2005-High power high efficiency AlGaN-GaN HEMT technology for wireless base station applications

High Power, High Efficiency, AlGaN/GaN HEMT Technology forWireless Base Station ApplicationsR.Vetury, Y.Wei, D.S.Green, S.R.Gibb, T.W.Mercier, K.Leverich, P.M.Garber, M.J.Poulton, J.B.Shealy RF Micro Devices, Infrastructure Product Line, Charlotte, NC, 28269, USAbstract — We report AlGaN/GaN high-electron-mobility-transistors (HEMT) on SiC substrates with field modulation plates (FP) of various dimensions and different gate widths. As a measure of the status of GaN technology achieved in this work, small periphery 150 µm HEMT demonstrated a continuous wave (CW) output power density of 22.7 W/mm at 2.14 GHz with power added efficiency (PAE) of 54% when biased at a drain-source voltage (V DS) of 80 V. As a demonstration of the scalability of this technology, a 20-mm-wide device exhibited 100 W CW output power and a simultaneous peak PAE of 55.3% at 2.14 GHz when biased at class AB and V DS=48V. WCDMA measurements on the 20mm part demonstrated ACP of –35 dBc at 42.5 dBm output power and 30% PAE under the same bias condition. Analysis of FP related performance tradeoffs are also presented in this work.Index Terms — Field plate, GaN, HEMT, WCDMAI. I NTRODUCTIONOver the past decade, AlGaN/GaN-based HEMT technology has emerged as a promising candidate for microwave power applications such as wireless base stations [1]. The competitive advantages of GaN HEMTs result from the material properties of the GaN/AlGaN material system and advantages afforded by various heterostructures possible in this material system [2-4]. Despite these material advantages over incumbent Si-LDMOS or GaAs technology, commercialization of GaN based HEMTs for wireless infrastructure applications has been hampered by the technological obstacles such as “current collapse” or “current slump” [3,5,6] and the high cost of SiC substrates. In this report, we present recent results and status of a manufacturable GaN technology developed at RFMD for wireless base station applications. AlGaN/GaN HEMT technology has made tremendous progress in the recent past, both in terms of maximum power density from small periphery devices [7] and total power output [8,9,10]. In AlGaN/GaN HEMT’s, the use of a field plate connected to the gate electrode has been shown to improve breakdown voltage and RF power performance [7,8]. Recently, 36 mm wide AlGaN/GaN HEMT based high power amplifiers (HPA’s) realized without FP’s were reported [9].However, there are few reports in the literature of a manufacturable GaN process technology achieving state-of-the-art power densities together with demonstrated scalability of the technology from small periphery “hero” devices to HPA’s, as characterized for base station applications. In this work, varied dimensions of field-plate are evaluated for understanding of tradeoffs between device performance metrics. We demonstrate AlGaN/GaN HEMTs on SiC developed for wireless infrastructure applications with state of the art device measurement results and high power amplifier WCDMA performance.II.D EVICE TECHNOLOGYFig 1 illustrates the AlGaN/GaN HEMT structure in our work. The AlGaN/GaN layer structure was grown by organometallic vapor phase epitaxy (OMVPE) on semi-insulating SiC. The GaN buffer layer was 2-µm thick. An undoped thin GaN cap layer was grown on the AlGaN barrier layer.There are generally two kinds of field plate structures reported. The so-called overlapping gate field plate structure [7] suffers from phase difference between gate and field plate because the field-plate is isolated from the gate over the device mesa. The phase difference between the two feed-in signals along the gate and field-plate cannot be neglected, especially for large periphery devices. The intimate contact of the gate and field plate in the structure of Fig. 1 eliminates this phase difference: a similar structure has been reported earlier by Ando [11]. Gate metal is formed from Ni/Au while a Ti/Al based metal scheme was used for source and drain ohmic contacts. On-wafer TLM demonstrated a contact resistivity of 0.18 Ω−mm. The devices were isolated by a mesa etch process.In this work, the gate length (L g ) and the gate to drain spacing (L gd ) was fixed at 0.6 um and 2.0 µm respectively . L fd is the field-plate dimension