PN Junction

At the junction, electrons diffuse into the p-side and holes into the n-side, recombining and leaving behind fixed ionised dopants. This depletion region has no free carriers and an electric field that stops further diffusion. The resulting built-in potential is about 0.6–0.8 V for silicon:

$V_{bi} = V_T \ln\frac{N_A N_D}{n_i^2}$

• Forward bias (p positive): the barrier shrinks and current rises exponentially.
• Reverse bias (n positive): the depletion region widens and only a tiny leakage current flows — until breakdown.