Overview
This lecture explains the differences between conductors, insulators, and semiconductors, focusing on band theory and the mechanism of electrical conductivity in materials.
Conductors, Insulators, and Semiconductors
- Conductors allow electric current to flow easily; insulators resist electrical flow; semiconductors have intermediate conductivity.
- Semiconductors are useful because their conductivity can be controlled for technological applications.
Band Theory and Molecular Orbitals
- In large networks of atoms, atomic orbitals combine to form an enormous number of molecular orbitals, creating bands.
- The lower-energy orbitals create the valence band, and higher-energy ones create the conduction band.
- In conductors, the energy gap between valence and conduction bands (band gap) is tiny or nonexistent, allowing free electron movement.
- Insulators have large band gaps that prevent electrons from flowing, blocking current.
- Semiconductors have small band gaps, enabling some electrons to move to the conduction band with added thermal energy.
Semiconductor Behavior
- Semiconductors conduct electricity better at higher temperatures due to more electrons being promoted to the conduction band.
- Common semiconductor materials include elements like silicon and germanium, and compounds like lead sulfide.
- Large band gaps, as in diamond or aluminum nitride, result in insulating behavior.
Doping and Types of Semiconductors
- Doping introduces impurities to increase semiconductor conductivity.
- N-type semiconductors have dopants with extra valence electrons, adding electrons to the conduction band.
- P-type semiconductors have dopants with fewer valence electrons, leaving "holes" in the valence band for current to flow.
- Diodes and transistors use both n-type and p-type semiconductors.
Key Terms & Definitions
- Conductor — Material allowing easy flow of electric current.
- Insulator — Material resisting the flow of electric current.
- Semiconductor — Material with electrical conductivity between a conductor and insulator.
- Band gap — Energy difference between the valence band and conduction band.
- Valence band — Band of lower-energy, filled molecular orbitals.
- Conduction band — Band of higher-energy, empty molecular orbitals that electrons move into to conduct electricity.
- Doping — Adding impurities to a semiconductor to modify its conductivity.
- N-type semiconductor — Semiconductor with more electrons (negative charge carriers).
- P-type semiconductor — Semiconductor with fewer electrons, creating positive charge carriers ("holes").
Action Items / Next Steps
- Review molecular orbital theory if unfamiliar.
- Prepare to study electronic components like diodes and transistors in future courses.