Thick-Film BST-Based Tunable Matching Network. Sebastian Preis1, Alex Wiens2, Enrico Lia3, Wolfgang Heinrich1, Rolf Jakoby2,. Holger Maune2, Olof ...
A High-Efficiency GaN Transistor Module with Thick-Film BST-Based Tunable Matching Network Sebastian Preis1, Alex Wiens2, Enrico Lia3, Wolfgang Heinrich1, Rolf Jakoby2, Holger Maune2, Olof Bengtsson1 1
Ferdinand-Braun-Institut, Leibniz-Institut fuer Hoechstfrequenztechnik, 12489 Berlin, Germany Technische Universitaet Darmstadt, Institut fuer Mikrowellentechnik und Photonik, 64283 Darmstadt, Germany 3 European Space Agency, ESTEC, Keplerlaan 1, 2200AG Noordwijk, The Netherlands
2
Abstract — Thick-film barium-strontium-titanate varactors package-integrated with GaN HEMTs are high-efficiency and robust tunable devices that enable frequency agility and efficiency optimization. The tunable transistors presented here achieve 44.4 dBm output power, a peak PAE of 77% and a PAE configurability of 5.6 percentage points. Tunability in saturation remains almost constant for temperature and frequency sweeps from 20 to 80°C and 1.5 to 2.45 GHz, respectively. LTE and WCDMA measurements of the modules used in a power amplifier show good linearity results with -45 dBc ACLR for signals with a high peak-to-average power ratio of 9 dB, no degradation due to the varactors is observed. Keywords — Barium-Strontium-Titanate (BST), Ferroelectric films, Gallium nitride (GaN), HEMTs, Power amplifiers, Power transistors, Tunable circuits, Varactors.
I. INTRODUCTION The ferroelectric ceramic barium-strontium-titanate (BST) is a promising material to realize reconfigurable RF circuits [1]. Compared to other technologies providing reconfigurability, such as micro-electro-mechanical systems [2] and semiconductor based varactors [3], BST offers high linearity and power handling [4] as well as high frequency capability [5]. Tunable devices based on BST can become key components for energy efficient and electronically configurable power amplifier (PA) systems, which is the reason why they have recently been given much attention [1],[5]. Additional applications besides load modulation are frequency agility and protection against mismatched loads, e.g. in case of high VSWR conditions [6]. This article presents reconfigurable GaN transistors integrated into a transistor package together with a new generation of thick-film BST varactors. Using modulated wideband signals up to 20 MHz, the devices show remarkable linearity results meeting the specifications for various telecom standards at 1.8 GHz. This was possible only through recent advances in the BST varactor design and fabrication [7]. An improved understanding of the thermal effects has been obtained by analyzing the materials used and performing thermal simulations, validated by thermo-reflectance measurements. Consequently, the insertion loss of the transistor module, which was in the range of 3 dB in early
work based on inter-digital structures [8], has been reduced with the new thick-film metal-insulator-metal (MIM) BST varactors to merely 0.5 dB. This tunable pre-matching (TpM) module utilizes a complete 10 mm gate width powerbar and provides more than 27 W of output power with record efficiency results for tunable transistors of 77% PAE. The transistor modules are characterized in terms of efficiency, linearity, tunability as a function of power back-off (BO) and frequency. II. THERMAL LIMITATIONS Ferroelectric materials are temperature dependent and BST is no exception, self-tuning effects occur when temperature changes. To estimate the power handling capability of BST thick-film varactors, first, the thermal conductivity of the thick-film layer needs to be determined. The light flash method [9] is used for this purpose here. The initial sample temperature is monitored with a thermocouple. The sample is exposed to a high-power laser pulse, which heats the backside of the sample, while the top-side temperature is monitored with a detector. From the transient behaviour of the detected temperature the thermal diffusivity α(T) and the thermal capacity cp(T) can be extracted. Knowing the specific density ρ(T) of the sample, the thermal conductivity κ(T) can be extracted as: (T) (T) (T) c p (T) (1) Since the BST thick-film sample consists of a 635µm thick Al2O3 carrier substrate coated with a 4 µm thick BST layer, three samples of each material were measured, pure BST bulk ceramic, pure Al2O3 ceramic, pure aluminum nitride ceramic and Al2O3 ceramic coated with 4 µm BST. Using the equivalent resistor model of layered substrates [10] the thermal conductivity of the BST layer is extracted as: 4 m BST (T) (2) 639 m 635 m
BST / Al O
2 3
Al O
2 3
Fig. 1a) summarizes the thermal conductivity of each sample. The thick-film layer has a factor 10 lower thermal conductivity compared to BST bulk sample due to higher porosity.
