THE AlGaN/GaN high electron mobility transistors

IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 55, NO. 1, JANUARY 2007 23 A Low Phase-Noise X-Band MMIC VCO Using High-Linearity and Low-Noise Composite-Channel Al 0:3 Ga 0:7 N/Al 0:05 Ga 0:95 N/GaN HEMTs Zhiqun Q. Cheng, Yong Cai, Jie Liu, Yugang Zhou, Kei May Lau, Fellow, IEEE, and Kevin J. Chen, Senior Member, IEEE Abstract A low phase-noise -band monolithic-microwave integrated-circuit voltage-controlled oscillator (VCO) based on a novel high-linearity and low-noise composite-channel Al 0 3 Ga 0 7 N/Al 0 05 Ga 0 95 N/GaN high electron mobility transistor (HEMT) is presented. The HEMT has a 1 m 100 m gate. A planar inter-digitated metal semiconductor metal varactor is used to tune the VCO s frequency. The polyimide dielectric layer is inserted between a metal and GaN buffer to improve the factor of spiral inductors. The VCO exhibits a frequency tuning range from 9.11 to 9.55 GHz with the varactor s voltage from 4 to 6 V, an average output power of 3.3 dbm, and an average efficiency of 7% at a gate bias of 3 V and a drain bias of 5 V. The measured phase noise is 82 dbc/hz and 110 dbc/hz at offsets of 100 khz and 1 MHz at a varactor s voltage ( tune )=5 V. The phase noise is the lowest reported thus far in VCOs made of GaN-based HEMTs. In addition, the VCO also exhibits the minimum second harmonic suppression of 47 dbc. The chip size is 1.2 1.05 mm 2. Index Terms Al 0 3 Ga 0 7 N/Al 0 05 Ga 0 95 N/GaN high electron mobility transistor (HEMT), monolithic, phase noise, voltage-controlled oscillator (VCO). I. INTRODUCTION THE AlGaN/GaN high electron mobility transistors (HEMTs), with their high breakdown voltage and high-frequency operation, are promising candidates for power amplifiers to be used in next-generation wireless base stations and military applications [1] [5]. The AlGaN/GaN HEMTs are also showing good linearity and excellent microwave noise performance [2], [6] [9], which are attractive not only for low-noise amplifier (LNA) development, but also for low phase-noise voltage-controlled oscillators (VCOs). The VCOs, if monolithically integrated with the power amplifiers, can provide flexible high-power signal sources for a broader range of applications [10] [12]. The VCOs are also indispensable in fully integrated AlGaN/GaN HEMT transceivers. For improved device performances, advanced device structures are also being investigated. For example, a composite channel high electron mobility transistor (CC-HEMT), which features Al Ga N as the main channel and GaN as the minor channel, was recently demonstrated. CC-HEMTs exhibit enhanced linearity and low noise [13], [14], both of which are favorable for low phase-noise VCOs. In particular, the enhanced linearity enables a lower noise up-conversion factor, which is favorable for phase-noise reduction. In this paper, we demonstrate a fully integrated -band VCO using the CC-HEMT technology that features improved noise characteristics. On-chip interdigitated metal semiconductor metal (MSM) varactors and planar spiral inductors form the tunable LC tank. The VCO with a 100- m-wide single HEMT exhibits a tuning range from 9.106 to 9.55 GHz with a maximum output power of 4.2 dbm at a supply voltage of 5.0 V. Low phase noise of 82 and 110 dbc/hz were obtained at offsets of 100 khz and 1 MHz, respectively. In addition, the VCO shows excellent second harmonic suppression of 47 dbc. To our best knowledge, this is the highest harmonic suppression reported on AlGaN/GaN HEMT VCOs. Manuscript received May 11, 2006; revised August 18, 2006. This work was supported in part by the Hong Kong Research Grant Council and National Science Foundation of China under Grant N_HKUST616/04 and by the National Science Foundation of China under Grant 60476035. Z. Q. Cheng is with the Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Hong Kong, and also with the Department of Information Engineering, Hangzhou Dianzi University, Hangzhou, 310018, China. Y. Cai and J. Liu were with the Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Hong Kong. They are now with the Material and Packing Technologies Group, Hong Kong Applied Science and Technology Research Institute Company Ltd., Hong Kong. Y. Zhou was with the Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Hong Kong. He is now with Advanced Packaging Technology Ltd., Hong Kong. K. M. Lau and K. J. Chen are with the Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Hong Kong (e-mail: eekjchen@ust.hk). