This thesis reports on the development of high current density InGaAs/AlAs/InP resonant tunnelling diodes (RTDs) grown by metal-organic vapour phase epitaxy (MOVPE) for terahertz (THz) applications. A wide variety of characterisation techniques are employed to investigate the material properties and quality on the wafer level allowing future device and growth optimisation. An optical characterisation technique based on photoluminescence (PL) spectroscopy is developed to non-destructively map the doping and structural composition of the RTD on the wafer level. A new non-destructive optical characterisation technique to resolve the absolute energy level position of the first electron state of RTDs through low temperature PL spectroscopy is also reported. The absolute energy levels are resolved by a combination of type-I and type-II quantum well (QW) recombination, allowing an investigation into important scattering mechanisms affecting device performance, and monitor the QW alloy content and thickness non-destructively. Details of the growth process and the characterisation techniques are discussed. A new fabrication technique based on conventional i-line photolithography for micron scale high current density RTD devices is also developed with accurate control over the final device area (and hence characteristics). This is achieved by measuring the V-I characteristic of the RTD during the fabrication process, which has not been previously possible. This was made possible by guiding the emitter current through the full RTD structure by a large second contact electrode on the collector side and using an air-bridge contact to the collector. Important information about the RTD performance is extracted by using this method. Temperature dependent V-I characterisation is also carried out to investigate the valley current of the RTD. Details of the design, fabrication, and characterisation of a room temperature operating THz emitter in the 300 GHz band are reported.