PN Junction Diode Characteristics A Complete Simple Guide

A pn junction diode is the simplest semiconductor device: a single crystal with a p-type region joined to an n-type region. It conducts electric current easily in one direction and blocks it in the other, which makes it the basic building block of rectifiers, LEDs and countless electronic circuits.

Its I-V characteristic curve, plotted forward and reverse in the lab, shows the knee where conduction begins, the tiny reverse leakage and the breakdown point, and its slope gives the diode's resistance.

Key Concepts

• Depletion region: the thin charge-free zone at the junction, widening in reverse bias and narrowing in forward bias.
• Barrier potential: the built-in voltage across the junction, about 0.7 V for silicon and 0.3 V for germanium.
• Forward bias: positive terminal to the p-side; the barrier collapses and large current flows once the applied voltage nears the knee voltage.
• Reverse bias: positive terminal to the n-side; only a tiny leakage current (microamperes) flows until breakdown.
• Knee (cut-in) voltage: the forward voltage where current starts rising steeply, roughly 0.7 V for silicon.
• Dynamic resistance: r_ac = ΔV / ΔI, the slope of the forward curve, which is small (a few ohms) in conduction.

Formulas

Diode equation: I = I₀ (e^(V / ηV_T) − 1), where I₀ is the reverse saturation current, V the applied voltage, η the ideality factor (1 to 2), and V_T = kT / q the thermal voltage, about 26 mV at room temperature (300 K). The exponential explains the steep forward rise and the flat reverse region near −I₀. Dynamic resistance from the curve: r_ac = ΔV / ΔI between nearby forward points. Assumptions: steady temperature and reverse voltage below breakdown.

Worked Example

A silicon diode's forward characteristic is measured. At V = 0.68 V the current is 10 mA; at V = 0.72 V the current is 30 mA. Find its dynamic resistance in this region.

1. Find the voltage change: ΔV = 0.72 − 0.68 = 0.04 V.
2. Find the current change: ΔI = 30 mA − 10 mA = 20 mA = 0.020 A.
3. Compute r_ac = ΔV / ΔI = 0.04 / 0.020 = 2.0 Ω.
4. Interpretation: once conducting, the diode behaves like a mere 2 Ω resistor in series with its 0.7 V knee drop.
5. A 40 mV rise tripling the current is exactly the steep exponential behaviour expected past the knee.

Common Misconceptions

• The diode does not conduct perfectly in forward bias. About 0.7 V (silicon) is always lost across it, and it has a small but real dynamic resistance.
• Reverse current is not exactly zero. A small leakage current of thermally generated carriers always flows; it is just thousands of times smaller than forward current.
• The knee is not a magic switch: conduction rises smoothly and exponentially; 0.7 V is where current becomes large enough to matter.

Real-World Applications

• Rectifiers in phone chargers and power supplies convert AC mains into DC.
• LEDs are diodes engineered to emit light instead of just passing current.
• Signal diodes demodulate radio signals in receivers.
• Protection diodes across relay coils and motor drivers absorb damaging voltage spikes.

The pn junction diode is a one-way street for current, and its I-V curve tells the whole story: a steep exponential climb past the 0.7 V knee in forward bias, near-silence in reverse, and a tiny dynamic resistance once it conducts. Master this curve and every diode circuit becomes readable.