4-Watt Ka-Band AlGaN/GaN Power Amplifier MMIC. A. M. Darwish, K. Boutros*, B. Luo*, B. Huebschman, E. Viveiros, and H. A. Hung. Army Research ...
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4-Watt Ka-B and AlGaN/GaN Power Amplifier MMIC A. M. Darwish, K. Boutros*, B. Luo*, B. Huebschman, E. Viveiros, and H. A. Hung Army Research Laboratory, 2800 Powder Mill Rd., Adelphi, MD 20783 *Rockwell Scientific Company LLC, 1049 Camino Dos Rios, Thousand Oaks, CA 91360 ABSTRA CT
A broadband Ka-band AlGaN/GaN on SiC HEMT power amplifier MMIC was developed for millimeter-wave antenna applications. The output stage is composed of a 1.2-mm-wide device with 0.18 ,Im gate length. The two-stage 50-ohm matched MMIC produces 13+1 dB of gain from 26 GHz to 36 GHz. At 35 GHz, the measured CW saturated output power was 4 W, indicating a power density of 3.3 W/mm. The power added efficiency was 23%. Across the band, the measured CW output power was > 2 W. While individual (or partially matched single stage) devices have been demonstrated with good output power, to the best of our knowledge, this is the first report of a 10GHzbandwidth Ka-band GaN MMIC with high output power, gain, and return loss.
I. INTRODUCTION
Gallium-Nitride devices have shown significant performance advantage over GaAs, and InP. The AlGaN/GaN on SiC is particularly desirable due to the high thermal conductivity of SiC. At the device level (or partially matched single stage circuits), record performance has been reported at the millimeter-wave [1][6] frequencies. Specifically, at 30 GHz, 3.4 W/mm [1], and 5.1 W/mm [2] have been reported. At 35 GHz, 3.3 W/mm [1], and 4.9 W/mm [2] have been reported. At 40 GHz, 2.8 W/mm [5] and recently 1OW/mm [4] have been reported. In spite of these excellent results at the device level, very few fully integrated MMIC were demonstrated [7]-[9]. In [7], a single stage MMIC with 9 dB of linear gain at 27 GHz, and 2.2 W of output power was reported. In [8], a LNA MMIC was reported along with a Ka-band PA MMIC with a saturated output power of 4 Watts at 28 GHz from a 3.2 mm output stage (1.25 W/mm power density) with 23.8% PAE. We report the development of a two stage broadband Ka-band MMIC power amplifier achieving 13 ±1 dB of gain from 26 GHz to 36 GHz. At 35 GHz, the measured CW output power was 4 watts, indicating a power density of 3.3 W/mm. The power added efficiency was 23%. The minimum output power across the band was 2 Watts, at the low frequency portion of the band. This first-pass design was successful due to high performance of the device and the precise full wave modeling and broadband design of the MMIC. To our knowledge, this is the best combination of output power, gain, and bandwidth reported in a two-stage GaN MMIC technology. This
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combination of high power and bandwidth represents one of the clear advantages of GaN technology over GaAs, where a relatively small device (here 0.6 mm) can produce a significant amount (about 2W) of output power, while having an optimum output load close to 50 ohms (see figure 4). II. DEVICE RESULTS
The GaN HIEMT devices used for this MMIC build were fabricated on 2" semi-insulating 4H-SiC substrates. The epitaxial layer consisted of an AlGaN/GaN HIEMT structure grown by metal-organic chemical vapor deposition (MOCVD). The devices were defined using implant isolation, Ta-based ohmic contacts with R,- 0.7 Q.mm, and 0.18 tm long T-gates. A Si3N4 layer was used as a device passivation layer, and as dielectric for MIM
capacitors.
Thin-film resistors were fabricated using Ni-Cr metal lines, and two-level interconnects with air-bridge technology were used to complete the front-end device fabrication process. Through-substrate vias with 50tm diameter were fabricated in the 3-mil thinned substrate to complete the MMIC back-end fabrication. Typical DC and RF device characteristics of a microstrip-loaded 400 tm gate-width device showed a saturation current Idss of 1 A/mm, a transconductance Gm max of 290mS/mm, a cut-off frequencyf, of 84 GHz and a unity-gain frequencyfmax of 114 GHz. The unit cell layout, DC and RF performance are shown in figures 1(a)-(c). The device model vs. measured S-parameters is shown in figure l(d) for a unit cell device (gate-width = 4x100 ptm) and indicates a good match between the modeled and measured characteristics. III. CIRCUIT DESIGN AND PERFORMANCE
The MMIC design consisted of a 2-stage layout with inter-stage matching. The first and second stages consisted of device unit cells with total gate periphery of 0.6mm and 1.2mm, respectively. A picture of the fully fabricated MMIC is shown in figure 2, and small-signal S-parameter measurements of the MMIC are shown in figure 3. The results indicate a gain of 13 ±1 dB from 26 to 36 GHz, and good isolation
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(S12) across this frequency band. The bias conditions were = 24 V, and ID= 40% Ids, for the first and second stages during small-signal and power measurements. For the second stage, the optimum power load and the designed output matching load are shown in figure 4, for frequencies in the Ka-band. Note that the optimum power load for the 0.6 mm 2W device is very close to 50 ohms. This facilitates the broadband performance shown in figure 3 and is a key advantage of the GaN technology over GaAs.
