Extrinsic Semiconductors
A doped semiconductor or a semi-conductor with suitable impurity atom added to it, is called extrinsic semiconductor. Extrinsic semiconductors are of two types:
(i) N-type semiconductors
(ii) P-type semiconductors
N-type semiconductors: When a pure semiconductor of silicon (Si) or germanium (Ge) in which each Di orGe atom has four valence electrons, is doped with a controlled amount of pentavalent atoms, say arsenic or phosphorus or antimony or bismuth, which have five valence electrons, the impurity atom will replace theSi or Ge atom. The four of the valence electrons of the impurity atoms will form covalent bonds by sharing the electrons with the adjoining four atoms of silicon, while the fifth electron is very loosely bound with the parent impurity atom is comparatively free to move. It is so because the force of attraction between electron and nucleus of donor atom becomes very small due to very high value of dielectric constant of Si semiconductor. The binding energy of electron becomes 0.045 eV. Thus each impurity atom added donates one free electron to the crystal structure. These impurity atoms which donate free electrons for conduction are called donor atoms. Since the conduction of electricity is due to the motion of electrons i.e. negative charges or n-type carries, therefore, the resulting semiconductors is called donor-type or n-type semiconductor. On giving up their fifth electron, the donor atom becomes positively charged. However, the matter remains electrically neutral as a whole. The extra electron of the donor atom orbits around the donor nucleus, in a hydrogen like manner. It has been found that 0.045 eV energy is required to remove this electron from the impurity atom and make it a free electron.
At room temperature, some of the covalent bonds may get ruptured, producing thereby free electrons and an equal number of holes in the n-type semiconductor. But overall, the total number of holes in n-type semiconductor is relatively low, hence in n-type semiconductor; electrons are majority carries and holes are minority carries.
For a silicon semiconductor with impurity atoms of arsenic or phosphorus, the energies of the free electrons are slightly less than the energies of the free electrons in the lowest energy level of conduction band. As a result of it, these electrons occupy discrete energy levels (called donor energy levels) between the valence and conduction band and the lowest donor electron energy level lies at 0.045 eV below the bottom of the conductions band.
When we add pentavalent impurity in a pure semiconductor of Ge or Si, the fermi level in forbidden gap shifts very close to conduction band. If doping is very large, the fermi level may more into the conduction band.
It is to be noted that this energy comparable to the thermal energy of electron at room temperature (= 0.03 eV). Thus a very small energy supplied can exit the electrons from donor levels to conduction band. Due to this, the conductivity of semiconductor is remarkably improved.
P-type semiconductor: when a pure semiconductor of Germanium (Ge) or Silicon (Si), in which each atom has four valence electrons is doped with a controlled amount of trivalent atoms say gallium, or Indium (In) or Boron (B) or Aluminium (Al) which have three valence electrons, the impurity atom will replace the Ge or Si atom. The three valence electrons of the impurity atom will form covalent bonds by sharing the electrons of the adjoining three atoms of Ge while there will be one incomplete bond with a neighbouring Ge=atom to the deficiency of an electron. This deficiency is completed by taking an electron from one of the Ge-Ge bonds, thus completing the In-Ge bond. This makes Indium ionised (negatively charged) and creates a hole. An electron moving from a Ge-Ge bond to fill a hole, leaves a hole behind. That is how, holes move in the semiconductor structure. The conduction of electricity occurs due to motion of holes i.e. positive charges or p-type carries. That is why the resulting semiconductor is called acceptor type or p-type semiconductor.
Also, at ordinary temperature, some of the covalent bonds may get ruptured, releasing equal number of holes and electrons and holes. Therefore, the total number of electrons is relatively small as compared to the number of the holes in the p-type semiconductor. Hence in the p-type semiconductor, electrons are minority carriers and holes are majority carriers.
For a Ge or Si semiconductor, the doping of impurity atoms of Indium or boron some allowed energy levels which are situated in the forbidden gap slightly above the valence bond. These levels are called acceptor levels.
When we add a trivalent impurity in a pure semiconductor of a Ge or Si, the fermi level in forbidden gap shifts very close to valence band. If doping is very large, the fermi level may move into the valence band.
At room temperature, due to thermal energy, the electrons from the valence band are easily transferred to the acceptor level until these levels are filled. This produces a large number of holes in the valence and thereby the valence band becomes a hole conduction band. When an external electric field is applied to a p-type semiconductor, these holes will act as carriers of current. Due to it, the p-type semiconductor shows its electrical conductivity much improved than what it was for pure semiconductor.
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