Modeling of the single- and dual-gate microwave field effect transistors for computer aided design

Abstract

The increasing need for advanced communication technologies in the 21 st century is leading to continuous development of new and more complex active devices and systems. Transistors such as BJTs, HBTs, FETs, and HEMTs are fundamental components in today's personal, corporate and global communication systems. The single- and dual-gate MESFETs are widely used devices with wide areas of RF/Microwave applications. Due to their superior performance, MESFETs have been extensively used in high frequency, high-gain, and low-noise amplifiers, oscillator, mixers, and many other applications. In this thesis, conventional and intelligent modeling methodologies are investigated for the applicability to today's computer-aided design methodologies and tools. Novel approaches of this work are applied to the area of small- and large-signal modeling of the GaAs MESFET (the single- and dual-gate.) Optimal models for the single-gate MESFET are obtained using conventional techniques. A CAD tool called, TopFinder, is developed for optimal small and large-signal model generation. The tool has been successfully tested on a commercial single-gate MESFET and HEMT transistors. Neural network modeling technique is applied to the modeling of the dual-gate MESFET. More specifically, neural network large-signal models for the dual-gate MESFET are developed. These models present the RF device performance behavior including temperature dependence. The neural network technique is successfully applied to two dual-gate MESFET devices; a discrete chip and an on-wafer chip. First, the drain current is modeled while presenting both isothermal (using pulsed measurements) and temperature dependent neural network model for the dual-gate MESFET. A comprehensive large-signal neural network model is also developed. This model includes the nonlinearity of conductances as well as those of the junction capacitances. The nonlinear dual-gate MESFET model is successfully incorporated into ADS. The model is verified by comparing the measurements of a nonlinear single-stage amplifier application based on the dual-gate MESFET to harmonic balance simulations in ADS showing satisfactory results.