SOLUTION

In an intrinsic semiconductor free electrons are thermally produced.

In an intrinsic semiconductor, the concentration of holes in the valence band and that of electrons in the conduction band is always the same at all temperatures, since the thermal excitation of an electron to the conduction band by breaking a covalent bond in the valence band creates one and only one hole.

• At room temperature (T> 0 K), the valence electron acquires a sufficient amount of thermal energy. As a result, breakage of covalent bonds takes place releasing free electrons.
• These free electrons create a vacancy in their initial position in the crystal.
• This vacancy is known as a hole and is assumed to carry a positive charge equivalent to the charge of the electron.
• These free electrons, due to acquiring sufficient thermal energy, cross the energy gap and enter into the conduction band from the valence band and occupy the energy levels in the conduction band.
• The electrons leaving the valence band create holes in their place. Thus, the valence band has holes while the conduction band has electrons