Intrinsic Silicon and Extrinsic SiliconPublished on 24/2/2012 & updated on Tuesday 22nd of May 2018 at 05:09:40 PM
In the intrinsic silicon crystal, the number of holes is equal to the number of free electrons. Since each electron when leaves the covalent bond contributes a hole in the broken bond. At certain temperature always new electrons-hole pairs are created by gaining thermal energy but at the same time, the same number of electrons-hole pairs are lost due to recombination. Hence at a particular temperature in a certain volume of insintric silicon number of electron-hole pairs remain same. This is an equilibrium condition. Hence, this is obvious that in the equilibrium condition, the free electrons concentration n and the holes concentration p are equal to each other, and this is nothing but intrinsic charge carrier concentration(ni). i.e, n = p = ni. The atomic structure is shown below.
Intrinsic Silicon at 0oK
Intrinsic Silicon at Room Temperature
Extrinsic Silicon with Pentavalent ImpurityIf a small amount of group III elements is added to an intrinsic semiconductor crystal, then they displace a silicon atom, group III elements like AI, B, IN have three valence electrons. These three electrons make the covalent bonds with neighboring atoms creating a hole. These kinds of impurity atoms known as acceptors. The semiconductor in known as p-type semiconductor as the hole is assumed to be positively charged.
Extrinsic Silicon with Trivalent ImpurityThe Fermi energy level moves down, closer to the valence bond in p-type semiconductors. Here the number of holes is increased, and the number of electrons is decreased over the intrinsic carrier concentration of silicon since here free electrons get plenty of holes in the crystal. In p-type semiconductors holes are the majority charge carriers.
Intrinsic Carrier Concentration of SiliconWhen an electron jumps from valence band to conduction band because of thermal excitation, free carriers are created in both bands that are electron in the conduction band and hole in the valence band. The concentration of these carriers is known as intrinsic carrier concentration. Practically in pure or intrinsic silicon crystal the number of holes (p) and electrons (n) are equal to each other, and they are equal to intrinsic carrier concentration ni. Therefore, n = p = ni The no. of these carriers depends on the band gap energy. For silicon, the band gap energy is 1.2 eV at 298oK intrinsic carrier concentration in silicon increases with the increase of temperature. Intrinsic carriers concentration in silicon is given by, Here, T = temperature in absolute scale The intrinsic carrier concentration at 300oK is 1.01 × 1010 cm-3. But the previously accepted value is 1.5 × 1010 cm-3.
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