A band gap is the energy range in a solid where no electron states exist. Electrons must acquire at least this much energy to move from the valence band to the conduction band. Its magnitude determines electrical conductivity and optical absorption properties.

How do you calculate band gap from UV-Vis absorption wavelength?

Determine the absorption edge cutoff wavelength (λ in nm) from the UV-Vis spectrum, then calculate E_g = hc / λ. In practical units, E_g (eV) ≈ 1239.84 / λ (nm). For example, a cutoff wavelength of 800 nm corresponds to E_g ≈ 1.55 eV.

Why does wavelength decrease as band gap increases?

Photon energy and wavelength are inversely related (E = hc/λ). A larger band gap requires a higher-energy photon to bridge it, and higher-energy photons have shorter wavelengths.

What is the boundary between semiconductor and insulator?

The boundary is conventional, not sharp. Most references treat E_g ≤ 4 eV as semiconductor territory (silicon: 1.12 eV, GaN: 3.4 eV). Diamond at 5.5 eV is a classic insulator. This tool uses 4 eV as the threshold.

Can I enter a band gap of zero?

Yes. Zero band gap corresponds to a metal or semimetal (e.g., graphene). The photon energy would also be zero, so wavelength and frequency are undefined — the tool will note this.

What common semiconductors can I test?

Try silicon (1.12 eV), germanium (0.67 eV), GaAs (1.42 eV), GaN (3.4 eV), or ZnO (3.37 eV).

Band Gap Calculator

Calculate semiconductor band gap energy from UV-Vis absorption wavelength or convert between eV and joules, find photon frequency, and classify material type.

Band Gap Input

Band gap energy (eV)

Absorption cutoff wavelength (nm)

Band gap energy (J)

Common semiconductors

Enter a band gap energy or absorption wavelength to see results

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Summary

The band gap (E_g) is the energy difference between the valence band and conduction band in a solid. Its size determines whether a material conducts electricity freely (metal), partially (semiconductor), or not at all (insulator). This calculator converts between band gap energy (eV or joules) and optical absorption cutoff wavelength (nm), using E = hf = hc/λ to calculate band gap from UV-Vis spectroscopy data or determine band-edge photon properties.

How it works

Enter a band gap energy value in eV or joules, or an optical absorption cutoff wavelength in nm (from UV-Vis spectroscopy).
The tool converts between eV and joules using 1 eV = 1.602176634 × 10⁻¹⁹ J.
Band gap energy and photon wavelength relate through E = hc / λ, where h = 6.62607015 × 10⁻³⁴ J·s and c = 2.99792458 × 10⁸ m/s.
Photon frequency is computed as f = c / λ = E / h.
Material class is determined: metals (E_g ≈ 0), semiconductors (0 < E_g ≤ 4 eV), insulators (E_g > 4 eV).

Use cases

Calculate band gap energy (eV) directly from UV-Vis spectroscopy absorption cutoff wavelength (λ_max or absorption edge).
Determine the minimum photon energy needed to excite electrons across a semiconductor band gap.
Find the cut-off wavelength for photodetectors or solar cells made from a given material.
Convert textbook band gap values between eV and SI joules for equations.
Classify an unknown material based on its measured band gap.

Frequently Asked Questions

Last updated: 2026-06-19 · Reviewed by Nham Vu