that was varied as 0.6, 1.0 and 1.4 µm.III. D EVICE PERFORMANCE AND ANALYSISA Cascade auto-probe station was used for on-wafer testing of all dc and small signal RF parameters using an Agilent 8720 Vector Network Analyzer and a 4145B semiconductor analyzer.a. Small signal RF Performance metricsThe maximum current-gain-cutoff frequency (f T ) and power-gain-cutoff frequency (f MAX ) were extracted using -20 dB/decade extrapolations from h 21 and Mason’s Gain respectively . A f T -f MAX map of 67 devices with gate width of 400 µm and field plate dimension, L fd of 0.6 µm on a 2” wafer is shown in Fig. 2. The devices were biased at V GS = -3.5 V and V DS = 10 V. As can be seen, Fig 2 shows excellent uniformity of f T and f MAX across the wafer.In order to understand the tradeoffs between frequency performance metrics with field plate dimension, the dependence of f T and f MAX on the FP dimension was investigated as a function of drain bias voltage for small periphery 2x200-µm wide devices. As shown in Fig 3, longer field-plate introduces larger C gd and results in lower f MAX and f T at all drain voltages. At higher drain bias voltage, the depletion layer under the field-plate extends toward the drain electrode: therefore f MAX increases with V DS due to the reduction of gate-drain capacitance (C gd ) and output conductance (G).Figure 2. f T -f MAX map of 67 devices, W g =0.4 mm, L fd =0.6 µm, V GS =-3.5 V and V DS =10 VDS fd L g =0.6 µm, W g =400 µm, biased at V GS =-3.5 V0.000.200.400.600.801.001.20246810121416V DS (V)I D C (A /m m ), I P _40 (A /m m )Figure 4 Pulsed IV plot of a 2x200 µm wide deviceb. Pulsed I-V performanceIn addition to dc and small signal RF measurements, pulsed IV test using an Accent DIVA 265 system were conducted, as shown in Fig. 4. A measure of the trapping effects that impact device performance can be obtained by examining the current recovery ratio (denoted CRR 540), which is defined as the ratio of dc current (I DC ) in the knee region (V DS =5V), to the pulsed current (I P-40) at the same V DS , but pulsed from a quiescent bias point of [Vds=40V and Vgs = -6V]. As can be seen from the inset in Fig. 4, this ratio is 0.89. While this ratio is relatively close to the ideal value of 1: it shows that there are transient and trapping effects to be overcome in this technology.c. Drain bias dependence of small periphery RF power performanceIn order to understand the capabilities of this technology, the large signal RF power performance of a small periphery (2x75µm) device was investigated over a range of drain bias : from 28V V DS to 80V V DS . RF power performance at 2.14 GHz was evaluated using a Maury loadpull system.The input and output match were tuned for the maximum output power at the 1-dB gain compression point. (P1-dB). Typically, devices could be biased up to 80Vand driven to (5-7) dB of compression, thus indicating that the breakdown voltage of this technology was more than adequate for 28V and 48V operation. Due to the soft rolloff exhibited by these GaN devices, saturated output power (P sat) was defined as the output power at the maximum PAE point, rather than at a fixed level of gain compression.satlinear gain, G T with V DS for a 2x75 µm device.Figure 6 Map of Psat and PAE of 150-µm wide devicesbiased at class-B and VDS=80 V.As can be seen from Fig 5, P sat increases linearly with V DS, while maximum PAE remained approximately constant (50-55%). This performance indicates that trapping effects that degrade PAE at higher drain biases [10], have been minimized to a large extent. The linear increase of P sat with V DS indicates that the maximum RF drain current stays constant over this range of drain bias, also indicating that trapping effects that cause RF current collapse have been minimized.To demonstrate across-wafer uniformity of this technology, large signal RF performance was measured on 14 2x75 µm devices distributed across a 2” wafer. The devices were biased at class B at a V DS of 80 V. As can be seen from the resulting wafer map of P sat and maximum PAE, shown in Fig 6, the maximum output power among this random selection of 14 devices is 22.7 W/mm with a corresponding maximum PAE of 54%.PAE (%) 0816243240-10-50510152025OutputPower(dBm),Gain(dB)Input Power (dBm)Fig. 7 Power performance of a 150-µm wide device with L fd of 