Conductors, Insulators and Semiconductors
Most solids can be placed in one of two classes, conductors and insulators. Conductors are those in which electric charges can flow easily, while insulators are those in which it is difficult for electric charges to flow.
This difference between conductors and insulators can be explained by the number of free electrons in them. There are a large number of free electrons in conductors, which act as charge carriers in the flow of current. In contrast, insulators have a negligible number of free electrons.
The resistance temperature coefficient of conductors is positive, that is, their electrical resistance increases or the electrical conductivity decreases with increase in temperature.
There are also some solid materials whose electrical conductivity lies between that of conductors and insulators. They are called Semiconductors. Carbon, silicon and germanium are examples of semiconductors.
In semiconductors, the most valence electrons are neither so strongly bound to an atom as in an indulator, nor as loosely bound as in a conductor.
The resistance temperature coefficient of semiconductors is negative, that is, their electrical resistance decreases or the electrical conductor increases with increase in temperature.
Conversely, the electrical resistance of semiconductors increases or the electrical conductivity decreases with decrease in temperature. At absolute zero temperature, a semiconductor behaves as an ideal insulator.
Intrinsic and Extrinsic Semiconductors
There are two types of semiconductor. intrinsic Semiconductor and electric Semiconductor
A pure semiconductor, in which no impurities are added, is called an intrinsic semiconductor. Thus, pure germenium and silicon in their natural state are intrinsic semiconductors.
Electric conduction in intrinsic semiconductors: Two well known pure semiconductors are germanium and silicon. Each of them has a valency of 4. We can assume that each atom of Ge or Si has 4 valence electrons around an inner core.
And the core has + 4e charge. In germenium crystal, these atoms are arranged in an ordered array such that each atom lies at some corner of the regular tetrahedron.
Atoms form covalent bonds by sharing one of the four valency electrons, one of the four closest electrons. These bonds provide binding force between neighboring atoms.
At temperatures near absolute zero, the atoms of pure germanium or silicon are strongly binding with all valence electron cores. Hence free electrons are not available for conduction.
But at normal temperatures, some covalent bonds are broken due to thermal agitation, so that some valent electrons are freed.
When an electric field is applied to the crystal, these free electrons move in the opposite direction of the field and the electric current starts to conduct in the crystal. On heating the crystal, more and more electrons are liberated due to which the conductivity of the crystal increases.
At room temperature, only one covalent bond is broken in germanium 109 atoms. Therefore, the conductivity of intrinsic semiconductors is very low, so that they cannot be of any practical use.
Intrinsic semiconductors have very low electrical conductivity. But if a very small amount of such a substance, whose valency is 5 or 3, is mixed in pure germanium or silicon crystal as impurity, then the conductivity of the crystal increases significantly.
This action is called doping. Example the conductivity of germanium increases up to 16 times when 1 impurity atom is mixed in 108 germanium atoms. Such impure semiconductors are called extrinsic or impurity or doped semiconductors. By controlling the amount of impurity to be mixed in these semiconductors, desired conductivity can be achieved.