Photoresistor VS Photodiode VS Phototransistor

Hey, in this article we are going to know the difference between Photoresistor, Photodiode, and Phototransistor. The working principles of all of them are related to light or photons but there are some differences. All of them work on the principle of the inner photoelectric effect. All of them are very useful and have their own advantages, disadvantages, and applications. All of them are used in different electronic circuits, and devices as per requirement. You can see their application in modern automation electronic circuits such as automatic light control, alarm systems, darkness sensing, etc.

Symbolic Difference Between Photoresistor, Photodiode, and Phototransistor

Photoresistor VS Photodiode VS Phototransistor, Difference Between Photoresistor, Photodiode, and Phototransistor

You can see in the above diagram, the symbol of the photoresistor is almost the same as a normal resistor. And the arrow towards the resistor indicates the light.

The symbol of the photodiode is almost the same as the normal PN Junction Diode. Here also the arrow towards the Diode symbol indicates the light.

The symbol of the Phototransistor is almost similar to the bipolar transistor with two arrows pointing towards the junction of the bipolar transistor.

Constructional Difference between Photoresistor, Photodiode, and Phototransistor

A photoresistor is made up of semiconductor material such as silicon or germanium. The photoresistor is a very simple semiconductor device, it does not have any junction like a photodiode or phototransistor. In normal conditions, its electrons are available in the valance band. When it is exposed to light, the electrons extract energy from the photons of the light source and move to the conduction band. Once the electrons are moved to the conduction band, the photoresistor allows the current to flow through it. So, basically, the resistance value changes with the variation of light that falls upon it.

A photodiode has the same construction as a normal PN junction diode. Just the difference is its junctions are exposed with a transparent material to allow the light to fall on the junction. So when light falls on it, it creates electron-hole pairs. Those electron-hole pairs produce an electric current. This is also known as the photoelectric effect. It also conducts current in one direction only according to falling light on it.

A phototransistor has almost the same construction as a normal bipolar transistor. It has a larger base and collector area than a normal transistor. Basically, the phototransistor is the combination of two photodiodes. It has two PN Junctions.

So, in simple words, the constructional difference is a photoresistor is a normal semiconductor device, it has no junction. The photodiode has one junction and the phototransistor has two junctions.

Characteristics Difference between Photoresistor, Photodiode, and Phototransistor

A photoresistor acts as a normal resistor when it is in a dark place. This means it opposes the flow of current through it. When it is exposed to light its resistance decreases and allows the flow of current through it. The resistance property is inversely proportional to light or illumination. This means increasing illumination causes a decrease in resistance.

The photodiode acts as a switch. When it is in a dark place it does not allow current in reverse bias, but when it is exposed to light it allows the current to flow through it. Like a photoresistor, there is no increase or decrease in the resistance of the photodiode. Either it allows to pass the current or does not allow the current. We can say, it acts as a gate valve controlled by the light source.

A phototransistor works like a normal transistor, just the difference is that a normal transistor requires a voltage to its base to allow the flow of current. However, a phototransistor does not require any voltage to its base. When light falls on its base it allows current to flow. In normal conditions, a photodiode allows current flow in one direction only but a phototransistor does not allow any current flow until light falls upon its base.