Dear readers,
This is my first blog on the internet regarding the Electronics and Telecommunication Engineering. Journey of modern electronics begins from the semiconductors. In this topic we will discuss how semiconductors play an important role in electronics.
We know that materials either can be conductor (such as silver, copper, aluminium etc.) or it can be insulator (glass, rubber, quartz etc.). Apart from them there is another group of material which is known as semiconductor. Semiconductors are neither good conductor nor good insulator. Thier conductivity lies between conductors and insulators. The restivity of semiconductor material lies in the range from 0.00001 ohm-meter to 0.5 ohm-meter.
An important property of semiconductors is that their resistivity can be controlled by adding a few ammount of impurity. The elimental semiconductors are germanium (Ge), Silicon (Si), tin (Sn), selenium(Se), boron (B) etc. Germanium and silicon are two of the widely used semiconductors.
The group IV element of modern periodic table show similar properties as well as graduation in properties with rise of atomic number. There is gradual change from non-metallic to metallic nature with the rise in atomic number. Thus carbon and silicon are non-metals germanium is a metalloid while tin and lead are metals. Carbon is the most electronegative element hence it can not be considered as a semiconductor material.
Enrgy Band Theory
As we discussed the semiconductor elements belong to group IV hence they are tetra valent elements that means these elements have 4 electrons in their valance shell (outer most shell). Let us consider a crystal of a material. Every electron of an atom consist of an specific energy level. This energy level is nearly equal to the energy level of another electron of neighbour atom having same orbits. All the electrons of first energy level consists energy in a small energy band this is called the first energy band there are many energy bands in a crystal. The electrons of second energy level lie on the second energy band and so on. The last energy band consists of electrons of outer most orbit (valance orbit or valance shell) this energy band is known as valance band. There is another energy band above from the valance band this band is called conduction band. Conduction band consists of free electrons.
An electron needs some energy to jump into conduction band from valance band. This energy is called forbidden energy gap. Forbidden energy gap is defined as the energy that a valance electron is required to break its bond and become free to move anywhere in the crystal. On the basis of energy gap types of material can be easily understood.
The energy gap for insulators is very large (hundreds of eV) that means they require very large amount of energy to release an electron from their atom.
Forbidden energy gap for semiconducting material is small in comparison to insulators( 1.1 eV in case of Ge and 0.72 eV in case of Si).
The third group of material i.e conductors do not have enrgy gap. Valance band and conduction band is overlap that means electrons of valance band can enters on conduction band without gaining the energy
Basics of Electronics 