Why does a p-n junction conduct in only one direction?

Forward bias lowers the built-in barrier so majority carriers flood across and current rises exponentially; reverse bias raises the barrier, leaving only a tiny minority-carrier current, so the junction acts as a one-way valve for current.

What is band bending?

Near the junction the built-in electric field of the depletion region shifts the local energy bands up or down with position; this bending is exactly what keeps the Fermi level flat across the device in equilibrium, as required for no net current.

p-n Junctions and Band Bending

Joining p-type and n-type semiconductors aligns their Fermi levels, bending the bands and creating a built-in field that lets current flow easily in one direction only, the essence of the diode.

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Definition

A p-n junction is the interface between p-type and n-type semiconductor; in equilibrium carrier diffusion creates a charge-depleted region with a built-in electric field that bends the bands so the Fermi level is constant, and an applied bias unbalances this field to give rectifying current flow.

Scope

This topic covers the physics of the p-n junction: the diffusion of carriers across the metallurgical junction, the resulting depletion region and built-in potential, the band bending that equalizes the Fermi level, and the rectifying current-voltage characteristic under forward and reverse bias. It treats the Shockley diode equation, the depletion-layer width and capacitance, and breakdown, providing the building block for diodes, transistors, and solar cells.

Core questions

Why does joining p-type and n-type material create a depletion region and a built-in potential?
How does band bending keep the Fermi level constant across the junction in equilibrium?
Why does the junction conduct readily under forward bias but block reverse bias?
What sets the depletion-layer width, junction capacitance, and breakdown voltage?

Key concepts

Depletion region and built-in potential
Band bending and Fermi-level alignment
Forward and reverse bias
Shockley diode equation and rectification
Junction capacitance and breakdown

Key theories

Shockley diode theory: Shockley derived the exponential current-voltage relation of an ideal p-n junction from the diffusion of minority carriers across the depletion region, explaining rectification and providing the quantitative model underlying diodes and bipolar transistors.

Clinical relevance

The p-n junction is the elementary building block of semiconductor electronics: rectifier and signal diodes, light-emitting diodes, photodiodes, and solar cells are junctions, and bipolar and field-effect transistors are built from combinations of them.

History

Ohl identified rectification at a silicon p-n boundary in 1939, and Shockley's 1949 theory of the junction explained its operation and led directly to the junction transistor, foundational work recognized by the 1956 Nobel Prize shared with Bardeen and Brattain.

Key figures

William Shockley
Russell Ohl
John Bardeen

Seminal works

shockley1949
sze2007

Frequently asked questions

Why does a p-n junction conduct in only one direction?: Forward bias lowers the built-in barrier so majority carriers flood across and current rises exponentially; reverse bias raises the barrier, leaving only a tiny minority-carrier current, so the junction acts as a one-way valve for current.
What is band bending?: Near the junction the built-in electric field of the depletion region shifts the local energy bands up or down with position; this bending is exactly what keeps the Fermi level flat across the device in equilibrium, as required for no net current.