As far as I have learnt, semiconductors are often made using elements from the 4th group, and their properties are often enhanced by doping with either pentavalent or trivalent elements. Take the case of pentavalent doping, which results in a n-type semiconductor. Well this would raise the Fermi energy since there are more electron donors and this enhances the properties of the semiconductor. Wouldn’t it be more effective if doped with an element like sulphur? There would be more electrons donated then. In addition, doping with group 2 elements would result in more holes too! Why is this currently not done? I would like an explanation that explains how these kinds of doping may affect the Fermi energy and why it is hence, not being done.
Silicon is doped with group III (B, hole doping) and group V (P, electron doping) elements because these form shallow impurities when residing at a silicon lattice position. Shallow impurities in silicon have binding energies of 40-50 meV and are ionised at room temperature so that their electrons reside in the conduction band. The cause of this low binding energy is that the excess nuclear potential is shielded by the high dielectric constant of 11.4.
It is also possible to dope silicon with group VI elements (S,SE,Te). These elements indeed produce double donors but with much larger binding energy of 300 meV for the first electron and 600 meV for the second. Such donors are therefor not ionised at room temperature and do not contribute to the conductivity.
I am not aware of successful group II (double acceptor) doping in silicon. Earth alkali atoms are unlikely to favour the silicon lattice position. Zinc appears to be a double acceptor, also with deep levels.