PN Junction Diode: Characteristics and Applications

A pn junction diode is the simplest semiconductor device: one piece of silicon with p-type material on one side and n-type on the other. It conducts current easily in one direction and blocks it in the other — a one-way valve for electricity. It rectifies AC into DC, protects circuits from reverse voltage, and sits inside nearly every power supply ever built.

It matters because the pn junction is the building block of all semiconductor devices. The LED, the Zener diode, the solar cell and even the transistor are variations or combinations of pn junctions. Understand this one device and you hold the key to the rest of electronics.

Key Concepts

• Depletion region — at the junction, electrons from the n-side diffuse into the p-side and recombine with holes, leaving uncovered ions behind: a thin zone with no free carriers that acts as a barrier.
• Built-in potential (Vbi) — the voltage across the depletion region at equilibrium, typically 0.6–0.7 V for silicon, which carriers must overcome to cross.
• Forward bias — positive voltage on the p-side shrinks the depletion region; above about 0.7 V (silicon) large current flows from p to n.
• Reverse bias — positive voltage on the n-side widens the depletion region; only a tiny reverse saturation current flows, until breakdown voltage is reached.
• Diode equation — I = Is(e^(V/(nVT)) − 1), with VT ≈ 25.85 mV at 300 K: current grows exponentially with forward voltage.

Formulas

• Built-in potential: Vbi = VT × ln(Na × Nd / ni²), where Na, Nd are doping concentrations. For Na = 10¹⁶ cm⁻³, Nd = 10¹⁵ cm⁻³: Vbi = 0.02585 × ln(4.44 × 10¹⁰) ≈ 0.63 V.
• Exponential current: from the diode equation, raising forward voltage by about 60 mV multiplies the current roughly tenfold (at ideality factor 1).
• Depletion width: W = √[2εs(Vbi − V)(Na + Nd)/(qNaNd)]. Forward bias shrinks W; reverse bias grows it — this voltage-controlled width is what makes varactor diodes work.

Worked Example

A silicon pn junction has Na = 10¹⁶ cm⁻³, Nd = 10¹⁵ cm⁻³. At 300 K (VT = 25.85 mV, ni = 1.5 × 10¹⁰ cm⁻³), what is the built-in potential?

1. Na × Nd / ni² = (10¹⁶ × 10¹⁵)/(1.5 × 10¹⁰)² = 10³¹/(2.25 × 10²⁰) = 4.44 × 10¹⁰.
2. ln(4.44 × 10¹⁰) = ln(4.44) + 10 × ln(10) ≈ 1.49 + 23.03 = 24.52.
3. Vbi = 0.02585 × 24.52 ≈ 0.63 V.
4. This internal barrier of ~0.63 V is exactly why a conducting silicon diode drops about 0.6–0.7 V — the applied voltage is overcoming this barrier.

Common Misconceptions

• "A diode has zero resistance when forward biased." It has small but real dynamic resistance plus the ~0.7 V drop; at 1 A that drop alone wastes 0.7 W as heat.
• "No current at all flows in reverse bias." A tiny saturation current (nanoamps for silicon) always flows, and it roughly doubles for every 10 °C of temperature rise.

Real-World Applications

• Rectifiers: bridge rectifiers in chargers and power supplies convert AC mains into DC.
• Reverse-polarity protection: a series diode saves gadgets when batteries are inserted backwards.
• Solar cells: a giant pn junction converts sunlight into current — a diode run in reverse, harvesting light.

The pn junction diode is electronics' one-way street: a depletion region that opens under forward bias and slams shut under reverse bias. Master its 0.7 V knee, its exponential current and its built-in potential, and every diode, LED and transistor that follows will make sense.