3.7 R ESISTIVITY OF V ARIOUS M ATERIALS

. R ESISTIVITY OF V ARIOUS M ATERIALS The resistivities of various common materials are listed in Table . . The materials are classified as conductors, semiconductors and insulators FIGURE . The dashed line represents the linear Ohm’s law. The solid line is the voltage V versus current I for a good conductor. FIGURE . Characteristic curve of a diode. Note the different scales for negative and positive values of the voltage and current. FIGURE . Variation of current versus voltage for GaAs. depending on their resistivities, in an increasing order of their values. Metals have low resistivities in the range of – Ω m to – Ω m. At the other end are insulators like ceramic, rubber and plastics having resistivities times greater than metals or more. In between the two are the semiconductors. These, however, have resistivities characteristically decreasing with a rise in temperature. The resistivities of semiconductors are also affected by presence of small amount of impurities. This last feature is exploited in use of semiconductors for electronic devices. T ABLE . R ESISTIVITIES OF SOME MATERIALS Material Resistivity, ρ Temperature coefficient ( Ω m) at °C of resistivity, α (°C) – at C d T ρ ρ   °     Conductors Silver . × – . Copper . × – . Aluminium . × – . Tungsten . × – . Iron × – . Platinum × – . Mercury × – . Nichrome ~ × – . (alloy of Ni, Fe, Cr) Manganin (alloy) × – . × – Semiconductors Carbon (graphite) . × – – . Germanium . – . Silicon – . Insulators Pure Water . × Glass – Hard Rubber – NaCl ~ Fused Quartz ~ Commercially produced resistors for domestic use or in laboratories are of two major types: wire bound resistors and carbon resistors . Wire bound resistors are made by winding the wires of an alloy, viz., manganin, constantan, nichrome or similar ones. The choice of these materials is dictated mostly by the fact that their resistivities are relatively insensitive to temperature. These resistances are typically in the range of a fraction of an ohm to a few hundred ohms.