P-Type and N-Type Semiconductor Materials and the Mechanism of Current Conduction

A semiconductor is a material whose electrical conductivity lies between that of a conductor and an insulator. Pure semiconductor materials such as silicon (Si) and germanium (Ge) have limited conductivity. Their conductivity can be greatly increased and controlled by adding a small amount of a suitable impurity. This process is known as doping.

Depending on the type of impurity added, a semiconductor becomes either an N-type or a P-type semiconductor.

1. N-Type Semiconductor

An N-type semiconductor is formed by adding a small quantity of a pentavalent impurity to a pure semiconductor such as silicon or germanium. Pentavalent impurities have five valence electrons. Examples include phosphorus, arsenic and antimony.

The pure semiconductor atom forms four covalent bonds with neighbouring atoms. When a pentavalent impurity atom replaces one of these atoms:

• Four of its five valence electrons form covalent bonds with neighbouring atoms.
• The fifth electron is weakly bound and becomes a free electron.
• This free electron can move through the crystal structure when an electric field is applied.

Therefore, an N-type semiconductor has an excess of free electrons.

• Majority charge carriers: Free electrons.
• Minority charge carriers: Holes.

The letter "N" denotes that the majority charge carriers are negative electrons. However, the semiconductor material as a whole remains electrically neutral.

Current Conduction in an N-Type Semiconductor

When a voltage is applied across an N-type semiconductor, an electric field is established.

The free electrons gain energy from the electric field and drift through the crystal towards the positive terminal (anode). Their movement through the conduction band constitutes the main mechanism of current conduction.

Thus:

Applied voltage → Electric field → Movement of free electrons → Current flow

Although conventional current is considered to flow from positive to negative, the actual electrons move in the opposite direction, from the negative terminal towards the positive terminal.

2. P-Type Semiconductor

A P-type semiconductor is formed by adding a small quantity of a trivalent impurity to a pure semiconductor. Trivalent impurities have three valence electrons. Examples include boron, gallium and indium.

When a trivalent impurity atom is introduced into the semiconductor crystal:

• Its three valence electrons form covalent bonds with neighbouring atoms.
• One bond remains incomplete because there is a shortage of one electron.
• This missing electron position is called a hole.

A hole behaves as a positive charge carrier because it represents a deficiency of an electron.

Therefore, a P-type semiconductor has an excess of holes.

• Majority charge carriers: Holes.
• Minority charge carriers: Free electrons.

The letter "P" denotes that the majority charge carriers are effectively positive holes. However, the semiconductor material as a whole remains electrically neutral.

Current Conduction in a P-Type Semiconductor

When a voltage is applied across a P-type semiconductor, an electric field is established.

The holes act as the main charge carriers. However, the actual physical movement is still carried out by electrons. An electron from a neighbouring covalent bond moves to fill a nearby hole. This movement leaves a new hole at the electron's original position.

The process continues as follows:

Electron fills a hole → A new hole is created → Another electron fills the new hole → Progressive movement of holes

As a result of this continuous electron movement, the holes appear to move through the material towards the negative terminal (cathode). The resulting progressive movement of holes in the valence band constitutes the main current flow in a P-type semiconductor.

Summary of P-Type and N-Type Semiconductors