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMTT.2006.888942 II. MONOLITHIC INTEGRATION TECHNOLOGY As shown in Fig. 1, the VCO was designed and fabricated in a fully integrated MMIC process that includes an active HEMT, air-bridge interconnects, spiral planar inductors, metal insulator metal (MIM) capacitors (with SiN as the dielectric), and the interdigitated MSM varactors [15]. A. Composite-Channel HEMT The epitaxial structure of the composite-channel HEMT was grown by metal-organic chemical vapor deposition (MOCVD) on a sapphire substrate. The epitaxial layer structure contains a 2.5- m undoped GaN buffer layer, a 6-nm undoped Al Ga N layer, 3-nm undoped spacer, a 21-nm doped 2 10 cm carrier supplier layer, and a 2-nm undoped cap layer. Different from the conventional AlGaN/GaN HEMT, 0018-9480/$25.00 2006 IEEE

24 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 55, NO. 1, JANUARY 2007 Fig. 1. Cross section of the GaN-based HEMT MMIC process. a thin layer (6 nm) of AlGaN with 5%Al composition is incorporated between the Al Ga N barrier and the GaN buffer, aimed at reducing the alloy scattering at the barrier interface. All the HEMTs have a gate length of 1 m and the fabrication details can be found in [13]. The reduced scattering leads to noise reduction, with a detailed explanation given in [13] and [14]. The HEMT devices exhibit a maximum drain current density of 910 ma/mm and dc extrinsic transconductance of 175 ms/mm. A maximum oscillation frequency of 35 GHz was obtained. An output third-order intermodulation intercept point (OIP3) of 33.2 dbm was obtained at 2 GHz. The minimum noise figure was measured to be less than 3.5 db up to 10 GHz [14]. B. On-Chip Passive Components Since the phase noise of the VCO is predominantly determined by the intrinsic noise of HEMT and the factor of the resonant tank, low-noise HEMT and high- inductors and varactors are essential. In order to improve the factor of inductors, it is essential to reduce the coupling to the GaN buffer layer, which still exhibit nonperfect insulating behavior with a resistivity in the range of 10 10 cm after many years optimization. A low- polyimide dielectric layer (5- m thick) was inserted between the major metal traces (transmission lines and inductor metal) and the GaN buffer to reduce buffer coupling. On-wafer -parameters were measured using Agilent Technologies 8722ES network analyzer and Cascade microwave probes. One-port inductors [as shown in Fig. 2(a)] ranging from 1 to 4.5 nh were designed and measured. The pad-only characteristics were measured on the open pad pattern [as shown in Fig. 2(b)] to extract the pad s parasitics. The pad s parasitics were then deembedded from the overall inductor characteristics by subtracting the -parameters of the open pad from the -parameter of the overall inductor. A one-port equivalent-circuit model, as shown in Fig. 2(c), was used to extract the inductance value and factor. Inductance is calculated using, and the factor is calculated using. As shown in Fig. 2(d), a factor of 11 was achieved at 10 GHz in a 2.5-nH inductor. A maximum factor of 14 was achieved at a frequency of 6.5 GHz. The self-resonance frequency of the inductor is over 20 GHz. For a 2.5-nH inductor with metal traces directly on top of the GaN buffer, the factor is approximately 5 at 10 GHz. Fig. 2. Photograph and high-frequency characteristics of the on-chip spiral inductor. Fig. 3. (a) Microphotograph of a fabricated MSM varactor with a close-up view of the interdigital fingers. (b) Capacitance and Q factor as a function of the varactor s bias, measured at 10 GHz. The MSM planar inter-digitated varactors [15], which showed an improved factor compared to conventional metal insulator semiconductor varactors, were also fabricated on the HEMT structure. The varactors were characterized by on-wafer -parameter measurement with a similar deembedding procedure as that used for inductors. In Fig. 3, an MSM

