A Cool, Sub-0.2 dB, Ultra-Low Noise Gallium Nitride Multi-Octave MMIC LNA-PA with 2-Watt Output Power Kevin W. Kobayashi, *YaoChung Chen, *Ioulia Smorchkova, *Benjamin Heying, *Wen-Ben Luo, *William Sutton,*Mike Wojtowicz, and *Aaron Oki RF MICRO DEVICES, Torrance, CA, 90505, [email protected], *Northrop Grumman Space & Technology, One Space Park, Redondo Beach, CA, 90278 I. INTRODUCTION Wide band, high dynamic range GaN HEMT LNAs can enable future/next generation systems such as multi-carrier base-stations, high definition CATV, broadband agile and software reconfigurable communication links. GaN HEMTs provide the wide bandwidth, low noise, and high linearity these future systems require. The wide band-gap, high electron mobility, and good thermal conductivity of GaN, combined with its 2-D gas hetero-structure allows GaN HEMT to achieve comparable noise performance, but with higher power density and linearity compared to GaAs PHEMT technology. This makes GaN HEMT very attractive as a new technology source for building wide dynamic range front-ends. Several of the recent GaN HEMT MMIC LNA demonstrations in literature have been motivated by a need for robustness and survivability under a harsh RF environment for military applications. Robustness is also critical for CATV applications where hardware may be exposed to large voltage transients induced by lightning. Previous work on GaN MMIC LNAs has resulted in broadband low noise amplifiers with high survivability and State-of-the-Art GaN MMIC LNA Noise Figure 3 Noise Figure (dB) Abstract — This paper reports on a S-,C-band LNA-PA which achieves a sub-0.2dB noise figure over a multi-octave band and a PSAT of 2 Watts at a cooled temperature of -30ºC. The GaN MMIC is based on a 0.2um AlGaN/GaN-SiC HEMT technology with an fT ~ 75 GHz. At a cool temperature of -30ºC and a power bias of 15V-400mA, the MMIC obtains 0.25-0.45dB NF over a 2-8GHz band and a linear P1dB of 32.8dBm (~2Watts) with 25% PAE. At a medium bias of 12V-200mA, the amplifier obtains 0.1-0.2dB NF across the same band and a P1dB of 32.2 dBm (1.66Watts) with 35% PAE. The corresponding PSAT is better than 2Watts. At a low-noise bias of 5V-200mA, 0.05-0.15dB NF is achieved with a P1dB > 24dBm and PAE~33%. These results are believed to be the lowest NF ever reported for a multioctave fully matched MMIC amplifier capable of > 2Watts of output power. The ultra-low noise, wide band, and high power obtained at modestly low temperature operation makes this an attractive and practical low-cost solution for applications such as WiMAX, CATV, base-stations, and broadband communication systems. Index Terms — Cryogenic, GaN HEMT, Low Noise Amplifier (LNA), Power Amplifier (PA), dynamic range. [1] HRL 2.5 [3] NGST 2 [4] NGST 1.5 [2] UCSB [5] NGST [6] SIRENZA-NGST 1 0.5 0.2dB 0 0 This Work 2 4 Frequency (GHz) 6 8 Figure 1 – State-of-the-art GaN MMIC LNAs. robustness properties. These devices have not, however, been developed for ultra-high linearity applications. These MMIC matched LNAs have until recently been limited to linear output power of 1 Watt or less and noise figures above 1-dB in the S- and C-band frequency range. Figure 1 illustrates the state-of-the-art noise figure performance for several GaNbased MMIC LNAs. An extremely wideband 3-18GHz LNA [1] was reported with a minimum NF of 2.4dB at the midband of 7GHz, and less than 4dB across a 4-18GHz broadband. A C-band LNA [2] obtained NF of 1.6dB at 6 GHz while also demonstrating 31 dBm of input survivability and a P1dB of 12.8dBm. A dual-gate LNA with a high P1dB of 30dBm achieved NF as low as 1.5dB at 1 GHz while also demonstrating high survivability