Chang, Da-Chiang

Chang, Yin-Cheng

Hsu, Shawn SH

Huang, Yi

Liu, Yeke

Wang, Jiayou

In this letter, a monolithic microwave integrated circuit (MMIC) cross-coupled oscillator with high operating frequency and high output power is proposed using 0.25-µm gallium nitride (GaN) high electron mobility transistor (HEMT) technology. The coupling capacitors in the core circuit are analyzed to obtain high output power with a suitable tank inductor. The π-type matching network is adopted to effectively extract the fundamental output signal. Also, a flip-transistor layout with a shared back via is proposed for reduced interconnect loss and a compact layout. With a chip area of only 0.71 mm2, the measured results demonstrate a maximum output power of 16 dBm at 24.3 GHz, and a phase noise (PN) of −137.9 dBc/Hz at a 10-MHz offset

University of Liverpool Repository

IEEE MICROWAVE AND WIRELESS TECHNOLOGY LETTERS 1A K -Band MMIC Cross-Coupled Oscillator WithHigh Output Power in 0.25-µm GaN HEMTJiayou Wang , Graduate Student Member, IEEE, Yi Huang , Fellow, IEEE, Yin-Cheng Chang , Member, IEEE,Yeke Liu, Da-Chiang Chang , Member, IEEE, and Shawn S. H. Hsu , Member, IEEEAbstract— In this letter, a monolithic microwave integratedcircuit (MMIC) cross-coupled oscillator with high operatingfrequency and high output power is proposed using 0.25-µmgallium nitride (GaN) high electron mobility transistor (HEMT)technology. The coupling capacitors in the core circuit are ana-lyzed to obtain high output power with a suitable tank inductor.The π -type matching network is adopted to effectively extractthe fundamental output signal. Also, a flip-transistor layout witha shared back via is proposed for reduced interconnect loss and acompact layout. With a chip area of only 0.71 mm2, the measuredresults demonstrate a maximum output power of 16 dBm at24.3 GHz, and a phase noise (PN) of −137.9 dBc/Hz at a10-MHz offset.Index Terms— Gallium nitride (GaN), high electron mobilitytransistor (HEMT), K -band, monolithic microwave integratedcircuit (MMIC), oscillator.I. INTRODUCTIONGALLIUM nitride (GaN) high electron mobility transistor(HEMT) has received great attention recently due to itscapability of high speed and high power operation [1]. Basedon GaN HEMT technology, power amplifiers (PAs) with goodperformance have been reported [2]. Utilizing GaN HEMTsfor high power oscillator design is also of great interest [3].The oscillator is an essential block in many microwave systemsfor different applications such as radars, communications,imaging, and sensing. With advanced GaN HEMTs of thegate length down to 0.1 µm or even 60 nm, oscillatorswith operating frequencies up to W -band and G-band havebeen reported [4], [5]. However, most of the reported worksare with relatively low frequencies in the range of severalgigahertz [6], [7], [8] using the 0.25-µm GaN technology,which could be attributed to the relatively low fT and fmax ofManuscript received 26 January 2023; revised 28 March 2023;accepted 24 April 2023. This work was supported by the National Sci-ence and Technology Council (NSTC) under Contract 110-2222-E-007-003-MY3 and Contract 111-2218-E-110-002. (Corresponding authors:Shawn S. H. Hsu; Yi Huang.)Jiayou Wang is with the Institute of Electronics Engineering, National TsingHua University, Hsinchu 30013, Taiwan, and also with the Department ofElectrical Engineering and Electronics, University of Liverpool, L69 3GJLiverpool, U.K.Yi Huang is with the Department of Electrical Engineering and Electronics,University of Liverpool, L69 3GJ Liverpool, U.K.Yin-Cheng Chang and Da-Chiang Chang are with the Taiwan Semiconduc-tor Research Institute (TSRI), Hsinchu 300091, Taiwan.Yeke Liu and Shawn S. H. Hsu are with the Department of ElectricalEngineering and Institute of Electronics Engineering, National Tsing