Photo diode

                      Photo diode

Photo diode is a p-n junction fabricated from a photosensitive semiconductor and provided with a transparent window so as allow light to fall on its junction. 

Basically it is a p-n junction diode operates in reverse biasing. 

Symbol

Construction

As shown in diagram  a resistance R is connected in series with a reverse biased photo diode. The voltage is kept slightly less than the breakdown voltage. When  no light is incident on the junction, a small reverse saturation current flows through the junction. This reverse current is due to thermally generated electron-hole pairs and is called dark current. When the photo diode is illuminated with light photons of energy hv greater than the energy gap Eg of the semiconductor, additional electron-holes pairs are generated in or near the depletion region due to the absorption of photons. Due to the electric field of the junction, electrons get collected on my side and holes on p-side giving rise to an end. This sets up a photocurrent in the circuit. The magnitude of the photocurrent is proportional to the intensity of incident light. 

When a photo diode is illuminated with light photons of energy hv is greater than Eg, and increasing intensities I1, I2, I3, etc., the value of reverse saturation current increases with the increase in intensity of incident light as shown in diagram. Hence, a measurement of the change in the reverse saturation current on illumination can give the values of light intensity.

Why photo diode is operated in reverse bias condition

Consider an n-type semiconductor. It's majority carrier (electron) density is much larger than the minority hole density   i.e., n is greater than p. When illuminated with light, both types of carriers Increase equally in number. 

n' = n + (change in n)      ;       p' = p + (change in p) 

Now    n is greater than p and (change in n) is greater than  ( change in p) 

So, 

                (Change in n) /n is smaller  than (change in p) /p

It means, the fractional increase in majority carriers is much less than the  fractional increase in minority carriers. Consequently, the fractional change due to the photo- effects  on the minority carrier  dominated reverse bias current is more easily measurable than the fractional change in the majority carrier dominated forward bias current. Hence, photodiodes are preferably used in the reverse bias condition for measuring light intensity. 

Uses of photodiodes

A photo diode can turn it's current ON and OFF  in nanoseconds. 

So it can be used as a fastest photodetector. 

The photo diodes are used for following purposes :

1. In detection of optical signals. 

2. In demodulation of optical signals. 

3. In light operated switches. 

4. In speed reading of computer punched cards. 

5. In electronic counters.