Page 950 - Basic Electrical Engineering

crystal. However, although the electrons are bound to their atoms due to
covalent bonding, a rise in temperature breaks some of the covalent bonding
and make some electrons free. A semiconductor material, silicon or

germanium where the electrons are bound to their respective atoms and are
not free to conduct electric current, are called intrinsic, or pure

semiconductors. When the temperature of the crystal is raised, external
energy in the form of heat gets applied to the semiconductor material. This

heat energy enables the valence electrons to acquire sufficient energy to
break away from the atoms and become free electrons. When an electron

leaves to become free, it leaves a vacant space called a hole. For every free
electron there will be a corresponding hole produced, which is called an
electron–hole pair. A large number of such electron–hole pairs are formed

due to rise in temperature of the semiconductor. When electrons become free
they get attracted and fall into a hole created by another electron. This

merging of free electrons and holes is called recombination. The time of
creation of a free electron and its falling into a hole is very quick and the time

taken is of the order of nanoseconds. Thus, pure silicon or germanium is not
of much use in electronics except for the manufacturing of heat- or light-

sensitive resistance.
The conductivity of semiconductor materials can be increased by adding
some amount of another material having either three or five valence

electrons. Adding such materials with the pure semiconductor material is
called doping. The material formed by doping is called extrinsic

semiconductors.