Thermal and small-signal characterisation of AlGaAs/InGaAs pHEMTs in 3D multilayer CPW MMIC

Rapid advancement in wireless communications over the years has been the driving force for many novel technologies providing compact and low cost solutions. Recent development of multilayer coplanar waveguide (CPW) MMIC technology promises realization of 3D MMIC in which large area-occupying passive components are translated from horizontal into vertical configuration resulting compact structure. The other main advantages of this technology are elimination of via-holes and wafer-thinning giving alternative performance solution, if not better, from the traditional MMIC. In this thesis, thermal and small-signal characteristics of prefabricated AlGaAs/InGaAs pseudomorphic high electron mobility transistors (pHEMTs) on semi-insulating (S.I.) GaAs substrate incorporated in the 3D MMIC technology have been analysed and modelled for the first time. A comprehensive small-signal parameter extraction procedure has been successfully developed which automatically determines the device small-signal parameters directly from the measured S-parameters. The developed procedure is unique since it provides a great deal of data on measured devices over a wide bias, temperature and frequency range for future incorporation of different active devices for the 3D MMIC technology and provides a first hand knowledge of how the multilayer structure will affect the performance of pre-fabricated pHEMTs. The extracted small-signal models of both pre- and post- multilayer processed pHEMTs have been compared and validated to the RF S-parameters measurements. The main focus was drawn upon the temperature dependent model parameters and how the underlying physics of the transistors behave in response to the change of temperature. These novel insights are especially valuable for devices designed specifically for high power applications like power amplifiers where tremendous heat could be generated. The data can also be interpreted as a way to optimise the multilayer structure, for example, alternative material with different properties can be implemented. The governing physics affecting device performance are also modelled and discussed empirically in details through extracted device parameters. These investigations would assist in the development of reliable, efficient and low cost production of future compact 3D multilayer CPW MMICs.