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Difference between photodiode and phototransistor | photodiode vs phototransistor

This page compares photodiode vs phototransistor and mentions difference between photodiode and phototransistor. It covers advantages and disadvantages of photodiode and phototransistor.

Introduction: As we know both are photocells. Photocell changes light signals into electrical signals. Light energy can be infrared or ultraviolet radiations. They are very useful for various applications such as fire alarms, counting systems and automatic control systems.

Photodiode

Photodiode

• Photodiode consists of a normal p-n junction housed in a small enclosure which a transparent window through which light can fall inside. Figure depicts photodiode symbol.
• 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 result of light energy which breaks the bonds in the crystal lattice of the semiconductor producing electrons and holes. This effect is similar to photo voltaic cell.
• Photodiodes are used as fast counters and used in light meters to measure the light energy.
• Figure-1 above depicts circuit symbol and photodiode manufactured by OSRAM.

Phototransistor

• It is a transistor controlled by exposure to light. It is similar to photodiode controlling an ordinary bipolar transistor.
• It can be either bipolar transistor or field effect transistor (FET).
• Its body is encased in resin or plastic or encased in a metal shell with window in it.
• Phototransistor has two leads which connect internally with its collector and emitter (or source and drain in FET). The base of transistor (or gate in FET) responds to light and controls the flow of current between the leads.

Phototransistor

It can resamble as photodiode giving current amplification due to transistor action. Few of these devices are moulded in transparent plastic cases with convex. This convex acts as a lens which focuses light on the transistor. As a result of this, extra minority carriers are liberated at the reverse biased CB junction (Collector to Base). This generated leakage current is later amplified. If used in this way,connection to base terminal is not needed. Hence many of the phototransistors do not have a base lead. Figure-2 above depicts circuit symbol and NPN phototransistor L14G2.

Difference between photodiode and phototransistor

Following points mentions difference between photodiode and phototransistor.
➨Photodiode is more responsive to incident light than phototransistor.
➨Photodiode has linear response over much wider range of light than phototransistor. Hence photodiode is used for precise measurement of light over wide range.
➨Photodiodes pass less current compare to phototransistor. Moreover photodiode consumes less current than phototransistor. Hence photodiode is used for battery powered devices requiring less power consumption.
➨A phototransistor is about one hundred times more sensitive than a photodiode.
➨Dark current of phototransistor is much higher than photodiode.
➨Phototransistor is used as solid state switch unlike photodiode. Saturation voltage of phototransistor is the voltage drop between collector and emitter, which seldom exceeds 0.5V.
➨Refer advantages and disadvantages of Photodiode >> and Phototransistor >> for more comparison between photodiode and phototransistor.



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