METALS, INSULATORS AND SEMICONDUCTORS

• The free electrons that exist in the conduction band of any material are capable of carrying charge from one point to another and so they are known as charge carriers. 
•  Depending on their concentration, materials classified as metals, insulators and semiconductors. 
• Metals (eg; copper or silver)- very large number of free electrons at room temperature. In fact in the energy band diagram, the valence and conduction bands are overlapping and so there is no forbidden energy gap. It is very easy for a valence electron to become a conduction (free) electron. Hence even in the absence of additional energy such as light or heat, they behave as a good conductor. 
• Insulating materials (eg: rubber) on the other hand, have a very wide forbidden energy gap (Eg=5eV). Because of this it is practically impossible for an electron in the valence band to jump to the conduction band at room temperatures. It may conduct if its temperature is very high or if a high voltage is applied across it
• . For a semiconductor, the forbidden energy gap. is lesser than that of insulators but more than that of metals (Silicon Eg=1.12eV, Germanium Eg=O.72eV). The heat at room, temperature is sufficient to lift the electrons across and hence they are capable of conducting some electric current

Energy bands in solids

• When atoms bond together to form a solid, the orbit of an electron is influenced not only by the charges in its own atom but by nuclei and electrons of every atom in the solid. Since each electron occupies a different position inside the solid, no two electrons can have the same pattern of surrounding charges. Now in the solid, there are millions of electrons belonging to the first orbit of the atoms in the solid. Each of them have different energies and all these closely spaced energy levels differing only slightly in energy, .form a cluster or band. Similarly the second orbit and higher orbit electrons form bands. Hence the energy-band diagram of a solid (silicon) is shown.
• Although the third shell of an isolated silicon atom is not completely filled (it has only four electrons while it could have accommodated up to eight) the third energy band shown above is completely filled. This is so because each atom positions itself between four other atoms and each of the neighbours share and electron with the central atom. Now the valence band is filled, meaning all the permissible energy levels are occupied by electrons. 
An additional band called the conduction band is shown above the valence band. This represents the next larger group of permissible energy levels. The gap between the conduction and valence band is called the forbidden energy gap. This is because there are no permissible energy levels in this region. An electron in the valence band can be lifted to the conduction band only if energy greater than or equal to this band gap energy is provided to the solid. Once they reach the conduction band, these electrons experience negligible nuclear attraction and move randomly throughout the solid. Hence they are called free electrons.

Electron Energies

Each isolated atom has only a certain number of orbits available which represent energy levels for the electrons and the electrons can exist at these levels only. greater the distance of the electron from the nucleus, the greater is its total energy. electron near to the nucleus and possesses only a small amount of energy. Hence it is difficult to knock out this electron.  electron  far from the nucleus would have a greater energy and hence it can be easily knocked out of its orbit. This is why' the valence electrons take part in chemical reactions and in bonding the atoms together to form solids.

SEMICONDUCTOR MATERIALS

group of materials that are neither good conductors nor good insulators.  At room temperature these materials have conductivities considerably lower than that of conductors but much higher than that of insulators  Examples - germanium, silicon  Semiconductors have a very important property that their conductivity can be changed to a very large extent by adding a very small amount of some specific materials (called impurities)  Consider a silicon atom as shown,

                       

nucleus contains 14 protons and 14 neutrons-  14 electrons revolving around the nucleus distributed in different orbits or shells  a shell can contain a maximum of 2n2 electrons, where n is the number of the shell  But there is an exception to this rule, That is, the outermost orbit cannot accommodate more than eight electrons ie., valence electrons  Applying this rule, the 14 electrons of.;the silicon atom are distributed as, 2 in first , 8  in  second orbit and the remaining 4 e in third . Similarly a germanium atom with atomic number 32 will have its 32 electrons distributed

as shown