(a)
(b)
(c)
Fig. 1. Measured thermal conductivity κ of AlN, Al2O3, bulk BST and BST-coated Al2O3 substrate and calculated thermal conductivity of the BST layer (a), shaded area denotes the uncertainty. Temperature gradient simulation results on Al2O3 carrier substrate (b) with κ = 25 W/(m∙K) and AlN substrate (c) with κ = 200 W/(m∙K)
With the extracted thermal conductivity, a 3D model of a BST MIM varactor was set up for different carrier substrate configurations and RF simulated. With the calculated field and current distribution, a thermal co-simulation of the component was performed. At an RF power of approximately 1 W the temperature of the BST layer on the Al2O3 substrate increases by ΔT = 36 K, while on AlN the temperature rises by ΔT = 20 K as can be seen in Fig. 1b), c). Even though the thermal conductivity of the carrier substrate increased by a factor 10, the resulting temperature difference ΔT is reduced a factor less than 2. Introducing the constraint that the temperature of the BST layer must not increase more than 10 K due to thermally induced permittivity change, the rated power handling capability Prated of the 50 × 50 µm² MIM varactor can be given as 110 W/mm² on Al2O3 and 200 W/mm² on AlN, respectively. Hence, in order to achieve maximum power handling capability, AlN substrates were used for the fabrication of the varactors. Further, the topology was designed such that the thermal heat sources are separated, leading to a saw-tooth like varactor shape [4].
III. RECONFIGURABLE MODULE Because of the π-configuration shown in Fig. 2a), DC could be supplied by external broadband bias tees. Load-pull measurements were performed to identify the optimum P out and PAE impedance of the module depicted in Fig. 2b). Fig. 3 shows the power added efficiency (PAE) of the TpM module at 2 GHz for a load impedance of ZL=9.0-j16.1 Ω. At this load the optimum shape for tunability is achieved. A diagonal combination of voltage combinations for Vvar1 and Vvar2 results in a maximum PAE for saturation (30 dBm) and a minimum for the BO state at 24 dBm Pin. This impedance was chosen because of the promising opportunity for load modulation of the PA given by such a PAE contour shape. A PAE improvement of 5.6% points is achieved. The overall highest measured power and PAE values for this type of TpM module were 44.4 dBm and 77%, respectively. The covered impedance space is shown in Fig. 4. A slight deviation between simulation and measurement was caused by higher bond-wire inductances than the simulation predicted.
(a)
(b)
(c)
Fig. 2. 3D model of the π-network, 5-cell GaN HEMT (green), two varactors (beige) and the bond-wires spanned over the varactors (a), a photograph of the manufactured module (b) and the power amplifier including the discrete level modulator (c).
PAE (%)
66 67 65 65
63 200
64 150
100
50
Uvar2 (V)
0
0
100
50
150
33
34
PAE (%)
63
Uvar1 (V)
32.5 32
30 200
32 31.5 150
100
50
Uvar2 (V)
0
0
100
50
150
31
Uvar1 (V)
Fig. 3. Measured PAE at 2 GHz, Vdd=28 V, Idq=400 mA, Pin at 30 dBm (top) and 24 dBm (bottom), ZL=9.0-j16.1 Ω.
A. Frequency adaptability Due to the low-pass characteristic of the reconfigurable network, a wideband solution is possible with the device. Table 1 presents PAE and Pout of a TpM module at frequencies between 1.5 and 2.45 GHz. With increasing frequency, the overall performance of the TpM modules degrades. Interesting is the higher tunability at 2.45 GHz compared to lower frequencies, especially in the BO case. This can be explained by the fact, that with operation close to the self-resonance of the varactors their permittivity variation increases. In contrast, the output power in saturation and under BO power is almost constant with frequency. B. Temperature dependency Confirming the results of the thermal simulation, thermoreflectance measurements [11] show that the temperature of the top metal plate is significantly higher than for the surrounding BST layer. The heat distribution within a MIM varactor during 50 ms CW pulses with 43 dBm peak power at 2 GHz, 20°C baseplate temperature and a duty cycle of 1:3 is plotted in Fig. 5. The measured varactor has a surface of 34 × 50 µm × 50 µm and was printed on Al2O3. With a power value in the range of the simulation, the measured rise of temperature ΔT of 39 K matches the expected increase in temperature. As the heat is generated inside the porous
Fig. 4. Measured and simulated impedances coverd by the tunable module at 2 GHz, Vdd=28 V, Idq=400 mA, Pin at 30 dBm and 24 dBm.