HuaUniversity, Hsinchu 30013, Taiwan (e-mail: shhsu@ee.nthu.edu.tw).Color versions of one or more figures in this letter are available athttps://doi.org/10.1109/LMWT.2023.3271989.Digital Object Identifier 10.1109/LMWT.2023.3271989Fig. 1. Circuit topology of the proposed cross-coupled GaN oscillator.the transistors, design limitations of inductive elements, andrestricted circuit topology and layout.In this letter, we report a K -band oscillator monolithicallyintegrated using 0.25-µm GaN HEMT technology. Analysisand design considerations are described in Section II, andsimulated and experimental results are reported in Section III.With a cross-coupled topology, π -type output matching, andflipped-transistor layout, the proposed oscillator achieves thehighest operating frequency at the fundamental tone with ahigh output power compared with prior works using similartechnology. The obtained figure of merit (FoM) are also amongthe best.II. OSCILLATOR DESIGNThe technology adopted in this design is WIN semicon-ductor 0.25-µm GaN-on-SiC HEMTs [9]. The AlGaN/GaNtransistor incorporates a T-gate, which utilizes the lift-offprocess to reduce gate resistance [10]. The Si3N4 film isemployed as the dielectric material for different metal layersand the capacitor. A thick top metal layer by Au (metal 2)of 4 µm is available, which can be used to design low-losstransmission lines and spiral inductors. The SiC substrate isthinned down to 100 µm and with vias connected to thebackside metal. More process details can be found in [9]. It isworth mentioning that the process utilizes a source-coupledfield plate to provide a breakdown voltage exceeding 120 Vrequired for reliable high drain bias operation. The nominalfT and fmax are 25 and 75 GHz, respectively.Fig. 1 shows the details of the proposed cross-coupledoscillator. The two GaN HEMTs T1 and T2 are cross-coupledby Cc1 and Cc2, which also act as dc blocking capacitors.RB1 and RB2 are used to provide negative bias voltage for thetransistors. L1 and L2 are the equivalent inductors, combiningthe resonant tank inductance and also the output-matching2771-957X © 2023 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See https://www.ieee.org/publications/rights/index.html for more information.This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. Authorized licensed use limited to: University of Liverpool. Downloaded on May 17,2023 at 08:02:59 UTC from IEEE Xplore.  Restrictions apply. 2 IEEE MICROWAVE AND WIRELESS TECHNOLOGY LETTERSFig. 2. (a) Simplified oscillator core circuit. (b) Real and imaginary parts ofZIN versus frequency with Cc = 50 f F.Fig. 3. (a) Simulated ZIN. (b) Tank inductor LT and output power POUTversus Cc at 25 GHz.inductance. The capacitors CM1 −CM4 construct the equivalentπ output matching network together with part of L1 and L2.The RC networks including RBP and CB are employed forbypass networks.A. Design of Cross-Coupled PairThe transistor periphery used in the design is 2 × 75 µmconsidering the trade-off between the output power level andthe desired operating frequency according to fT and fmax.Fig. 2(a) shows the simplified oscillator core circuit, includinga cross-coupled pair, coupling capacitors, and an equiva-lent resonant tank inductor LT. The design of the couplingcapacitor Ce is critical to the oscillator characteristic whichcan be seen from ZIN looking into the cross-coupled pair.Fig. 2(b) shows the simulated real and imaginary parts of ZINas a function of frequency. The real part Re(ZIN) becomesless negative as frequency increases. Also, the imaginarypart Im(ZIN) is mostly negative, which indicates a capacitiveimpedance of ZIN.Fig. 3(a) shows the simulated ZIN as a function of Cc atthe center frequency of 25 GHz. As can be seen, the negativeresistance disappeared if Cc is very small since T1 and T2 areno