We report on the fabrication and the characterization of low-loss, single-mode GaAs/AlGaAs single heterostructure ridge waveguides and a linear electro-optic phase modulator on silicon substrate. The waveguides and the phase modulator were grown by molecular beam epitaxy and were characterized at a 1.3 μm wavelength. The average TE mode propagation loss of 1.24 dB/cm, obtained for a 6-μm-wide ridge waveguide, is the lowest loss so far reported. The measured phase shift efficiency of the phase modulator was 3.5°/V mm.