Silicon and Germanium

Silicon and Germanium are semiconducting materials. They are used in many semiconductor devices. They are indirect bandgap semiconductors. By doping with pentavalent or trivalent impurities, they can be converted to p-type and n-type. Commonly used pentavalent impurities are Arsenic, Antimony, Bismuth and phosphorous. Commonly used trivalent impurities are Aluminium, Boron, Indium and Gallium. Excessive doping results in degenerate semiconductors where Fermi level lies inside the conduction band (in n-type) and inside the valence band (in p-type). Silicon and Germanium are covalent crystals. Their crystal structure is similar to that of a diamond and grey tin. Silicon, Germanium, Diamond and Grey Tin have four electrons in the outermost orbit. These electrons are shared by two atoms in a covalent bonding and plays major role in chemical compound formation. Inner orbit electrons which are bound to nucleus strongly, have negligible role in various properties of these materials. Diamond has least lattice constant value of 3.56 Å.  For Silicon, it is 5.42 Å and Germanium 5.62 Å. Lattice constant of Tin is 6.46 Å.  Bandgap energy of diamond is around 6 eV and it acts as a perfect insulator. Bandgap energy values of Silicon, Germanium and Tin are 1.1 eV, 0.72 eV and 0.08 eV respectively.