Energy Bands and Band Gap in Solids
Energy bands are continuous ranges of allowed electron energies in a solid. The valence band contains electrons involved in bonding, the conduction band contains electrons that can move through the solid, and the band gap is the forbidden energy separation between them.
In one line: Small gap means semi-conduction: not free like a conductor, not blocked like an insulator.
Concept Practice Coming SoonUnderstand it
- In an isolated atom, electrons have separate energy levels.
- In a solid, a very large number of atoms come close together, so these levels split into closely spaced groups called bands.
- Electrical conduction depends on whether electrons can reach the conduction band.
- Conductors have overlapping or partly filled bands, semiconductors have a small band gap, and insulators have a large band gap.
- At room temperature, some semiconductor electrons can gain enough energy to cross the small gap, leaving holes behind in the valence band.
Example
Formula and rule
Key rule: conductivity increases when more electrons are available in the conduction band or more holes are available in the valence band.
Qualitative comparison: conductor has overlapping or partly filled bands, semiconductor has a small forbidden gap, and insulator has a large forbidden gap.
The band gap energy is commonly denoted Eg and measured in electron volt, eV.
See it worked out
Follow the example line by line, then use the same method on new questions.
Worked example
Classify three materials from band descriptions: Material A has overlapping valence and conduction bands, Material B has Eg about 1 eV, and Material C has a large Eg of several eV.
- 1A is a conductor because electrons can move easily into available states.
- 2B is a semiconductor because a small gap allows thermally excited carriers.
- 3C is an insulator because the large gap prevents ordinary thermal excitation.
Method you can reuse
- 1Identify the valence band and conduction band.
- 2Check whether they overlap, have a small gap, or have a large gap.
- 3Link this to the availability of mobile electrons.
- 4State the classification and give one reason based on Eg.
Check yourself
Answer the question in your head before opening the answer.
Try it yourself
Why does a semiconductor conduct better than an insulator but worse than a conductor?
Think first, then show answerHide answer
Common wrong answer
Calling every material with a forbidden gap an insulator.
Why it loses marks: Semiconductors also have a forbidden gap, but it is small enough for carrier generation at ordinary temperatures.
Don't mix this up
Exam tip
Write it in the exam
Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.
How CBSE asks it
- 1
Enough for 1 mark
Energy bands are continuous ranges of allowed electron energies in a solid. The valence band contains electrons involved in bonding, the conduction band contains electrons that can move through the solid, and the band gap is the forbidden energy separation between them.
- 2
Add for 2 marks
Example: Silicon and germanium are semiconductors because their band gaps are small enough for some electrons to reach the conduction band at ordinary temperatures. Copper behaves as a conductor because electrons are readily available for conduction, while glass behaves as an insulator because its band gap is large.
- 3
Add for 3 marks
In an isolated atom, electrons have separate energy levels.
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