The most important question of Semi Conductor Physics in Electronic Devices and Circuits; Explain the concept of majority and minority carriers?, is being answered here.
Majority and Minority Carriers:
In the intrinsic state, the number of free electrons in Ge or Si is due only to those few electrons in the valence band that have acquired sufficient energy from thermal or light sources to break the covalent bond or to the few impurities that could not be removed. The vacancies left behind in the covalent bonding structure represent our very limited supply of holes. In an n-type material, the number of holes has not changed significantly from this intrinsic level. The net result, therefore, is that the number of electrons far outweighs the number of holes. For this reason: In an n-type material (Fig. 1.13a) the electron is called the majority carrier and the hole the minority carrier.
For the p-type material the number of holes far outweighs the number of electrons, as shown in Fig. 1.13b. Therefore: In a p-type material the hole is the majority carrier and the electron is the minority carrier. When the fifth electron of a donor atom leaves the parent atom, the atom remaining acquires a net positive charge: hence the positive sign in the donor-ion representation. For similar reasons, the negative sign appears in the acceptor ion.
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| Fig. Positions of Fermi level in (a) n-type and (b) p-type semiconductors |
In N type material Fermi level is just below the conduction band. In P type material Fermi level is just above the valence band.
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