- There are many semiconductors available, but very few of them have a practical application in electronics. The two most frequently used materials are germanium (Ge) and silicon (Si). It is because the energy required to break their covalent bonds (i.e. energy required to release an electron from their valence bands) is very small; being about 0.7 eV for germanium and about 1.1 eV for silicon.
(i) Germanium: Germanium has become the model substance among the semiconductors; the main reason being that it can be purified relatively well and crystallised easily. Germanium is an earth element and was discovered in 1886. It is recovered from the ash of certain coals or from the flue dust of zinc smelters. Generally, recovered germanium is in the form of germanium dioxide powder which is then reduced to pure germanium.
- The atomic number of germanium is 32. Therefore, it has 32 protons and 32 electrons. Two electrons are in the first orbit, eight electrons in the second, eighteen electrons in the third and four electrons in the outer or valence orbit [See Fig. 2.2 (i)]. It is clear that germanium atom has four valence electrons i.e., it is a tetravalent element. Fig. 2.2 (ii) shows how the various germanium atoms are held through covalent bonds. As the atoms are arranged in an orderly pattern, therefore, germanium has crystalline structure.
(ii) Silicon: Silicon is an element in most of the common rocks. Actually, sand is silicon dioxide. The silicon compounds are chemically reduced to silicon which is 100% pure for use as a semiconductor.
- The atomic number of silicon is 14. Therefore, it has 14 protons and 14 electrons. Two electrons are in the first orbit, eight electrons in the second orbit and four electrons in the third orbit [See Fig. 2.3 (i)]. It is clear that silicon atom has four valence electrons i.e. it is a tetravalent element.
- Fig. 2.3 (ii) shows how various silicon atoms are held through covalent bonds. Like germanium, silicon atoms are also arranged in an orderly manner. Therefore, silicon has crystalline structure.
Wilfykil answered the question on
August 13, 2019 at 12:18