Mumbai University > Electronics ana telecommunication engineering > Sem 3 > Analog electronics 1

Marks: 5M

Years: Dec 14, Dec 15

Photodiode:

The photodiode is a semiconductor p-n junction device whose region of operation is limited to the reverse biased region. The Fig. 1.1(a) shows the symbol of photodiode while the Fig. 1.1(b) shows the working principle of photodiode.

The photodiode is connected in reverse biased condition. The depletion region width is large. Under normal condition, it carries small reverse current due to minority charge carriers.

When light is incident through glass window on the p-n junction, photons in the light bombard the p-n junction and some energy is imparted to the valence electrons. Due to this, valence electrons are dislodged from the covalent bonds and become free electrons. Thus more electron-hole pairs are generated.

Thus total number of minority charge carriers increases and hence the reverse current increases. This is the basic principle of operation of photodiode.

Photodiode Characteristics:

The photodiode is designed such that it is sensitive to the light.

When there is no light, the reverse biased photodiode carries a current which is very small and called dark current. It is denoted is $I_λ$.

It is purely due to thermally generated minority carriers. When light is allowed to fall on a p-n junction through a small window, photons transfer energy to valence electrons to make them free.

Hence reverse current increases. It is proportional to the light intensity. The Fig. 5.46 shows the photodiode characteristics. The Fig. 1.2 (a) shows the relation between reverse current and light intensity while the Fig. 1.2 (b) shows relation between reverse voltage and reverse current at different light intensities.

It can be seen that reverse current is not dependent on reverse voltage and totally depends on light intensity.