How it works
At the PN junction, diffusion of electrons into the p-side and holes into the n-side creates a space charge region. The fixed positive donor ions on the n-side and negative acceptor ions on the p-side produce an electric field directed from n to p, which opposes further diffusion — equilibrium is reached when drift current exactly cancels diffusion current. Forward bias of 0.6–0.7 V on silicon reduces this barrier and allows exponential current increase described by the Shockley equation: ID = IS(e^(VD/ηVT) − 1), where VT = 26 mV at 300 K and η is 1 for Ge, 2 for Si at low currents. Reverse bias widens the depletion region and allows only the tiny reverse saturation current IS, typically nanoamperes.
Key points to remember
The built-in potential is 0.7 V for silicon and 0.3 V for germanium — these are threshold voltages for forward conduction, not the built-in contact potential itself, but examiners often use them interchangeably in circuit problems. The Shockley diode equation ID = IS(e^(VD/ηVT) − 1) is derived assuming ideal diode behaviour; η = 1 for germanium and approaches 2 for silicon at small forward currents. Reverse saturation current IS doubles approximately for every 10°C rise in temperature. Depletion width varies as √(V0 − V) — it narrows under forward bias and widens under reverse. The junction capacitance CT = ε·A/W is exploited in varactor diodes used in FM tuners.
Exam tip
Every VTU and Anna University paper asks you to apply the Shockley equation to find diode current — remember VT = kT/q = 26 mV at 300 K and substitute it directly without deriving it again.