Photoconductive Cell | Photoresistor | Light Dependent Resistor

In this lecture, we are going to learn about the Photoconductive Cell, how Photoconductive Cell is working, its construction, working principles, and applications of Photoconductive Cell.

What is a Photoconductive Cell?

• A photoconductive cell (PC) is a light-sensitive semiconducting device whose electrical conductivity varies with the amount of light falling on it.

• If voltage is applied across the cell it is found to vary with the light intensity.

• With no illumination, the conductivity decreases and increases with illumination.

Note: The photoconductive cell is also referred to as a photoresistor or light-dependent resistor.

Working Principle of Photoconductive Cell

The photoconductive cell works based on the principle of the photoconductive effect.

• The photoconductive effect is a process in which the conductivity of a semiconductor material changes according to the wavelength and intensity of the radiation.

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The circuit symbol of the Photoconductive Cell

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Photoconductive cell construction and working

• The photoconductive cell (PC) is a two-terminal light-sensitive semiconductor device.

• The semiconductor materials are made in the form of zig-zag strips with their ends being attached to external pins. The whole assembly is enclosed in a glass cover or protected in transparent plastic.

• Cadmium sulfide (CdS) and cadmium selenide (CdSe) are the materials that are normally used in photoconductive cell manufacture.

• Nowadays, other materials like lead selenide and lead telluride are also used in the manufacture of photoconductive cells.

The simple circuit for a photoconductive cell is shown in the figure below:

• In the circuit shown, the resistance of the photoconductive cell is in series with an external resistance(R) which limits the current. A bias voltage is also used in series with the photoconductive cell.

• In the absence of light, the resistance of the photoconductive cell is very high and hence the current as measured using an ammeter is also very low. This current is called dark current and the resistance corresponding to this is called dark resistance.

• This reduction in current leads to a voltage drop across the load resistor R.

• When the photoconductive cell is exposed to external light, the resistance of the photoconductive cell decreases, resulting in a large current flow called the joint current. This is because the light energy supplied to the photoconductive cell causes its covalent bonds to be broken, resulting in the creation of electron-hole pairs.

• Due to this increase in the number of charge carriers, the resistance of the material is proportional to the number of charge carriers.

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Photoconductive cell Applications

1. The photoconductive cells are used in light-sensitive alarms, fast recording situations, automatic street lights, Lightning control, etc.
2. The photoconductive cells can be used for detecting the location of aircraft and ships with the help of radiation coming out from their exhaust.
3. They are used to either switch on or off the transistors in electronic circuits.
4. Photoconductive cells are used at the LED current level.

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