Intrinsic Semiconductor

A semiconductor is a material whose electrical conductivity lies between that of a conductor and an insulator. Common examples of semiconductor materials are silicon and germanium. When these materials are chemically pure and contain no intentional impurity atoms, they are called intrinsic semiconductors.

A pure semiconductor in which no impurity is added intentionally.

The concentration of charge carriers in an intrinsic semiconductor, where the number of electrons equals the number of holes.

The vacancy left in the valence band when an electron leaves it behaves like a positive charge carrier in semiconductor theory.

At very low temperatures, a pure semiconductor behaves almost like an insulator because very few electrons are available for conduction. As temperature increases, some valence electrons gain enough energy to move into the conduction band, leaving behind vacancies called holes.

Each electron that enters the conduction band leaves one hole behind in the valence band. Therefore, in an intrinsic semiconductor, electrons and holes are created in equal numbers.

Important idea

• One electron promoted to the conduction band produces one hole in the valence band.
• Hence, in intrinsic semiconductors, electron concentration equals hole concentration.
• This equality is the basis of the relation n_e = n_h = n_i​

The conductivity of an intrinsic semiconductor depends strongly on temperature. At 0 K, it behaves like an insulator because the valence band is full and the conduction band is empty.

When the temperature rises, some electrons cross the energy gap and become free charge carriers. As a result, the number of electrons and holes increases, and conductivity also increases.

In an intrinsic semiconductor, the valence band is nearly full, and the conduction band is nearly empty at low temperature.

Flow sequence

Valence electron gains heat energy → moves to the conduction band → free electron is formed → hole is left behind → both contribute to conduction.

• It is chemically pure; no impurity atoms are intentionally added.
• The number of electrons equals the number of holes.
• Conductivity is low at low temperatures and increases with temperature.
• Conduction occurs due to both free electrons and holes.
• Pure silicon and pure germanium are common examples.

C, Si and Ge have the same lattice structure. Why is C an insulator while Si and Ge are intrinsic semiconductors?

Carbon (C), silicon (Si), and germanium (Ge) have the same crystal structure, but their electrons are held with different strengths. Carbon has a much larger energy gap, so its electrons cannot easily move to the conduction band. As a result, almost no free electrons are available for conduction, making carbon an insulator. Silicon and germanium have smaller energy gaps, so some electrons gain enough energy at room temperature to move into the conduction band. Therefore, Si and Ge behave as intrinsic semiconductors.

Imagine a classroom in which every student is seated properly, so no one is free to move around. This is similar to a semiconductor at very low temperature, where electrons are tightly bound and conduction is minimal.

If some students get up and move to empty spaces elsewhere, they create both moving students and vacant seats. Similarly, when an electron gains energy and moves into the conduction band, it creates a free electron and a hole simultaneously.

Everyday significance

Pure semiconductors are not usually used directly in most electronic devices because their conductivity is limited. However, understanding intrinsic semiconductors is essential because extrinsic semiconductors and all basic semiconductor devices are built on this concept.