The Bandgap Rainbow

Plate 59 speaks for itself: “Colour is a gap.”

The Physics

An LED emits photons with energy close to its semiconductor's band gap, so the gap sets the colour. GaN emits near-UV, GaP green, GaAs infrared.

λ [nm] = 1239.84 / E_g [eV]

• λ: emission wavelength, nm
• E_g: band gap energy, eV (1 eV = 1.602176634×10⁻¹⁹ J)

An electron crossing the gap emits E_g. GaP: indirect gap, so it emits weakly.

Honest caveat: Emission sits near, not exactly at, the gap; doping and temperature shift it.

Sources

• S. M. Sze, K. K. Ng, Physics of Semiconductor Devices, 3rd ed., Appendix.

On the poster

Plate 059, “The Bandgap Rainbow”: the pixel-art plate with its slogan “Colour is a gap.”, the key equation and the facts panel shown above. Pixelated Physics. Physics. Personified.