CHENG et al.: LOW PHASE-NOISE -BAND MMIC VCO 25 Fig. 5. Circuit schematics of the X-band VCO. Fig. 4. Photograph and high-frequency characteristics of the on-chip spiral inductor. planar varactor s microphotograph together with its characteristics at 10 GHz were presented. The dimensions of the varactor include finger length of 2 m, finger width of 20 m, finger-to-finger spacing of 1.5 m, and number of fingers of 20. The varactor s capacitance varies from 2.0 to 0.5 pf when its bias voltage changes from 4 to 5 V. MIM capacitors are made of bottom metal (0.3 m), upper metal (3 m) and SiN dielectric layer (0.2 m). The capacitor [as shown in Fig. 4(a)] and open pad], as shown in Fig. 4(b)] for deembedding are designed and measured such as that used for inductors. The capacitance and factor are extracted using Fig. 4(c) shows the measured result for a 2-pF capacitor according to the above equations. It should be noted that the factor of both the varactor and MIM capacitor are relatively low, between 8 10 GHz, seemingly creating obstacles in achieving low phase noise. However, in our VCO circuit, both of them are in series with the input of the transistor, which presents a small capacitance value and small resistance. As the overall capacitance of series-connected capacitors is dominated by the smaller capacitor, the overall capacitance in the resonant tank is reduced. The overall effective capacitor together with an inductor form an LC tank that resonates in -band. The reduced effective capacitance also results in a higher factor in the effective capacitor according to, even though the resistive loss ( ) remains the same. More discussions are provided in Section III when the design of the VCO is discussed. III. DESIGN OF VCOS The VCO circuit was designed using Agilent Technologies Advanced Design System (ADS). The schematic circuit of the Fig. 6. Stability factor K as a function of the length of T with and without the shunt capacitor C. -band VCO is shown in Fig. 5. In the design of VCO, the measured small-signal -parameters of the CC-HEMT was used. A stub was connected between the CC-HEMT s source and the ground, providing a positive feedback to make the HEMT more unstable. A small MIM shunt capacitor pf was in parallel with the stub in order to shorten the stub s length and reduce the size of the VCO chip. Fig. 6 shows the simulated stability factor s ( s) variation with the length of the stub with or without the shunt capacitor. Without the capacitor, the factor is large (not favorable for oscillator circuits) and the minimum value is larger than 0.2 at a length of 5 mm. With the capacitor, the factor is reduced and less than 0.2 at a length of 1 mm. As a result, the addition of the shunt capacitor not only makes the oscillation more likely, but also helps shorten the length of the feedback stub. The resonant tank was designed and connected to the gate of the CC-HEMT. The resonator circuit includes the spiral inductor, the MIM capacitor, transmission line (, ), and an interdigitated MSM varactor. The MIM capacitor pf is used to decouple the negative gate bias of the HEMT and the varactor s control voltage.,, and are applied through off-chip inductors of 10 nh. In the design of the VCO, in order to reduce the phase noise of the VCO, the tuning gain (tuning frequency divided by tuning voltage) of the VCO must be minimized [16]. However, minimizing tuning gain usually results in reduced frequency tuning range. Therefore, the tuning gain was capped limited to 299.5 MHz/V (from small-signal simulation) when the tuning range was specified to be approximately 500 MHz in our design. After tuning the elements values (it is mainly inductance)

