In the energy band diagram of a material shown below, the open circles and filled circles denote holes and electrons respectively. The material is a/an: [AIPMT (S) 2007] |
A) p-type semiconductor
B) insulator
C) metal
D) n-type semiconductor
Correct Answer: A
Solution :
The given figure represents p-type semiconductor as described below: |
When one of the silicon atom (valence = 4) has been replaced by an atom of aluminium (valence = 3), the aluminium atom can bond covalently with only three silicon atoms, so there is now a missing electron (a hole) in one aluminium-silicon bond with a small expanditure of energy, an electron can be torn from a neighbouring silicon-silicon bond to fill this hole, thereby creating a hole in that bond. Similarly, an electron from some other bond can be moved to fill the second hole. In this way, the hole can migrate through the lattice. |
The aluminium atom is called an acceptor atom because it readily accepts an electron from a neighbouring bond that is from the valence band of silicon. As figure suggests, this electron occupies a localized acceptor state that lies within the energy gap, at an average energy interval \[{{E}_{a}}\] above the top of the valence band. |
By adding acceptor atoms, it is possible to increase very greatly the number of holes in the valence band. |
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