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Explain n and p-type semiconductor based on band theory. |
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Answer» Solution :In EXTRINSIC semiconductor because of the abundance of majority current carriers the minority carriers produced thermally have more chance of meeting majority carriers and thus getting destroyed and indirectly helps to REDUCE the intrinsic concentration of minority carriers. The semiconductor.senergy band structureis affected by doping. In extrinsic semiconductor, additionalenergy states due to donor impurities `E_(D)` and acceptor impurities `E_(A)` also exist. In n-type of Si semiconductor, the donor energy level `E_(D)` is slightly below the bottom `E_(C )` of the CONDUCTION band and electrons from this level move into the conduction band with very small of energy. At room temperature most of donor atoms get ionised but very few `(~10^(12))` atoms of Si get ionised so the conduction band will have most electrons coming from the donor impurities as shown in figure (a).  For p-type semiconductor the acceptor energy level `E_(A)` is slightly above the top `E_(V)` of the valence band. With very small supply of energy an electron from the valence band can jump to the level `E_(A)` and ionise the acceptor negatively. In other words,withvery small supply of energy the hole from level `E_(A)` sinks down into the valence band `E_(V)` and electron comes up. This is shown in figure (b).  At room temperature, most of the acceptor atoms get ionised leaving holes in the valence band `E_(V)`. Thus, at room temperaturethe density of holes in the valence band is mostly due to impurity in the extrinsic semiconductor. The above description is grossly approximate and hypothetical it helps in understanding the difference between metals, insulators and semiconductors (extrinsic and intrinsic) in a simple manner. The energy difference between `E_(V)` and `E_(C )` for C (diamond) is 5.4 eV, for Si is 1.1 eV and for Ge it is 0.7 eV and Sn is also a GROUP IV element but it is a metal because the energygap in its case is 0 eV. |
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