26 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 55, NO. 1, JANUARY 2007 Fig. 7. Microphotograph of the X-band VCO chip. Its size is 1.221.05 mm. Fig. 8. Measured output spectrum at V =5V, V = 03 V, and V = 5 V. of the resonant tank in simulation, a large negative resistance is presented at the drain output. An output matching network ( pf and ) connecting the drain port of the CC-HEMT and a 50- load was then designed according to the oscillation conditions as follows: (1) Real (2) where Real is the real part of the negative resistance at the drain output of the HEMT and is the imaginary part. and are the real and imaginary parts of the impedance looking into the load network with the 50- load included. is the resonant frequency. As mentioned in Section III, the impedance looking into the gate of the HEMT is at 9.4 GHz, yielding a small effective capacitance of 0.108 pf. When is combined with and in series, the overall capacitance of the resonant loop at the transistor s input is dominated by, which indicates that the tuning range of will be much less than that of. However, the reduced capacitance improves the factor of the resonant circuit significantly, leading to low phase noise in the -band VCO, as shown in Section IV. The microphotograph of the fabricated -band VCO is shown in Fig. 7. The feedback stub is designed with the meander layout to minimize the chip size. IV. MEASUREMENT RESULTS The VCO was characterized with Agilent Technologies E4440A PSA spectrum analyzer. The HEMT s gate and drain bias were set to 3 and 5 V, respectively, which was experimentally found to be optimal for both phase noise and the tuning frequency range. A typical output power spectrum is shown in Fig. 8 at a tuning voltage of 5 V. The output power at the resonance is 3.3 dbm. In general, the VCO s phase noise is dominated by two major factors: the thermal noise of the passive components (e.g., inductors and capacitors) and the noise of the active transistor. The thermal noise contribution from the passive elements is minimized by reducing the loss of the passive elements through implementation of thick metal and the insertion of a low- (polyimide) dielectric between the metal traces and Fig. 9. Oscillation frequency and output power versus the varactor control voltage. substrate. As for the noise of the AlGaN/GaN HEMTs, it can affect the phase noise of RF/microwave VCOs after frequency up-conversion. As a result, high-linearity HEMTs are preferred for VCO application because they provide a lower noise up-conversion factor. It has also been reported that the AlGaN/GaN HEMTs have a corner frequency in the range of 70 100 khz [17]. The Hooge parameter of AlGaN/GaN HEMTs have been reported to be in the range of 10 [18], comparable to or slightly lower than that in conventional Al- GaAs/GaAs HEMTs. The phase noise realized in this study is approximately 10 dbc/hz higher than that achieved in an -band VCO using a 0.6- m GaAs MESFET [19]. It can be expected that further improvement in the material quality for suppressing the surface states can lead to competitive noise performance in AlGaN/GaN HEMT VCOs. The output power at the fundamental resonance and the oscillation frequency at a different tuning voltage are plotted in Fig. 9. The tuning gain was measured to be 222 MHz/V. Compared to the previously reported AlGaN/GaN HEMT VCOs [11], the VCO reported here features a smaller active HEMT, a lower supply voltage of 5 V, and eventually lower output power. It should be pointed out that while high-power AlGaN/GaN HEMT VCOs takes advantage of the power-handling capability of AlGaN/GaN HEMTs, low-power VCOs also have their own suitable applications. For example, for a single-chip AlGaN/GaN HEMT transceiver, the low-power VCOs can be used in the receiver front-end. In addition, compared to high-power VCOs, low-power VCOs integrated with amplifier buffers can provide lower frequency pulling.

