We have theoretically studied the phase modulation in InGaAsP/InP and GaAs/AlGaAs double heterostructure waveguides in the P-i-N and P-p-n-N configurations at wavelengths of 1.3 and 1.55 μms. The carrier induced effects (plasma, band filling, and many body) and the field-induced effects (linear electrooptic and electro-refractive) are considered to calculate the change in the refractive index. Both structures made of InGaAsP/InP show higher modulation efficiency than the corresponding structures made of GaAs/AlGaAs in almost all the cases considered due to a larger index change in InGaAsP as its band-gap wavelength is closer to both 1.3 and 1.55 μms. © 1992 IEEE