Biasing of P-N Junction

Forward biasing reduces the barrier width of the depletion region by applying a voltage that opposes the junction potential. This narrowing enables majority carriers to diffuse across the junction more easily, increasing current flow. The energy band diagram shows the energy bands bending upwards, indicating the reduction in barrier potential that facilitates carrier transport.

Reverse biasing widens the depletion region and increases the barrier potential, making it difficult for majority carriers to cross the junction. The resulting current is a small reverse saturation current from minority carriers generated thermally within the depletion region. The energy band diagram shows bands bending downwards, reflecting the increased barrier potential.

During forward bias, electrons and holes diffuse across the junction and recombine in the junction region. This carrier generation and recombination process emits photons, causing the LED to glow. The reduced barrier width enables sufficient majority carrier diffusion to sustain continuous recombination and light emission.

At equilibrium, the built-in potential is created by immobile charges in the depletion region—acceptors on the p-side and donors on the n-side. This potential generates an electric field that prevents further diffusion of carriers across the junction. The Fermi levels on both sides align, indicating equilibrium and zero net carrier flow.

The polarity of the applied voltage determines conduction direction. Forward bias connects the positive terminal to the p-side and negative to the n-side, reducing the barrier and enabling majority carrier diffusion. Reverse bias reverses the polarity, increasing the barrier and restricting carrier flow to a small minority carrier saturation current.

Junction breakdown occurs when the reverse bias voltage exceeds a critical threshold, causing a sudden large increase in current flow. At this point, the electric field becomes strong enough to accelerate minority carriers to energies sufficient for impact ionization, creating additional carriers and triggering avalanche multiplication.

In forward bias, energy bands bend upwards, indicating reduced barrier potential and easier carrier crossing. In reverse bias, bands bend downwards, showing increased barrier potential and suppressed carrier flow. These band bending patterns reflect how the applied voltage modifies the junction's electrostatic environment and carrier transport capability.