430 IEEE MICROWAVE AND GUIDED WAVE LETTERS, VOL. 8, NO. 12, DECEMBER 1998 High-Gain Cascode MMIC’s in Coplanar Technology at W-Band Frequencies Axel Tessmann, William H. Haydl, Axel H¨ ulsmann, and Michael Schlechtweg, Member, IEEE Abstract— Compact high-gain -band multistage amplifier MMIC’s have been developed in coplanar technology using 0.15- m AlGaAs/InGaAs/GaAs PM-HEMT’s. The conventional dual-gate HEMT has been modified to include an additional interstage network between the common-source and the common-gate HEMT. The effect of stabilizing circuit elements has been investigated. A gain of 10 dB per cascode stage is obtained at 94 GHz. Multistage amplifier MMIC’s with up to 40-dB gain have been realized. Index Terms— Cascode, coplanar waveguide, MMIC, PM- HEMT, -band. I. INTRODUCTION I N THE past, dual-gate field-effect transistors (FET’s) have been used successfully in a number of MMIC applica- tions, such as gain-controlled low-noise amplifiers [1], power amplifiers [2], ultrawide-band distributed amplifiers [3], and highly integrated millimeter-wave circuits [4]. The MMIC’s described below make extensive use of a cascode amplifier, illustrated schematically in Fig. 1. It differs from a dual-gate device [1]–[4], since now an interstage network physically separates the common-source and the common-gate high- electron mobility transistors (HEMT’s). This amplifier is easier to stabilize and still offers significant advantages in gain versus chip area, compared to a conventional two-stage common- source amplifier. This cascode connection has been practiced in the past and was applied to vacuum tubes and recently to HEMT’s [5]. We have systematically investigated the effect of circuit and substrate on this type of cascode circuit, and demonstrate its application in high-gain MMIC’s with gains up to 40 dB at 94 GHz. II. TECHNOLOGY An MBE-grown double-doped pseudomorphic (PM) Al- GaAs/InGaAs/GaAs HEMT structure was used to realize the active devices. The 0.15- m mushroom gates were written with e-beam, and the recess was dry etched [6]. The HEMT’s typically had an of 110 GHz and an of 160 GHz. With 25% indium in the channel, a current density of 900 mA/mm and an extrinsic transconductance of 800 mS/mm were achieved. Coplanar lines of 3- m thickness and 50- m ground-to-ground spacing were used. Broad-band models of the coplanar components are described in detail in [7]. Manuscript received August 11, 1998. This work was supported by the German Ministry for Defense (BMVg). The authors are with the Fraunhofer Institute for Applied Solid State Physics (IAF), 79108 Freiburg, Germany. Publisher Item Identifier S 1051-8207(98)09827-4. Fig. 1. Schematic representation of a cascode HEMT single-stage amplifier with an interstage matching network between the common-source and the common-gate HEMT’s and feedback for improved stability. (a) (b) Fig. 2. (a) Coplanar dual-gate HEMT and its schematic representation and (b) coplanar cascode HEMT with interstage network, a short length of transmission line . III. CIRCUITS The conventional dual-gate HEMT, illustrated in Fig. 2(a), inherently suffers from a large feedback capacitance, the drain–source capacitance of the common-gate transistor. This makes it more prone to be unstable and thus more difficult to use in an amplifier circuit than a common-source HEMT. However, since the cascode amplifier configuration clearly requires less chip area per unit gain, we investigated the effect of an interstage network as well as various feedback networks on stability. The coplanar cascode HEMT with pos- sible matching and feedback networks is illustrated in Fig. 1. Feedback A was used for low-noise applications, resulting in a pronounced decrease in gain. Feedback B has not been considered, because of its difficulty of implementation in a coplanar environment. Simulations with resistors in the gate or drain line of the common-gate HEMT indicated unconditional stability, but resulted in severe gain reduction. Good results were obtained with a capacitive element in the gate line of the 1051–8207/98$10.00  1998 IEEE