PN Diode

PN DIODE

i. In a piece of semiconductor, one half is doped by P-type impurity and the other half is doped by N-type impurity. Thus PN junction is formed.

ii. This formation of PN junction is called diode as it has two electrodes one for P region called as Anode and the other for N region called as Cathode.

N-Type Material

The majority charge carriers are electrons and has higher concentration of free electrons than holes as shown in Fig.1.7 (a).

The majority charge carriers are holes and P type material has higher concentration of holes than electrons, as shown in Fig. 1.7 (b).

Diffusion

The tendency of the free electrons to diffuse from the n-side to p-side and holes from p- region to n-region is called diffusion.

Diffusion is the process by which charge carrier moves from high concentration region to low concentration region.

Depletion Region

The free electrons diffuse from 'n' side to 'p' side region and recombine with the holes in P region. Thus negative charged immobile ions are formed near the junction.

Also, the holes diffuse from P-region to n-region and recombine with the electrons in n-region and leaves positively charged immobile ions near the junction of n-side.

This process continues over certain time duration. After certain extent, the net negative charge on P-side prevents further diffusion of electrons into P-side. Similarly, the net positive charge on N-side repels holes crossing from P-side to N- side. These immobile positive and negative ions form a region called as depletion region.

Depletion region is defined as the region over which all the free charge carriers are depleted. This region is also known as space charge region since there are no free charge carriers available for conduction. In equilibrium condition, the depletion region gets widened upto a point where no further electrons or holes can cross the junction. The physical distance from one side to other side of the depletion region is called width of the depletion region.

Barrier Potential

The electric filed is developed across the depletion region due to the potential difference between P and N regions. This potential acts as barrier for further conduction between the junction. This potential is called as barrier potential or diffusion potential or contact potential or cut in voltage.

i. It depends on doping levels and temperature

ii. The value of contact potential is 0.3 V for Germanium and 0.72V for Silicon at 25° C.

The barrier potential depends on

i. The type of semiconductors

ii. Concentration of donor impurity on n- side

iii. Concentration of acceptor impurity on p - side

iv. Intrinsic concentration of semiconductors

v. Temperature ܀

OPERATION OF A PN JUNCTION DIODE 

Biasing

Applying any external voltage to electronic device is called biasing. Biasing refers to provide minimum external voltage and current to activate the device.

There are two types of biasing for PN junction.

Forward Bias

The positive terminal of the battery is connected to P-type semiconductor and negative terminal is connected to n-type semiconductor material.

Reverse Bias

The positive terminal of the battery is connected to n - type semiconductor and the negative terminal is connected to P-type semiconductor material.