Elemental Semiconductor Materials:
Group IV includes five elements viz., Carbon (C), Silicon (Si), Germanium (Ge), Tin (Sn) and lead (Pb). The elements of this subgroup show marked similarity as well as gradation in properties with rise of atomic number and on these grounds, their inclusion in the same subgroup is perfectly justified. Following are the points which justify the inclusion of these Elemental Semiconductor Materials in the same subgroup.
1. Electronic Configuration:
The electronic configuration of these elements is indicated in Table 6.1.
The given configurations show that these elements have four electrons in their valence shell, two of which are in s-orbital while the remaining two are in p-orbital. Hence these Elemental Semiconductor Materials possess s2p2 configuration in the valence shell. The penultimate shell of C contains s2 electrons, of Si contains s2p6 electrons, of Ge contains s2p6d10 (saturated) electrons while Sn and Pb contain s2p6d10 (unsaturated). This shows why C differs from Si and both from the other members of this subgroup—carbon in crystalline form (diamond) is an insulator, silicon and germanium solids are semiconductors, and tin is a metal.
2. Similarities in Physical Properties:
- There is a gradual change from non-metallic to metallic nature with the rise in atomic number. Thus carbon and silicon are typical non-metals, germanium is a metalloid while tin and lead are distinctly metallic in nature.
- Their ionisation potential is high and decreases gradually from carbon to lead.
- Carbon is the most electronegative element of this subgroup and the electronegativities decrease with the rise of atomic number but not in a regular manner.
- All are tetravalent elements.
- The size of atoms increases gradually from carbon to lead.
- Gram atomic volume, (i.e., the volume occupied in the solid state by one gram-atom of the element) increases gradually from carbon (= 5) to lead (= 18).
- The tendency of an element to form long chains of identical atoms is called catenation and decreases with the decrease in the value of bond energy. The values of bond energies, as given below, indicate that carbon has the maximum tendency to form chains, silicon has lesser tendency, germanium has much lesser tendency while tin and lead have small tendency. [C-C = 85 kcal/mole; Si-Si = 54; Ge-Ge = 40; Sn-Sn = 37 kcal/mole]
3. Similarities in Chemical Properties:
- All the elements of this group form covalent hydrides. The number of stable hydrides and the ease with which these elements form the hydrides, decreases from carbon to lead.
- All the elements give dioxides of MO2 type. The acidity of these oxides decreases as we move down the group. Thus CO2 and SiO2 are acidic and GeO2, SnO2, and PbO2 are amphoteric. All these oxides dissolve in alkalies to give respectively carbonates, silicates, germanates, stannates and plumbates.
- All the elements give acids of H2MO3 type, e.g. H2CO3 (carbonic acid), H2SiO3 (silic acid), H2SnO3 (meta stannic acid), H2PbO3 (meta plumbic acid). Sodium salts of these acids are stable.
- All, except lead, form oxy-chlorides, e.g.
- Except carbon, all are attacked by caustic alkalies with the evolution of hydrogen (H2).
Elemental carbon occurs in widely varied forms from the transparent diamond with its extreme hardness to graphite, which is grey and soft. Electrical carbon materials are manufactured from graphite and other forms of carbon (coal etc.). Graphite occurs in nature as a mineral with high content of carbon (up to 90% or more).
Silicon does not occur free in nature. It is the most abundant element after oxygen in the earth’s crust. It occurs in combination with oxygen as silica (SiO2) and silicates. The important minerals containing silicon are Feldspar or orthoclase [K2O.A12O3.6SiO2]; Mica [KH2A13(SiO4)3]; Kaolinite [A12O3,2SiO2, 2H2O]; Asbestos [CaMg3(SiO3)4].
Free silicon exists in the allotropic form viz., (i) Amorphous form and (ii) Crystalline form. These two forms are prepared by different methods.
Germanium is an earth element and 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 reduced to pure germanium.
Tin does not occur free in nature. Its important ores are (i) Cassiterite (also called tinstone), SnO2 (ii) Wolframite.
The chief ore of lead is galena, Pbs, which is associated with Zinc blend, iron pyrites and traces of silver. Lead is mainly extracted from galena ore.
Uses of Elemental Semiconductor Materials:
- Carbon is crystalline in structure and has a very high melting point (about 3,900°C). Pure carbon is a semiconductor with negative temperature coefficient of resistance. Carbon has conductivity slightly less than that of metals and their alloys. In electrical engineering, carbon elements are extensively used as (i) brushes for electrical machines, (ii) carbon electrodes for electric arc furnaces, electrolytic baths and arc welding (iii) non-wire resistors (iv) arc light (v) battery cell elements (vi) microphone powders and other components of telecommunication equipment. etc.
- Silicon is used in the manufacture of alloy ferra-silicon and carborundum (SiC). It is used as a deoxidiser in the manufacture of steel. High purity silicon is used for making semiconductor devices.
- Germanium is used mainly for making semiconductor devices.
- Tin is used in the preparation of a number of alloys, such as solder, white metal, bell metal, babbit metal, etc; which are very useful. It is used in tinning of household utensils and in tin plating of iron sheets. Tin amalgam is used for making mirrors and tin compounds are used as mordants in dyeing—calico printing.
- Lead is used for making cable coverings, protective sheets for roofs and drains, water pipes and for lining the chambers in sulphuric acid manufacture. It is employed in making lead-acid accumulator plates, lead shots, fuse wire and compounds like red lead, litharge, lead tetraethyl and white lead.