Abstract: There is disclosed an optoelectronic device, particularly an optoelectronic modulator (5a) including a resonant tunnelling diode (RTD) (15a) and operating by the electro-optic effect. The optoelectronic modulator device (5a) comprises a waveguide means (10a) including at least one resonant tunnelling diode (RTD) (15a), and wherein a change in absorption coefficient of a semiconductor material of the device with applied electric field is negligible at a wavelength of operation. In this way the device (5a) operates substantially solely by the electro-optic effect providing a change in refractive index of the waveguide (10a). The device (5a) may therefore act as a phase modulator. The device (15a) is conveniently termed a Resonant Tunnelling Diode Electro-optic Modulator (RTD-EOM).