Conductors, Insulators & Semiconductors
The spectrum of electrical conductivity and where semiconductors fit
The Conductivity Spectrum
The Conductivity Spectrum
All materials fall somewhere on a spectrum of electrical conductivity:
| Category | Resistivity (Ω·cm) | Examples |
|---|---|---|
| Conductors | 10⁻⁶ to 10⁻⁴ | Copper, aluminum, gold |
| Semiconductors | 10⁻³ to 10⁶ | Silicon, germanium, GaAs |
| Insulators | 10⁸ to 10¹⁸ | Glass, rubber, SiO₂ |
The magic of semiconductors is that their conductivity can be precisely controlled — by adding impurities (doping), applying voltage, or changing temperature. This controllability is what makes transistors possible.
Analogy: The Water Valve
Think of conductors as open pipes (water flows freely), insulators as sealed pipes (no flow), and semiconductors as pipes with adjustable valves. You can control exactly how much current flows — and that's the basis of all electronics.
What Makes Semiconductors Special
What Makes Semiconductors Special
Three properties make semiconductors uniquely useful for electronics:
- Tunable conductivity: By adding tiny amounts of impurities (dopants), we can change silicon's conductivity by a factor of a million or more.
- Temperature sensitivity: Unlike metals (which conduct worse when hot), semiconductors conduct better at higher temperatures as more electrons gain enough energy to move.
- Light sensitivity: Semiconductors can absorb and emit photons, enabling LEDs, solar cells, and image sensors.
Key Concept: The Bandgap
The key difference between conductors, semiconductors, and insulators is the bandgap — the energy barrier electrons must overcome to conduct. Conductors have no bandgap, insulators have a huge one (>5 eV), and semiconductors have a small, useful one (silicon = 1.1 eV).
Knowledge Check
Knowledge Check
1 / 2What distinguishes a semiconductor from a conductor?