Semiconductors- Short story

          All of you may aware of "Conductors" and "Insulators". It means things which conduct electric current well called 'Conductors', ex: Copper, Gold, Iron, etc and things don't called as 'Insulators', ex: Glass, Ceramic, Plastic, etc. Wait. A simple question. Which characteristic defines an object? Obviously it is "Electrical Conductivity". Objects have more conductivity become conductors (Silver has 63000000 S/m) and objects has lesser conductivity become insulators (Glass has 10e-11 to  10e-15).

            How conductors conduct electric current is another phenomena. To explain this thing clearly you have to aware of Energy Levels, Electron orbitals, valance band and conduction band. Please read here: http://hyperphysics.phy-astr.gsu.edu/hbase/solids/semcn.html

          Got something?? OK fine. Simply say, energy level of the outermost electron orbital is called as Valance band and energy level of electrons required for electrical conduction is called as conduction band. In atoms, electrons occupy in orbitals in according to the probability function (You have to read Quantum Mechanics guys !!!). So depending upon no of electrons in that conduction band energy level, we can say whether it is conductor or insulator. In another way, we can differentiate objects based upon where the conduction band resides. Look at this picture.

              So in conductor, there are plenty of free electrons in conduction band so it easily conducts. But look at insulator. For conduction, every electron needs to jump from valance band to conduction band and that's why it doesn't conduct electric current and it exhibits great electrical resistance!

           Now where 'Semiconductors' comes into the picture?? The name itself it has the meaning and the few basic characteristics  are

• They have conductivity less than conductors and more than insulators
• Band gap between conduction and valance band is less than insulator.
• At room temperature, few electrons occupy conduction band but still it doesn't conduct electric current.
• These semiconductors are called as "Intrinsic Semiconductors".
• Ex: Silicon (Si), Germanium(Ge)

        But for conduction, these intrinsic semiconductors are not suitable and our aim is make the conductivity of semiconductor more means to have many free electrons. One of the way is to add impurities to intrinsic semiconductor. These semiconductors are known as "Extrinsic Semiconductor". We see bit brief about extrinsic semiconductor.

       Adding impurities in intrinsic semiconductor is known as 'Doping'. Lets take Silicon atom. It has 4 electrons in outer orbit. So if we add a Phosphorus (has 5 electron in outer orbit) atom with it , 4 electrons of Phosphorus and Silicon make covalent bond and remaining one electron of Phosphorus become free electron. At room temperature, we have electrons above valance band. This type semiconductor is known as "N-Type" semiconductor. Look at the following figures.

         If we add materials which has 3 electrons in outer orbit (Ex: Boron[B]), the outcome is "P-Type" semiconductor. 3 of both Si and B make covalent bond and deficiency in Si atom creates a "Hole". This hole is not permanent. Electrons from nearby atoms try to replace it but atom which replaces nearby hole will create hole. So at room temperature, "P-Type" material has free electrons at quite higher level than valance band. Look at this pictures.

 

          When compare with intrinsic semiconductors, these P and N type extrinsic semiconductors has better conductivity and obviously lesser electrical resistance.

           Now we came to know a bit about P and N type semiconductors. Our next goal how a PN junction diode is created and how it works..

Thanks and Regards,

Arun Alagarsamy @ arunvasan@gmail.com