n - type Semiconductor

n-type semiconductor

When a small amount of pentavalent impurity (group V element) is doped to a pure semiconductor, it becomes n-type semiconductor.

Such impurities are known as donor impurities because they donate free electrons to semiconductor crystal.

Typical examples of pentavalent impurities are phosphorus, (Atomic No. 15) and antimony (Atomic No. 51).

Covalent bond in n-type semiconductor

A pentavalent impurity (phosphorus) having five valence electrons is added to a pure semiconductor having four valence electrons (silicon or germanium).

Now, four electrons of germanium form a covalent bond with four valence electrons of phosphorus (impurity atom).

The fifth electron which is now free finds no place in covalent bond structure as shown in fig. 3.7 (a).

We have one electron left free. This acts as a conduction electron. A very small amount of energy (0.01 eV for germanium and 0.05 eV for silicon) is needed to detach this fifth electron.

The addition of pentavalent impurity gives a large number of free electrons (negative charges) in semiconductor. Therefore, it is called n-type semiconductor where n stands for negative type.

Since every pentavalent atom contributes one free electron, in addition to thermally generated electron-hole pairs, the number of free electrons is more than the number of holes in n-type semiconductor.

Thus in this case, electrons are majority charge carriers and holes are minority charge carriers.

Energy band of n-type semiconductor

The energy band diagram of n-type semiconductor is shown in fig.3.7 (b). When the donor impurities are added, the allowable energy levels (donor energy levels) are introduced.

These donor energy levels are slightly below the conduction band. They are discrete and do not form a band because the impurity atoms are far away in the crystal and hence their interaction is small.

The donor energy level for germanium is 0.01 eV and for silicon it is 0.05 eV below the conduction band. Therefore, even at room temperature, almost all the fifth electrons enter into the conduction band.