This paper presents the linear modelling of high breakdown InP pseudomorphic High Electron Mobility Transistors (pHEMT) that have been developed and fabricated at the University of Manchester (UoM) for low noise applications mainly for the Square Kilometre Array (SKA) project. The ultra-low leakage properties of a novel InGaAs/InAlAs/InP pHEMTs structure were used to fabricate a series of transistor with total gate width ranging from 0.2 mm to 1.2 mm. The measured DC and S-Parameters data from the fabricated devices were then used for the transistors’ modelling. The transistors demonstrated to operate up to frequencies of 25 GHz. These transistors models are used in the design of Low Noise Amplifiers (LNAs) using fully Monolithic Microwave Integrated Circuit (MMIC) technology.