Question

Explain the working process of a forward biased p-n junction diode with the help of a circuit diagram. Show the dynamic resistance by making a graph between the forward voltage and forward current.

Hint:

In a forward biased diode, current increases exponentially with the forward voltage once the threshold voltage is surpassed.

Answer 1

Step 1: Working of a Forward Biased p-n Junction Diode.
In a forward biased p-n junction diode, the positive terminal of the battery is connected to the p-type material, and the negative terminal is connected to the n-type material. This reduces the width of the depletion region and allows current to flow through the diode once the forward voltage exceeds the threshold (or "cut-in" voltage).
Step 2: Circuit Diagram.
Here is the circuit diagram of a forward biased p-n junction diode:

Step 3: Dynamic Resistance.
The dynamic resistance \( r_d \) of the diode is defined as the change in voltage \( \Delta V \) divided by the corresponding change in current \( \Delta I \): \[ r_d = \frac{\Delta V}{\Delta I} \]
Step 4: Graph Between Forward Voltage and Forward Current.
The current through a forward biased p-n junction diode increases exponentially with the applied forward voltage after the threshold voltage. The relationship between the forward voltage \( V \) and the forward current \( I \) is given by the Shockley diode equation: \[ I = I_0 \left( e^{\frac{V}{nV_T}} - 1 \right) \] where \( I_0 \) is the saturation current, \( n \) is the ideality factor, and \( V_T \) is the thermal voltage.
Step 5: Conclusion.
Thus, as the forward voltage increases, the current increases exponentially. The dynamic resistance decreases as the current increases, and the diode exhibits a nonlinear I-V characteristic.