GaN RF Testbed for Space Applications

Transcription

1 GaN RF Testbed for Space Applications Microwave Technology and Techniques Workshop May 200 ESA-ESTEC, Noordwijk, The Netherlands Hugo Mostardinha (), Pedro Miguel Cabral (), Nuno Borges Carvalho (), J. C. Pinto (2) and François Garat (3) () Instituto de Telecomunicações - Universidade de Aveiro Campus Universitário de Santiago, , Aveiro, Portugal Tel: Fax: , hmostardinha@av.it.pt (2) EFACEC Sistemas de Electrónica, S. A. Aerospace Activity Manager R. Eng. Frederico Ulrich, apartado 3078, Moreira Maia, Portugal Tel: Fax: , cpinto@efacec.pt (3) ESA/ESTEC European Space Research and Technology Center Keplerlaan - P.O. Box AG Noordwijk ZH The Netherlands Tel: Fax: francois.garat@esa.int ABSTRACT This paper addresses the design, implementation and test of a RF board that will be included in the next Alphasat program. The design and construction of a GaN oscillator, that will be used in space applications, will be presented. Actually, to the best knowledge of the authors, this is the first test made solely to study the GaN technology applicability in space and one of the first tests of European GaN, onboard of an ESA satellite. INTRODUCTION The replacement of travelling wave tubes by solid state devices has always been an objective in satellite communications as a way to perform the necessary revolution in communication systems. This future goal has now been made possible by the appearance of a new transistor technology: Gallium Nitrade (GaN). Its rapid growth will have a tremendous impact on the PA design for high power applications in space industry and its radiation hardness should be studied. This work is exactly focused on this issue, since the technology robustness on space environments will be tested. The presented experience consists in a series of four GaN oscillators, controlled by a motherboard that will be continuously monitoring the oscillator power. The experiment is, at this time, in a pre-flight stage and the reported material in this paper follows the engineering approach. The paper will be organized by first presenting the oscillator circuit to be tested and then by presenting the final motherboard.

2 GAN TRANSISTORS GaN High Electron Mobility Transistor (HEMT) devices constitute a very promising technology for high power applications. Its unique physical properties such as: high bandgap (3. ev), high breakdown field ( MV/cm), enables the construction of devices capable of operating with higher supply voltages and thus to provide higher output powers, when compared with GaAs and Si FETs, with similar channel-lengths, []. Besides that, superior transport characteristics, in terms of electron peak velocity (3x0 7 cm/s), saturation velocity (.5x0 7 cm/s), field mobility (500cm 2 /Vs) [2], and parasitics makes it suitable for high-frequency applications (f t =0GHz and f max =55GHz, with a 0.2µmx00µm GaN/Al 0.20 Ga 0.80 N HEMT, reported by Lu et al. in [3]). GaN grown on SiC offers a thermal conductivity of.5w/cmk, making this technology appropriate for high-temperature (Daumiller et al. [] reported GaN-based FETs operating up to 750ºC) and high-power applications (Pout=9.8W/mm at 8GHz, obtained with a 0.5µm x 50µm Al 0.38 Ga 0.62 N/GaN HEMT, reported by Wu et al. in [5]). In this work the transistors used were 2-finger GaN HEMTs (device geometry 2x50 µm, Gate length 0.25 µm), see Fig. for a graphical representation of the devices. The devices were designed, epi-grown and processed at the Ferdinand Braun-Institut (FBH) in Berlin in the frame of the ESA funded GaN benchmarking project. The device periphery is specially designed to account for rugged and reliable mounting as required for space applications. Fig.. GaN transistor OSCILLATOR DESIGN In order to test GaN in space applications, an RF oscillator, running at near 2.5 GHz, was built and, when launched into space, its output power variation over time will be object of intense measurements. The adopted oscillator topology follows a Colpitz configuration and assumes that the feedback loop is made by a capacitive/resistive network. The inductor for the Colpitz arrangement, and thus for the resonance loop, is made by a coaxial resonator. The use of a coaxial resonator determines that the oscillation frequency must be comfortably below its first resonance frequency, in this case 3.5GHz. Accordingly, and taking into account the excess inductance introduced by the resonator tab and its connection to the active device input, we designed the oscillator for a selected frequency of around 2.5 GHz. Due to onboard DC power supply availability and, despite we are using GaN devices [6], we had to limit the V DS voltage to 5V. Besides the RF Oscillator itself, system test board also includes a temperature compensated power detector. This being the case, due to electromagnetic compatibility reasons the overall system s outputs are only two DC voltages: one corresponds to the oscillator power and the other to a calibration voltage. Using the device nonlinear model [6], which proved to be very robust and reliable, we designed a first oscillator. Fig. 2 presents the final simulated circuit in computer aided design software.

