Question

Explain the effect of biasing on the depletion region of a p-n junction. Using a p-n junction diode, draw the circuit diagram of a half-wave rectifier and show the shape of the input and output voltage.

Hint:

Forward bias lowers the barrier and narrows the depletion region; reverse bias widens it. A half-wave rectifier uses one diode that conducts only in the positive half cycle, giving a pulsating DC output.

Answer 1

Step 1: What the depletion region is.
When p-type and n-type semiconductors are joined, electrons and holes diffuse across the junction and recombine, leaving behind immobile charged ions. This carrier-free layer of immobile charges is the depletion region, and it sets up a built-in potential barrier that opposes further diffusion.

Step 2: Effect of forward bias.
In forward bias the p-side is connected to the positive terminal and the n-side to the negative terminal. The applied field opposes the built-in barrier field, so the depletion region becomes thinner, the potential barrier is lowered, and a large forward current flows.

Step 3: Effect of reverse bias.
In reverse bias the p-side is connected to the negative terminal and the n-side to the positive terminal. The applied field is in the same direction as the barrier field, so the depletion region becomes wider, the barrier increases, and only a very small leakage current flows.

Step 4: Half-wave rectifier circuit.
The AC supply is fed to the primary of a transformer; the secondary is connected in series with a p-n junction diode and a load resistor \( R_L \), and the output is taken across \( R_L \). (Circuit path: transformer secondary \( \to \) diode \( \to R_L \to \) back to the secondary.)

Step 5: Working and waveforms.
During the positive half cycle the diode is forward biased and conducts, so current flows and a voltage appears across \( R_L \). During the negative half cycle the diode is reverse biased and does not conduct, so the output is zero.
Input voltage: a full sine wave with both positive and negative halves.
Output voltage: only the positive half cycles appear across \( R_L \); the negative halves are missing (flat at zero). The output is therefore a unidirectional, pulsating DC voltage.

\[\boxed{\text{Forward bias narrows, reverse bias widens the depletion region; half-wave output = positive half cycles only}}\]