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The research work in this thesis is concerned with improving the output RF power of Uni-Travelling Carrier Photodiodes (UTC-PDs) by increasing their bias voltage capability and improving their thermal performance, targeting the frequency range of 200 - 300 GHz for ultra-high-speed wireless communications. Antenna integrated UTC-PDs with higher output power will enable sub-THz signals to be transmitted over longer distance and with higher signal to noise ratio (SNR). At the beginning of this thesis, types of continuous wave THz sources are reviewed in the literature to compare their suitability for wireless communications. Photonic techniques, benefiting from signal transmission over optical fibre cables, allow THz modulated optical signals to be distributed over long distances, superior to electronic techniques in this aspect. With respect to frequency bandwidth, tuneability and room temperature operation, UTC-PDs have exhibited advantages over competing photonic devices, and were chosen as the signal source for sub-THz wireless communications to investigate in this thesis. Following this, the first waveguide coupled phosphide-based UTC-PDs grown by Solid Source Molecular Beam Epitaxy are reported. Coplanar Waveguide (CPW) integrated UTC-PDs and antenna integrated UTC-PDs were fabricated and characterised. Techniques were developed to reduce the leakage current of UTC-PDs from several µA to less than 100 nA (biased at -1V) for different sizes of devices up to 7×15 μm², and bias voltage capability was improved to enable UTC-PDs work up to -5V voltage. Limitations due to thermal effects in UTC-PDs are studied in detail. Experiments with pulsed heterodyne input signals were used to separate the thermal effects on the output power of CPW devices. Pulse measurement results indicate that UTC-PD output saturation power can be increased by more than 3 dB by reducing thermal effects. Thermal modelling of UTC-PDs is presented to show the temperature distribution inside devices. The modelling shows the restrictions preventing heat dissipation mainly come from two factors: the small cross-sectional area of the P-type contact and the low thermal conductivity of semiconductor materials. Three solutions are proposed to reduce thermal resistance and improve heat dissipation inside UTC-PDs. In addition, verification experiments were conducted to measure UTC-PD temperature and compare with the simulated results. Good agreement was achieved, demonstrating the validity of the thermal modelling presented in this thesis.