EXTRINSIC SEMICONDUCTOR.ppt (Size: 352 KB / Downloads: 196)
In an extrinsic semiconducting material, the charge carriers originate from impurity atoms added to the original material is called impurity [or] extrinsic semiconductor.
This Semiconductor obtained by doping TRIVALENT and PENTAVALENT impurites in a TETRAVALENT semiconductor. The electrical conductivity of pure semiconductors may be changed even with the addition of few amount of impurities.
N – type semiconductor
When pentavalent impurity is added to the intrinsic semiconductors, n type semi conductors are formed.
When small amounts of pentavalent impurity such as phosphorous are added during crystal formation, the impurity atoms lock into the crystal lattice[ see above Fig).
Consider a silicon crystal which is doped with a fifth column element such as P, As or Sb.
Four of the five electrons in the outermost orbital of the phosphorus atom take part in the tetrahedral bonding with the four silicon neighbours.
The fifth electron cannot take part in the discrete covalent bonding. It is loosely bound to the parent atom.
It is possible to calculate an orbit for the fifth electron assuming that it revolves around the positively charged phosphorus ion, in the same way as for the “1s” electron around the hydrogen nucleus.
The electron of the phosphorus atom is moving in the electric field of the silicon crystal and not in free space, as is the case in the hydrogen atom.
This brings in the dielectric constant of the crystal into the orbital calculations, and the radius of the electron orbit here turns out to be very large, about 80 Å, as against 0.5 Å for the hydrogen orbit. Such a large orbit evidently means that the fifth electron is almost free and is at an energy level close to the conduction band.
P -Type Semiconductor
When trivalent impurity is added to intrinsic semiconductor, P type semi conductors are formed.
Al has three electrons in the outer orbital. While substituting for silicon in the crystal, it needs an extra- electron to complete the tetrahedral arrangement of bonds around it.
The extra electron can come only from one of
the neighbouring silicon atoms, thereby creating a vacant electron site (hole) on the silicon.
The aluminium atom with the extra electron becomes a negative charge and the hole with a positive charge can be considered to resolve around the aluminium atom, leading to the same orbital calculations as aboveT.