CHENG et al.: LOW PHASE-NOISE -BAND MMIC VCO 27 The sensitivity of the oscillation frequency upon the load impedance variation is characterized by a pulling figure measurement. The change of oscillation frequency is measured as a function of the phase of the load reflection coefficient for a constant return loss of 12 db, corresponding to a constant magnitude of 0.25 for the load reflection coefficient. The measurement was carried out using Maury Microwave s 982B01 load pull system, and the results are plotted in Fig. 12. The maximum frequency shift from the frequency in the matched load situation is 5 MHz. Fig. 10. Oscillation frequency and the phase noise (at 100-kHz and 1-MHz offset, respectively) versus varactor control voltage. V. CONCLUSION A MMIC -band VCO using Al Ga N Al Ga N/GaN composite-channel HEMTs was designed, fabricated, and characterized. The MMIC features monolithically integrated passive components including planar inductors, MSM varactors, and MIM capacitors. The VCO exhibits phase noise of 82- and 110-dBc/Hz at offsets of 100 khz and 1 MHz. In addition, the VCO also exhibits a maximum second harmonic suppression of 47 dbc. The low phase noise and high harmonic suppression of the CC-HEMT-based VCOs are attributed to the inherent high linearity and low noise of the CC-HEMT. Fig. 11. Measured output spectrum of fundament and second harmonic at V =5V, V = 03 V, and V =5V. Fig. 12. Pulling-figure measurement of the AlGaN/GaN CC-HEMT VCO. V =5V, V = 03 V, V = 05 V. The return loss of the load is set to be 012 db, corresponding to j0j =0:25: The phase noise at 100-kHz and 1-MHz offset is characterized within the entire tuning range, as shown in Fig. 10. It is also observed that the second harmonic suppression of 47 dbc was obtained within the entire tuning range. One such example is shown in Fig. 11. To our best knowledge, this is the highest second harmonic suppression reported in VCOs based on GaN HEMTs. The high harmonic suppression and the low phase noise of the VCOs reported here can be attributed to the high linearity and low noise performances of the CC-HEMTs. REFERENCES [1] K. Kasahara, N. Miyamoto, Y. Ando, Y. Okamoto, T. Nakayama, and M. Kuzuhara, Ka-band 2.3 W power AlGaN/GaN heterojunction FET, in Int. Electron Devices Tech. Dig., Dec. 2002, pp. 667 680. [2] K. Joshin, T. Kikkawa, H. Hayashi, S. Yokogawa, M. Yokoyama, N. Adachi, and M. Takikawa, A 174 W high-efficiency GaN HEMT power amplifier for W-CDMA base station applications, in Int. Electron Devices Tech. Dig., Dec. 2003, pp. 983 985. [3] Y. F. Wu, A. Saxler, M. Moore, R. P. Smith, S. Sheppard, P. M. Chavarkar, T. Wisleder, U. K. Mishra, and P. Parikh, 30-W/mm GaN HEMTs by field plate optimization, IEEE Electron Device Lett., vol. 25, no. 3, pp. 117 119, Mar. 2004. [4] J.-W. Lee, L. F. Eastman, and K. J. Webb, A gallium nitride push pull microwave power amplifier, IEEE Trans. Microw. Theory Tech., vol. 51, no. 11, pp. 2243 2249, Nov. 2003. [5] Y. Chung, C. Y. Hang, S. Cai, Y. Chen, W. Lee, C. P. Wen, K. L. Wang, and T. Itoh, Effects of output harmonic termination on PAE and output power of AlGaN/GaN HEMT power amplifier, IEEE Microw. Wireless Compon. Lett., vol. 12, no. 11, pp. 421 423, Nov. 2002. [6] W. Lu, V. Kumar, R. Schwindt, E. Piner, and I. Adesida, DC, RF, and microwave noise performances of AlGaN/GaN HEMTs on sapphire substrates, IEEE Trans. Microw. Theory Tech., vol. 50, no. 11, pp. 2499 2504, Nov. 2002. [7] J.-W. Lee, A. Kuliev, V. Kumar, R. Schwindt, and I. Adesida, Microwave noise characteristics of AlGaN/GaN HEMTs on SiC substrates for broadband low-noise amplifiers, IEEE Microw. Wireless Compon. Lett., vol. 14, no. 6, pp. 259 261, Jun. 2004. [8] A. Minko, V. Hoel, S. Lepolliet, G. Dambrine, J. C. De Jaeger, Y. Cordier, F. Semond, F. Natali, and J. Massies, High microwave and noise performance of 0.17-m AlGaN-GaN HEMTs on high-resistivity silicon substrates, IEEE Electron Device Lett., vol. 25, no. 4, pp. 167 169, Apr. 2004. [9] Y. Ando, A. Wakejima, Y. Okamoto, T. Nakayama, K. Ota, K. Yamanoguchi, Y. Murase, K. Kasahara, K. Matsunaga, T. Inoue, and H. Miyamoto, Novel AlGaN/GaN dual-field-plate FET with high gain, increased linearity and stability, in IEEE Int. Electron Devices Meeting Tech. Dig., 2005, pp. 576 579. [10] H. Xu, C. Sanabria, A. Chini, S. Keller, U. K. Mishra, and R. A. York, A C-band high-dynamic range GaN HEMT low-noise amplifier, IEEE Microw. Wireless Compon. Lett., vol. 14, no. 6, pp. 262 264, Jun. 2004. [11] V. S. Kaper, R. M. Thompson, T. R. Prunty, and J. R. Shealy, Signal generation, control, and frequency conversion AlGaN/GaN HEMT MMICs, IEEE Trans. Microw. Theory Tech., vol. 53, no. 1, pp. 55 65, Jan. 2005.

