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GR9677 #78
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Advanced Topics$\Rightarrow$}Solid State Physics

A n-type semiconductor is a material with negative-charge carriers, such as electrons. A p-type semiconductor is a material with positive-charge carriers, such as holes (positrons).

In band theory, n-type semiconductor impurities are (electron) donors, while p-type semiconductor impurities are (electron) acceptors.

The setup is as follows:

Impurities add in more levels to the energy bands. Without impurities, one has just a valance band and a conduction band with an energy gap in between. The impurities supply an extra energy level in between the conduction and valance bands. In an n-type semiconductor, the material becomes conducting when there are electrons in the conduction band; the impurity helps the material become conducting by supplying it with electrons.

Essentially, one starts with a lattice of pure semiconductor atoms, say Silicon. Silicon has four valance electrons and forms a decent crystal lattice. Pluck out a few silicon atoms and replace them with some impurities, like Arsenic, which five valance electrons. The extra electron from each impurity atom is free to roam around. In fact, these extra electrons act as donors to the conduction band. This is choice (E).

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mianghazanfar786
2010-07-22 16:17:22
We should remember the difference b/w n-type and p-type materials. We know that n-type materials are those materials which can donate electrons so we eliminate A, B and C. In choice D and E, They donate electrons to the conduction band, therefore "E" is the choice
CaspianXI
2009-04-03 13:34:55
Here's a simple way to help you remember the difference between n-type and p-type semiconductors for those who are trying to learn just enough solid state physics for the GRE.

"n" stands for negative, so n-type silicon is doped with negatively charged atoms (say, phosphorus) . This means that these atoms have extra electrons, and can easily part with the extra electron. Hence, they are donors.

"p" stands for positive, so p-type silicon is doped with positively charged atoms (say, boron). This means that these atoms have missing electrons, so they can easily accept new electrons to fill the vacancy. Hence, they are acceptors.

In this problem, we're dealing with n-type silicon. Hence, we expect the correct answer to include a statement about donors.
 ramparts2009-10-01 12:36:30 Great! That just prevented me from not touching semi-conductors for the GRE :) Thanks!
petr1243
2008-03-17 18:59:52
It would be choice A, if we dealt with a p-type semiconductor.

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