sintered BST with poor heat transfer, the thermal conductivity of the substrate has only a limited effect and CW power measurements show comparable performance for structures on both substrates. Thus the simulations explain why the sawtooth design of the varactor spreads the heat less than expected on either Al2O3 or on AlN samples. Table 2 compares the variation in tunability of the TpM module with temperature, which seems to be significant only for BO operation. The remaining tunability at 80°C and 40 dBm output power is less than half of that for the 17°C measurement. Since the tunability in saturation increases slightly, the temperature stability is not only related to the varactor tunability change, but also to the impedance variation of the transistor which seems to be more pronounced at BO power levels. C. PA design To verify functionality and linearity under modulated conditions a PA using one of the manufactured TpM modules was fabricated. PA and discrete level modulator [12] for the modulated measurements are shown in Fig. 2c). The design frequency was 1.8 GHz. A 0.8 mm thick RO4003C substrate was used to realize microstrip matching networks to match the source and load impedance of the TpM module to 50 Ω. D.
Modulated measurements
TABLE 1. Output power and PAE over frequency
f (GHz) 1.5 1.8 2.0 2.14 2.45
PAE
PAE
PAE
PAE
max,sat
min,sat
max,BO
min,BO
(%) 48.7 46.9 51.7 46.4 46.7
(%) 47.4 45.0 49.2 42.6 41.7
(%) 73.9 65.0 72.2 64.1 56.4
(%) 70.9 61.6 68.7 59.6 50.8
Pout,sat (dBm)
Pout,BO (dBm)
43.1 42.4 43.1 42.9 42.8
39.9 39.4 40.1 39.3 40.3
Measurements were taken at different load impedances, Vdd=28 V, Idq=400 mA.
Fig. 5. Temperature distribution of a BST varactor on Al2O3 at 2 GHz, with 43 dBm CW pulses of 50 µs, measured by thermo-reflectance, baseplate temperature at 20°C.
TABLE 2. Output power and PAE over frequency
Temp (°C) 17 40 60 80
PAE
PAE
PAE
PAE
max,sat
min,sat
max,BO
min,BO
(%) 51.7 50.1 47.8 46.2
(%) 49.2 47.9 46.9 45.3
(%) 72.2 68.9 68.5 67.3
(%) 68.7 64.7 64.5 63.5
TABLE 3. Output power and PAE over frequency BW (GHz)
ACLR (dB)
Pout,sat (dBm)
Pout,BO (dBm)
Signal
43.1 42.8 42.7 42.7
40.1 39.9 39.7 39.3
Wideb. CDMA
5
-33.0
LTE LTE LTE
5 10 20
-35.0 -34.8 -32.1
Measurements were taken at Vdd=28 V, Idq=400 mA, ZL=4.2-j11.9 Ω, f=2.0 GHz.
The manufactured PA was tested under modulated conditions to proof linearity also for high envelope bandwidth and PAPR. Centre frequency of the measurement was 1.81 GHz. For validation a WCDMA signal with 5 MHz bandwidth and 6.5 dB PAPR was used, as well as an LTE equivalent signal with 9 dB PAPR and various bandwidths. The results of these measurements are summarized in Table 3. The average output power level was chosen in each case to allow a resulting adjacent channel leakage ratio (ACLR) after digital pre-distortion (DPD) of -45 dB or less. For both signal types the resulting peak power was in the range of 44 dBm, which corresponds to a significant compression level. As DPD, an algorithm implementing iterative learning control was used [13]. Other methods, e.g., a Parallel-Hammerstein model showed slightly less linearity improvement. Remarkable is the improvement in case of a WCDMA signal. Despite the higher average power level, which would imply more spectral regrowth, the ACLR after DPD improved by 16.4 dB and the normalized mean square error (NMSE) decreased from -24.6 dB to -42 dB. This is probably due to the higher PAPR of the LTE signal. IV. CONCLUSION A tunable pre-matching (TpM) GaN-HEMT module using advanced thick-film metal-insulator-metal barium-strontiumtitanate (BST) varactors is presented, with an insertion loss of ≤0.5 dB and a power handling of >50 W. It allows for frequency agility and efficiency optimization from saturation to BO. With an output power of 44.4 dBm, a peak PAE of 77% and a PAE improvement thanks to the varactor tunability of 5.6 percentage points, excellent