Q.On which factors does the wavelength of light emitted by a LED depend?

[!TLDR] The emitted wavelength depends on the semiconductor material's band gap $E_g$ (via $\lambda=hc/E_g$), which is set by the compound semiconductor chosen and the proportions of its constituent elements. [!ANSWER] The LED's emitted wavelength depends on the band gap of the semiconduct …

When an LED's forward current causes electron-hole recombination at the junction, each recombination event releases energy approximately equal to the semiconductor material's BAND GAP $E_g$, in the form of an emitted photon. Since a photon's energy and its wavelength are related by $E=\dfrac{hc}{\lambda}$, the wavelength (and hence perceived colour) of the light an LED emits is fixed almost entirely by the band gap of the material used to fabricate it -- a larger $E_g$ gives shorter-wavelength (bluer/higher-energy) light, a smaller $E_g$ gives longer-wavelength (redder/lower-energy or infrared) light. The band gap itself is controlled by which COMPOUND SEMICONDUCTOR material is chosen, and, within a given compound, by the exact PROPORTIONS of its constituent elements: for instance, aluminium gallium arsenide (AlGaAs) emits infrared radiation; gallium arsenic phosphide (GaAsP) emits red or yellow light; aluminium gallium phosphide (AlGaP) emits red or green light; and zinc selenide (ZnSe) emits blue light -- by varying the re …

Connect the photon-energy relation $E=hc/\lambda$ to the recombination energy (≈ band gap $E_g$), then note that $E_g$ itself is set by the s …

Naming only the forward current/voltage as the controlling factor (it controls the INTENSITY/brightness of the emitted light, via how many recombination events occur per second) rather than the WAVELEN …