Conductor
A conductor is a material that lets electric charge flow easily, because it contains a large population of charge carriers that are free to move. Metals are the standard examples: copper, aluminum, silver and gold carry current with little resistance, which is why they form wires, circuit-board traces and the interconnects inside chips. Conductors sit at one end of a spectrum that runs through semiconductors to insulators, and the contrast with the middle of that spectrum is what makes the transistor possible.
Metals conduct because their outer electrons belong to the whole crystal rather than to single atoms. In the free-electron picture, introduced by Paul Drude around 1900 and refined quantum-mechanically by Arnold Sommerfeld in 1927 to 1928, each atom gives up one or a few valence electrons to a shared sea that moves through a lattice of positive ions. Copper has about 8.5 × 10^28 free electrons in every cubic meter. In the language of band theory the highest occupied band is only partly filled, so there is no gap to cross and even a small electric field shifts the electrons. Silicon at room temperature, by contrast, has only about 10^16 thermally freed carriers per cubic meter, which is why it conducts so much worse until doped.
Resistivity puts numbers on how good a conductor is. Resistance depends on the shape as R = ρ × L / A, where ρ is the resistivity of the material, L the length and A the cross-sectional area. Rough values at room temperature, in ohm-meters: silver about 1.6 × 10^-8, copper about 1.7 × 10^-8, gold about 2.4 × 10^-8, aluminum about 2.7 × 10^-8 and iron about 10^-7. Nichrome, an alloy used in heaters, is near 10^-6. Pure silicon is around 2 × 10^3, and glass and rubber exceed 10^10, so the whole range spans some twenty-five or more powers of ten, with metals at the bottom. The numbers also show why copper wins for wiring, since only silver beats it and silver is expensive, while gold, only slightly worse in resistivity, resists corrosion and so coats connectors and bond wires.
A metal's resistance rises with temperature, the opposite of a semiconductor. In a metal the number of carriers is essentially fixed, but heating makes the ions vibrate more strongly and scatter the moving electrons, so resistivity climbs; for copper it grows by about 0.4 percent per degree Celsius, which is why a hot wire is a poorer conductor and why the filament of an incandescent lamp has a far higher resistance when lit than when cold. In a semiconductor, warming frees more carriers and the resistance falls. Impurities in a metal also raise resistivity, which is why electrical-grade copper is refined to high purity and why alloys such as nichrome are used when resistance is wanted.
Conductors that vanish into the background of a circuit diagram carry real engineering stakes. In an integrated circuit, the wires linking transistors were made of aluminum for decades, with an alloy of a little silicon or copper to prevent spiking and electromigration, until IBM announced a copper interconnect process in 1997, taking advantage of copper's lower resistivity and better resistance to electromigration. As wires shrink to tens of nanometers, electrons scatter off their surfaces and grain boundaries, so thin copper has a considerably higher resistivity than bulk copper, and interconnect delay and heat have become limits on processors and AI accelerators comparable to the transistors themselves. Alternative metals such as cobalt and ruthenium are used for the smallest lines.
Superconductors are the exceptional conductors that drop to zero resistance below a critical temperature. Heike Kamerlingh Onnes found the effect in mercury cooled to about 4 kelvin in 1911. Discoveries of copper-oxide ceramics beginning with Georg Bednorz and Alex Müller in 1986 raised the critical temperature above the boiling point of liquid nitrogen, 77 kelvin, with yttrium barium copper oxide at about 93 kelvin in 1987. Superconductors carry the large currents of magnetic-resonance imaging magnets and particle accelerators, and quantum computers use superconducting circuits, but ordinary good conductors such as copper are not close to being superconductors: they merely have low, finite resistance.
Several misconceptions cluster around conductors. A good conductor is not one in which electrons move fast: the drift velocity in a copper wire carrying a few amperes is typically a fraction of a millimeter per second, while the electromagnetic signal travels along the wire at a large fraction of the speed of light. Conductors are not the same as perfect conductors, since finite resistance turns current into heat. Finally, the best conductor is not always the best choice: aluminum, with roughly 60 percent of copper's conductivity but about a third of its density, dominates overhead power lines.
Read conductor as the baseline against which semiconductors are defined: a material with so many free carriers that its behavior is fixed by the lattice and the temperature, so that all of the interesting control, doping, junctions and gates, had to be found in the materials that conduct only a little.