Compact semi-physical modelling for HBT circuit simulation

With the maturing of III-V heterojunction bipolar transistor's (HBT's) design and processing teclinology, development of a truly physical and predictive III-V HBT compact model for circuit simulation that covers geometry, bias, temperature, DC and RF characteristics becomes a major challenge. Many of the existing HBT models are based on solid physical backgroimd, but again because of the complexity of the device physics they end up with many empirical coefficients that are difficult to extract. In this thesis, aspects of InGaP/GaAs double HBTs (DHBTs) pertaining to large signal models (LSMs) are addressed. A new LSM generation methodology for III-V HBTs that is physics and process oriented, which reduces parameter extraction efforts moderately is presented. As an initial step in the LSM development methodology, an algorithm based on T-topology HBT equivalent circuit (EC) is developed using IC-CAP software to effectively generate multi-bias small-signal parameter sets. Here analytical approximations and numerical optimisation procedui-es are utilised to accurately fit measured s-parameter data, extracting the maximimi amount of information, parameter values and constraints. Based on this technique, continuously differentiable constitutive relationships are established that takes into accoimt dynamic and static temperature effects as well as high ciuTent effects such as Kirk effect and self-heating, which are important for the operation of InGaP/GaAs DHBTs. It combines the basis of the Ebers-Moll model with elements of the vertical bipolar inter-company model (VBIC) and the high current model (HICUM) as well as including specific extensions for InGaP/GaAs DHBTs. In order to assess the validity and accuracy of the LSM development methodology, an eight port symbolically defined device (SDD) model is constructed in Agilent's ADS circuit simulator. In addition, a novel teclmique for extracting directly the temperature dependence of terminal resistances using the observed kink effect is presented. Using a theoretical model for tunnelling tlirough metal-semiconductor barriers, incoiporating Fermi-Dirac statistics and Wentzel-Kramers-Brillouin (WKB) approximations, it was demonstrated that InGaAs capping layers provided stable, temperature insensitive contacts to the HBT. The LSM development methodology presented in this thesis gives a complete set of model equations that is applicable to III-V HBTs together with parameter extraction teclmiques suitable for the implementation in commercial simulators and characterisation programmes. These provide intuitive imderstanding in the development of modern III-V HBT models for circuit simulations.