• Tags: junction, depletion, barrier
• Difficulty: Moderate

When p-type and n-type semiconductors are joined, a p-n junction forms. Near the junction: electrons from n-side diffuse to p-side (high to low electron concentration), holes from p-side diffuse to n-side. This diffusion leaves behind immobile ions: positive donor ions on the n-side and negative acceptor ions on the p-side. These ions create the depletion region — a thin zone (~1 um) with no free carriers. The exposed ions create a built-in electric field E pointing from n to p (positive to negative ions). This field opposes further diffusion. Equilibrium: diffusion current = drift current (due to built-in field). The potential barrier (contact potential) $V_0$ ≈ 0.7 V for Si and 0.3 V for Ge at room temperature. The depletion width is wider on the lightly doped side (to maintain charge neutrality: $N_A$$x_p$ = $N_D$$x_n$). Higher doping = thinner depletion region.