EASA / CAR Part-66 · Module 4 · 4.1 Semiconductors · Cat B1/B2
Join P-type to N-type and carriers diffuse across, leaving a depletion layer of fixed ions with a barrier potential across it. Bias it forward and the barrier collapses; bias it reverse and the layer widens until only leakage flows.
With no bias the junction still does something. Holes diffuse into the N material and electrons into the P material, and where they meet they recombine, stripping the region next to the junction of mobile carriers. What is left is the depletion layer: fixed negative acceptor ions on the P side, fixed positive donor ions on the N side. That charge sets up the barrier potential, about 0.7 V for silicon and 0.3 V for germanium, which opposes further diffusion.
Forward bias connects positive to the P side. The applied field opposes the barrier field, the depletion layer narrows and, once the bias approaches the knee voltage, majority carriers pour across and recombine. Current then rises exponentially, so a small voltage change makes a large current change, which is why a diode is always operated with a current-limiting resistor. Above the knee the curve straightens out as the bulk resistance of the silicon takes over.
Reverse bias connects positive to the N side. Majority carriers are pulled away from the junction, the depletion layer widens and the barrier grows, so only the tiny reverse saturation current of thermally generated minority carriers flows. Raise the temperature and watch that leakage climb: it roughly doubles every 10 °C. The same heating lowers the forward knee by about 2 mV per °C.
Keep increasing reverse voltage and you reach breakdown, where current rises steeply. In an ordinary rectifier that exceeds the peak inverse voltage (PIV) rating and destroys the junction through heating. A Zener diode is designed to work there safely, which is the subject of the Zener regulator simulation.