All solids can be broadly grouped into one of three categories:
- Metals and metallic conductors - this includes all metals and carbon. ρ (resistivity) is low and increases as temperature increases.
- Insulators - most of them are non-metals, plastics and much are organic materials if dry.
- Semi-conductors - this includes Silicon, germanium, Gallium arsenide etc...With these, ρ (resistivity) decreases as temperature rises.
- The electrons strictly are not moving but jumping from hole to hole.
- As an electron moves from the Valence band to the Conduction band, it will leave a hole.
- As an electron moves to the Conduction band or Valence band, it will land on a hole and then jump from hole to hole.