What is the working principle of forward and reverse biased pn junction diode?
2 Answers
A **PN junction diode** is a semiconductor device that allows current to flow in one direction while blocking it in the opposite direction. It consists of two types of semiconductor material, **P-type** (positively charged) and **N-type** (negatively charged), joined together to form a PN junction. The behavior of the diode depends on how the external voltage is applied across the junction, leading to two modes of operation: **forward bias** and **reverse bias**.
### 1. **Forward Biased PN Junction Diode**
In forward bias, the positive terminal of the external voltage source is connected to the P-type material, and the negative terminal is connected to the N-type material.
**Working Principle:**
- When the diode is forward biased, the external voltage reduces the barrier potential of the PN junction. The barrier potential is the internal electric field created at the junction due to the difference in charge carriers (holes in the P-type and electrons in the N-type).
- The reduced barrier potential allows charge carriers to overcome the junction's potential barrier. Electrons from the N-region move towards the P-region, while holes from the P-region move towards the N-region.
- As a result, current starts to flow across the junction. The flow of electrons (from N to P) and holes (from P to N) constitutes the electric current. This flow is typically large when the diode is forward biased because the barrier potential is minimal.
- **Threshold Voltage:** For the current to flow significantly, the applied voltage must exceed a certain threshold. For silicon diodes, this threshold voltage is typically around 0.7V; for germanium diodes, it is around 0.3V.
- Once the threshold is crossed, the diode conducts electricity with very low resistance, allowing a significant current to flow through it.
### 2. **Reverse Biased PN Junction Diode**
In reverse bias, the positive terminal of the external voltage source is connected to the N-type material, and the negative terminal is connected to the P-type material.
**Working Principle:**
- When the diode is reverse biased, the external voltage increases the barrier potential of the PN junction. The electric field at the junction becomes stronger, making it difficult for charge carriers to move across the junction.
- The increased barrier potential prevents the majority charge carriers (electrons in the N-region and holes in the P-region) from crossing the junction. As a result, the diode ideally does not conduct current, and it acts as an insulator.
- However, a small leakage current, known as reverse saturation current, may still flow due to the movement of minority charge carriers (holes in the N-region and electrons in the P-region). This current is usually negligible.
- **Breakdown Voltage:** If the reverse voltage is increased beyond a certain limit, the junction can break down, leading to a large reverse current. This breakdown can be either **Zener breakdown** or **avalanche breakdown**. Zener breakdown occurs at low reverse voltages in heavily doped diodes, while avalanche breakdown occurs at higher reverse voltages in lightly doped diodes.
- In normal operating conditions, the reverse biased diode does not conduct current, effectively blocking any flow of current in the reverse direction.
### Summary
- **Forward Bias:** The PN junction diode conducts current when the external voltage reduces the barrier potential, allowing charge carriers to move across the junction.
- **Reverse Bias:** The diode blocks current flow as the external voltage increases the barrier potential, preventing charge carriers from crossing the junction, except for a small leakage current.
This unique behavior of PN junction diodes is what makes them essential components in circuits for controlling the direction of current flow.
### 1. **Forward Biased PN Junction Diode**
In forward bias, the positive terminal of the external voltage source is connected to the P-type material, and the negative terminal is connected to the N-type material.
**Working Principle:**
- When the diode is forward biased, the external voltage reduces the barrier potential of the PN junction. The barrier potential is the internal electric field created at the junction due to the difference in charge carriers (holes in the P-type and electrons in the N-type).
- The reduced barrier potential allows charge carriers to overcome the junction's potential barrier. Electrons from the N-region move towards the P-region, while holes from the P-region move towards the N-region.
- As a result, current starts to flow across the junction. The flow of electrons (from N to P) and holes (from P to N) constitutes the electric current. This flow is typically large when the diode is forward biased because the barrier potential is minimal.
- **Threshold Voltage:** For the current to flow significantly, the applied voltage must exceed a certain threshold. For silicon diodes, this threshold voltage is typically around 0.7V; for germanium diodes, it is around 0.3V.
- Once the threshold is crossed, the diode conducts electricity with very low resistance, allowing a significant current to flow through it.
### 2. **Reverse Biased PN Junction Diode**
In reverse bias, the positive terminal of the external voltage source is connected to the N-type material, and the negative terminal is connected to the P-type material.
**Working Principle:**
- When the diode is reverse biased, the external voltage increases the barrier potential of the PN junction. The electric field at the junction becomes stronger, making it difficult for charge carriers to move across the junction.
- The increased barrier potential prevents the majority charge carriers (electrons in the N-region and holes in the P-region) from crossing the junction. As a result, the diode ideally does not conduct current, and it acts as an insulator.
- However, a small leakage current, known as reverse saturation current, may still flow due to the movement of minority charge carriers (holes in the N-region and electrons in the P-region). This current is usually negligible.
- **Breakdown Voltage:** If the reverse voltage is increased beyond a certain limit, the junction can break down, leading to a large reverse current. This breakdown can be either **Zener breakdown** or **avalanche breakdown**. Zener breakdown occurs at low reverse voltages in heavily doped diodes, while avalanche breakdown occurs at higher reverse voltages in lightly doped diodes.
- In normal operating conditions, the reverse biased diode does not conduct current, effectively blocking any flow of current in the reverse direction.
### Summary
- **Forward Bias:** The PN junction diode conducts current when the external voltage reduces the barrier potential, allowing charge carriers to move across the junction.
- **Reverse Bias:** The diode blocks current flow as the external voltage increases the barrier potential, preventing charge carriers from crossing the junction, except for a small leakage current.
This unique behavior of PN junction diodes is what makes them essential components in circuits for controlling the direction of current flow.