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Power testing of packaged MMIC was performed at a drain bias of 24V. The chip was mounted on a THERMKONR metal block with Au-Sn eutectic die-attach for heat sinking. Constant temperature was maintained using a controlled temperature re-circulating chiller. The temperature of the metal block was maintained at approximately 25°C during power measurements.
The measured output power, gain, and power added efficiency at 35 GHz are shown in Fig. 5, and the P3dB output power is shown as a function of frequency in Fig. 6. At 35GHz, the MMIC showed a linear gain of 12 dB. At the peak PAE (3 dB into compression), a Pout of 3.8 W was measured, corresponding to a power-density of 3.2 W/mm (with 9 dB of gain). And a saturated output power of 4 W was measured. The output power (at peak PAE) was measured across the band (see figure 6). The minimum output power measured was > 2W, at the low frequency portion of the band. With GaN technology, producing 5 to 10 W of output power across the entire Kaband (26.5 - 40 GHz) may be demonstrated soon. IV. CONCLUSION We have demonstrated a broadband Ka-band MMIC PA. This may pave the way for the replacement of the bulky traveling wave tube amplifiers with GaN solid state amplifiers, if the reliability and efficiency of the technology can be improved. The MMIC has a bandwidth of 32%, covering most of the Ka-band, while producing output powers of 2-4 Watts across the band. The development of broadband power amplifier MMICs is critical for cost reduction in multi-band multi-function phased array systems.
ACKNOWLEDGEMENT Work performed at Rockwell Scientific was partially funded by Army CTA program, Contract # DAAD19-012-0008
REFERENCES [1] Wu, Y.-F.; Moore, M.; Saxler, A.; Smith, P.; Chavarkar, P.M.; Parikh, P., "3.5-watt AlGaN/GaN HEMTs and amplifiers at 35 GHz," Proceedings of EEE International Electron Devices Meeting, pp. 23.5.1-2, 2003. [2] Y.-F. Wu, M. Moore, A. Saxler*, T. Wisleder, U. K. Mishra, P. Parikh, "8-Watt GaN HEMTs at Millimeter-Wave Frequencies," Proceedings of EEE International Electron Devices Meeting, 2005. [3] Quay, R.; Tessmann, A.; Kiefer, R.; Weber, R.; van Raay, F.; Kuri, M.; Riessle, M.; Massler, H.; Muller, S.; Schlechtweg, M.; Weimann, G., "AlGaN/GaN HEMTs on SiC: towards power operation at V-band," Proceedings of EEE International Electron Devices Meeting, pp. 23.2.1-2, 2003. [4] T. Palacios, A. Chakraborty, S. Rajan, C. Poblenz, S. Keller, S. P. DenBaars, J. S. Speck, and U. K. Mishra "High-Power AlGaN/GaN HEMTs for Ka-Band Applications," WEEE Electron Device Letters vol. 26, No. 11, pp. 781-783, 2005. [5] Boutros, K.; Regan, M.; Rowell, P.; Gotthold, D.; Birkhahn, R.; Brar, B., "High performance GaN HEMTs at 40 GHz with power density of 2.8W/mm," Proceedings of WEEE International Electron Devices Meeting, pp. 12.5.1-2, 2003. [6] J. Moon, S. Wu, D. Wong, I. Milosavljevic, P. Hashimoto, M. Hu, M. Antcliffe, and M. Micovic, "Deep submicron gate-recessed and fieldplated AlGaN/GaN HFETs for mmwave applications," in Proc. Materials Research Society Fall Meeting, vol. E6-1, pp. 119, Dec. 2004. [7] Micovic, M.; Kurdoghlian, A.; Moyer, H.P.; Hashimoto, P.; Schmitz, A.; Milosavljevic, I.; Willadesn, P.J.; Wong, W.S.; Duvall, J.; Hu, M.; Delaney, M.J.; Chow, D.H., "Kaband MMIC power amplifier in GaN HFET technology," IEEE Int'l Microwave Symp., pp. 1653-1656, June, 2004. [8] Micovic, M.; Kurdoghlian, A.; Moyer, H.P.; Hashimoto, P.; Schmitz, A.; Milosavljevic, I.; Willadesn, P.J.; Wong, W.S.; Duvall, J.; Hu, M.; Wetzel, M.; Chow, D.H., "GaN MMIC Technology for Microwave and Millimeter-wave Applictions," IEEE Compound Semiconductor Integrated Circuit Symp., Session J, October, 2005. [9] Green, B.M.; Sungjae Lee; Chu, K.; Webb, K.J.; Eastman, L.F., "High efficiency monolithic gallium nitride distributed amplifier," IEEE Microwave and Guided Wave Letters, vol. 10, Issue 7, pp. 270 - 272, July, 2000.
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Device S-parameters (Meas. vs Model)
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Figure Id: Device model vs. measured S-parameters for a unit cell device showing a good match between the simulated and measured characteristics.
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Figure lb: Drain current and transconductance as a function of gate voltage for a 400 gm unit device. Typical DC characteristics indicate a saturation current IdSS of 1
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Figure ic: RF characteristics of a 400 gm unit device indicate a cut-off frequencyj, of 84 GHz and a unity-gain frequencyf,,x of 114 GHz.
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Figure 5. Measured MMIC CW output power at 3 5 GHz showing a linear gain of 13 dB, a saturated power of 36dBm, and a peak efficiency of 2300. Pout at peak PAE
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Frequency (GHz) Figure 6. Measured P3dB of MMIC under CW conditions across the frequency band.
