High Electron Mobility Transistors: Performance Analysis, Research Trend and Applications

In recent years, high electron mobility transistors (HEMTs) have received extensive attention for their superior electron transport ensuring high speed and high power applications. HEMT devices are competing with and replacing traditional field‐effect transistors (FETs) with excellent performance at high frequency, improved power density and satisfactory efficiency. This chapter provides readers with an overview of the performance of some popular and mostly used HEMT devices. The chapter proceeds with different structures of HEMT followed by working principle with graphical illustrations. Device performance is discussed based on existing literature including both analytical and numerical models. Furthermore, some notable latest research works on HEMT devices have been brought into attention followed by prediction of future trends. Comprehensive knowledge of up‐to‐date results, future directions, and their analysis methodology would be helpful in designing novel HEMT devices

RF Coil Design, Imaging Methods and Measurement of Ventilation with 19F C3F8 MRI

This thesis attempts to address the challenge of low signal in fluorinated gas ventilation imaging and optimize imaging methods considering the particular MR parameters of C3F8 by the following approaches: (i) Exploration of coil designs capable of imaging both proton (1H – 63.8 MHz at 1.5T) and fluorine (19F – 60.1 MHz at 1.5T) nuclei involved: 1. The novel use of microelectromechanical systems to switch a single transceive vest coil between the two nuclei was compared to hard-wired or PIN diode switching. 2. The design of an 8 element transceive array with an additional 6 receive only coils for 19F imaging. MEMs was utilized for broadband transmit-receive switching. 3. The amalgamation of a ladder resonator coil with a 6-element transceive array to reduce SAR and improve transmit homogeneity when compared to standard vest coil designs. (ii) Development of imaging methods involved: 1. The optimization and comparison of steady-state free precession and spoiled gradient 19F imaging with C3F8 at 1.5T and 3T. Simulation of the optimal SNR was verified through comprehensive phantom and in-vivo imaging experiments. 2. The investigation of compressed sensing via incoherent sparse k-space sampling to maximize the resolution in 19F ventilation imaging under the constraint of low SNR. Retrospective simulation with hyperpolarized gas images were corroborated by prospective 19F imaging of a 3D printed lung phantom and in-vivo measurements of the lungs. (iii) In-vivo ventilation metrics obtained by 19F ventilation imaging were explored by: 1. The in-vivo mapping of T1 at 1.5T and 3T and mapping of FV and T2* at 3 T. The apparent diffusion coefficient (1.5T) and the evaluation of ventilated volume (1.5T and 3T) was also compared to imaging performed with 129Xe (1.5T). 2. The optimization of imaging for the evaluation of percent ventilated volume with 19F at 3T with a commercial birdcage coil