Over the past 15 years, much progress has been made in the development of active optical devices in silicon on insulator. It is now, widely accepted that the free carrier effect is one of the most efficient modulation and switching mechanism in silicon, and also facilitates the potential advantages of combining optical and electronic devices onto a single substrate rather than using discrete components. Moreover, the providence of using existing silicon CMOS microelectronics fabrication technology and silicon's wide abundance has proved to be attractive. The aim of this thesis is to report the results of an investigation into the design and fabrication of high speed silicon on insulator phase modulators using a pn junction inserted in a rib waveguide. An in depth analysis of the performance of several modulators operating using this principle is given. We propose a micrometer sized polarisation independent rib waveguide modulator based on this principle. The device is predicted to require a pi phase shift of voltage of 10 volts for a length of 2.5mm, with an operating bandwidth of over 15 GHz. Another key result is the development of an improved modulator based on the same principle of operation but inserted in a submicrometer size rib waveguide. This specific device is also predicted to require a pi phase shift of voltage of 10 volts for a length of 2.5mm, but shows a much improved operating bandwidth of over 40 GHz. In order to simplify the fabrication of a depletion type modulator, a third modulator design is proposed and fabricated. The junction is positioned laterally in order to avoid the need for a top contact and is formed using ion implantation. The device is based on a nano-wire rib waveguide and is inserted into a ring resonator. The reported bandwidth is 19 GHz in a ring with a diameter of 40 microns.