longer cross-coupled. On the other hand, Re(ZIN) becomesless negative if Cc increases gradually after a certain point.The value of Cc used in the final design is approximately60 fF for obtaining maximum negative resistance. It should bementioned that Im(ZIN) becomes more negative monotonicallywith reduced Cc. This indicates a relatively small equivalentcapacitance of ZIN with small Cc. As a result, a largerinductor is allowed for the LC tank of the oscillator evendesigned at high frequency. This is beneficial for practicaldesign, especially using the adopted GaN technology. Differentfrom using a typical CMOS process, small inductors could bechallenging to implement considering the relatively large sizeof GaN transistors. Increasing the required inductance for theLC tank can ease the design of the circuit layout. Based onthe final design, Fig. 3(b) shows how the required equivalentFig. 4. Simplified small-signal model of the proposed GaN oscillator.tank inductor LT and output power POUT change as a functionof Cc. At Cc of ∼60 fF, POUT is close to the maximum value,and the corresponding tank inductance is ∼600 pH.B. Design of Output Matching NetworkUsing the output buffer stage is a typical approach forconnecting the output to the 50  load in high-frequencyoscillator design, which is usually seen in CMOS-based RFICswith a cross-coupled topology [11]. However, this is unsuitableif using GaN technology considering the large transistor sizeand the area-consuming through wafer vias (TWV). The bufferstage also requires a considerable amount of extra dc power.In the proposed design, the π -type matching network isemployed for matching the circuit to 50  load. Fig. 4shows the small-signal equivalent circuit of the oscillatorincluding the matching network CM1–CM4 and LM1–LM2.The design starts with establishing the small-signal modelof the transistor under the chosen biasing conditions. In thesmall-signal model, the drain–source resistance Rds extractedbased on the S-parameters is employed to design output-matching networks. Once oscillation starts, the reactive part isresonated out, and the matching network is mainly designedby converting Rds to the 50- load. As illustrated in Fig. 4, theπ -matching network composed of CM1–CM2 (CM3–CM4) andLM1(LM2) is designed as wideband matching for maximumpower transfer. Note that the circuit is differentially operatedat the fundamental frequency, the center of LT is a virtualground. Thus, in practical implementation, only one inductorL1 (L2) is required (see Fig. 1) on each side which combinesLM and LT/2, as expressed in (1). Note that L1 and L2 alsoserve as the paths where dc power is fed to the transistorsL1 =LMLT2LM + LT. (1)In our final design, L1 and L2 are both 270-µm long and35-µm wide. Based on EM simulation, the correspondinginductance and quality factor (Q) are 160 pH and 56, respec-tively, at 25 GHz. Compared with the typical CMOS processon a silicon substrate, a relatively high-quality factor can beobtained for the adopted GaN HEMT technology due to thelow-loss SiC substrate and the thick metal layer. We alsosimulate the phase noise (PN) with varying values of Q byadjusting the geometry of L1 and L2 while maintaining aconstant frequency. The results show that the PN varies from−117.7 dBc/Hz (Q = 26) to −119 dBc/Hz (Q = 70) ata 1 MHz offset. The PN can be improved with increased Q asexpected. Despite this, the PN is not a strong function of theThis article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. Authorized licensed use limited to: University of Liverpool. Downloaded on May 17,2023 at 08:02:59 UTC from IEEE Xplore.  Restrictions apply. WANG et al.: K -BAND MMIC CROSS-COUPLED OSCILLATOR WITH HIGH OUTPUT POWER 3TABLE IPERFORMANCE COMPARISON WITH PRIOR WORKS USING 0.25-µm GaN HEMT TECHNOLOGYFig. 