Charge Carrier in Semiconductors

Definition

A charge carrier in a semiconductor is any mobile particle or quasiparticle that can transport electric charge through the material when an electric field, temperature gradient, or concentration difference is applied. In semiconductors, the two primary charge carriers are:

Electrons, which carry negative charge
Holes, which behave as positive charge carriers

Unlike metals, where conduction is mainly due to free electrons, semiconductors conduct by the movement of both electrons and holes. The availability and concentration of these carriers depend strongly on temperature, impurity doping, and the energy band structure of the semiconductor.

Main Content

1. Charge Carriers in Intrinsic Semiconductors

In a pure semiconductor such as silicon or germanium, charge carriers are generated only when electrons gain enough energy to jump from the valence band to the conduction band.
When an electron leaves the valence band, it creates a hole in the valence band. Thus, intrinsic semiconductors always produce electron-hole pairs in equal numbers.

In an intrinsic semiconductor:

The number of electrons in the conduction band is equal to the number of holes in the valence band.
Both types of carriers contribute to current flow.
At low temperature, carrier concentration is very small, so the material behaves almost like an insulator.

Energy band idea for intrinsic semiconductor:

Conduction Band
--------------------  e- can move here freely

        ↑
        | thermal energy

Band Gap
--------------------

        ↓
        hole created here

Valence Band
--------------------  electrons normally stay here

Important features:

The generation of carriers depends on thermal excitation.
Higher temperature means more electron-hole pairs.
The conductivity of intrinsic semiconductors increases rapidly with temperature.

Example:
In pure silicon at room temperature, some covalent bonds break due to thermal energy, producing a small number of free electrons and holes. These carriers allow a tiny current to flow.

2. Charge Carriers in Extrinsic Semiconductors

In an extrinsic semiconductor, impurity atoms are intentionally added to increase the number of charge carriers.
Doping creates either electron-rich or hole-rich semiconductors depending on the dopant type.

N-type Semiconductor

Created by doping silicon or germanium with pentavalent impurities such as phosphorus, arsenic, or antimony.
Each dopant atom donates an extra electron, which becomes a free electron.
In n-type material:
Electrons are majority carriers
Holes are minority carriers

P-type Semiconductor

Created by doping silicon or germanium with trivalent impurities such as boron, aluminum, or gallium.
Each dopant atom creates a missing electron position, called a hole.
In p-type material:
Holes are majority carriers
Electrons are minority carriers

Energy band idea for doped semiconductors:

N-type:
Donor level close to conduction band
Electrons easily move to conduction band

P-type:
Acceptor level close to valence band
Electrons leave valence band, creating holes

Key observations:

Doping greatly increases electrical conductivity.
The majority carrier determines the main conduction behavior.
Minority carriers still exist and are essential in devices such as diodes and transistors.

Example:
A silicon crystal doped with phosphorus has one extra valence electron from each phosphorus atom. That extra electron is weakly bound and can move freely, making the material n-type.

3. Motion and Behavior of Charge Carriers

Charge carriers move through a semiconductor under the influence of an electric field, concentration gradient, or thermal energy.
Their motion gives rise to current, and the way they move determines the electrical properties of the semiconductor.

Electron Motion

Electrons move through the conduction band.
They drift opposite to the direction of the applied electric field because of their negative charge.
Electron movement is usually faster than hole movement.

Hole Motion

A hole is not a physical particle in the usual sense; it is the absence of an electron in a covalent bond.
When neighboring electrons move to fill the hole, the hole appears to move in the opposite direction.
Holes behave as if they have positive charge.

Drift and Diffusion

Drift: Motion of carriers due to an external electric field.
Diffusion: Motion of carriers from a region of high concentration to low concentration.

Simple carrier movement diagram:

Applied electric field →→→

Electrons: move ←←←
Holes:     move →→→

Why this matters:

Current in semiconductors is the result of both drift and diffusion.
The balance of electron and hole motion determines device operation in junctions and circuits.
Carrier mobility and lifetime affect speed and efficiency of semiconductor devices.

Example:
In a forward-biased p-n junction, electrons from the n-side and holes from the p-side move across the junction, enabling current flow.

Working / Process

Carrier generation
Energy from heat, light, or doping supplies electrons with enough energy to move into the conduction band.
In intrinsic materials, this creates electron-hole pairs.
In extrinsic materials, dopants supply extra electrons or create holes.
Carrier movement
Once generated, carriers move through the crystal lattice.
Under an electric field, electrons drift toward the positive terminal, while holes effectively drift toward the negative terminal.
Carriers may also diffuse from regions of high concentration to low concentration.
Recombination and steady state
Electrons can fall back into holes, a process called recombination.
Recombination reduces the number of free carriers.
In normal operation, semiconductors reach a balance between generation and recombination, maintaining a steady carrier concentration.

Advantages / Applications

Controlled conductivity: By adjusting carrier concentration through doping, semiconductors can be engineered for specific electrical behavior.
Foundation of electronic devices: Charge carrier control is the basis of diodes, transistors, LEDs, solar cells, and integrated circuits.
Efficient switching and amplification: Precise manipulation of electrons and holes enables fast and low-power operation in modern electronics.

Summary

Charge carriers in semiconductors are mainly electrons and holes.
Their concentration and behavior depend on temperature, doping, and band structure.
Controlled movement of these carriers makes semiconductor devices possible.
Important terms to remember: electron, hole, intrinsic semiconductor, extrinsic semiconductor, n-type, p-type, drift, diffusion, recombination