--- title: "Energy bands: conductor, insulator and semiconductor" subject: "Electrical Engineering Material" chapter: 1 key: "semester_3/electrical_engineering_material:energy-bands-conductor-insulator" source: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/energy-bands-conductor-insulator site: BCT Engineering --- # Energy bands: conductor, insulator and semiconductor Electrical Engineering Material · Chapter 1: Theory of Metals - Asked in: 2073 Shrawan, 6 marks - Video: [Conductors, insulators, and semiconductors | Class 12 (India) | Physics | Khan Academy](https://www.youtube.com/watch?v=y865nLVyQsY) — Khan Academy India - English - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/energy-bands-conductor-insulator - Whole chapter: https://www.bctengineer.com/md/semester_3/electrical_engineering_material/chapter-1 ## Question What are energy bands? Distinguish between a conductor, an insulator and a semiconductor on the basis of energy diagram. Write two characteristic features to [?] (rest of the question is cut off in the scan). ## Short answer Energy bands are quasi-continuous allowed energy ranges separated by gaps. In conducters, bands overlap or are partly filled, giving high conductivity and positive temperature coefficient. Insulators have a large band gap (>5 eV) and extremely high resistivity. Semiconductors have a small gap (~1 eV), conductivity increasing with temperature, and can be doped. ## Answer An **energy band** is a set of very closely spaced allowed energy levels in a solid. When $N$ atoms come together, each atomic level splits into $N$ levels because the outer orbitals overlap (Pauli principle); these form a nearly continuous band. Bands are separated by **forbidden gaps** $E_g$ in which no electron states exist. The highest band containing valence electrons is the **valence band (VB)** and the next higher band is the **conduction band (CB)**. #### Conductor, semiconductor and insulator ```text Conductor Semiconductor Insulator +---------+ +---------+ +---------+ | CB | | CB | | CB | |#########| +---------+ +---------+ +---------+ Eg ~ 1 eV |#########| overlap +---------+ Eg > 5 eV | VB | |#########| +---------+ |# VB #| +---------+ (or partly +---------+ |#########| filled band) |# VB #| +---------+ ### = filled with electrons ``` | Point | Conductor | Semiconductor | Insulator | |---|---|---|---| | Band gap $E_g$ | None (bands overlap or band half-filled) | Small, about 1 eV (Si 1.1, Ge 0.67) | Large, > 5 eV (diamond 5.5) | | Electrons in CB at 300 K | Very many | Few (thermally excited) | Practically none | | Resistivity (Ω m) | $10^{-8}$ to $10^{-6}$ | $10^{-4}$ to $10^{3}$ | $10^{10}$ to $10^{20}$ | | Temperature coefficient | Positive | Negative | Negative (very high R) | | Examples | Cu, Al, Ag | Si, Ge, GaAs | Glass, mica, diamond | - **Conductor:** VB and CB overlap (e.g. Mg) or the highest band is only half filled (e.g. Na, Cu). Free electrons exist even at 0 K, so a small field produces a large current. - **Semiconductor:** at 0 K the VB is full and the CB empty, so it behaves as an insulator. At room temperature some electrons gain enough thermal energy to jump the small gap, leaving holes, so conduction increases with temperature. - **Insulator:** the gap is so large that thermal or ordinary electric field energy cannot lift electrons into the CB. #### Two characteristic features of each (The rest of the question is cut off; the usual demand is two features of each type.) - **Conductors:** (1) high conductivity due to a large number of free electrons; (2) resistance rises with temperature, because lattice vibrations scatter electrons more. - **Semiconductors:** (1) conductivity rises sharply with temperature (negative temperature coefficient); (2) conductivity can be controlled by doping, producing n-type or p-type material, with both electrons and holes as carriers. - **Insulators:** (1) very high resistivity and high dielectric strength; (2) they break down and conduct only under very high fields or temperatures. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Energy bands form because: A. Atoms lose their electrons B. When N atoms come together each atomic level splits into N closely spaced levels C. Only valence electrons exist D. Electrons stop moving Answer: B. Overlap of outer orbitals splits each level into a band. (Key point: Band formation) 2. In a conductor: A. A wide gap separates VB and CB B. The VB is full and CB empty C. The highest band is half filled or bands overlap D. There is no valence band Answer: C. Free electrons exist even at 0 K. (Key point: Conductor) 3. The band gap of an insulator is typically: A. Zero B. About 1 eV C. Less than 0.1 eV D. Greater than 5 eV Answer: D. Diamond is about 5.5 eV. (Key point: Insulator gap) 4. Conductivity of a semiconductor with rising temperature: A. Increases because more electrons cross the gap B. Decreases C. Stays constant D. Becomes zero Answer: A. Thermal excitation creates more electron-hole pairs. 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