Band Theory of Solids: To understand the origin of the formation of energy levels and energy bands (leading to a band gap) in a semiconductor, we need to analyze the periodic potentials present in a semiconductor. If we solve the periodic wave functions (Bloch functions), the result is that the energy levels are grouped in bands, separated by energy band gaps. The behavior of electrons at the bottom of such a band is similar to that of a free electron and the electrons are affected by the presence of the periodic potential. The effect of a periodic arrangement on the electron energy levels is shown as the energy levels of electrons in a carbon crystal with the atoms arranged in a diamond lattice. These energy levels are plotted as a function of the lattice constant, a . Isolated carbon atom has six electrons in 1s, 2s and 2p orbital in pairs. The energy of an electron occupying the 2s and 2p orbital is indicated in the figure. As the lattice constant is...