How Does a P-N Junction Work?

What you will learn

• Explain how a depletion region forms.
• Compare forward and reverse bias on one junction.
• Distinguish equilibrium from an absence of carrier motion.

Before you start

Read the linked prerequisite first. This article explains principles and does not require assembling or opening equipment.

From diffusion to an electrical barrier

The n side has many electrons and the p side many holes. Concentration differences drive electrons toward p and holes toward n. As carriers move, charges at dopant sites are exposed.

The n side leaves positive ions and the p side negative ions. These charges are fixed in the lattice, with the field directed from n toward p. They create an electric field across the boundary, leaving the nearby region depleted of mobile carriers. A depletion region is not an empty gap without atoms; it lacks free carriers.

• Depletion region
• Semiconductor on both sides
• Left — holes in the p region
• Right — electrons in the n region

What does an applied voltage change?

Forward bias reduces the net field at the junction, making carrier diffusion across the barrier easier and increasing diffusion current. In this diode model, p is positive relative to n.

Reverse bias increases the field and reduces diffusion current. “Reduced” does not mean every current becomes zero: the source distinguishes diffusion current from drift current.

Equilibrium does not mean every carrier stops

Without an external input such as light or applied voltage, component currents balance and net current is zero. Opposing contributions that cancel are a better picture than frozen carriers.

Diffusion, the field and external conditions together explain junction behaviour. Simply naming positive and negative sides without the barrier mechanism cannot explain forward versus reverse bias.

A worked example

Follow one illustrative p-n junction through three states. This is a model interpretation, not a measurement on a real diode. State one: the p and n regions have just been joined. Electrons diffuse from the n side toward p and holes diffuse from p toward n, leaving fixed positive ions on the n side and fixed negative ions on the p side. These ions set up a field directed from n toward p, so the region near the junction is depleted of mobile carriers.

State two: an external voltage is applied with the p side positive relative to n (forward bias). The net field at the junction falls, carriers diffuse across more easily and the diffusion current increases. State three: reverse the voltage (reverse bias). The field grows and diffusion current falls, although the source does not say all current becomes zero. With no external voltage, the component currents balance and the net current is zero; ask yourself why that does not mean the carriers are standing still.

Frequently asked questions

Is the depletion region empty?

No. The lattice remains; mobile carriers are depleted.

Does reverse bias mean current is always zero?

No. Reducing diffusion current does not eliminate every current contribution.

Summary

• Diffusion exposes fixed charges and forms a field.
• Applied voltage changes the carrier barrier.
• Compare biases through fields and currents.