The fabrication and characterisation of AlxGa1-xN (0 less than or equal to x less than or equal to 0.35) photodetectors grown on Si(1 1 1) by molecular beam epitaxy are described. For low Al contents (< 10%), photoconductors show high responsivities ( similar to 10A/W), a non-linear dependence on optical power and persistent photoconductivity (PPC). For higher Al contents the PPC decreases and the photocurrent becomes linear with optical power. Schottky photodiodes present zero-bias responsivities from 12 to 5 mA/W (x=0-0.35), a UV/visible contrast higher than 10(3), and a time response of similar to 20 ns, in the same order of magnitude as for devices on sapphire substrate. GaN-based p-n ultraviolet photodiodes on Si(1 1 1) are reported for the first time.