CBSE NOTES CLASS 12 PHYSICS
CHAPTER 14 SEMICONDUCTORS
An arrangement, consisting of a p-type semiconductor brought into a close contact with n-type semiconductor, is called a p -n junction.
Diffusion of charge and diffusion current
During the formation of p-n junction, and due to the concentration gradient across p-, and n- sides, holes diffuse from p-side to n-side (p→n) and electrons diffuse from n-side to p-side (n→p) and a diffusion current is setup across the junction.
p-n junction formation
Due to diffusion of holes p→n and electrons n→p, a –ve region is created on p-side and a +ve region is created on n-side.
This space-charge region on either side of the junction together is known as depletion region and net charge in this region is 0.
Due to the positive space-charge region on n-side of the junction and negative space charge region on p-side of the junction, an electric field directed from positive charge towards negative charge develops. Due to this field, an electron on p-side of the junction moves to n-side and a hole on n-side of the junction moves to p-side.
Drift of charge carriers and drift current
The motion of charge carriers due to the electric field is called drift and current is called drift current.
Initially, diffusion current is large and drift current is small. As the diffusion process continues, the space-charge regions on either side of the junction extend, thus increasing the electric field strength and hence drift current. This process continues until the diffusion current equals the drift current and p-n junction is formed. In a p-n junction under equilibrium there is no net current.
The loss of electrons from the n-region and the gain of electrons by the p-region; causes a difference of potential across the junction of the two regions. The polarity of this potential is such as to oppose further flow of carriers so that a condition of equilibrium exists.
Potential barrier across a p-n junction
The potential difference across the depletion layer is called potential barrier (Vo).