Semiconductor and Band-gap

Some of the semiconductors like silicon (Si) and germanium (Ge) have band gap energy of 1.1 and 0.7 eV respectively, they absorb visible radiation. from the statical law of thermal energy distribution in solid, at room temperature there are significant numbers of electrons in valance band which acquire sufficient energy to cross the energy gap, this is semiconductivity. Unlike in metal, semiconductivity increases with the temperature not with the increased lattice vibration. The properties of semiconductor depends on the number and types of charge carriers n-type (negative- electrons), p-type (holes, positive) and can be controlled by the apropriate dopant elements. For more information;
http://ocw.mit.edu/courses/materials-science-and-engineering/3-091sc-introduction-to-solid-state-chemistry-fall-2010/syllabus/MIT3_091SCF09_aln03.pdf