1.0 µm.Fig. 7 shows a typical power performance of a 2x75 µm wide device with L fd of 1.0 µm. The input and output match were tuned for the maximum output power at the 1-dB gain compression point. (P1-dB). Output power density at the peak PAE point is 20.1 W/mm. The linear gain of the device was23.5 dB and the maximum PAE was 45.9 %.d. Large periphery GaN HPA PerformanceAs a demonstration of the scalability of this technology to large periphery HPA’s, devices with gate periphery of 20 mm were fabricated and tested at a Vds of 48V. Fig. 9 shows a die photograph of a 20-mm part. 20mm die were mounted in a flanged CuW package and pre-matched at 2.14 GHz for optimum gain and bandwidth. The HPA’s were biased into deep class AB (I ds=3 % I dmax). At V DS=48 V, 100 W output power was obtained with a simultaneous PAE of 55.3% (Fig. 10). The linear power gain was found to be 13.3 dB. At a drain voltage of 28V, the same part displayed 63 W P sat along with a peak PAE of 57.2%. T hese power performance metrics demonstrate the state of the art scalability of this technology to large periphery HPA’s.Figure 9 Die photograph of 20-mm HEMTFigure 10 Power performance of 20-mm HEMT biased at deep class-AB and V DS =48 V2.14 GHz WCDMA measurements employing TM1-16DPCH were conducted on a loadpull station . Biased in deep class AB, the PAE and ACPR of the 20-mm device versus output power at V DS =28 V and V DS =48 V are plotted in Fig. 11. At ACPR of –35 dBc and V DS of 28 V, the output power was 39 dBm and the PAE was 27.5%. At the same ACPR level when increased V DS to 48 V, 43-dBm output power was obtained with PAE of 30%. This indicates higher voltage operation at 48V should provide improved linearity Figure 11 WCDMA results of 20-mm HEMT biased at deep class-ABReliability analysis of this technology will be reported separately.IV. C ONCLUSIONIn summary, AlGaN/GaN HEMTs on SiC substrate with field-plate technology have been presented. 150-um wide HEMT demonstrated a continuous wave (CW) output power density of 22.7 W/mm at 2.14 GHz with power added efficiency (PAE) of 54% at V DS of 80 V . A 20-mm-wide device exhibited 100 W CW output power and a simultaneous PAE of 55.3% at 2.14 GHz when operated at deep class AB and V DS of 48V . Under the same testing condition, WCDMA measurement demonstrated ACPR of –35 dBc at 43-dBm output power and 30% PAE. These results demonstrate that AlGaN/GaN HEMTs can become a truly disruptive technology in the wireless base station arena.R EFERENCES[1] J. B. Shealy, , et al , “Gallium nitride (GaN) HEMT's: progress and potential for commercial applications ” Gallium Arsenide Integrated Circuit (GaAs IC) Symposium, 2002. 24th Annual Technical Digest , 20-23 Oct. 2002.[2] L. F. Eastman, et al, “The toughest transistor yet [GaN transistors],” Spectrum, IEEE , Vol: 39 , Issue: 5 , May 2002.[3] U.K. Mishra, et al , “AlGaN/GaN HEMTs-an overview of device operation and applications,” Proceedings of the IEEE , vol. 90, issue 6, pp.1022 – 1031, June 2002.[4] R.J. Trew, “SiC and GaN transistors - is there one winner for microwave power applications?” Proceedings of the IEEE , vol. 90, issue 6, pp.1032 – 1047, June 2002.[5] R. Vetury, et al , “The impact of surface states on the DC and RF characteristics of AlGaN/GaN HFETs,” Transactions on Electron Devices, Volume: 48 , Issue: 3, pp. March 2001.[6] L. F. Eastman, et al, “Undoped AlGaN/GaN HEMTs for microwave power amplification,” Electron Devices, IEEE Transactions on, Volume: 48 , Issue: 3 , March 2001.[7] Y.-F. Wu, et al , “30-W/mm GaN HEMTs by field plate optimization.,” IEEE Electron Device Letters , vol. 25, issue 3, pp.117-119, March 2004.[8] Y. Okamoto, “179 W recessed-gate AlGaN/GaN heterojunction FET with field-modulating plate,” IEEE Electronics Letters , vol. 40, issue: 10 , pp. 629 – 631, May 2004.[9] T. Kikkawa, et al , “An over 200-W output power GaN HEMT push-pull amplifier with high reliability,” Microwave Symposium Digest, 2004 IEEE MTT-S International , vol. 3, pp.1347–1350, June 2004.[10] P. Parikh , , et al , “AlGaN-GaN HEMTs: material, device, circuit technology and applications,” Compound Semiconductors, 2003. International Symposium , 25-27 Aug. 2003.[11] Y. Ando, et al , ”10-W/mm AlGaN-GaN HFET with a field modulating plate,” IEEE Electron Device Letters , vol. 24 , issue 5, pp.289–291, May 2003.。