Sotoodeh, Mohammad; (2000) Design, characterisation, and numerical simulation of double heterojunction bipolar transistors for microwave power applications. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

GaAs-based heterojunction bipolar transistors (HBTs) are very attractive candidates for digital, analogue, and power applications due to their excellent switching speed combined with high current driving capability. Recently, there has been a widespread interest in replacing the conventionally used wide bandgap material AlxGa1-xAs with the more physically and technologically advantageous In0.49Ga0.51P. Additionally, employing a wide bandgap material as the collector of double HBTs (DHBTs) gives the designer another degree of freedom in tailoring the device behaviour for high power and/or high temperature applications. In this thesis, InGaP/GaAs DHBTs are studied in details for microwave power applications. A comprehensive numerical simulation code is developed using FORTRAN 90 to investigate some important physical phenomena in HBTs. Particular attention is paid to tunnelling of electrons through conduction band potential barriers, and design issues of base-collector heterojunction in DHBTs. Material properties of a wide range of III-V compounds are studied extensively and very useful empirical relations are presented to model parameters like energy bandgaps, carrier mobilities, and minority carrier lifetimes. DC and small-signal characterisation methods of fabricated HBTs are also considered in details. A novel technique is proposed to measure the base and collector series resistances of HBTs using only DC measured data. Also a new small-signal parameter extraction procedure is introduced in which all the equivalent circuit elements are extracted analytically without reference to numerical optimisation. This method is shown to have a wide range of applicability, which makes it appropriate for GaAs- and InP-based single and double HBTs. Moreover, a novel approach is presented to accurately determine the HBT total delay time and its components. Both the vertical layer structure and horizontal layout of HBTs are optimised to improve the cutoff frequency-breakdown voltage product in the fabricated devices. These include design of a new mask set, optimisation of a multiple energy ion implantation process for device isolation to minimise device parasitics, proposing a new structure for the base-collector heterojunction of DHBTs, and improving the processing yield of the existing fabrication technology. As a result, DHBTs with simultaneously high cutoff frequency and maximum frequency of oscillation, large breakdown voltage, and low offset voltage are realised and demonstrated in this work.

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