Diode I-V Characteristics

A p-n junction diode has an asymmetric I-V curve: ALMOST NO current in reverse, EXPONENTIAL current in forward.

Forward characteristic: current is negligible until V ≈ V_threshold (knee voltage, ~0.7 V Si, ~0.3 V Ge), then rises steeply.

Reverse characteristic: tiny saturation current I₀ (nA-μA) until BREAKDOWN voltage, where current suddenly grows.

Static resistance R = V/I (depends on operating point). Dynamic (small-signal) resistance r_d = dV/dI = k_BT/(eI) at room temperature.

Ideal diode: zero forward resistance, infinite reverse resistance — used in idealized circuit analysis.

Real diode has a 'knee' due to V_threshold and finite forward slope (a few Ω).

Breakdown mechanisms: Zener (tunnelling, low V_BR, < 5V) and avalanche (impact ionisation, high V_BR).

Special diodes: LED (light emission), photodiode (photocurrent), Zener (voltage reference), Schottky (low V_F), tunnel diode.