Visualize depletion region formation, carrier drift, diffusion, and real-time current flow — from equilibrium to breakdown
When p-type and n-type semiconductors are joined, electrons from the n-side diffuse into the p-side and recombine with holes. Simultaneously, holes diffuse into the n-side. This leaves behind fixed ions — negative on the p-side, positive on the n-side.
These fixed charges create a built-in electric field pointing from N to P, which opposes further diffusion. Eventually diffusion and drift reach balance — this is the depletion region, a carrier-free zone at the junction.
Applying positive voltage to the p-side and negative to the n-side reduces the built-in barrier. The depletion region narrows. When applied voltage exceeds ~0.7V (silicon), the barrier is overcome and large current flows.
Majority carriers — holes from p-side and electrons from n-side — rush toward the junction and recombine, sustaining continuous current. This is how LEDs emit light — energy released during recombination becomes photons.
Applying negative voltage to the p-side widens the depletion region and increases the barrier height. Majority carriers are pulled away from the junction. Only a tiny reverse saturation current flows due to minority carriers.
If reverse voltage is increased beyond the breakdown voltage, avalanche or Zener breakdown occurs — current surges dramatically. This effect is exploited in Zener diodes for voltage regulation.
A p-n junction is formed when a p-type semiconductor and an n-type semiconductor are brought into contact. It is the fundamental building block of all modern semiconductor devices — including diodes, transistors, solar cells, LEDs, and integrated circuits. Understanding the p-n junction is essential for any student of electronics, physics, or materials science.
In a p-type semiconductor, the majority charge carriers are holes (positive), created by doping with acceptor atoms like boron. In an n-type semiconductor, the majority carriers are electrons (negative), created by doping with donor atoms like phosphorus or arsenic.
When p-type and n-type materials are joined, electrons from the n-side and holes from the p-side diffuse across the junction and recombine. This leaves behind a region depleted of free carriers — the depletion region or space charge region. Fixed ionized donor atoms (positive) remain on the n-side and fixed ionized acceptor atoms (negative) remain on the p-side, creating a built-in electric field directed from n to p. This field opposes further diffusion until equilibrium is reached. The built-in potential for silicon is approximately 0.7 V.
When the positive terminal of a battery is connected to the p-side and the negative terminal to the n-side, the junction is said to be forward biased. The applied electric field opposes the built-in field, reducing the barrier height. The depletion region narrows. When the applied voltage exceeds the threshold (approximately 0.6–0.7 V for silicon), current flows freely. This is the operating condition for diodes conducting current, LEDs emitting light, and solar cells in circuit operation.
When the negative terminal is connected to the p-side, the junction is reverse biased. The applied field adds to the built-in field, widening the depletion region and increasing the barrier. Majority carrier flow is blocked. Only a tiny reverse saturation current flows due to thermally generated minority carriers. At a sufficiently large reverse voltage — the breakdown voltage — avalanche multiplication or Zener tunneling causes a sudden surge of current. Zener diodes are designed to operate precisely at this breakdown voltage for voltage regulation applications.
The p-n junction is a core concept in the Physics and Chemistry for Renewable and Clean Energy curriculum under the Kerala University Four Year Undergraduate Programme (FYUGP). Students need to understand depletion region formation, built-in voltage, bias conditions, and carrier dynamics for their semester examinations. This simulation makes abstract band diagrams and carrier flows visually intuitive and interactive.