The operation of a p-n junction diode involves various biasing conditions, including forward bias, reverse bias, and equilibrium.

In equilibrium, no external voltage is applied across the p-n junction. The depletion region is formed at the junction interface due to the diffusion of carriers, which leaves behind charged dopants, acceptors on the p-side, and donors on the n-side. These immobile charges create an electric field that prevents further diffusion of carriers. The related energy band diagram shows that the Fermi levels on both sides are aligned, indicating equilibrium. The built-in potential across the junction prevents the net flow of carriers across the junction.

When the diode is forward biased the applied voltage reduces the barrier potential and narrows the width of the depletion region. As a result, carriers can easily cross the junction, and the corresponding band diagram shows the energy bands bending upwards, indicating the reduction of the barrier potential. The current across the diode under forward bias increases exponentially with the applied voltage.

In reverse bias, the external voltage is applied in the opposite direction, widening the depletion region, increasing the barrier potential, making it difficult for carriers to cross the junction, and reducing the current flow to a very small reverse saturation current. The energy band diagram for reverse bias shows the bands bending downwards, indicating increased barrier potential. The current in reverse bias is a small constant value. It is close to the saturation current but with the opposite sign, reflecting the minor flow of carriers due to thermal generation within the depletion region.