^Extrinsic or doped semiconductors
By adjusting the amount & distribution of doping the conductivity of a SC can be increased to desired amount. Generally the amount of doping is about ppm (i.e. part per million) i.e. one impurity atom is added in 106 pure atoms (i.e. 1016 impurity atoms in 1023 pure atoms per cc). This increases the conductivity more than 103 times.
^Electrical mobility
The drift velocity acquired by a charge carrier in a unit electric field is called its electrical mobility and is denoted by μ. i.e. 
. Mobility of electrons in the VB is less than the mobility of electrons in the CB.
Current in a SC is, I = [ve ne + vh nh] e
Conductivity of SC is, σ = e [ne μe + nh μh],
σSi = 10–11 σCu = 10+13sfused quartz
Due to very small number density of charge carriers the conductivity of intrinsic SCs is very low & they are of no practical utility.
^Explosion of a bomb
Before explosion, suppose the bomb is at rest i.e., initial total momentum is zero. Now when it explodes, it breaks up into several pieces of masses m1, m2, m3, etc., which fly off in different directions with velocities 
etc. in such a manner that vector sum of linear momentum of all the parts after explosion is zero or the linear momentum of any piece is nullified by the resultant of the linear momenta of the remaining. During explosions total energy is always conserved however KE after explosion increase. This is possible because the chemical energy of the explosive is converted into KE. Thus we can say that the internal forces acting on a system can never change the linear momentum of a system but can change the KE of the system.
^Intrinsic SCs
At 0 K the VB of an intrinsic semiconductor is completely filled with the electrons while the conduction band is completely empty as a result a SC behaves like an insulator at 0 K temperature.
However at higher temperatures say at room temperature some of the electrons from the VB gain sufficient thermal energy & jump to CB & occupy some level in the CB in accordance with the extent of the excitation energy. This is called thermal agitation. The vacant spaces left behind in the VB are called holes.
The vacancy of an electron with an effective positive electronic charge is called a hole. It behaves as an apparent free particle with a charge +e. For a intrinsic SC ne = nh = ni
On applying external electric field electrons in the CB start drifting in a direction opposite to the electric field & holes in the VB start drifting in the direction of the electric field due to the force exerted by the electric field as shown in the diagram.
^Energy bands in solids
Energy levels of electrons in an isolated atom are well defined & discrete, however the valence energy levels get modified drastically when atoms interact to form the lattice. If N atoms interact, then total 8 N energy valence energy levels are available which split in two bands (called VB & CB) separated by energy gap (called Eg) as shown in the diagram.
As Eg (Ge) < Eg (Si), thus Ge is preferred in making semi conducting devices.
^Semiconductors
Semiconductors (e.g. Si, Ge) belong to 14th group, thus have 4 valence electrons & hence from covalent bonds (temperature sensitive).
^Absorption radiations
Radiations are absorbed by the materials according to the relation: I = I0 e-μ X
Here μ = absorption coefficient
I = intensity after penetrating the material through ‘x’ thickness of material
I0 = intensity of the radiation before penetrating
Absorption coefficient is highest for Pb.
1. R: Roentgen
2. Rad: Radiation absorbed dose
3. rem: Roentgen equivalent in man
4. RBE: Relative biological effectiveness
^Moderator
The neutrons produced in fission of 235U nuclei have average KE about 2 MeV. Such neutrons are called fast neutrons. These fast neutrons have more tendency to escape instead of triggering another fission reaction. Slow neutrons are more efficient in inducing fission in 92U235 nuclei than fast neutrons. By the use of a moderator, the fast neutrons are slowed to thermal velocities i.e. velocities » 2200 m/s & energies » 0.0235 eV, it is same as that of atoms and molecules at room temperatures, such slow moving neutrons are called thermal neutrons. Light target are better moderators. The commonly used moderator are water, heavy water (D2O), graphite and beryllium. About 25 collisions with deutrons (present in heavy water) or 100 collisions with carbon or beryllium are sufficient to slow down a neutron from 2 MeV to thermal energies.
A good moderator must have:
Call 9872662552