William L. Pribble

A Review of GaN on SiC High Electron-Mobility Power Transistors and MMICs

Gallium-nitride power transistor (GaN HEMT) and integrated circuit technologies have matured dramatically over the last few years, and many hundreds of thousands of devices have been manufactured and fielded in applications ranging from pulsed radars and counter-IED jammers to CATV modules and fourth-generation infrastructure base-stations. GaN HEMT devices, exhibiting high power densities coupled with high breakdown voltages, have opened up the possibilities for highly efficient power amplifiers (PAs) exploiting the principles of waveform engineered designs. This paper summarizes the unique advantages of GaN HEMTs compared to other power transistor technologies, with examples of where such features have been exploited. Since RF power densities of GaN HEMTs are many times higher than other technologies, much attention has also been given to thermal management-examples of both commercial “off-the-shelf” packaging as well as custom heat-sinks are described. The very desirable feature of having accurate large-signal models for both discrete transistors and monolithic microwave integrated circuit foundry are emphasized with a number of circuit design examples. GaN HEMT technology has been a major enabler for both very broadband high-PAs and very high-efficiency designs. This paper describes examples of broadband amplifiers, as well as several of the main areas of high-efficiency amplifier design-notably Class-D, Class-E, Class-F, and Class-J approaches, Doherty PAs, envelope-tracking techniques, and Chireix outphasing.

Thermal analysis and its application to high power GaN HEMT amplifiers

A systematic and consistent approach to the thermal modeling and measurement of GaN on SiC HEMT power transistors is described. Since the power density of such multilayered wide bandgap structures and assemblies can be very high compared with other transistor technologies, the application of such an approach to the prediction of operating channel temperatures (and hence product lifetime) is important. Both CW and transient (i.e. pulsed and digitally modulated) thermal resistances are calculated for a range of transistor structures and sizes as a function of power density, pulse length and duty factor and compared with measured channel temperatures and RF parameters. The resulting thermal resistance values have then been imported into new “self-heating” large signal models so that transistor channel temperatures and the resulting effects on RF performance such as gain, output power and efficiency can be determined during the amplifier design phase. Some practical examples are included in the paper including the temperature rises in the carrier and peaking transistors of a high power Doherty amplifier.

AlGaN/GaN-on-SiC HEMT Technology Status

GaN-based devices offer significant advantages for next generation military and commercial systems. Military systems benefit from high power densities of 4 to 7 W/mm depending on bias conditions along with efficiencies over 60% at frequencies through X-band, and commercial systems take advantage of excellent linearity as well. In this paper, we will review a number of commercial products that only GaN technology can achieve. In addition to narrow-band circuits for highly linear commercial applications, results will be shown for two commercial GaN MMIC products that have been developed for general-purpose applications in the 2.5-6.0 GHz and DC-6.0 GHz bands. Additionally, results are shown for a 2-stage high efficiency S-band switch mode amplifier operating from 3.1-3.5 GHz. Significant progress has also been made in the development of 100-mm SiC substrates. Micropipe densities as low as 2.5 cm-2 have been demonstrated for 100-mm HPSI substrates.

A 2.5 Watt 3.3-3.9 GHz Power Amplifier for WiMAX Applications using a GaN HEMT in a Small Surface-Mount Package

A 2.5 watt average power (15 watt peak power) amplifier for application with WiMAX signal protocols has been constructed using small surface-mount GaN HEMT transistors. The GaN HEMT characteristics are uniquely suitable for producing high peak power in very small (3times3 mm) surface-mount packages. The PA produces 12.5 dB of gain over 3.3-3.9 GHz, with EVM under 2.5% with 2.5 watts average output. A design methodology for optimizing performance for the WiMAX protocol is presented.

DEVELOPMENT OF NITRIDE-BASED TECHNOLOGY FOR SPACE: ENABLING NEW FRONTIERS

Gallium nitride based microwave technology is being developed in an Air Force Dual-Use Science and Technology project. The technology has previously demonstrated significant characteristics for high power operation and robustness in extreme environments. Its desirable features (such as high breakdown voltage), overall development issues, and systems implications are reviewed. In this context, the programs recent advancements in state-of-the-art performance are presented, such as 14.8 W pulsed at 10 GHz with a 6mm HEMT with 27% PAE and 11.7 dB gain.