Conductor vs Insulator vs Semiconductor
Three-panel side-by-side band comparison with resistivity labels.
Key Notes
CLASSIFICATION of solids by electrical conductivity / band structure:
Conductors (metals): partially filled or overlapping bands; resistivity ~10⁻⁸ Ω·m; conductivity DECREASES with temperature.
Insulators: large bandgap (E_g > 3 eV); resistivity 10⁸-10¹⁶ Ω·m; essentially no free carriers at room temperature.
Semiconductors: small bandgap (0.5-2 eV); resistivity 10⁻⁴-10⁴ Ω·m; conductivity INCREASES with temperature.
Conductivity range spans ~24 orders of magnitude across these classes — most extreme variation in all of solid-state physics.
Doping makes semiconductors functionally indispensable — controllable conductivity by orders of magnitude.
Examples: Cu, Ag, Au (conductors); diamond, rubber, glass (insulators); Si, Ge, GaAs (semiconductors).
Formulas
Resistivity ranges (rough)
Reference values at room temperature.
Temperature dependence
Opposite behaviour: metals heat up = harder; semiconductors heat up = easier.
Important Points
Conductivity differs by 20+ orders of magnitude across classes — the most extreme range in physics.
Metals: resistance rises with T (phonon scattering). Semiconductors: resistance FALLS with T (more carriers).
Carbon: graphite is a (semi-)metal; diamond is an insulator. Same element, very different bandgaps.
Doping a semiconductor moves it dramatically toward metallic conductivity without changing it crystalographically.
Superconductors: zero resistance below T_c. Distinct from metals — not just better conductors.
Plastic / amorphous semiconductors are now used in flexible electronics, OLEDs, etc.
Conductor vs Insulator vs Semiconductor notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 12 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.