3 Oscillator Power Detector VT C7 VDD Vdc=5 C8 C6 C= nf L L=68.0 nh R=0. Ohm VD Vdc=0.38 C2 C C R6 R=70 Ohm D R7 R=7 kohm C C3 Ref Temex E=5 F=3.5 GHz C C=0.8 pf R R= kohm C2 C=0.7 pf R2 R=0 Ohm AngFBH2 FET C5 C R R=50 Ohm C9 C=0.3 pf C0 R5 R=70 Ohm D2 R8 R=7 kohm C5 V_GaN C6 C3 C=0.9 pf R3 R=00 Ohm Fig. 2. Oscillator and power meter DC circuit. After this, we passed directly to the implementation stage. The oscillator printed circuit board (PCB) was built using Duroid RT600 substrate, and attached with electrical and thermal conductive adhesive glue (ATI-ESP8350) to the external housing box. To avoid cracks due to temperature dilatations, the box material needs to have a thermal expansion coefficient (CTE) near the one of the substrate. The box also needs to have good electrical and thermal properties and to be a light and robust material. Due to all the above mentioned reasons, we chose a special alloy: CE- 7 with NI 6-0µm + Au 0,5-µm finish. Fig. 3 presents the final housing box. Fig. 3. Oscillator prototype housing box. In order to improve the chip thermal dissipation and ground connection, we glued it directly to the box using an epoxy glue: AIT (EG8050). Besides that, we also used an RF absorber material glued on the housing cover inside part with silicon base glue (ECCOSORB BSR-2-SS6M). Thermal and vacuum test cycles were conducted to study the behavior on hostile environments.

4 D2 D Fig. presents the final oscillator PCB board contour view with and without lumped components. a) b) Fig.. Oscillators PCB board contour view: a) with and b) without lumped components. Fig. 5 presents the final implemented GaN oscillator. Fig. 5. Implemented GaN oscillator. Fig. 6 presents a captured picture obtained from the Spectrum Analyzer. The measured oscillation frequency was 2.55GHz, with an associated power of near 7dBm. Fig. 6. Oscillator Output Spectrum.

5 This prototype is one of the four that will be included in the overall testbed, providing redundancy and statistical nature to the experiment. To perform the interconnection and control of all circuits, a motherboard was also designed. This board plays a very important role in powering circuits, controlling the oscillators and measuring data in this experience as well as the communicating with the satellite experience unit. It allows independently powering or shutting down each of the oscillators; it also feeds the power detector, measures the oscillator power while making diodes thermal compensation and allows measuring the board temperature and the radiation level to which it will be exposed to. Fig. 7 presents the motherboard oscillator supply and measuring blocks. TP30 EB_CTRL TP25 R7 0K R8 33K R8 0K QA 2N2920 R9 33K 0R R0 C nf Q7 IRF5305 EB+5V R R Vp D N6677UR C3 2.2uF R2 22K OSC VDS 3 VD CON6A VG 5 R 0K VGAN GND 6 VT C nf TP UA LM2 UB 7 LM2 R3 0K UC 8 LM2 R6 5k6 R5 5k6 C3 68nF C2 68nF TP35 Fig. 7. Oscillator supply and measurement block. R 75K The complete experiment assembly can be found in Fig. 8. Fig.8. GaN RF testbed for integration test. CONCLUSION In this paper we presented the engineering model of an experiment to test GaN devices in space applications. Despite these are initial results, the already obtained data allows the team to be confident with the final results.

6 ACKNOWLEDGEMENTS We would like to thank the collaboration and the transistors from FBH, specially to Mattias Rudolph, Paul Kurpas and J. Würfl, to A. Barnes from ESA, to Prof. José Carlos Pedro, Prof. Nuno Matos and Eng. Cupido for the fruitful discussions, to Prof. Mendiratta and Eng. Jorge Monteiro for the vacuum tests and finally to Portuguese Communications Authority (ANACOM) for the electromagnetic compatibility tests. REFERENCES [] Y. Ohno and M. Kuzuhara, Application of GaN-based heterojunction FETs for advanced wireless communication, IEEE Trans. Electron Devices, vol. ED-8, pp , Mar [2] U. K. Mishra, Y.-F.Wu, B. P. Keller, S. Keller, and S. P. Denbaars, GaN microwave electronics, IEEE Trans. Microwave Theory andtech., vol. MTT-6, pp , June 998. [3] W. Lu, J. Yang, M. A. Khan, and I. Adesida, AlGaN/GaN HEMTs on SiC with over 00 GHz f and low microwave noise, IEEE Trans. Electron Devices, vol. ED-8, pp , Mar [] Y.-F. Wu, D. Kapolnek, J. Ibbetson, P. Parikh, B. Keller, and U. K. Mishra, Very high power density AlGaN/GaN HEMTs, IEEE Trans. Electron Devices, vol. ED-8, pp , Mar [5] I. Daumiller, C. Kirchner,M. Kamp, K. Ebeling, L. Pond, C. E.Weitzel, and E. Kohn, Evaluation of AlGaN/GaN HFET s up to 750 C Device Res. Conf. Proc., pp. 5,998. [6] Matthias Rudolph, Documentation of FBH HBT Model, Ferdinand-Braun-Institut für Höschst frequeztechnik (FBH), Gustav-Kirchhoff-Str., D-289 Berlin, Germany