VI Characteristics of Diodes

A diode is a two-terminal electronic device that allows current to flow in only one direction. The voltage-current (VI) characteristics of a diode describe the relationship between the voltage across the diode and the current flowing through it. These characteristics are crucial in understanding the behavior and applications of diodes. Let's delve into the details of diode VI characteristics.

Forward Bias Region

In the forward bias region, the diode is connected in such a way that the positive terminal of the voltage source is connected to the anode (p-side) of the diode and the negative terminal is connected to the cathode (n-side). In this state, the diode allows current to flow easily, and the VI characteristics can be described as follows:

1. Forward Voltage: Initially, as the voltage across the diode increases, the current remains nearly zero until the voltage reaches a certain threshold called the forward voltage (Vf). Once the forward voltage is reached, the diode starts conducting and the current increases rapidly.

2. Forward Current: Once the diode enters the conduction state, the current flowing through it increases exponentially with the increase in voltage. The relationship between the forward current (If) and the forward voltage follows the diode equation: If = Is * (e^(Vf/Vt) - 1), where Is is the reverse saturation current and Vt is the thermal voltage (approximately 26 mV at room temperature).

Reverse Bias Region

In the reverse bias region, the diode is connected in such a way that the positive terminal of the voltage source is connected to the cathode (n-side) of the diode and the negative terminal is connected to the anode (p-side). In this state, the diode acts as an insulator, preventing the flow of current. The VI characteristics in the reverse bias region can be described as follows:

1. Reverse Breakdown: As the reverse voltage (Vr) across the diode increases, a point is reached where the diode breaks down and allows current to flow in the reverse direction. This breakdown can occur in two ways:

- Zener Breakdown: In this type of breakdown, the reverse voltage is below a certain threshold called the breakdown voltage (Vz). Once this voltage is reached, the diode enters the breakdown region and allows reverse current to flow while maintaining a relatively constant voltage across its terminals.

- Avalanche Breakdown: In this type of breakdown, the reverse voltage is above a certain threshold called the avalanche breakdown voltage (Va). Once this voltage is reached, the diode experiences a rapid increase in reverse current due to the collision of charge carriers, leading to a breakdown.

2. Reverse Leakage Current: In the reverse bias region, a small amount of current, known as the reverse leakage current, flows due to the minority charge carriers present in the diode. The reverse leakage current increases with an increase in reverse voltage, but it is usually very small.

Applications of VI Characteristics

Understanding the VI characteristics of diodes is crucial for their various applications, such as:

- Rectification: Diodes are widely used in rectifier circuits to convert alternating current (AC) to direct current (DC) by