A PN junction is a fundamental semiconductor device formed by joining a p-type semiconductor (which has an abundance of holes or positive charge carriers) and an n-type semiconductor (which has an abundance of electrons or negative charge carriers). The behavior of a PN junction under different types of biasing—forward bias and reverse bias—determines its electronic properties and applications.
### 1. **Formation of the PN Junction**
When a PN junction is formed, a depletion region or zone is created at the junction due to the diffusion of charge carriers (electrons and holes) across the junction. Electrons from the n-type side will diffuse into the p-type side, where they recombine with holes. Similarly, holes from the p-type side will diffuse into the n-type side, where they recombine with electrons. This diffusion creates a region around the junction where there are no free charge carriers, creating an electric field. This electric field establishes a built-in potential that opposes further diffusion of charge carriers.
### 2. **Forward Bias**
In forward biasing, the positive terminal of the external power supply is connected to the p-type side, and the negative terminal is connected to the n-type side. This reduces the built-in potential barrier of the junction. Here's how it works:
- **Reduction of Barrier Potential**: The external voltage applied reduces the potential barrier created by the built-in electric field.
- **Increase in Carrier Injection**: With a lower barrier, electrons from the n-type side gain enough energy to overcome the junction potential and move into the p-type region. Similarly, holes from the p-type side can move into the n-type region.
- **Current Flow**: The movement of charge carriers across the junction constitutes a current. The current increases exponentially with the applied voltage due to the exponential nature of carrier injection and recombination processes.
**Key Points in Forward Bias:**
- The depletion region narrows, allowing current to flow more easily.
- The current increases significantly with an increase in forward voltage.
- The diode acts as a closed switch when forward biased, allowing current to flow.
### 3. **Reverse Bias**
In reverse biasing, the positive terminal of the external power supply is connected to the n-type side, and the negative terminal is connected to the p-type side. This increases the potential barrier of the junction. Here's what happens:
- **Increase in Barrier Potential**: The external voltage increases the potential barrier, making it harder for charge carriers to move across the junction.
- **Depletion Region Widens**: The depletion region widens as the external voltage pulls more electrons away from the junction on the n-type side and more holes away on the p-type side.
- **Current Flow**: In ideal conditions, very little current flows in reverse bias because the majority carriers are prevented from crossing the junction. However, a small leakage current, known as reverse saturation current, still flows due to thermal generation of electron-hole pairs.
**Key Points in Reverse Bias:**
- The depletion region widens, and the current is minimal.
- The diode acts as an open switch when reverse biased, blocking the current flow.
- If the reverse voltage exceeds a certain threshold (known as breakdown voltage), a large current can flow through the junction, leading to breakdown phenomena such as avalanche or Zener breakdown.
### **Summary**
- **Forward Bias**: Reduces the potential barrier, allowing current to flow through the junction.
- **Reverse Bias**: Increases the potential barrier, preventing current flow except for a small leakage current.
The ability to control the flow of current in this manner makes PN junctions essential in various electronic devices, including diodes, transistors, and integrated circuits.