Photoresistor – Definition, Working, Symbol, Types, Uses & Characteristics

What is a Photoresistor?

Photoresistor is the combination of words “photon” (meaning light particles) and “resistor”. True to its name, a photo-resistor is a device or we can say a resistor dependent on the light intensity. For this reason, they are also known as Light Dependent Resistors a.k.a. LDRs. So in single line we can write it as:
“Photoresistor is a variable resistor whose resistance varies inversely with the intensity of light”

In other words, the flow of electric current through the photoresistor increases when the intensity of light increases. Photoresistor changes its resistance only when it is exposed to light.

Symbol of Photoresistor

In order to represent a Photoresistor in a circuit diagram, the symbol chosen was that would indicate it to be a light dependent device along with the fact that it is a resistor. While mostly the symbol used is shown in figure 2a (two arrows pointing to a resistor), some prefer to encase the resistor in a circle like that shown in figure 2b.

Complete Details About Working principle of a Photoresistor

How photoresistor works?

When the light falls on the photoresistor, some of the valence electrons absorbs energy from the light and breaks the bonding with the atoms. The valence electrons, which break the bonding with the atoms, are called free electrons.

When the light energy applied to the photoresistor is highly increased, a large number of valence electrons gain enough energy from the photons and breaks the bonding with the parent atoms. The large number of valence electrons, which breaks the bonding with the parent atoms will jumps into the conduction band.

The electrons present in the conduction band are not belongs to any atom. Hence, they move freely from one place to another place. The electrons that move freely from one place to another place are called free electrons.

When the valence electron left the atom, a vacancy is created at a particular location in an atom from which the electron left. This vacancy is called as hole. Therefore, the free electrons and holes are generated as pairs.

The free electrons that are moving freely from one place to another place carry the electric current. In the similar way, the holes moving in the valence band carry electric current. Likewise, both free electrons and holes will carry electric current. The amount of electric current flowing through the photoresistor is depends on the number of charge carriers (free electrons and holes) generated.

When the light energy applied to the photoresistor increases, the number of charge carriers generated in the photoresistor also increases. As a result, the electric current flowing through the photoresistor increases.

Increase in electric current means decrease in resistance. Thus, the resistance of the photoresistor decreases when the intensity of applied light increases.

Photoresistors are made of high resistance semiconductor such as silicon or germanium. They are also made of other materials such as cadmium sulfide or cadmium selenide.

In the absence of light, the photoresistors acts as high resistance materials whereas in the presence of light, the photoresistors acts as low resistance materials.

Types of Photoresistors

Photoresistors are classified into two types based on the material used to construct them:

(i) Intrinsic photoresistor
(ii) Extrinsic photoresistor

Define Intrinsic photoresistor
Intrinsic photoresistors are made from the pure semiconductor materials such as silicon or germanium. The outermost shell of any atom is capable to hold up to eight valence electrons. However, in silicon or germanium, each atom consists of only four valence electrons. These four valence electrons of each atom form four covalent bonds with the neighboring four atoms to completely fill the outermost shell. As a result, no electron is left free.

When we apply light energy to the intrinsic photo resistor, only a small number of valence electrons gain enough energy and becomes free from the parent atom. Hence, a small number of charge carriers are generated. As a result, only a small electric current flows through the intrinsic photo resistor.

We already have known that increase in electric current means decrease in resistance. In intrinsic photoresistors, the resistance decreases slightly with the increase in light energy. Hence, intrinsic photoresistors are less sensitive to the light. Therefore, they are not reliable for the practical applications.

Each silicon atom consists of four valence electrons and each phosphorus atom consists of five valence electrons. The four valence electrons of the phosphorus atom form four covalent bonds with the neighboring four silicon atoms. However, the fifth valence electron of the phosphorus atom cannot able to form the covalent bond with the silicon atom because the silicon atom has only four valence electrons. Hence, the fifth valence electron of each phosphorus atom becomes free from the atom. Thus, each phosphorus atom generates a free electron.

Define Extrinsic Photoresistor
Extrinsic photoresistors are made from the extrinsic semiconductor materials. Let us consider an example of extrinsic photoresistor, which is made from the combination of silicon and impurity (phosphorus) atoms.

The free electron, which is generated will collides with the valence electrons of other atoms and makes them free. Likewise, a single free electron generates multiple free electrons. Therefore, adding a small number of impurity (phosphorus) atoms generates millions of free electrons.

In extrinsic photoresistors, we already have large number of charge carriers. Hence, providing a small amount of light energy generates even more number of charge carriers. Thus, the electric current increases rapidly.

Increase in electric current means decrease in resistance. Therefore, the resistance of the extrinsic photoresistor decreases rapidly with the small increase in applied light energy. Extrinsic photoresistors are reliable for the practical applications.

What are the Main Advantages and Disadvantages of photoresistor?

Advantages of Photoresistor
1. Small in size
2. Low cost
3. It is easy to carry from one place to another place.
Disadvantages of Photoresistor
1. The accuracy of photoresistor is very low.

What are the Uses and Applications of Photoresistor?

Automatic Street Lights: One of the prominent uses of Photoresistor that we experience in daily life is in the circuits of automatic street lights, as already hinted in the introductory paragraph. Here they are so used in a circuit that the street lights turn on as it starts getting dark and turns off in the morning.

Some of the Photoresistors are used in some of the consumer items like light meters in camera, light sensors like in robotic projects, clock radios etc.
They are also used to control the reduction in gain of dynamic compressors. They are also considered as a good infra-red detector and hence find application in infrared astronomy. With this we come to the conclusion of the article, let’s rewind what we learnt in this short tutorial.

Short and Important Notes on Photoresistor

“Photons” + “Resistor” = Photoresistor: A special type of variable resistor whose resistance depends on the intensity of light falling on it.
Other Names: Photoconductor, Photocell , Light dependent resistor(LDR)
Willoughby Smith : First scientist to discover the photoconductivity in Selenium(a semiconductor)
Construction: Made of semiconductor material that is photosensitive. They do not have any PN junction.
Working Principle: When light falls on the photosensitive material (or on the Photoresistor), the valence electrons absorb the light energy and break free from the nucleus to become free electrons. These electrons lead to flow of current when an external force like an electric field is applied.

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