Characterization of aluminum gallium nitride/gallium nitride high electron mobility transistors and their application in microwave push-pull amplifiers

Wide bandgap devices such as AlGaN/GaN high electron mobility transistors (HEMTs) have emerged as a next generation vehicle suitable for high power and high temperature applications. In order to apply this new device in microwave power amplifiers, an accurate large-signal model is essential. A nonlinear model suitable for high-power, large periphery AlGaN/GaN HEMTs on SiC was developed that incorporates thermal effects associated with high power density operation. The model was constructed based on DC, pulsed I-V, and small-signal microwave measurements at different temperatures, and was verified using continuous wave and pulsed load-pull measurements.; Baluns were designed for application in high efficiency, linear, push-pull (Class B) power amplifiers. The synthesis strategy developed was based on the use of a simplified analytic model, followed by a more accurate coupled line model, and finally validation using numerical simulation. The challenge of realizing low-loss microwave baluns and the complex biasing necessary for a push-pull amplifier was addressed by using a planar three-coupled-line balun with an integrated biasing scheme. This new balun design simplified both amplifier design and testing, and fabricated structures provided a low insertion loss of less than 0.5 dB over 5--11 GHz and 8--13 GHz bands, making them suitable for use in a push-pull amplifier.; The impact of balun bandwidth and the out of band load impedance presented by the output balun on push-pull amplifier performance was investigated, and addressed in realizing broadband linear GaN power amplifiers. A broadband push-pull microwave amplifier constructed with two discrete 1.5 mm GaN HEMTs and input/output baluns achieved a small signal gain of 9 dB with 42% power-added-efficiency at 5.2 GHz. A new concept of broadband power combining where two GaN MMIC amplifiers are operated in high efficiency Class B push-pull mode was demonstrated for high power applications.