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photo diode vs photo transistor-difference between photo diode and photo transistor

This page on photo diode vs photo transistor covers difference between photo diode and photo transistor. 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.

Photo diode

photo diode,photo transistor

Photo diode consists of a normal p-n junction housed in a small enclosure which a transparent window through which light can fall inside. Figure depicts photo diode symbol.

A photo diode 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.

Photo diodes are used as fast counters and used in light meters to measure the light energy.

Figure-1 above depicts circuit symbols of photodiode and phototransistor.

Photo Transistor

photo diode and photo transistor

It can resamble as photo diode 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 boased 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 photo transistors do not have a base lead.

A photo transistor is about one hundred times more sensitive than a photo diode.

The figure-2 depicts actual photodiode and phototransistor components.


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