The phase modulation due to the linear electro‐optic effect of GaAs&sngbnd;AlxGa1−xAs double‐heterostructure (DH)p‐njunction diode light modulators can be a strong function of the applied bias voltage. The observed nonlinear voltage dependence of the phase difference &Dgr;&phgr; between the phases of the TE and TM modes is in good agreement with the predictions of a mathematical dielectric slab waveguide model with five stepwise constant dielectric layers, if a suitably defined average junction electric field is used. The magnitude of &Dgr;&phgr; and its nonlinear voltage dependence measurably depend on the properties of the optical dielectric waveguide and on thep‐njunction doping profile. Within the accuracy of our measurements, we have not been able to identify contributions due to higher‐order electro‐optic effects. Our results emphasize that large modulation effects and good modulation linearity of &Dgr;&phgr; modulation can be achieved by appropriately controlledp‐njunction doping profiles. Conversely, &Dgr;&phgr; measurements as a function of bias provide a novel tool for studyingp‐njunction doping profiles and for determining the Al concentration of the AlxGa1−xAs layers of DH diodes.