Abstract: Example embodiments relate to monolithic III-V/Si waveguide phase modulators. One embodiment includes a monolithic integrated phase modulator that includes a waveguide for propagating light. The waveguide for propagating light includes a waveguide base made of a first conductivity type Si-based semiconductor material. The waveguide for propagating light also includes at least one groove formed in a surface of the waveguide base. Further, the waveguide for propagating light includes an epitaxial region formed on the waveguide base in the at least one groove. The epitaxial region is made of a second conductivity type III-V semiconductor material. The waveguide base and the epitaxial region form a monolithically integrated junction diode that is a phase modulation region for light propagated through the waveguide.