Planar photodiodes for the 1.0–1.3 μm wavelength range have been fabricated by diffusing Zn into vapour-phase epitaxial InxGa1−xAs grown on a GaAs substrate. For compositions x=0−0.31, the dark current and spectral response are reported. The dark-current density for x=0.17 was as low as 5.4 × 10−6 A/cm2 at VB/2. Planar photodiodes for the 1.0–1.3 μm wavelength range have been fabricated by diffusing Zn into vapour-phase epitaxial InxGa1−xAs grown on a GaAs substrate. For compositions x=0−0.31, the dark current and spectral response are reported. The dark-current density for x=0.17 was as low as 5.4 × 10−6 A/cm2 at VB/2.