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How does a p-n junction diode work?

Class 12 PhysicsNCERT chapter: Semiconductor Electronics: Materials, Devices and Simple Circuitsइसे हिंदी में पढ़ें

Exam-ready answer

A p-n junction diode allows current mainly in one direction. At the junction, electrons diffuse from n to p and holes from p to n, leaving a depletion region with a barrier potential. In forward bias (p-side to the positive terminal) the barrier is lowered and a large current flows; in reverse bias the barrier rises and only a tiny current flows.

Explanation

When a p-n junction forms, the n-side has many free electrons and the p-side has many holes. Electrons diffuse into the p-side and holes into the n-side, giving a diffusion current. Each electron leaving the n-side leaves behind an immobile positive donor ion, and each hole leaving the p-side leaves an immobile negative acceptor ion. This thin layer, emptied of free charge carriers, is the depletion region.

The layers of positive and negative ions create an electric field directed from n to p. This field sweeps minority carriers across, producing a drift current opposite to the diffusion current. At equilibrium the two currents are equal and there is no net current. The potential difference across the junction that opposes further diffusion of majority carriers is called the barrier potential.

In forward bias, the p-side goes to the positive terminal. The applied voltage opposes the barrier, the depletion layer narrows, and beyond the threshold voltage (about 0.7 V for silicon) the current rises sharply, in milliamperes. In reverse bias, the barrier increases, the depletion layer widens, and only microamperes flow, due to minority carriers. This one-way behaviour lets the diode work as a rectifier, converting AC into DC.

Points to remember

  1. Depletion region: the layer near the junction with no free charge carriers, containing only immobile ions.
  2. Barrier potential: the potential difference across the junction that opposes further diffusion of majority carriers.
  3. Forward bias: p-side to positive terminal; depletion layer narrows; large current (mA) beyond the threshold voltage (about 0.7 V for Si).
  4. Reverse bias: p-side to negative terminal; depletion layer widens; very small current (μA) due to minority carriers.
  5. Main application: rectifier (half-wave and full-wave) that converts AC into DC.

Exam tip

Draw the circuit diagrams for forward and reverse bias and sketch the I–V characteristic, marking the threshold voltage, the mA scale for forward current and the μA scale for reverse current.

Questions students also ask

What is the depletion region?
It is a thin region on both sides of a p-n junction that has no free electrons or holes. It contains only immobile positive ions on the n-side and negative ions on the p-side. These ions produce the electric field and barrier potential that stop further diffusion.
Why does a small current flow in reverse bias?
In reverse bias, majority carriers are pushed away from the junction, but minority carriers (electrons on the p-side and holes on the n-side) are swept across by the field. Their number is small, so the reverse current is only microamperes and nearly independent of voltage until breakdown.
How does a p-n junction diode work as a half-wave rectifier?
AC is applied to a diode in series with a load resistor. During the half-cycle when the diode is forward biased, current flows through the load. During the other half-cycle it is reverse biased and practically no current flows. So the load gets current in only one direction.

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