Explain the VI characteristics of a PN junction diode and show that PN diode works as a rectifier.

VI Characteristics of a PN Junction Diode

A PN junction diode is a fundamental electronic component that allows current to flow in one direction but blocks it in the opposite direction. This rectifying behaviour is essential for many electronic circuits, such as power supplies, radios, and computers. The VI characteristics of a PN junction diode describe the relationship between the voltage applied to the diode and the current that flows through it.

Forward Bias

When a positive voltage is applied to the P-side of the diode and a negative voltage is applied to the N-side, the diode is said to be forward-biased. In this case, the potential barrier at the PN junction is reduced, allowing electrons to flow from the N-side to the P-side and holes to flow from the P-side to the N-side. This results in a forward current that increases exponentially with increasing forward voltage.

The relationship between forward voltage and forward current can be described by the following equation:

I_F = I_S (e^{V_F / (n * V_T)} - 1)

where:

• (I_F) is the forward current
• (I_S) is the saturation current
• (V_F) is the forward voltage
• (n) is the ideality factor
• (V_T) is the thermal voltage

The saturation current is a small, temperature-dependent current that flows through the diode even when there is no forward bias. The ideality factor is a measure of the quality of the diode, with a value of 1 indicating an ideal diode. The thermal voltage is a constant that depends on the temperature of the diode.

Reverse Bias

When a negative voltage is applied to the P-side of the diode and a positive voltage is applied to the N-side, the diode is said to be reverse-biased. In this case, the potential barrier at the PN junction is increased, making it very difficult for electrons and holes to flow across the junction. This results in a very small reverse current that is typically in the nanoampere range.

The relationship between reverse voltage and reverse current can be described by the following equation:

I_R = I_S (e^{-V_R / (n * V_T)})

where:

• (I_R) is the reverse current
• (I_S) is the saturation current
• (V_R) is the reverse voltage
• (n) is the ideality factor
• (V_T) is the thermal voltage

Breakdown Voltage

If the reverse voltage is increased beyond a certain value, the reverse current will start to increase rapidly. This is known as the breakdown voltage. The breakdown voltage is typically in the range of a few hundred volts for a silicon diode.

PN Diode as a Rectifier

The rectifying behaviour of a PN junction diode can be used to convert alternating current (AC) to direct current (DC). In a simple rectifier circuit, a diode is connected in series with a load resistor. When the AC waveform is positive, the diode is forward-biased and current flows through the load resistor. When the AC waveform is negative, the diode is reverse-biased and no current flows through the load resistor. As a result, the output of the rectifier circuit is a DC waveform.

PN junction diodes are used in a wide variety of rectifier circuits, including:

• Half-wave rectifiers
• Full-wave rectifiers
• Bridge rectifiers

These circuits are used in many electronic devices, such as power supplies, radios, and computers.

Conclusion

The VI characteristics of a PN junction diode are essential for understanding how the diode works as a rectifier. By understanding these characteristics, engineers can design rectifier circuits that can efficiently convert AC to DC.