of 30dBm [3]. A wideband flat gain C-band MMIC [4] achieved ~1.5dB across 2-5GHz using dual-gates. That device also obtained a P1dB of 20dBm and input survivability of 28dBm. A 1-12GHz wideband dual-gate GaN LNA using a similar 0.2um GaN technology [5] of this work achieved NF between 1.03-2.4dB over a multi-octave 2-8GHz BW and a P1dB less than 25dBm. In a previous work by the present authors [6], we described what was believed to be the lowest NF achieved from a (GaN) MMIC LNA that demonstrated ~0.9dB NF over a 2-8GHz band and delivered an average P1dB of 2Watts from 1-4GHz. In this document, we further explore the enhanced ultralow noise performance capability of the GaN LNA of [6] at lower but modest temperatures down to -30ºC. For the first time we demonstrate sub-0.2dB noise figure across a 2-8GHz band by cooling the amplifier down to -30ºC while also providing output power > 2Watts. To our knowledge this is the lowest noise figure recorded for a multi-octave GaN HEMT fully-matched MMIC LNA. It is believed that operation at these modestly low temperatures can enable practical low-cost solutions for new high performance datacom systems. Minimum Device NF Vd=10V, Id = 80 mA/mm 0.8 Figure 3a) – Photograph of the GaN MMIC LNA-PA. Chip size is 1.7mm2. NFmin (dB) 0.7 0.6 0.5 0.4 0.3 Wg = 500 um Wg = 200 um 0.2 0.1 0 2 4 6 8 Frequency (GHz) Figure 2 – GaN HEMT NFMIN. II. GAN MMIC DESIGN Figure 3b) –Schematic of GaN MMIC Amplifier. Wg = 600um Γopt 1 GHz 8 GHz 8 GHz S11 1 GHz Wg = 1200 um S(1,1) Sopt S(1,1) Sopt The MMIC was fabricated using NGST’s AlGaN/GaN HEMT process technology. The AlGaN/GaN material is grown on a 3-inch semi-insulating SiC substrate formed by metal organic chemical vapor deposition (MOCVD). The details of the device structure are similar to those reported in [6]. Room temperature measurements show a typical 2-DEG of 1.2x1013cm-2 and a mobility of 1600cm2/V-s. HEMT devices were fabricated with a 0.2-um T-gate, 2-um source-todrain spacing, and 750Å SiN passivation. Peak transconductance calculated from the DC transfer curve and cutoff frequency (fT) extracted from s-parameters are 285mS/mm and 75GHz, respectively. Figure 2 shows the device NFMIN for both 200um and 500um width GaN devices. Minimum noise figures (NFMIN) measured from a 4-finger 500um device at 2GHz and 8GHz are ~0.3 and ~0.6dB, respectively at room temperature. A 0.15um x 200um GaAs PHEMT with similar fT fabricated in the same foundry achieves an NFmin of 0.4dB and 0.5dB, respectively. On-wafer noise parameter testing was not available over temperature. Figure 3 shows a photograph and schematic of the GaN MMIC LNA-PA. The chip size is 1.7mm2. The design consists of a common-source amplifier topology with series inductive source feedback to achieve an optimum 50ohm noise match. The inductance was realized by a microstrip transmission line seen in the chip photograph. RC feedback is also used in order to tailor the gain bandwidth, stability, and output return-loss response across a wide bandwidth. A total device periphery of 1.2mm width is used. The periphery was optimized to facilitate broadband noise match as shown in Γopt 8 GHz 1 GHz 8 GHz S111 GHz Figure 4 – Device periphery optimization for noise. Figure 4, as well as to handle the targeted +2Watt output power. ADS simulations were carried out and the source inductance and device size/bias were further tuned to achieve a minimum of 10dB of input return loss in order to maximize amplifier gain and minimize noise figure across an 8GHz bandwidth at a bias of 15V and 400mA. A target P1dB of 2Watts and sub-dB NF were the general goals. III. MEASURED PERFORMANCE