Understanding the PN Junction: From Structure to Diode Behavior

The PN junction is the cornerstone of modern semiconductor devices, turning a single crystal into a self‑sustaining voltage barrier that governs diode behavior. By doping one side of a crystal with acceptor atoms (P‑type) and the other with donor atoms (N‑type), a region of immobile charge forms at their interface. This depletion region acts as an intrinsic insulator, yet it is the very feature that allows the PN junction to function as a one‑way valve for electrical current.

Structure of a PN Junction

When a P‑type block is placed next to an N‑type block, the two materials simply touch and share no unique electrical properties—each remains a conductor with no net charge. The difference emerges when a single crystal is doped with P‑type material on one end and N‑type material on the other. The majority carriers are holes in the P side and electrons in the N side, and they are free to move through the lattice.

Formation of the Depletion Region

Near the junction, electrons diffuse from the N side into the P side, where they recombine with holes. The N side loses electrons and becomes positively charged; the P side gains electrons and becomes slightly negative. This creates a thin, immobile layer—called the depletion region—that is free of majority carriers and behaves like an insulator.

(a) Separate blocks of P and N semiconductor show no exploitable properties. (b) A single crystal doped with P and N material forms a potential barrier.

Potential Barrier and Biasing

The charge separation at the junction creates a built‑in potential barrier. The magnitude of this barrier depends on the semiconductor material: ~0.6–0.7 V for silicon, lower for germanium. An external voltage must exceed this barrier to allow current flow.

Forward Bias

Connecting the battery so that its negative terminal feeds electrons to the N side and its positive terminal removes electrons from the P side pushes majority carriers toward the junction. Once the applied voltage surpasses the barrier (≈0.6 V in silicon), electrons and holes recombine, and the depletion region shrinks, allowing current to flow.

(a) Forward bias drives carriers into the junction, enabling recombination and current flow. (b) Reverse bias pulls carriers away, widening the depletion region and blocking current.

Reverse Bias

Reversing the battery polarity pulls majority carriers away from the junction, increasing the depletion width. No significant current flows except for a tiny leakage current, and the device is protected from destructive breakdown by selecting a reverse voltage rating above any operating voltage.

Diode Characteristics

The diode symbol reflects this behavior: the arrow (anode) points toward the P side, and the bar (cathode) points toward the N side. Forward bias current rises steeply once the barrier is overcome. In silicon diodes, the forward voltage (V_F) is 0.6–0.7 V; in germanium it is ~0.2 V; in LEDs it can reach several volts.

The forward current range varies widely—from milliamps for small signal diodes to thousands of amperes for power diodes. Reverse leakage currents are typically <1 µA for silicon small‑signal devices, increasing with temperature. Breakdown occurs at high reverse voltages (50–800 V for silicon), necessitating a series resistor to protect the device.

Leakage is partly intrinsic—thermal generation of electron‑hole pairs—and partly due to surface conduction caused by contamination. Germanium diodes exhibit much higher leakage and are rarely used today.

Key Takeaways

• PN junctions are created by doping a monocrystalline semiconductor with both P‑type and N‑type regions.
• Electron diffusion across the junction creates a voltage barrier of ~0.6–0.7 V in silicon.
• Forward bias overcomes this barrier, enabling current flow through recombination at the junction.
• Reverse bias widens the depletion region, limiting current to a small leakage that rises with temperature.
• Leakage currents are <1 µA in small silicon diodes and are temperature dependent.

Further Reading

• PN Junctions Worksheet
• Electrical Conduction in Semiconductors Worksheet