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SEMICONDUCTOR PHYSICS In solid substances electricity is conducted with the help of charge carriers which are the free electrons, that move and transfer electric charge from one place to another (from lower potential to higher potential). At 0 K no thermal energy is present so electrons remain in their valence band. As temperature increases these electrons gain sufficient kinetic energy to cross the energy gap between valence band and conduction band and move to the conduction band. Once energy reaches a sufficient level these electrons move in bulk and conduct charge. Based on charge carrier concentration, solid substances can be classified as metals, semiconductors or insulators. In metals electrons are the only charge carriers and their concentration is very high because of the overlapping that takes place between the valence and the conduction band. In semiconductors however, this concentration depends on the energy gap between bands. In insulators this gap is too high for electrons to jump into the conduction band and so insulators do not conduct electricity. At room temperature these electrons that are conducted move in random motion. Their motion is like that of gas molecules since in conductors a large number of electrons move randomly hence they do not have a net motion in any particular direction. Thus no current is generated. As some potential difference is applied across the two ends of a conductor, an electric field is set up inside the conductor and electrons start moving from the lower potential to the higher potential point. An external force accelerates the motion of electrons inside the conductor. Drift velocity is the small velocity imposed due to the random motion of electrons on the application of an electric field. It is also defined as the velocity with which the free electrons get drifted with at the positive potential on the application of an electric field. Given by, vd=(eE/m)τ , τ: Time instant between successive collisions. Relation between drift velocity and electric current density (J) ΔQ=neAvdΔt Or, I = ΔQ/Δt = neAvd Current density J is the total flow of charge per time over cross section area A. In a semiconductor the concentration of the charge carrier depends on the energy gap between the two bands (i.e. valence band and conduction band). When the electrons leave the valence band in a semiconductor, it leaves behind a positively charged vacancy, referred to as ‘hole’. These holes are mobile in the valence band. And these electrons pair with the corresponding hole, known as electron hole pair. Hence semiconductors have two types of charge carrierselectrons in conduction band and holes in valence bands. An intrinsic semiconductor is one, which is composed purely of only one type of element and is free from any defects or impurities. The electrons and holes are always created in pairs by thermal agitation; the concentration of both of these is the same. Whereas in extrinsic semiconductors, it is possible to generate electrons and holes by the process known as doping in which impurities are inserted into the substance. These exhibit greater conductivity than the intrinsic semiconductors. Extrinsic semiconductors can be further classified as n-type and p-type. N-type semiconductors - doped by infusing a small amount of donor element. Donor elements are elements with 5 valence electrons. Hence n-type semiconductors have an excess of electrons in them. P-type semiconductors- doped by infusing a small amount of acceptorelements with three valence electrons. Hence p-type semiconductors have an excess of holes. The process of infusing the impurities in the semiconductor in order to increase the charge carrier concentration and hence increase conductivity is the process of diffusion. This process plays an important role in semiconductor physics.