The GaN HEMT LNA design of this work had been previously RF characterized at room temperature and reported in [6]. Here we present the lower temperature RF characteristics and compare the results with the room temperature performance. Noise figure and s-parameters measurements have been taken at various temperatures. Temperature measurements were taken within a few minutes 0 T = -30C 2 1 2 3 4 5 Frequency (GHz) 3 4 5 6 7 Gain (dB) Noise Figure (dB) 8 Gain & NF over Temperature 12V/200mA 20 18 16 14 12 10 8 6 4 2 0 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Gain @ -30C Gain Gain @ 25C NF @ -30C NF @ 25C T = 25C NF NF ~ 0.1-0.2 dB T = -30C 2 3 4 5 6 7 Frequency (GHz) Figure 8 – Noise figure and associated gain at a medium bias of 12V-200mA for T=-30ºC and 25ºC. -30 0 NF ~ 0.25-0.45 dB NF 1 S22 -20 T = 25C Figure 7 – Noise figure and associated gain at a power bias of 15V-400mA for T=-30ºC and 25ºC. S11 -10 NF @ 25C Frequency (GHz) S21 10 NF @ -30C 6 7 Gain & NF over Temperature 5V/200mA 8 Noise figure and associated gain were measured at T=25ºC and -30ºC for high power (15V/500mA), medium (12V/200mA), and low-noise (5V/200mA) biases. On-wafer thru calibration indicates a noise and loss uncertainty of +0.02/-0.05dB across the band excluding an anomalous +0.08dB glitch at 3GHz. The amplifier noise figure measurements at T=25ºC and T=-30ºC are shown in Figures 7, 8, and 9. At all three biases the noise figure reduced by as much as 0.4dB by cooling the baseplate carrier of the amplifier to -30ºC. The dramatic noise reduction is believed to be a result of a reduction in access resistance (RD, RS) as the amplifier is cooled. This access resistance has been shown to dominate the extrinsic noise temperature dependent characteristics of GaN HEMT transistors [7]. At high power bias the average noise figure is between 0.25-0.45dB across a 2-8GHz multi-octave band. At medium bias the average NF is 0.1-0.2dB and at low-noise bias the average NF is a remarkable 0.05-0.15dB across the band. These are impressive results considering the modest low temperature cooling of the amplifiers. As far as we know, these are the lowest noise GAN amplifiers ever demonstrated and may be the lowest noise fully-MMIC matched LNAs operating over a multi-octave bandwidth and providing > 1Watt of power. Gain (dB) Figure 6 – Medium bias (12V/200mA) S-parameters at T=25ºC, 0ºC, and -30ºC. 20 18 16 14 12 10 8 6 4 2 0 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Gain @ -30C Gain @ 25C Gain NF @ -30C NF @ 25C T = 25C NF NF ~ 0.05-0.15 dB T = -30C 1 2 3 4 5 6 Frequency (GHz) 7 Noise Figure (dB) Gain, Return-Loss (dB) 20 Gain @ 25C Gain 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 Noise Figure (dB) Vdd= 12V, Idd= 200mA Temperature= -30C, 0C, 25C Gain @ -30C 1 Gain (dB) of each other using the same calibration. This is to ensure an apples-to-apples comparison within the same test calibration. For small-signal s-parameter measurements, we took data over temperature for medium-bias (12V/200mA) and ultralow noise bias (5V/200mA) conditions at chuck temperatures of T=25ºC, 0ºC, and -30ºC. The medium-bias s-parameters are shown in figure 6. At the medium bias, close to where the design was optimized, the amplifier achieves roughly 10dB return-loss or better, and gain greater than 10dB up through 5GHz. At this frequency the gain changes by 0.22dB from 25ºC to -30ºC, corresponding to a low temperature coefficient of -0.004dB/C. This is about 50% smaller than the typical 0.01dB/ºC for GaAs PHEMTs. At the low-noise bias, the amplifier gain and return-loss match degrades slightly, but preserves its excellent temperature