charge density on hole and electron in valence and conduction bands.

for details look into 

http://ecee.colorado.edu/~bart/book/intrinsi.htm

http://lamp.tu-graz.ac.at/~hadley/psd/weblectures/Ef_intrinsic/index.php

Fermi-level is strongly dependent on carrier density and hence defines the material type. Because carrier density alters Fermi-level, it approaches to conduction band (valance band)  by doping donor (acceptor) atoms which increase electron (hole) concentration.

thanks a lot ...