Depletion Region in P-N Diode (Semiconductor Diode)

The discovery of electrons was a landmark in physics and gave significant technological advantages to electronics. In semiconductors, electron flow is controlled; they are called electronic devices, such as P-N Diodes, Transistors, Integrated Circuits, etc. These devices are changing the field of science and technology. They have small heat dissipation, low voltage operation, and small sizes. A p-n diode is the basic building block of semiconductor devices.

1.0Define depletion region in P-N Junction diode

The space charge region on either side of the junction, where an accumulation of immobile ions forms a layer near the junction, is called the depletion region, depletion layer, or space charged region. When a P-type semiconductor is joined to a N-type semiconductor such that crystal structure remains continuous at the boundary, the resulting arrangement is called a P-N junction diode.

2.0Formation of Depletion region

There are two important processes in the P-N junction: Diffusion and Drift

1. Diffusion: It occurs for majority charge carriers due to concentration gradient. Holes diffuses from p-side to n-side of the junction. Electrons diffuses from n-side to p-side of the junction. Direction of diffusion current is from p-side to n-side. In an n-type semiconductor, the concentration of electrons (electrons per unit volume) exceeds that of holes, whereas in a p-type semiconductor, the concentration of holes is greater than that of electrons. When a p-n junction is formed, a concentration gradient develops across the junction. Consequently, holes diffuse from the p-side to the n-side (p → n), and electrons diffuse from the n-side to the p-side (n → p). This movement of charge carriers generates a diffusion current across the junction.

2. Drift: It occurs for minority charge carriers due to electric fields in the depletion region. Electron drift from p-side to the n-side of the junction. Holes drift from n-side to the p-side of the junction. Electrons and holes drift to their respective majority sides. Direction of drift current is from n to p, opposite to that of diffusion current. Initially, the diffusion current in a p-n junction is large while the drift current is small. As diffusion progresses, the electric field strength within the junction increases, which in turn amplifies the drift current. This process continues until the diffusion current balances out with the drift current. At this point, the p-n junction reaches equilibrium, and no net current flows through the junction.

• It means $\left|I_{\text{Drift }}\right|=\left|I_{\text{Diffusion }}\right|$

3.0Depletion Region Diagram

4.0Width of Depletion Layer

• The thickness of the depletion layer is one micron, d = 1µ = 10^-6m.
• Width of depletion layer $\propto$ Doping.

(∵Doping: The process of adding a desirable impurity to an intrinsic semiconductor is called doping. By adding impurity, the conductivity of the semiconductor increases)

• Depletion is directly proportional to Temperature
• P-N junction diode is equivalent to the capacitor having dielectric between them.

5.0Barrier Potential (VB) of Depletion Layer

An electric field is set up from n-side to p-side. Thus n-regions have higher potential than p-regions. The difference in potential between p and n regions across the junction makes it difficult for the holes and electrons to move across the junction. This act as a barrier and hence called ‘potential barrier’. Depends on temperature, doping and nature of semiconductor material. The potential barrier opposes the motion of the majority carriers and helps the movement of minority carriers.

Points to be Remember:

• This barrier potential opposes the diffusion of electrons and holes across the junction.
• Only electrons and holes with energy of at least EvB can cross the barrier, and some diffusion occurs.
• The movement of most charge carriers across the junction creates an electric current flowing from the p-side to the n-side, known as diffusion current.
• The barrier potential set up barrier field in the direction $\mathrm{n}\to p\text{ side }.$
• Electron hole pairs are continuously produced in the depletion region due to thermal or electric field conditions.
• The electric field EB immediately pushes the electrons towards the n-side and holes towards the p-side. This current set up by the barrier field from the n→p side is called drift current.
• The drift current and diffusion current are in opposite directions.
• In the Equilibrium state, the diffusion current is equal to the drift current, and there is no net flow of charges across the junction.
• No mobile charge carriers in the depletion region.

6.0Dependence of Barrier Potential

Barrier Potential depends on

• Nature of semiconductor
• Temperature
• Amount of Doping

7.0Variation Of Barrier Potential