5. (a) Chip micrograph of the proposed GaN oscillator. (b) Simulatedand measured oscillating frequency and POUT versus VGS.Fig. 6. Measured results of the GaN HEMT oscillator. (a) Output spectrum.(b) PN.Q of L1 and L2 since these inductors are part of the outputmatching network. The overall quality factor of the matchingnetwork has a more significant influence on the oscillator PN.III. EXPERIMENTAL RESULTSFig. 5(a) shows the chip micrograph of the proposed GaNcross-coupled oscillator. The chip size is 0.71 mm2 includingthe testing pads. It should be emphasized that a typical layoutof cross-coupled transistors needs relatively long intercon-nects, which makes it difficult for a practical layout usingthe adopted GaN technology. As indicated in the dashed-linebox in the chip photo, one transistor is flipped by 180◦ andthe cross-coupled route can form a loop to effectively reducethe length of the interconnect. Also, a TWV at the center isshared by two transistors to further reduce the circuit size.The oscillating frequency and output power of the fabricateddesign were measured on-wafer using a spectrum analyzer(Keysight N9041B UXA), and the PN was measured usinga signal source analyzer (Keysight E5052A). Fig. 5(b) showsthe comparison of the simulated and measured results ofoscillating frequency and output power which show a goodagreement in general. The relatively small discrepancy couldbe mainly attributed to the active device model, given thatrigorous full electromagnetic (EM) simulations of the entirecircuits, including the probing pads and all interconnects,have been performed. Note that one possible reason for theinaccuracy of the transistor model is the thermal effect [15],which is challenging to model precisely under different biasconditions. This also makes it more difficult to predict the Poutof the GaN oscillator. The operating frequency can be tunedfrom 23.9 to 24.4 GHz with VGS varying from −2 to −0.6 V.Fig. 6(a) shows the measured spectrum with the peak outputpower of 16 dBm at 24.3 GHz after calibration of cable loss.The measured values of PN were −109.4 and −137.9 dBc/Hzat 1 and 10 MHz, respectively, as shown in Fig. 6(b). Table Isummarizes and compares the performance of the proposedoscillator with previously reported oscillators using 0.25-µmGaN HEMT technology. To the best of our knowledge, theproposed design achieves the highest oscillation frequencywith a higher or comparable output power level based on asimilar GaN HEMT technology. The proposed design alsoobtains comparable or better FoMs [3], [6], [11] even witha higher operating frequency.IV. CONCLUSIONIn this letter, a K -band MMIC cross-coupled oscillator using0.25-µm GaN HEMT technology has been demonstrated. Thecoupling capacitors in the core circuit were analyzed anddesigned carefully to achieve high output power with a suitabletank inductor at the desired frequency. The π -type outputmatching network was employed instead of a buffer stage toextract the fundamental signal to 50  load. Also, the flippedtransistor with shared TWV of the cross-coupled pair wasproposed for reduced interconnect loss and a compact layout.The experimental results showed that the proposed oscillatorachieved the highest frequency with a high output powercompared with prior works using similar GaN technology.The results indicate the potential of GaN technology for high-power millimeter-wave signal source applications.ACKNOWLEDGMENTThe authors would like to thank the Taiwan SemiconductorResearch Institute (TSRI), Hsinchu, Taiwan, for the chipmeasurements.This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. Authorized licensed use limited to: University of Liverpool. Downloaded on May 17,2023 at 08:02:59 UTC from IEEE Xplore.  Restrictions apply. 