capacitor basic

Capacitor: A capacitor is a passive two terminal electronic component that stores electrical energy in an electric field.

E = k*Q/r^2
V = k*Q/r
& Q is directly proportional to V
Q = C*V
Q/V = C (capacitance is measured in Farad,F)

So,a capacitance is the charge required to increase the voltage of 1volt.
For Ex> If capacitance  of a material is 50 F it means that it needs 50 coulomb of charge to increase 1 volt.

Have you ever thought that how much charge can be given to a conductor??
> A conductor can store charge until the surrounding mediam  does not get ionised.

 V = k*Q/R &  C =Q/V
so C = k*R
from above equation  it's clear that  capacitance  depends on size of sphere.

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Semiconductor basics part (a)

An isolated or a single atom has some energy level which become different when combine with any other atom.

Note: When two charges are near to each other they experience a force of attraction/repulsion.
i.e, F=k*q1q2/r^2 and U=k*q1*q2/r.

so during formation of crystal, electrons of two atoms will repel each other and their corresponding +ve charge will attract their electrons but the distance between the electrons are not the same so every electron of an atom will experience some different energy and hence, energy will not be the same what it was earlier.

 As we know one silicon atom has 4 electrons in outermost orbit each having different energy level. Now, after forming crystal each atom has 4 level of energy.  when N number of atoms comes closer to form a crystal. So, N atoms would have 4N energy level.(i.e,  one atom>4energy level

Si atomic configuration is:
1s2 2s2 2p6 3s2 3p2
so, probable energy level is 8N (two energy level of 3s which is completely filled and six energy level of 3p  where  2N is filled and 4N is unfilled). so, we can say that out of 8N, 4N is filled and 4N is vacant and if electron get that possible energy then they may excite to vacant 4N.

 After actual formation of crystal it was observed that  there remains only two energy levels.Some electrons occupy  higher energy level(4N) and some electrons occupy lower energy level (4N) forming a high band and low band energy level. It was observed that between high band and low band there is no electron(vacant space was observed between high and low energy band).

Valance band: The lower energy band occupied by electrons after formation of crystal is called valance band.

Conduction band: The higher energy band occupied by the electrons after formation of crystal is called conduction band. Electrons in conduction band are free electrons.

NOTE: After formation of crystal  one electron revolves around its own orbit and  also around its neighboring orbit.

          Formation of free electrons:
Nucleus attract the electron towards  its center with a force of attraction  and an electron has some energy but that is not sufficient to come out from its respective orbit so they keep on revolving around its respective nucleus.
If electron gain the enough energy from external applied source  then that electron will escape the orbit and becomes free.

Types of semiconductor:
a) Intrinsic semiconductor
b) extrinsic semiconductor

a) Intrinsic semiconductor:
 --> At 0 k all electrons are in valance band and conduction band is empty so there is no free electron.
Hence, it is said that at 0 k semiconductor behaves like an insulator.

-->At room temperature,  some electrons (few electrons)  gain energy equal to or more than Eg and become conduction or free  electrons.

Semicondutor behaviour at room temp.