• Tags: IV-curve, dynamic-resistance, diode-equation
• Difficulty: Moderate

The diode equation I = $I_0$*(e^$\frac{eV}{nkT}$ - 1) describes the complete I-V characteristic. n = ideality factor (1 for Ge, ~2 for Si at low currents). At room temperature, kT/e = 26 mV, so the exponent = V/26 mV (for n=1). Forward bias: exponential rise above threshold. At V = 0.7 V (Si): I jumps dramatically. Dynamic (AC) resistance $r_d$ = $\frac{dV}{dI}$ = $\frac{nkT}{eI}$ decreases as current increases. Typical values: $r_d$ ~ 1-10 ohm at forward operating point. Static (DC) resistance R = $\frac{V}{I}$ is larger. Reverse bias: I = -$I_0$ (constant, ~nA for Si, ~uA for Ge). Dynamic resistance in reverse is extremely high (~M$\Omega$). At breakdown: current increases sharply while voltage remains nearly constant (low dynamic resistance). This is why Zener diodes make good voltage regulators. JEE questions often give the I-V graph and ask for dynamic resistance at a specific operating point.