coefficient. Gain & Noise Figure 15V/400mA 20 18 16 14 12 10 8 6 4 2 0 8 Figure 9 – Noise figure and associated gain at a low noise bias of 5V-200mA for T=-30ºC and 25ºC. What is impressive about these record NFs is the linear output power that this LNA amplifier can achieve, providing P1dB > 0.25Watt, > 1.6Watts, and 2Watts at the respective high-, medium-, and low-noise biases. Figure 10 shows the cool T=-30ºC P1dB and PAE performance of the MMIC at the three bias levels. The power characteristics at T=25ºC is within a few tenths of a dB and is not shown. At the time of this writing, some of the measurements were frequencylimited by drive capability of our test equipment. At high power bias the P1dB~33dBm (2Watts) with a PAE of 2428%. The saturated power is ~3Watts. At medium bias the P1dB~32dBm with an associated PAE ~32-36%. The corresponding saturated power is > 2Watts. At low-noise bias the P1dB is > 24dBm with a PAE~32%. These are P1dB (dBm) 30 ~33 dBm (15V) ~32 dBm (12V) 20 ~32-36% (12V) 15 PAE 10 5 ~28% (15V) 70 45 Pout = -30C 40 Gain T=-30C 35 PAE T=-30C Pout = 25C 60 > 24 dBm (5V) 25 50 50 40 ~32% (5V) P1dB=(T=-30C, 5V/200mA) 30 P1dB (T=-30C, 12V/200mA) P1dB (T=-30C, 15V/400mA) 20 PAE (T=-30C, 5V/200mA) PAE (T=-30C, 12V/200mA) 10 Pout (dBm), Gain(dB), PAE(%) P1dB 35 Pout @ 2 GHz 12V/200mA 80 PAE @ P1dB (%) P1dB (dB) & Linear PAE Temperature = -30C 40 PAE (T=-30C, 15V/400mA) 0 1 2 3 Frequency (GHz) 4 PAE Pout Gain T= 25C 25 PAE T= 25C 20 Gain 15 10 5 0 0 0 30 P1dB = 32.2 dBm PAE@P1dB= 35% 1.35 W/mm @ P1dB -20 5 -15 -10 -5 0 5 10 15 20 25 Pin (dBm) Figure 10 – P1dB and PAE at a cool temperature of -30ºC. Figure 11 – P1dB, PAE, & Gain for power bias of 12V-200mA operating at T=-3º0C and T=25ºC. remarkable power levels for a MMIC which also can achieve sub-0.2dB NF. Figure 11 gives the detailed output power characteristics at a medium bias of 12V-200mA for both T=25ºC and T=-30ºC. The measurement shows that there is very negligible change in the output characteristics by cooling it down from 25ºC to 30ºC. The P1dB is 32.2dBm with a PAE of 35% and a modest power density of 1.35W/mm. The saturated power is > 2Watts. Table 1 gives a summary of the performance of this work implications to future high performance linear front-end systems which require ultra-wide bandwidth, sensitivity, and dynamic range. ACKNOWLEDGEMENT The authors wish to acknowledge the key contribution of Tony Sellas for on-wafer RF characterization, and the support and assistance of Curtis Kitani, B. Bayuk, J. Johnson, and J. Ocampo. The authors also recognize the late Dr. Barry R. Allen of TRW/NGST for inspiring the pursuit of ultra-low noise amplifiers for space applications. Table 1 REFERENCES Summary of GaN MMIC LNA Performance (S-, C-band) Reference Ellis et.al. H. Xu et.al. S. Cha, et.al. Shih, et.al. Temperature (C) Noise Figure (dB) P1dB (dB) PAE (%) 25 2.4 25 1.6 12.8 25 1.5 25 1.5 20 < 25 dBm [5] MV. Aust, et.al. 25 1.03 (Psat) [6] Kobayashi, et.al. 25 0.75-0.9 32.9 29 -30 0.25-0.45 32.8 25 This Work Kobayashi, et.al. -30 0.1-0.2 32.2 35 -30 0.05-0.15 24.8 33 [1] [2] [3] [4] with respect to other GaN S-, C-band MMIC LNAs. This work achieves the best results to date on GaN LNA noise performance. World record noise figure has been recorded at modestly cool temperatures while providing 2Watts of output power. IV. Conclusion What is exceptional about this work is that it reveals both the low noise and high power capability of the GaN HEMT device technology. It also shows we can achieve an incredible amount of improvement in noise figure at modestly cooled temperatures. 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