Photo Diode

Photo Diode

Photo diode is a PN junction device that works in the ‘reversed biased’ condition when light falls on it

Advantages

These are small in size and durable

Their life time is very long

These are more efficient

Circuit diagram:

Symbol:

Construction:

A photo diode is construct just like a semiconductor conventional PN junction diode. Except one thing, that a transparent Lenz is attached over its PN junction to focus light rays on it. Light passes through this transparent Lenz and falls on PN junction.

Operation of photo diode

Photo diode normally operates in reverse bias mode.

We know that when a junction is reverse biased than ‘reverse saturation current’ passes through the junction, that flows due to pairs of holes & electrons produced due to heat.

When small amount of is light is falling on the junction, then it resistance is very high, at this condition, a minor current passes through junction that is called ‘dark current’

But when the intensity of light increase, the minority carriers obtain thermal energy.

As a result, the resistance of junction is decreased and high quantity of covalent bond breaks, it means more pairs of holes and electrons are produced and reverse current is increase.

As the light intensity increases in same speed

Materials used for photo diode

(Photo diode _ conductive materials)

Cadium sulphide

Cadium selenoide

Cadium taloride

Lead sulphide

Thallium sulphide

Selenium

V/I Characteristics curves:

Uses

Detection

Demodulation

Switching

Optional communication devices

Encoder

CD players

Punched card readers

Movies devices

Logic circuit