# How to determine donor or acceptor impurity

I am working on a practice exam for semiconductor physics. Can anyone help me figure out how to determine whether we have donor or acceptor impurity in Gallium Arsenide?

Gallium is column 3 and (I think) it has a valence of 3.
Arsenide is column 5 and (I think) it has a valence of 5.
But it has a valence of 4 when it is GaAs?


Based on the information I believe I will be provided in the exam proper, I will know only the periodic group and/or atomic number, and/or valence.

I am given these choices as impurities:

Si <-- Donor impurity --> n-type semiconductor
S  <-- Donor impurity --> n-type semiconductor
P  <-- ??
Zn <-- Acceptor impurity --> p-type semiconductor


So you can see I have the answers, I just don't know how to get there.

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 1) I believe you should exchange n and p types in your last table. Doping with acceptors makes the p-type semiconductor and vice versa. 2) For Si it seems you should also know which atom (Ga or As) it replaces in the lattice to know the impurity type. 3) what about deep centers? – Misha Nov 21 '11 at 16:16 Let me stress 2nd point. According to this table Si is both donor and acceptor. If you have an access to the book referenced there you may find more information. – Misha Nov 22 '11 at 9:12 Thanks - pretty sure it is Ga when in GaAs, with a valence of four - what worries me is this information is not available in the question... As for part 3, that isn't in the exam syllabus – eggonlegs Nov 22 '11 at 13:15