phototransistor
working of phototransistor, phototransistor working
  • Source voltage usually applied between the emitter lead of the transistor and load, the negative potential being at the emitter lead, the collector ic is nearly proportional to the intensity of the light falling on the collector–base junction of the transistor. Hence, ic can be used as a measure of the light intensity.
  • Germanium or silicon is the semiconductor material that is commonly used in phototransistors.

Difference Between Photodiode and Phototransistor

Basis For Comparison




It is a type of PN-junction diode which generates electric current when light or photon is incident on their surface.

It is a type of transistor which converts the light energy into an electrical energy


 Shown above. Shown above.



Current and Voltage

Output Response







Both forward and reversed biasing.

Forward biasing (emitter is more negative as compared to the collector. )


For generating solar power, for detecting ultraviolet or infrared rays, for measuring light etc.

Smoke detector, compact disc players, invisible light receiver, in laser etc.


A photo-field effect transistor is similar to a conventional FET. The symbol shown in Figure is for an n-channel photo-FET.

Photo FET symbol


This consists of an n-type semiconductor element (e.g., silicon doped with boron), called channel. A much smaller element of p-type material is attached to the n-type element.  

The lead on the p-type element forms the gate (G). The drain (D) and the source (S) are the two leads on the channel.


The operation of a FET depends on the electrostatic fields created by the potentials applied to the leads of the FET.

Under operating conditions of a photo-FET, the gate is reverse-biased i.e., a negative potential is applied to the gate of an n-channel photo-FET.

When light is projected at the gate, the drain current id will increase. Hence, drain current can be used as a measure of light intensity.



Photocells are similar to photosensors except that a photocell is used as an electricity source rather than a sensor of radiation.

For more details click here and read section Photovoltaic transducer or Photocell.

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