A Semi-conductor is a material which has a resistance in between that of a conductor and an insulator. A semi-conductor in its purest form is known as intrinsic semi-conductor. The main features of intrinsic semi-conductors are:
1. R more than metal conductors and less than insulators.
2. Negative temperature coefficient as R decreases with rise in temperature.
3. Electron valence of ±. This valence means the atom has four Electrons in the outermost ring-halfway to the goal of 5.
Doping:
The special feature of the semi-conductor elements, however, is that the atomic structure allows their conductivity to be increased by adding Impurity elements, a process called doping. The purpose is to increase the number of free charges that can be moved by an external applied voltage. When the number of free electrons is increased, the doped semi-conductor is negative or N type; reducing the number of free electrons makes the material P –type. Both germanium (Ge) and silicon (Si) can be used with either P-type or N-type doping, but silicon semi-conductor devices are most common.
Element | Symbol | Atomic number | No: of valence electrons | Applications |
Aluminium Boron Gallium Indium | Al | 13 | 3 | Acceptor impurity elements Take electrons to form P-type semi conductor, Ga and In used for Ge, Al, and B for Si. |
B | 5 | 3 | ||
Ga | 31 | 3 | ||
In | 49 | 3 | ||
Germanium Silicon | Ge | 32 | 4 | Intrinsic semi-conductors Used in pure crystal form to be doped with impurity elements. Doped semi conductors are extrinsic. |
Si | 14 | 4 | ||
Antimony Arsenic Phosphorous | Sb | 51 | 5 | Donor impurity elements Give electrons to form N-type semi conductors, As and Sb used for Ge and phosphorous for Si |
As | 33 | 5 | ||
P | 15 | 5 |
Silicon and germanium have 5.0 × 10^28 and 4.42 × 10^28 atoms per m^3 respectively and an addition of only one impurity atom per 10^7 or 10^8 semi-conductor atoms is sufficient to yield the desired properties.
