A Balanced 150-240 GHz Amplifier MMIC using Airbridge Transmission Lines J. Uingst* t , S. DieboId*, H. MassIer+, A. Tessmann+, A. Leuther+, T. Zwick*, I. Kallfass*+
*Karlsruhe Institute of Technology (KIT), Kaiserstrasse 12, 76131 Karlsruhe, Gennany t
[email protected] +Fraunhofer Institute for Applied Solid State Physics (IAF),
Tullastrasse 72, 79108 Freiburg, Germany
Abstract-A
compact balanced amplifier
monolithic
milli
meter-wave integrated circuit (MMIC) for the frequency range from
150 to 240GHz is developed using airbridge microstrip
(AirMS) transmission lines in a 35nm gate-length metamorphic high electron mobility transistor
(mHEMT) technology. The
hybrid couplers for the balanced design are realized using airbridge-stacked coupled lines. The balanced three-stage cascode amplifier achieves a small-signal gain of more than 15dB between 150 and 240GHz, from 210 to 230GHz even more than 23dB.
and offer a simple and direct connection to the coplanar embedded transistors. A transmission line that combines these aspects can be built using an airbridge process [1], [5]. This airbridge microstrip (AirMS) is used for the circuit presented in this paper. Furthermore the airbridge process is used to built a 90° hybrid coupler applying broadside coupled transmission lines.
In the latter frequency range the variation of the measured gain curve is less than 0.8 dB. Due to the balanced design, the input and output reflections are lower than - 12dB up to 227GHz, and lower than -6 dB in the whole frequency range.
Index Terms-Airbridge, balanced amplifier, hybrid coupler, metamorphic high electron mobility transistor (mHEMT), mono lithic millimeter-wave integrated circuit (MMIC).
I. INTRODUCTION Wireless communication systems with very high data rates require a large absolute bandwidth that can be realized only at high frequencies. For long range communication links, high power transmitters and sensitive receivers are required as well as a low atmospheric attenuation of the transmitted signal. A wide transmission window with low atmospheric attenuation is located between 200 and 300 GHz. Therefore, this frequency range is suitable for wireless communication with high data rates. Also, high-resolution radar systems operate at these frequencies. For generating the high transmission power as well as for amplification of the received signal, amplifier cir cuits are needed, which provide a wide bandwidth, high gain, and high output power. Compact amplifier circuits that meet these requirements are realized as monolithic millimeter-wave integrated circuits (MMICs) [1], based on GaAs technologies that offer the capabilities to build metamorphic high electron mobility transistors (mHEMTs), as reported in [2]. In this paper, a three-stage balanced amplifier MMIC using a 35 nm gate-length mHEMT process is presented. Compared to single-branch amplifiers, the balanced design reduces input and output reflections significantly, and thus allows for very broadband impedance matching. Also, as an advantage to conventional single-ended unbalanced designs [3], [4], the balancing of two individual amplifier branches is a way of power combining to increase the amplifier's output power. A major issue of MMIC design are transmission lines. Well suited transmission lines are compact, have low attenuation,
II. TECHNOLOGY The amplifier circuit is fabricated using an InAlAslInGaAs transistor technology on GaAs substrate at the Fraunhofer IAF [2]. Within this technology two Au metallization layers are available on the wafer front side. The lower metal layer, de noted as Metl, is electron beam evaporated and has a thickness of 0.31J111. The plated upper layer has a thickness of 2.7 f.lm and is referred to as MetG. An airbridge process for MetG is used to build strip conductors through the air with a distance of 1.6 f.lm above Met1. In this airbridge process, the minimum and maximum widths of MetG are 4 and 20 f.lm, respectively. The maximum length of an airbridge strip conductor is ten times its width, but not more than 100 f.lill. The minimum length is one half its width, but not less than 51J111. III. AIRBRIDGE MICROSTRIP (AIRMS) With the airbridge process a microstrip line whose dielectric layer consists of air can be built as shown in Fig. 1 and reported in [5] and [6]. Compared to grounded coplanar waveguides (GCPW) [7], this structure is independent of the GaAs substrate thickness and does not require complex pro cessing of the wafer back side, such as thinning, metallizing,
GaAs Fig. 1. AirMS transmission line. Due to the closed metallization layer Metl, the structure is independent of the wafer thickness.
978-1-4673-2949-1/12/$31.00 ©2012 IEEE
and the placement of vias, because it is completely located on the wafer front side. Further advantages are compactness, low attenuation, and a simple connection to the coplanar embedded mHEMTs. This connection is made by changing the strip conductor's level from the air onto the surface of the wafer. As opposed to conventional microstrip lines, through-substrate vias are not needed to connect to ground, thus parasitic effects, such as inductances caused by long vias, do not appear. The strip conductor is surrounded by air, there is no boundary between different dielectric layers. This forms a homogeneous medium around the strip conductor. Therefore, a pure TEM wave can propagate on this line. With the possible line widths between 4 and 20l1m, as imposed by the process design rules, lines with impedances from 62 to 22 S1 and a very compact cross section can be realized. Because of the air dielectric, the velocity factor equals 1. Nevertheless, AirMS based designs are compact since the narrow transmission lines can easily be folded. To hold the strip conductor in the air, posts have to be placed periodically. I V. H Y BRID COUPLER A 90° hybrid coupler is developed and simulated using CST MICROWAVE STUDIO®I. It is a broadside-coupled line coupler, whose lines are stacked by the use of the airbridge process. Similar couplers that are designed for much lower frequencies are reported in [8], [9] and [10]. The structure of the coupler is shown in Fig. 2. The upper conductor has to be propped up by MIM-posts, which additionally increase the capacitive coupling, as shown in Fig. 3. The simulation results for the transmission, coupling and phase difference between 150 and 240GHz can be seen in Fig. 4. In this frequency range, return loss is greater than 17 dB, isolation greater than 32 dB. IOnline available at http://www.cst.com
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