28 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 55, NO. 1, JANUARY 2007 [12] C. Sanabria, H. Xu, S. Heikman, U. K. Mishra, and R. A. York, A GaN differential oscillator with improved harmonic performance, IEEE Microw. Wireless Compon. Lett., vol. 15, no. 7, pp. 463 465, Jul. 2005. [13] J. Liu, Y. G. Zhou, R. M. Chu, Y. Cai, K. J. Chen, and K. M. Lau, Highly linear Al Ga N/Al Ga N/GaN composite-channel HEMTs, IEEE Electron Device Lett., vol. 26, no. 3, pp. 145 147, Mar. 2005. [14] Z. Q. Cheng, J. Liu, Y. G. Zhou, Y. Cai, K. J. Chen, and K. M. Lau, Broadband microwave noise characteristics of high-linearity composite-channel Al GaN N/Al Ga N/GaN HEMTs, IEEE Electron Device Lett., vol. 26, no. 8, pp. 521 523, Aug. 2005. [15] C. S. Chu, Y. G. Zhou, K. J. Chen, and K. M. Lau, Q-factor characterization of radio-frequency GaN-based metal semiconductor metal planar inter-digitated varactors, IEEE Electron Device Lett., vol. 26, no. 7, pp. 432 434, Jul. 2005. [16] B. Razavi, RF Microelectronics. Englewood Cliffs, NJ: Prentice-Hall, 1998, p. 223. [17] V. S. Kaper, V. Tilak, H. Kim, A. V. Vertiatchikh, R. M. Thompson, T. R. Prunty, L. F. Eastman, and J. R. Shealy, High-power monolithic AlGaN/GaN HEMT oscillator, IEEE J. Solid-State Circuits, vol. 38, no. 9, pp. 1457 1461, Sep. 2003. [18] A. Balandin, S. V. Morozov, S. Cai, R. Li, K. L. Wang, G. Wijeratne, and C. R. Viswanathan, Low flicker-noise GaN/AlGaN heterostructure field-effect transistors for microwave communications, IEEE Trans. Microw. Theory Tech., vol. 47, no. 8, pp. 1413 1417, Aug. 1999. [19] C.-H. Lee, S. Han, B. Matinpour, and J. Lasker, A low phase noise X-band MMIC GaAs MESFET VCO, IEEE Microw. Guided Wave Lett., vol. 10, no. 8, pp. 325 327, Sep. 2000. Zhiqun Q. Cheng received the B.S. and M.S. degrees in microelectronics from Hefei University of Technology, Hefei, China, in 1986 and 1995, respectively, and the Ph.D. degree in microelectronics and solid-state electronics from the Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai, China, in 2000. He is currently a Professor with the Department of Information Engineering, Hangzhou Dianzi University, Hangzhou, China, and a Senior Visiting Scholar with the Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology (HKUST), Hong Kong. Prior to joining HKUST, he was a Lecturer with the Hefei University of Technology, Hefei, China (1986 1997) and an Associate Professor with the Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai, China (2000 2005). He has carried out research on GaAs HBTs and GaN HEMTs and has been involved with the design of compound semiconductor digital integrated circuits and RF transceivers. He has authored or coauthored 50 technical papers in journals and conference proceedings. His current interests focus on III V high-power and low-noise devices and circuits for microwave and millimeter applications and high-speed Si and SiGe devices and T/R systems for wireless communications. Yong Cai was born in Nanjing, Jiangsu Province, China, in 1971. He received the B.S. degree in electronics engineering from Southeast University, Nanjing, China, in 1993, and the Ph.D. degree from the Institute of Microelectronics, Peking University, Beijing, China, in 2003. From 2003 and 2006, he was a Post-Doctoral Research Associate with the Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, where he was involved with wide-bandgap GaN-based devices and circuits. In August 2006, he joined the Material and Packing Technologies Group, Hong Kong Applied Science and Technology Research Institute (ASTRI) Company Ltd., Hong Kong, where he is a Senior Engineer. Jie Liu received the B.S. and M.S. degrees in physics from Nanjing University, Nanjing, China, in 2000 and 2003, respectively, and the Ph.D. degree in electrical and computer engineering from the Hong Kong University of Science and Technology (HKUST), Hong Kong, in 2006.. His master s thesis concerns the Schottky contacts of III-nitride devices. In August 2003, he joined the Department of Electronic and Computer Engineering, HKUST, where he is involved with device technologies and high-frequency characterization techniques of III-nitride HEMTs. In particular, he has focused on the channel engineering of III-nitride HEMTs and developed highly linear composite-channel HEMT and low-leakage current AlGaN/GaN/InGaN/GaN double-heterojunction HEMT. In October 2006, he joined the Hong Kong Applied Science and Technology Research Institute (ASTRI) Company Ltd., Hong Kong, where he is involved with advanced wireless packaging technologies. Yugang Zhou was born in Hubei Province, China, in 1975. He received the B.S. and Ph.D. degrees in physics from Nanjing University, Nanjing, China, in 1996 and 