Question

Explain the working of a forward biased (\( p - n \)) junction diode by making circuit diagram and by drawing its \( V - I \) characteristic graph. Define its dynamic resistance.

Hint:

For a silicon diode, the threshold voltage is typically around 0.7 V. The diode's dynamic resistance decreases as the current increases in forward bias.

Answer 1

A p-n junction diode is formed by joining a p-type semiconductor and an n-type semiconductor. When the diode is forward biased (i.e., the p-side is connected to the positive terminal of the battery and the n-side to the negative terminal), the majority charge carriers (holes from the p-side and electrons from the n-side) move towards the junction, reducing the width of the depletion region. As the applied voltage increases, the current increases rapidly after a threshold voltage (called the forward voltage).
Circuit Diagram:
In a forward biased condition, the circuit diagram is as shown below:
\[ \text{(Diagram with p-n junction diode, load resistance \( R_L \), and the power supply)} \] The diode conducts only when the voltage exceeds a certain threshold (typically 0.7V for a silicon diode).
\( V-I \) Characteristic Graph:
The \( V-I \) characteristic of a p-n junction diode shows the relationship between the voltage across the diode and the current through it. The graph has the following features:
- For small forward voltages (less than 0.7V for silicon diodes), the current is almost zero (the diode does not conduct). - Once the forward voltage exceeds 0.7V, the current rises exponentially. - In reverse bias, the current is negligible until the breakdown voltage is reached, after which the current increases sharply.
Dynamic Resistance of the Diode:
The dynamic resistance \( r_d \) of the diode in the forward biased condition is defined as the slope of the \( V-I \) curve at any given point. It is given by:
\[ r_d = \frac{dV}{dI} \] At high currents, the dynamic resistance is very small (almost zero), as the diode behaves like a short circuit.