Question

With the help of circuit diagrams, briefly explain the forward biasing and the reverse biasing of a p-n junction diode.

Hint:

Remember the simple rule for connections: - Forward Bias = Same polarities connected together (Positive to P-type, Negative to N-type). - Reverse Bias = Opposite polarities connected together (Positive to N-type, Negative to P-type).

Answer 1

Concept: A p-n junction diode acts as a one-way valve for electric current. Its behavior is modified by applying an external voltage, a process called biasing. Biasing changes the width of the depletion region and alters the internal barrier potential.

Step 1: Explaining Forward Biasing.
In a forward bias configuration, the positive terminal of an external DC battery is connected to the p-type region of the diode, and the negative terminal is connected to the n-type region. Mechanism: The positive terminal repels the holes in the p-region toward the junction, while the negative terminal repels the free electrons in the n-region toward the junction. This direct opposition counters the internal built-in electric field, shrinking the width of the depletion layer. Once the applied voltage exceeds the barrier potential threshold (e.g., \(0.7\text{ V}\) for Silicon), the barrier is overcome, allowing majority carriers to cross the junction. This creates a significant forward current that increases exponentially with voltage.

Step 2: Explaining Reverse Biasing.
In a reverse bias configuration, the positive terminal of the external DC battery is connected to the n-type region, and the negative terminal is connected to the p-type region. Mechanism: The positive terminal attracts free electrons away from the junction in the n-region, while the negative terminal attracts holes away from the junction in the p-region. This widens the depletion layer and increases the height of the potential barrier. Majority charge carriers can no longer cross the junction. The current drops nearly to zero, leaving only a tiny leakage current (on the scale of microamperes or nanoamperes) driven by minority carriers.