Variation of Carrier Concentration with Temperature and Impurity

VARIATION OF CARRIER CONCENTRATION WITH TEMPERATURE AND IMPURITY

In extrinsic semiconductor, the resistivity decreases linearly with increase in temperature. This variation is considered under three different regions.

i) Extrinsic or impurity range

ii) Exhaustion range

iii) Intrinsic range

For a n-type semiconductor, the variation of carrier concentration n and p with temperature is shown in fig. 3.11.

At OK, both conduction and valence bands are free from any charge carriers and hence, the electrical conductivity is zero.

With increase in temperature, the donor atoms get ionised and hence electron concentration in conduction band increases with temperature until all the donor atoms are ionised. This range is known as impurity or extrinsic range.

When the temperature is further increased to room temperature, there are no more donor atoms to be ionised and hence the concentration of electrons in conduction band remains constant over a certain temperature range. This region is known as exhaustion range.

As the temperature is increased further, the electrons in valence band are lifted across the forbidden gap to conduction band. Thus, electron concentration increases in conduction band considerably.

With further increase in temperature, more and more electrons from valence band reach conduction band and completely out-number the donor electrons.

The material practically becomes intrinsic and so this range is called intrinsic range. All these ranges are shown in fig. 3.11.

The dotted curve indicates hole concentration in an intrinsic range.

Table 3.4

Differences between intrinsic and extrinsic semiconductors

Table 3.5

Differences between n-type and p-type semiconductors