Raymond Sydney Pengelly

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.

Highly Efficient Saturated Power Amplifier

In this article, a high-efficiency saturated amplifier has been presented. This amplifier takes advantage of the nonlinear output capacitor to shape the voltage waveform. The nonlinear capacitor generates substantial second harmonic voltage with small higher order harmonics. Thus, using √2 times larger fundamental load and the proper second harmonic termination, the resultant voltage waveform can be half-sinusoidal. For high efficiency, the PA should be driven by the large input power. Therefore, the current of the PA is bifurcated, resulting in the quasirectangular current shape. The resultant output waveforms are similar to those of class-F1 amplifiers, whose waveforms are optimal for high efficiencies. Based on harmonic source/ load-pull simulation, the saturated amplifier has been designed using Crees GaN HEMT CGH40006P packaged model. The matching networks for the input and output were optimized using the Momentum simulator. The implemented amplifier delivered a PAE of 80.1% at 3.475 GHz. The simulation and measurement results verify that the saturated amplifier is suitable for a high-efficiency PA over a relatively high frequency band.

A saturated PA with high efficiency [Technical Committee]

The IEEE MTT-5 student design competition for high-efficiency PAs provides the opportunity for the student to do an in-depth study of a PA from theoretical concept and analysis to fabrication and testing. This competition motivates many students to have a strong interest in a highly efficient PA design and development. The contest rules require the PA to operate at a frequency greater than 1 GHz but less than 20 GHz and produce an output power of greater than 5 W but less than 100 W into a 50 Ω load with a power input of less than 25 dBm. Aparticipant demonstrates the PA at the International Microwave Symposium (IMS), and the designer of the PA with the highest power-added efficiency (PAE) becomes the winning entry of the contest. The PAs designed by previous winners showed PAE with >75% at about 1 GHz frequency range [1]¿[3]. In the 2008 contest, a new rule, which has a frequency weighting factor of the PAE multiplied by (GHz)0.25, was introduced to encourage PA design at higher frequencies.

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.