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The research work presented in this thesis is concerned with the design, fabrication and characterisation of Continuous Wave (CW) Photonic Terahertz (THz) Emitters employing antenna-integrated Uni-Travelling-Carrier Photodiodes (UTC-PDs), based on the Indium Phosphide (InP) materials system. The solution employing photonic techniques for the generation of sub-millimetre and THz waves, via photomixing of lasers operating at 1550 nm, is a major candidate for the realisation of tuneable, power efficient, compact and cost effective THz sources operating at room temperature. The availability of sources endowed with such properties would make many important applications possible in this frequency range, such as ultra broad band wireless communications, spectroscopic sensing and THz imaging. The UTC-PDs enable high optical to electrical (O-E) conversion efficiency and are key components for the realisation of a photonic terahertz emitter. In this thesis the fabrication and characterisation of test vertically illuminated UTC-PDs, achieved using materials grown by Solid Source Molecular Beam Epitaxy at UCL and the fabrication of high performance waveguide UTC-PDs are reported, as milestones towards the development of a simple, repeatable and high yield fabrication process. A comprehensive study of UTC-PD impedance and frequency photo response, carried out using experimental techniques, circuit analysis and 3D full-wave electromagnetic modelling, is presented. The results of this investigation provide valuable new information for the optimisation of the UTC-PD to antenna coupling efficiency. New THz antenna and antenna array designs, obtained by means of full wave modelling, are also presented, and shown to be suitable for integration with both standard silicon lenses and a novel solution employing a ground plane. The accurate antenna design, along with the results of the UTC-PD impedance investigation, enables the prediction of the power radiated by antenna integrated UTC-PDs, not only in terms of trend over the frequency range but also of absolute level of emitted power. 3D full-wave modelling has also been used at optical frequencies, to address the problem of the optical fibre-to-chip coupling efficiency, which is another fundamental factor for the optimisation of a photonic THz emitter. Among other features, this analysis enables a better understanding of how the light is absorbed throughout the device structure and provides key information for future realisation of travelling-wave photodetectors. An additional experimental tool for the analysis of THz emission, namely the sub-wavelength aperture probe, has been modelled and characterised, revealing interesting properties for the characterisation of antenna far-fields and near fields, and hence providing a valuable tool for THz antenna analysis and design.