How Do Electrons and Holes Move Through Material?

• Direction of the electric field
• Electron: goes against the field
• Hole: goes with the field

What you will learn

• Explain how electrons move in a semiconductor.
• Describe why a hole seems to move even though it is a missing electron.
• Say which way an electric field sends each kind of carrier.

Before you start

No prior knowledge is needed. You only need to know that matter is made of atoms and that an electron is a tiny negatively charged particle.

How do electrons move in a semiconductor?

Electrons move freely only after they leave the bonds between atoms. According to PVEducation's page on conduction in semiconductors, electrons held in covalent bonds stay in place and do not take part in current flow. At higher temperature, electrons around the atoms can break away from their bonds and move freely about the lattice. At room temperature a semiconductor has enough free electrons to conduct current.

• Atoms in the lattice
• Bonds between atoms
• Free electron
• The spot the electron left

A free electron does not run in a straight line. The page on carrier movement describes each carrier as moving in a random direction at a certain velocity. It travels a distance called the scattering length, then collides with an atom in the lattice and moves off in a different direction.

What is a hole and why does it shift?

A hole is a place in the crystal where an electron is missing. Electronics Tutorials calls it a positively charged carrier sitting where an electron is effectively missing.

That gap attracts a neighbouring electron. The electron moves in to fill it and leaves another hole behind. The result, according to the same source, looks like holes moving as a positive charge through the crystal structure.

• Electrons stepping into the gap
• The gap (hole) at each step
• Electrons go right
• The hole goes left

PVEducation treats both electrons in the conduction band and holes in the valence band as free carriers, because they can move through the crystal lattice.

How does an electric field change the motion?

An electric field adds a common direction to the random motion. According to PVEducation's page on drift, carriers keep their random motion but also move in a net direction. Electrons move in the net direction opposite to the field. Holes move in the net direction of the field.

Without an electric field or other special circumstances, the carrier-movement page says there is no net movement of carriers in any direction.

• Equilibrium: no net motion
• With a field: drifts one way
• Electric field

A field is not the only route. PVEducation's page on diffusion says that through random motion carriers move from areas of high concentration to areas of low concentration.

Real-world example

Picture a comparison: a row of seats full of people with one empty seat. The person beside it shifts into the empty seat, then the next person shifts into the seat just vacated. Each person only takes one step, yet the empty seat drifts the other way. This is only a comparison to help picture it, not an exact description of the inside of a crystal.

Common misunderstandings

A free electron runs in a straight line. It moves in a random direction, collides with an atom in the lattice and changes direction.

A hole is a particle that flies off. A hole is a missing electron. It shifts because a neighbouring electron jumps in to fill it and leaves a gap behind.

Electrons and holes always go the same way. In an electric field electrons go against the field and holes go with it.

Frequently asked questions

What is a hole in a semiconductor?

A place in the crystal where an electron is missing, acting like a positively charged carrier.

Why does a hole seem to move?

Because a neighbouring electron fills the gap and leaves another gap behind, so the gap shifts opposite to the electron.

Which way do electrons and holes go in an electric field?

Electrons go against the field. Holes go with the field.

Why can a semiconductor conduct at room temperature?

Because at room temperature it has enough free electrons to conduct current.

Summary

• Electrons move freely only after leaving their bonds, and they move in random directions.
• A hole is a missing electron and shifts when a neighbouring electron fills it.
• An electric field adds net drift: electrons against the field, holes with it.