There are two types of semiconductors (i) Intrinsic or pure semiconductor (ii) Extrinsic or impure semiconductor
(i) Intrinsic or pure semiconductor: The conduction band of silicon and germanium is empty and the valence band is fully filled up with electrons at very low temperature. Germanium and silicon have four valence electrons. Each atom of germanium silicon shares one electron with its neighboring atom. Thus covalent bond is made. So, there is no free electron in germanium and silicon. For this reason there is no conduction of electricity in them. Such pure semiconductors are known as intrinsic semiconductors. If pure semiconductors are heated at high temperature due to thermal agitation electrons of pure semiconductors becomes free by breaking the bonds. The electrons can pass forbidden gap if the energy of their electrons is very large and transferred into conduction band. When an electron goes into conduction band from valence band there a vacancy occurs. The vacancy makes a hole and this hole is equal to a positive charge.
Due to thermal vibration a bound electron next to a hole can move across to fill the gap. The net motion of the negative charge from a bound position to another being in effect equivalent to the motion of a hole in the opposite direction. This means the transfer of a hole is the transfer of positive charge. A hole is positive charge so flowing of holes mean flowing of current. Holes and electrons acts as charge carriers. Transferring of a hole and electron is shown in a figure. The number of electrons and holes are equal in pure on intrinsic semiconductor. Electron-hole bond pair forms due to thermal agitation. Hence the number of holes and electrons becomes equal in pure semiconductor.
(ii) Extrinsic semiconductors or impure: We know that due to thermal agitation charger carrier are produced in pure semiconductor. At normal temperature the charge carrier in pure semiconductors is very small. So the electrical conductivity is also very low in this situation. The small current has no importance practically. It has been found that if a small amount of impurity is added with the pure semiconductor the charge carrier increases. The mixing amount is one part in million. Hence the conductivity of the semiconductor increases. This system of mixing suitable impurity is called doping or doping semiconductor. Semiconductor doping is the process of making impure semiconductor.
This type of mixed semiconductor is called extrinsic semiconductor or impure semiconductor.
Aluminum, arsenic, antimony, gallium etc impurities are mixed in silicon or germanium.
Extrinsic or impure semiconductors are classified into two types, viz n-type and p-type semiconductors. n-type semiconductor means negative type semiconductor and p-type semiconductor means positive type semiconductor.