Energy Bands in Solids

PHY-118 · Semester I · Physics

intermediate25 minAvailable

Energy bandsConductor · Semiconductor · Insulator

Semiconductor · Eg = 1.10 eV

BandsValenceConductionGap

Material

Semiconductor

Band gap

1.10eV

Temperature

300K

Conduction

Negligible

Lab brief

Objective

See how overlapping atomic levels spread into energy bands and why the gap between bands decides whether a material is a conductor, insulator or semiconductor.

Theory & concepts

In a crystal, many atoms sit close together and their discrete energy levels split into bands — ranges of allowed energy separated by gaps. Electrons fill the lowest bands first. If the highest filled band (valence) overlaps the next (conduction) band, electrons can move freely and the material conducts. A small gap gives a semiconductor; a large gap gives an insulator.

Rendering diagram…

From atomic levels to energy bands

Band gap  →  Eg ≈ 0 eV   conductor (overlap)
          Eg ~ 1 eV   semiconductor (Si 1.1 eV)
          Eg > 5 eV   insulator (diamond 5.5 eV)
Conductivity  σ ∝ e^(−Eg / 2kT)   (intrinsic)

Why silicon?

Silicon's 1.1 eV gap is small enough that a tiny thermal nudge excites a few electrons into the conduction band, yet large enough that devices can stay off until switched — the perfect balance for transistors.

Prerequisites

Topic: Conductors, insulators, semiconductors and effective mass

Instructions

1Pick a material class — conductor, semiconductor or insulator — and watch the band diagram build.
2Compare the gap size and the position of the Fermi level in each case.
3Raise the temperature and watch the number of excited conduction electrons grow for the semiconductor.
4Move the electron markers to the conduction band and see the holes left behind.
5Read the band-gap energy and its electrical consequence from the legend.

Tasks

Classify by gap
For a gap of 0, 1.1 and 5.5 eV, name the material class and why.
Thermal excitation
Explain why raising T raises conductivity in a semiconductor but barely in an insulator.
Electron-hole pairs
Describe what a hole is and why moving an electron creates one.
Fermi level
State where the Fermi level sits for each material class.

Self-check

Band-gap size is correctly mapped to conductor/semiconductor/insulator.
The temperature dependence of semiconductor conductivity is explained.
The electron-hole pair concept is stated correctly.
The role of the Fermi level in band filling is described.

Environment

material presetsband diagramgap readouttemperature controlelectron & hole markers