Conventional pseudomorphic High Electron Mobility Transistor (pHEMTs) with lattice-matched InGaAs/InAlAs/InP structures exhibit high mobility and saturation velocity, and are hence attractive for the fabrication of three-terminal low noise and high frequency devices, which operate at room temperature. The major drawbacks of conventional pHEMT devices are the very low breakdown voltage (< 2 V) and the very high gate leakage current (~ 1 mA/mm), which degrade device, performance especially in MMIC LNAs. These drawbacks are caused by the impact ionization in the low band gap, i.e. the InxGa(1-x)As (x = 0.53 or 0.7) channel material plus the contribution of other parts of the epitaxial structure. The capability to achieve higher frequency operation is also hindered in conventional InGaAs/InAlAs/InP pHEMTs, due to the standard 1 μm flat gate length technology used. A key challenge in solving these issues is the optimization of the InGaAs/InAlAs epilayer structure through bandgap engineering, without affecting the device RF characteristics. A related challenge is the fabrication of sub-micron gate length devices using I-line optical lithography, which is more cost-effective, compared to the use of e-Beam lithography. The main goal for this research involves a radical departure from the conventional InGaAs/InAlAs/InP pHEMT structures by designing new and advanced epilayer structures, which significantly improves the performance of conventional low-noise pHEMT devices, and at the same time preserves the RF characteristics. To achieve this, modified epilayer structures were fabricated and characterized, including solving the standard 1 μm gate length processing issue. DC and RF results are then carefully analysed, and compared with those of the conventional pHEMT. Optimization of the submicron T-gate length process is then performed, by introducing a new technique to further scale-down the bottom gate opening. A new material, SoG, is also explored to simplify the submicron process flow even further. The results of this work show outstanding performance compared to the conventional pHEMT. The breakdown voltage and gate current leakage are significantly improved, by ≥ 70 % and ≥ 90 % respectively, with no detrimental effect on the RF characteristics, while the new technique of the submicron process shows a 58 % increase in fT, and 33 % increase in fmax. The SoG material shows suitability for use in a soft-reflow process, but due to some constraints, its development is left as future work; however, at present it could be used for passivation and production of capacitor dielectrics. The success of the modification and optimisation of the InGaAs/InAlAs material system, coupled with the gate length reduction into sub-μm regime enable high breakdown and ultra-high speed low noise devices to be fabricated, especially for low-noise amplifiers (LNAs) and low-noise receivers operating in the microwave and millimetre wave regime.