energy of both the orbitals is the same. When the third atom of the same element is brought to this system, the valence orbitals of all the three atoms are split into three.The unoccupied orbitals also will split into three. In reality, a solid is made up of millions of atoms. When millions of atoms are brought close to each other, the valence orbitals and the unoccupied orbitals are split according to the number of atoms.
In this case, the Conduction band Valence band Forbidden energy gap Electron (a) E V E C E g Conduction band Valence band E g = 6eV E g = 1eV Free electrons Holes Insulator Semiconductor Conductor (b) (c) (d) CB CB CB VB VB VB Figure . (a) Schematic representation of valence band, conduction band and forbidden energy gap. Energy band structure of (b) Insulator (c) Conductor (d) Semiconductor - - - - - - - - Unit electronics and Communication (Figure . (d)).
At a finite temperature, thermal agitations in the solid can break the covalent bond between the atoms (covalent bond is formed due to the sharing of electrons to attain stable electronic configuration). This releases some electrons from valence band to conduction band. Since free electrons are small in number, the conductivity of the semiconductors is not as high as that of the conductors. The resistivity value of semiconductors is from – Ωm to Ωm.
In semiconductors, electrons in the valence band are bound electrons which cannot move. Hence, they cannot contribute for conduction. Note When the temperature is increased further, more number of electrons are promoted to the conduction band and they increase the conduction. Thus, we can say that the electrical conduction increases with the increase in temperature.
In other words, resistance decreases with increase in temperature. Hence, semiconductors are said to have negative temperature coefficient of resistance. The most important commonly used elemental semiconducting materials are silicon (Si) and germanium (Ge). The values of forbidden energy gap for Si and Ge at room temperature are .
eV and . eV respectively.