AlGaN/GaN microwave power transistors: Focussed optimization of linearity and efficiency
Fig. 1: SEM micrograph of GaN power transistor cell
Fig. 2: Linearity (C/I3) in dependence on output power
The linearity of AlGaN/GaN heterojunction field-effect transistors (HFETs) is one of the key parameters in modern telecom systems. This feature needs to be optimized to design GaN-based RF circuits fulfilling modern system requirements. These type of amplifiers are of paramount interest not only for terrestrial but also for space-borne communication applications. At the FBH, the ESA-funded project "Linearity Assessment of GaN Technology" investigates possible approaches in epitaxy, processing and transistor design to optimize the linearity at transistor level. This leads to GaN power devices with optimized inherent linearity properties which will reduce linearizing effort in microwave power amplifiers and thus increase energy efficiency at a given linearity level. Together with its partners TESAT Spacecom, Germany QinetiQ, United Kingdom and Technical University of Denmark (TUD), the institute aims at delivering highly linear C- and L-band amplifier prototypes for space applications.
The first phase’s primary goal is a cross-comparison of epitaxial structures supplied by FBH and QinetiQ with particular emphasis on linearity. The investigated epitaxial structures compare different buffer versions and diverse design concepts of the active transistor layers. In detail, the GaN standard buffer has been compared with a buffer structure encompassing an AlGaN potential barrier. Variations of the AlGaN barrier composition and thickness in the active transistor layers as well as the GaN cap layer have been investigated in terms of different carrier concentrations in the channel [1]. Linearity performance was evaluated by means of two-tone intermodulation distortion (IMD) measurements at different classes of amplifier operation (Class A, AB and B). The third order intermodulation ratio (C/I3) was calculated from the two-tone measurements and used to compare the transistors on different epitaxial structures. The measurements being performed on individual power cells were then combined to larger power bars showing power levels of typically 30 W at 2 GHz. This class of power devices is designated for the subsequent demonstrator amplifiers scheduled for final phase of the project.
The linearity results strongly depend on the specific epitaxial designs. In this connection, Fig. 1 compares the C/I3 ratio of devices from different epi designs in dependence on output power level for class A operation. It is evident that for given power levels the linearity can be improved by almost 20 dB just by selecting the proper epitaxial design. This also means that for a given linearity value the amplifier efficiency can be significantly increased by using optimized epi designs. Therefore, these concepts are considered to be extremely attractive for future amplifier concepts showing good linearity and high efficiency at the same time.
Publication:
[1] S. A. Chevtchenko, O. Bengtsson, F. Brunner, E. Bahat-Treidel, J. Würfl, M. J. Uren, A. Barnes, "Linearity evaluation of AlGaN/GaN HFETs fabricated on different epitaxial structures", Proc. 5th Space Agency – MOD Round Table Workshop on GaN Component Technologies, Noordwijk, The Netherlands, September 02-03 (2010).
FBH research: 19.01.11