IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 18, NO. 3, MARCH 2008 215 A 220 GHz (G-Band) Microstrip MMIC Single-Ended Resistive Mixer Sten E. Gunnarsson, Student Member, IEEE, Niklas Wadefalk, Iltcho Angelov, Member, IEEE, Herbert Zirath, Member, IEEE, Ingmar Kallfass, and Arnulf Leuther Abstract—This letter presents the design and characterization of a 220 GHz microstrip monolithic microwave integrated circuit single-ended resistive mixer in a 0.1 GaAs mHEMT tech- nology. A conversion loss as low as 8.7 dB is obtained, limited by the available local oscillator (LO) power (1.5 dBm) in the measurement setup. The radio frequency (RF) bandwidth is also limited by the measurement setup, but the mixer demonstrates a flat response over the measured 200 to 220 GHz frequency range. Furthermore, measured intermediate frequency bandwidth, 1-dB input compression point, LO-to-RF isolation, and reflection coef- ficients are presented and discussed. Index Terms—GaAs, G-band, imaging, mHEMT, microstrip, millimeter-wave, monolithic microwave integrated circuit (MMIC), single-ended resistive mixer, 220 GHz. I. INTRODUCTION M ONOLITHIC microwave integrated circuits (MMICs) and systems operating beyond 100 GHz have gained increased academic and commercial interest over the recent years. Applications include radiometers measuring the absorp- tion lines of e.g., oxygen (118 GHz) and water (183 GHz) for environmental studies. The atmospheric windows at 94, 140, and 220 GHz have been proposed and used for high-resolu- tion passive and active millimeter-wave (mm-wave) imaging applications such as through-wall imaging and detection of concealed weapons, plastic explosives, and biological weapons. Clearly, such systems should ideally be small and lightweight mobile units which easily could be moved to the place of operation. Thus, a MMIC based solution would be favorable over a traditional waveguide solution due to its smaller size and weight. Furthermore, a MMIC solution with its lower price per unit and high manufacturing repeatability opens up the possibility of multi-pixel systems with several receivers and/or transmitters for enhanced system performance. Manuscript received October 1, 2007; revised November 15, 2007. This work was supported by the Swedish Defense Agency (FOI) through the NanoComp Project (a Swedish/German Research Collaboration together with the Swedish Foundation for Strategic Research (SSF) through the High Speed Electronics and Photonics (HSEP) Program), and by the Fraunhofer Institute for Applied Solid-State Physics (IAF). S. E. Gunnarsson, N. Wadefalk, and I. Angelov are with the Microwave Elec- tronics Laboratory, Department of Microtechnology and Nanoscience - MC2, Chalmers University of Technology, Göteborg SE-412 96, Sweden (e-mail: sten. gunnarsson@chalmers.se) H. Zirath is with the Microwave Electronics Laboratory, Department of Mi- crotechnology and Nanoscience - MC2, Chalmers University of Technology, Göteborg SE-412 96, Sweden and also with the Microwave and High Speed Electronics Research Center, Ericsson AB, Mölndal SE-431 84, Sweden. I. Kallfass and A. Leuther are with the Fraunhofer Institute for Applied Solid State Physics (IAF), Freiburg 79108, Germany. Digital Object Identifier 10.1109/LMWC.2008.916819 Regarding MMIC design for very high frequencies, a few MMICs operating at H-band (220–325 GHz) have been pub- lished and these include voltage controlled oscillators (VCOs) [1] and low noise amplifiers (LNAs) [2]. Large-signal de- signs such as power amplifiers (PAs), multipliers, and mixers operate at much lower frequencies compared to VCOs and small-signal amplifiers for a given technology. Very few G-band (140–220 GHz) and no H-band large-signal MMIC designs are presented in the open literature but the existing G-band publications include for example [3] on multipliers. Mixers are equally rare, with only one mixer characterized up to 210 GHz, [4]. Thus, there is a need to explore the performance of MMIC mixers operating at G-band and beyond. Furthermore, almost all of the mentioned designs are made in coplanar waveguide (CPW) technologies due to its simplified processing and better high frequency performance due to the reduced source induc- tance compared to microstrip line designs. However, to sim- plify the layout and to shrink the size of complex multi-func- tional MMICs, microstrip designs are of particular interest. This work presents a microstrip MMIC mixer which operates up to 220 GHz. The upper frequency is limited by the measurement setup rather than the mixer itself. II. MIXER TOPOLOGY AND DESIGN The mixer was manufactured in a 0.1 gate length GaAs metamorphic high electron mobility transistor (mHEMT) MMIC process offered by the Fraunhofer Institute for Applied Solid-State Physics (IAF) in Germany, [5]. A 2 20 mHEMT microstrip device on the standard 50 thick GaAs substrate from this process has and of 170 and GHz, respectively. In circuits like resistive mixers when the HEMT device operates as a gate-voltage controlled resistor with zero drain bias, versus is an important parameter. For maximum conversion efficiency in such circuits, the HEMT would ideally switch between infinite off- and zero on-resistance. Particularly, the on-resistance needs to be low since the off-resistance in practical circuits is high enough for efficient switching. The on-resistance for the mHEMT device is 0.5 . A more comprehensive description of this process can be found in [3]. The devices are modeled and characterized within our group at Chalmers. The Chalmers FET model (also known as the An- gelov model) was chosen due to its suitability for large signal designs such as power amplifiers, multipliers, and mixers. The model predicts not only and capacitance characteristic, but also its derivatives in a correct way without discontinuities for positive and negative drain voltages. The device model 1531-1309/$25.00 © 2008 IEEE