Semiconductors are face centered cubic crystals (FCC).FCC has in addition to eight atoms in corner, six atoms, one at each face centered. In these cubic crystals there are thousands of unit cells, a unit cell contains 4 atoms. In semiconductors, Ge and Si are FCC. Diamond is also FCC but is insulator.
This bonding is present Group IV elements especially in semiconductors i.e. Si and Ge.
This bond is formed by sharing two electrons, one by each atom (neighboring) in crystal lattice. In atomic structure of Silicon there are 14 electrons according to Bohr’s Model of atom. In outermost shell there are four electrons, this is also the case with Germanium So these elements are tetravalent.
Every atoms of Silicon Share all its 4 electrons with neighboring atoms of lattice.
Semiconductor As Insulator:
At room temperature,In covalent bonding, the valence shell of each atom, in solid silicon appears to be filled and consequently there are no holes and no free electrons available for conduction, due to low ionization potential. That’s why, semiconductors are insulator at room temperature.
Difference Between Semiconductors and Conductors:
- For Conductors, with the increase in temperature increases the resistance, because numbers of carriers in a conductor don’t increase significantly with temperature, whereas in Semiconductors, conductivity increases with the increase in temperature and resistance decreases
- With the increase in temperature valence electrons absorb sufficient energy to break covalent bond and contribute to number of free electrons.
Due to this semiconductors have negative temperature coefficient.
Semiconductor in pure form is intrinsic and has low level of doping. In this form it is insulator.
To increase the electrons or holes a little amount of impurity is added so that to increase the conductivity. A one part of atom is added with 10^6 of semiconductor. So, adding impurity in pure form of semiconductor atom is called doping.
Majority and Minority Carriers:
In diodes, in N-type Material, that formed by tetravalent atoms thus outermost five electrons form them one electron from each atom is set free and has high ionization potential, electrons are in majority and holes are in minority.
In P-Type Materials, trivalent atoms, 3 electrons of these atoms form covalent bond with 3 electrons of semiconductor leaving behind a hole to be filled by an electron, so we say that holes are in majority in P-Type and electrons are minority carriers.
So, electrons move in P-Type material leaving behind Positive ions in N-Type and as holes are taking these electrons so a layer of negative ino is accomplished along side the potential barrier.
Department Of Electronics,FET
International Islamic University,Islamabad.