performance is demonstrated. The known weakness of BST regarding temperature dependence has been studied thoroughly and was significantly reduced by careful material composition, varactor layout and module design. The presented transistor module was then used to realize a complete PA which, for the first time, allows to demonstrate the performance for modulated measurements. The results verify usability of the concept. The tunable transistor modules can be used in transmitters with multi-band capability, eliminating the necessity of multiple transmitters and thus reducing system cost. Since GaN is known to be robust against radiation and ceramics are expected to fulfill this condition, too, the TpM modules are interesting not only for terrestrial, but also for spaceborne
ACLR
NMSE,
PAE,avg (%)
Pout,avg (dBm)
-49.4
26.4
38
-42.0
-46.0 -44.6 -46.8
18.0 18.1 17.5
35 35 35
-39.1 -37.9 -40.6
,DPD
(dB)
DPD
(dB)
applications, where weight reduction and energy efficiency are key features as well. ACKNOWLEDGMENT This work was supported by ESA ESTEC as an Innovation Triangle Initiative (ITI) project under contract number No. 4000111787/14/NL/SC. Further acknowledged are the KIT (IAM-WPT) for preparation of BST layers as well as Microsanj LLC and BSW AG for providing the thermoreflectance measurement equipment. REFERENCES [1] C. Kong, H. Li, X. Chen, S. Jiang, J. Zhou and C. Chen, “A Monolithic AlGaN/GaN HEMT VCO Using BST Thin-Film Varactor,” IEEE Trans. Microw. Theory Techn., vol. 60, no. 11, pp. 3413-3419, Nov. 2012. [2] G. Kahmen, M. Wietstruck, M. Kaynak, B. Tillack and H. Schumacher, “Static and dynamic characteristics of a MEMS Varactor with broad analog capacitive tuning range for wideband RF VCO applications,” in European Microw. Conf., Paris, 2015, pp. 1011-1014. [3] H. M. Nemati, C. Fager, U. Gustavsson, R. Jos and H. Zirath, “Design of Varactor-Based Tunable Matching Networks for Dynamic Load Modulation of High Power Amplifiers,” IEEE Trans. Microw. Theory Techn., vol. 57, no. 5, pp. 1110-1118, May 2009. [4] S. Preis et al., “Discrete RF-power MIM BST thick-film varactors,” in European Microw. Conf., Paris, 2015, pp. 941-944. [5] R. De Paolis, S. Payan, M. Maglione, G. Guegan and F. Coccetti, “High-Tunability and High- Q-Factor Integrated Ferroelectric Circuits up to Millimeter Waves,” IEEE Trans. Microw. Theory Techn., vol. 63, no. 8, pp. 2570-2578, Aug. 2015. [6] S. Preis et al., "Response time of VSWR protection for GaN HEMT based power amplifiers," in European Microw. Conf., London, 2016, pp. 401-404. [7] C. Kohler et al., “Effects of ZnO–B2O3 Addition on the Microstructure and Microwave Properties of Low-Temperature Sintered Barium Strontium Titanate (BST) Thick Films,” Int. J. Appl. Ceramic Technol., vol. 10, no. S1, pp. E200-E209, Sept./Oct. 2013. [8] O. Bengtsson et al., “Discrete tunable RF-power GaN-BST transistors,” in European Microw. Conf., Amsterdam, 2012, pp. 703-706. [9] W. J. Parker, R. J. Jenkins, C. P. Butler, and G. L. Abbott, “Flash method of determining thermal diffusivity, heat capacity, and thermal conductivity,” J. Appl. Physics, vol. 32, no. 9, pp. 1679-1684, 1961. [10] J.R. Anderson, E. Charles, Donald E. Ketchum, and William P. Mountain. “Thermal conductivity of intumescent chars.” J. of Fire Sciences, vol. 6, no. 6, pp. 390-410, 1988. [11] K. Yazawa, D. Kendig, A. Shakouri, “Time-Resolved Thermoreflectance Imaging for Thermal Testing and Analysis,” in Int. Symp. Testing Failure Analysis, San Jose, CA, Nov. 2013, pp. 194-202. [12] N. Wolff; W. Heinrich; O. Bengtsson, “100-MHz GaN-HEMT Class-G Supply Modulator for High-Power Envelope-Tracking Applications,” IEEE Trans. Microw. Theory Techn., vol. 65, no. 3, pp. 872-880, Mar. 2017. [13] A. Hyo-Sung, C. Yang Quan, and K. L. Moore, “Iterative Learning Control: Brief Survey and Categorization,” IEEE Trans. Syst. Man, Cybern. C, vol. 37, no. 6, pp. 1099-1121, Nov. 2007.