4 IEEE MICROWAVE AND WIRELESS TECHNOLOGY LETTERSREFERENCES[1] L. Li et al., “GaN HEMTs on Si with regrown contacts and cut-off/maximum oscillation frequencies of 250/204 GHz,” IEEE ElectronDevice Lett., vol. 41, no. 5, pp. 689–692, May 2020.[2] S.-H. Li, S. S. H. Hsu, J. Zhang, and K.-C. Huang, “Design of a compactGaN MMIC Doherty power amplifier and system level analysis withX-parameters for 5G communications,” IEEE Trans. Microw. TheoryTechn., vol. 66, no. 12, pp. 5676–5684, Dec. 2018.[3] D. Kim and S. Jeon, “W- and G-band GaN voltage-controlled oscillatorswith high output power and high efficiency,” IEEE Trans. Microw.Theory Techn., vol. 69, no. 8, pp. 3908–3916, Aug. 2021.[4] R. Weber, D. Schwantuschke, P. Bruckner, R. Quay, F. van Raay, andO. Ambacher, “A 92 GHz GaN HEMT voltage-controlled oscillatorMMIC,” in IEEE MTT-S Int. Microw. Symp. Dig., Tampa, FL, USA,Jun. 2014, pp. 1–4.[5] T. N. Thi Do, Y. Yan, and D. Kuylenstierna, “A low phase noise W-bandMMIC GaN HEMT oscillator,” in Proc. IEEE Asia–Pacific Microw.Conf. (APMC), Hong Kong, Dec. 2020, pp. 113–115.[6] H. Liu et al., “Design of ultra-low phase noise and high power integratedoscillator in 0.25 µm GaN-on-SiC HEMT technology,” IEEE Microw.Wireless Compon. Lett., vol. 24, no. 2, pp. 120–122, Feb. 2014.[7] S. Lai et al., “Accurate phase-noise prediction for a balanced ColpittsGaN HEMT MMIC oscillator,” IEEE Trans. Microw. Theory Techn.,vol. 61, no. 11, pp. 3916–3926, Nov. 2013.[8] C.-C. Chuang and H.-K. Chiou, “A low phase noise X band class Epower VCO in 0.25 µm GaN/SiC technology,” in Proc. IEEE Int. Symp.Radio-Freq. Integr. Technol. (RFIT), Taiwan, Aug. 2021, pp. 1–3.[9] M.-H. Weng, C.-K. Lin, J.-H. Du, W.-C. Wang, W.-K. Wang, andW. Wohlmuth, “Pure play GaN foundry 0.25 µm HEMT technol-ogy for RF applications,” in Proc. IEEE Compound Semiconduc-tor Integr. Circuit Symp. (CSICS), Monterey, CA, USA, Oct. 2013,pp. 1–4.[10] C.-W. Tsou, C.-Y. Lin, Y.-W. Lian, and S. S. H. Hsu, “101-GHz InAlN/GaN HEMTs on silicon with high Johnson’s figure-of-merit,” IEEE Trans. Electron Devices, vol. 62, no. 8, pp. 2675–2678,Aug. 2015.[11] P.-Y. Wang, G.-Y. Su, Y.-C. Chang, D.-C. Chang, and S. S. H. Hsu,“A transformer-based current-reuse QVCO with an FoM up to−200.5 dBc/Hz,” IEEE Trans. Circuits Syst. II, Exp. Briefs, vol. 65,no. 6, pp. 749–753, Jun. 2018.[12] M. Hörberg and D. Kuylenstierna, “Low phase noise power-efficientMMIC GaN-HEMT oscillator at 15 GHz based on a quasi-lumped on-chip resonator,” in IEEE MTT-S Int. Microw. Symp. Dig., Phoenix, AZ,USA, May 2015, pp. 1–4.[13] S.-L. Jang, Y.-H. Chang, and W.-C. Lai, “A feedback GaN HEMToscillator,” in Proc. Int. Conf. Microw. Millim. Wave Technol. (ICMMT),Chengdu, China, May 2018, pp. 1–3.[14] T. N. T. Do, S. Lai, M. Horberg, H. Zirath, and D. Kuylenstierna,“A MMIC GaN HEMT voltage-controlled-oscillator with high tuninglinearity and low phase noise,” in Proc. IEEE Compound SemiconductorIntegr. Circuit Symp. (CSICS), New Orleans, LA, USA, Oct. 2015,pp. 1–4.[15] M.-J. Liu and S. S. H. Hsu, “A miniature 300-MHz resonant DC–DCconverter with GaN and CMOS integrated in IPD technology,” IEEETrans. Power Electron., vol. 33, no. 11, pp. 9656–9668, Nov. 2018.This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. Authorized licensed use limited to: University of Liverpool. Downloaded on May 17,2023 at 08:02:59 UTC from IEEE Xplore.  Restrictions apply. 

A $K$-Band MMIC Cross-Coupled Oscillator With High Output Power in 0.25-$\mu $m GaN HEMT

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A $K$ -Band MMIC Cross-Coupled Oscillator With High Output Power in 0.25- $\mu$ m GaN HEMT

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A KKK-Band MMIC Cross-Coupled Oscillator With High Output Power in 0.25-μ\mu μm GaN HEMT

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A $K$ -Band MMIC Cross-Coupled Oscillator With High Output Power in 0.25- $\mu$ m GaN HEMT