2001, respectively. From September 2001 to September 2004, he was a Post-Doctoral Research Associate with the Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology. In September 2004, he joined Advanced Packaging Technology Ltd., Hong Kong. He was mainly involved with MOCVD growth, device fabrication, and device physics of GaN-based heterostructure field-effect transistors (HFETs) prior to September 2004. Since that time, he has been focused on the fabrication of GaN-based high-power LEDs. Kei May Lau (S 78 M 80 SM 92 F 01) received the B.S. and M.S. degrees in physics from the University of Minnesota at Minneapolis St. Paul, in 1976 and 1977, respectively, and the Ph.D. degree in electrical engineering from Rice University, Houston, TX, in 1981. From 1980 to 1982, she was a Senior Engineer with M/A-COM Gallium Arsenide Products Inc., where she was involved with epitaxial growth of GaAs for microwave devices, development of high-efficiency and millimeter-wave IMPATT diodes, and multiwafer epitaxy by the chloride transport process. In Fall 1982, she joined the faculty of the Electrical and Computer Engineering Department, University of Massachusetts at Amherst, where, in 1993, she became a Full Professor. She initiated metalorganic chemical vapor deposition (MOCVD) and compound semiconductor materials and devices programs at the University of Massachusetts at Amherst. Her research group has performed studies on heterostructures, quantum wells, strained-layers, and III V selective epitaxy, as well as high-frequency and photonic devices. In 1989, she spent her first sabbatical leave with the Massachusetts Institute of Technology (MIT) Lincoln Laboratory. From 1995 to 1006, she developed acoustic sensors with the DuPont Central Research and Development Laboratory, Wilmington, DE, during her second sabbatical leave. In Fall 1998, she was a Visiting Professor with the Hong Kong University of Science and Technology (HKUST), Hong Kong, where, in Summer 2000, she joined the regular faculty. She established the Photonics Technology Center for research and development efforts in wide-gap semiconductor materials and devices. In July 2005, she became a Chair Professor of electronic and computer engineering with HKUST. Prof. Lau served on the IEEE Electron Devices Society Administrative Committee and was an editor for the IEEE TRANSACTIONS ON ELECTRON DEVICES (1996 2002). She also served on the Electronic Materials Committee of the Minerals, Metals and Materials Society (TMS) of AIME (American Institute of Materials Engineers). She was a recipient of the National Science Foundation (NSF) Faculty Award for Women (FAW) Scientists and Engineers in the U.S.

CHENG et al.: LOW PHASE-NOISE -BAND MMIC VCO 29 Kevin J. Chen (M 96 SM 06) received the B.S. degree in electronics from Peking University, Beijing, China, in 1988, and the Ph.D. degree from the University of Maryland at College Park, in 1993. From January 1994 to December 1995, he was a Research Fellow with NTT LSI Laboratories, Atsugi, Japan, where he was engaged in the research and development of functional quantum effect devices and heterojunction field-effect transistors (HFETs). In particular, he developed device technologies for monolithic integration of resonant tunneling diodes and HFETs on both GaAs and InP substrates for applications in ultrahigh-speed signal processing and communication systems. He also developed the Pt-based buried gate technology that is widely used in enhancement-mode HEMT and pseudomorphic high electron-mobility transistor (phemt) devices. From 1996 to 1998, he was an Assistant Professor with the Department of Electronic Engineering, City University of Hong Kong, where he carried out research on high-speed device and circuit simulations. In 1999, he joined the Wireless Semiconductor Division, Agilent Technologies, Santa Clara, CA, where he was involved with enhancement-mode phemt RF power amplifiers used in dual-band global system for mobile communication (GSM)/digital communication system (DCS) wireless handsets. His work with Agilent Technologies has covered RF characterization and modeling of microwave transistors, RF integrated circuits (ICs), and package design. In November 2000, he joined the Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology (HKUST), Hong Kong, as an Assistant Professor and, in 2006, became an Associate Professor. He has authored or coauthored over 140 publications in international journals and conference proceedings. With HKUST, his group has carried out research on novel III-nitride device technologies, III-nitride and silicon-based microelectromechanical systems (MEMS), silicon-based RF/microwave passive components, RF packing technology, and microwave filter design.