Doping 19.4
A semi-conductor, in its purest form is not such a good conductor, so it is of little practical value. But it can be rendered useful by increasing the number of holes and free electrons according to our requirement. This is done by adding specified quantities of a trivalent or a pentavlent atoms as impurity while a single crystal of silicon or germanium is being grown. This is done by adding specified quantities of a trivalent or pentavalent atom as impurity while a single crystal of silicon or germanium is being grown. This drastically increases the current conducting properties of germanium or silicon. The impurity added crystal are of two types- one is n-type and the other is p-type.
N-Type Semi-Conductor 19.5
When a pentavalent impurity such as arsenic (As), phosphorous (P), bismuth (Bi) or antimony (Sb) is added to crystals of silicon or germanium, the number of free electrons increases in it. The impurity is usually in the ratio of 
atoms of silicon or germanium there is one atom of the impurity. Such a germanium or silicon crystal is known as n-type semi-conductor. Now let us see that how the process of adding impurity increases the number of free electrons. Take the example of silicon crystal doped with antimony. In fig. 19.6 an antimony atom surrounded by four silicon atoms is shown.
Antimony, being a pentavalent atom, has five valence electrons. Four of them get bound by forming covalent bonds by sharing one electron from each of the four silicon neighbors. But the fifth electron of antimony atom does not find any electron from its four neighbors to form a covalent bond. As such it becomes a free electron. Thus, each impurity atom gives birth to a free electron. This increases the number of free electrons as compared to pure crystal. By controlling the number of impurity atoms during doping, the required number of free electrons can be obtained. In an n-type semi-conductor, current passes mostly due to free electrons.
P-Type Semi-Conductor 19.6
if a trivalent impurity atom such as aluminum (Al), indium (in), boron (b) or gallium (Ga) is doped while preparing single crystal of germanium or silicon, the number of holes in these impure crystals is much greater as compared to the number of holes in pure crystal. Such doped semi-conductors are known as p-type.
Take the example of a crystal of silicon which is doped by an aluminum atom. Fig. 19.7 shows aluminum as impurity atom with its four silicon neighbors. Aluminum being trivalent, has three valence electrons which form convent bonds by sharing one electron from each of its three silicon neighbors, but the aluminum atom does not provide any electron for sharing to its fourth neighboring atom. Thus a vacancy for an electron is created here i.e., a hole has been formed (fig. 19.7). as every impurity atom creates a hole, therefore the number of holes in such crystals is much larger as compared to that in a pure crystal. N a p-type semi-conductor, nearly the whole of current flows due to holes.
