Band Theory
Every solid has allowed energy bands (formed from atomic orbitals) separated by band gaps.
| Type | Band gap | Conductivity | Examples |
|---|---|---|---|
| Conductor (metal) | Zero (bands overlap) | High (10^{6}–10^{8} S/m) | Cu, Ag, Al |
| Semiconductor | Small (~1 eV) | Intermediate; increases with T | Si, Ge |
| Insulator | Large (>3 eV) | Very low | Diamond, |
Intrinsic Semiconductors
Pure Si or Ge — limited conductivity from thermally excited electrons crossing the small band gap. At higher temperatures, more electron-hole pairs form → conductivity increases.
Extrinsic (Doped) Semiconductors
n-type (Group 15 dopant):
- Dopants: P, As, Sb (5 valence electrons)
- 4 electrons form bonds; 5th electron is free (excess electron → majority carrier)
- Majority carriers: electrons
- Examples: Si doped with P or As
p-type (Group 13 dopant):
- Dopants: B, Ga, In (3 valence electrons)
- Only 3 bonds form; 1 bond site is empty (hole) → majority carrier
- Majority carriers: holes (behave as positive charges)
- Examples: Si doped with B or Ga
Temperature Effect
- Metals: Conductivity decreases with T (more lattice vibrations → more scattering)
- Semiconductors: Conductivity increases with T (more thermally excited charge carriers)
Semiconductor Applications
- n-p junction → diode, transistor, LED, solar cell
- F-centres in ionic crystals → n-type semiconductor behaviour (metal excess defect)
- Metal deficiency compounds (FeO) → p-type behaviour