IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 59, NO. 12, DECEMBER 2011 3419 Linearity Characterization and Optimization of Millimeter-Wave GaN HEMTs Joe X. Qiu, Member, IEEE, Ali M. Darwish, Senior Member, IEEE, Edward A. Viveiros, Khamsouk Kingkeo, and H. Alfred Hung, Senior Member, IEEE Abstract—This paper presents the first comprehensive study of the linearity characteristics of GaN/SiC high-electron mobility transistors (HEMTs) at millimeter-wave (mmW). Similar size de- vices from three sources are compared using AM–AM, AM–PM, two-tone, and 16–quadrature-amplitude modulation (16QAM) modulated waveforms measurements at 31.5 GHz. Additionally, error vector magnitude (EVM) and spectrum regrowth [adja- cent channel power ratio (ACPR)] data are also presented. The results were measured, using a unique digital waveform system integrated with a mmW (50-GHz) load-pull system, for HEMTs at different classes of operation: A, AB, and B. In all devices, it is observed that a “balanced AB” (b-AB) class of device bias condi- tion where linearity and efficiency are simultaneously optimized. A linearity figure of merit , based on single-tone power measurements, is defined and calculated for each class. The closely correlates with linearity performance of the HEMT under two-tone and digitally modulated drive conditions. Compared with class A, at a fixed output power, a b-AB class showed a 5–10–dB improvement in intermodulation distortions, combined with 1.2 to 1.9 enhancement in drain efficiency. The characterization methodology and derived should benefit monolithic microwave integrated circuit (MMIC) designers to optimize the operation of GaN HEMTs. The results also indicate that GaN HEMTs can provide linearity character- istics suitable for applications such as satellite communications, while maintaining high efficiency and power density. Index Terms—AlGaN, GaN, high-electron mobility transistor (HEMT), intermodulation distortion (IMD), linearity, mil- limeter-wave (mmW), wide bandgap, 16 quadrature-amplitude modulation (16QAM). I. INTRODUCTION W ITH THE increasing need for high-data-rate satellite communication (SATCOM) links and the over- crowding spectrum at -band and below, millimeter-wave (mmW) frequencies have become an appropriate solution to provide wider bandwidth and higher data throughput for both commercial and defense satellite applications [1], [2]. The availability of high-power, high-efficiency, linear amplifiers at these frequencies is important for the successful deployment of these systems. Traditionally, SATCOM applications have relied on traveling-wave-tube amplifiers (TWTAs) to satisfy their Manuscript received July 16, 2011; revised September 26, 2011; accepted September 30, 2011. Date of publication November 16, 2011; date of current version December 14, 2011. This paper is an expanded paper from the IEEE International Microwave Symposium, Baltimore, MD, June 5–10, 2011. The authors are with the Army Research Laboratory, Adelphi, MD 20783 USA (e-mail: joe.x.qiu.civ@mail.mil). 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.2011.2171986 transmitter power requirement [3]. During the past decade, the emergence of wide-bandgap semiconductor devices has enabled solid-state power amplifiers (SSPAs) to compete with TWTAs for these applications. GaN on SiC high-electron mobility transistors (HEMTs) and monolithic microwave inte- grated circuits (MMICs) have been demonstrated at microwave and mmW frequencies with power densities, ranging from an order of magnitude at microwave frequencies to several times at mmW, higher than earlier technologies [4]. GaN technology has showed continuous improvements in output power, efficiency, bandwidth, and reliability [5]. A decade ago, most of the focus in the GaN technology has been on improving lifetime, repeatability, reliability, and on proving the technology’s merits. The early focus was at low frequencies ( - through -band) and achieving high-power density. Currently, many of the reliability and lifetime goals have been met, failure mechanism are better understood [6], [7], the technology matured further, and attention is shifting towards applications, system issues, and high-frequency per- formance. The use of GaN technology at mmW frequencies has witnessed a significant increase in the past few years starting from devices with record power densities [8], [9] to high-perfor- mance MMICs with excellent bandwidth (1–50 GHz [10], and 2–20 GHz [11]), high power [5], and high levels of integration [12]. The GaN technology’s impressive performance has led to consideration for many applications in which linearity and efficiency are critical. Recent efforts have focused on improving linearity [13]–[15] and efficiency [16] of GaN amplifiers. This paper presents the first comprehensive study of the lin- earity characteristics of a number of GaN/SiC HEMT devices at mmW. The performance of similar size devices (200 m) from three foundries are compared. Measurements of single- tone AM–AM and AM–PM, two-tone third-/fifth-order inter- modulation distortions (IMD3/IMD5), and spectral regrowth of 16–quadrature–amplitude modulation (16QAM) waveforms are measured for each device at 31.5 GHz. In addition, error vector magnitude (EVM) and adjacent channel power ratio (ACPR) data are also presented. The results were measured for various classes of operation, A, AB, and B (50%, 37.5%, 25%, 12.5%, 10%-2% ). In all devices, a “balanced AB” (b-AB) class where linearity and efficiency are simultaneously maximized can be observed. A b-AB class is a gate bias condition slightly above class B, where the power gain at small and large signals are roughly equal, as will be shown later. A linearity figure of merit, , based on single-tone power measurements, can be defined for each class. The closely corre- lates with linearity performance of the HEMT under two-tone and digitally modulated drive conditions. Compared with class U.S. Government work not protected by U.S. copyright.