IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 56, NO. 12, DECEMBER 2008 2685 Electrical Characterization and Small-Signal Modeling of InAs/AlSb HEMTs for Low-Noise and High-Frequency Applications Mikael Malmkvist, Eric Lefebvre, Malin Borg, Ludovic Desplanque, Xavier Wallart, Gilles Dambrine, Member, IEEE, Sylvain Bollaert, and Jan Grahn, Senior Member, IEEE Abstract—Electrical characterization and modeling of 2 50 m gatewidth InAs/AlSb HEMTs with 225 nm gate- length have been performed. The fabricated devices exhibited a transconductance of 650 mS/mm, an extrinsic cutoff frequency and an extrinsic maximum frequency of oscillation of 120 and 90 GHz, respectively, already at a low of 0.2 V. A min- imum noise figure less than 1 dB between 2–18 GHz was achieved at a dc power consumption of only 10 mW/mm. This demonstrates the potential of InAs/AlSb HEMTs for low-power, low-noise appli- cations. To account for the elevated gate-leakage current in the narrow-bandgap InAs/AlSb HEMT, the conventional field-effect transistor small-signal model has been extended. The relatively high was modeled by shunting both and with and , respectively. As a result, the small-signal -parameters were more accurately modeled, especially for frequencies below 10 GHz. Utilizing this modeling approach, excellent agreement was obtained between measured and modeled -parameters, unilateral power gain (Mason’s gain) and stability factor . Index Terms—Gate-leakage current, HEMT, heterostructure, InAs/AlSb, noise, small-signal model. I. INTRODUCTION T HE InAs/AlSb high electron-mobility transistor (HEMT) exhibits superior mobility and electron velocity compared to the more mature InGaAs/InAlAs/InP HEMT (InP HEMT) Manuscript received December 06, 2007; revised May 09, 2008. First pub- lished November 18, 2008; current version published December 05, 2008. This work was supported by the Swedish Research Council (VR), the Swedish Foun- dation for Strategic Research (SSF) (Strategic Research Centre for High-Speed Electronics and Photonics), the IRCICA Laboratory, the European Union, the French Government, and the French Regional Council. M. Malmkvist was with the Microwave Electronics Laboratory, Department of Microelectronics and Nanoscience—MC2, Chalmers University of Tech- nology, SE 412 96 Göteborg, Sweden. He is now with Ericsson AB, 431 84 Mölndal, Sweden. E. Lefebvre was with the Microwave Electronics Laboratory, Department of Microelectronics and Nanoscience—MC2, Chalmers University of Technology, SE 412 96 Göteborg, Sweden. He is now with OSRAM Opto Semiconductors GmbH, 93055 Regensburg, Germany. M. Borg was with the Microwave Electronics Laboratory, Department of Mi- croelectronics and Nanoscience—MC2, Chalmers University of Technology, SE 412 96 Göteborg, Sweden. She is now with Saab Microwave Systems, 417 56 Göteborg, Sweden. L. Desplanque, X. Wallart, G. Dambrine, and S. Bollaert are with the Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN), 59652 Villeneuve d’Ascq, France. J. Grahn is with the Microwave Electronics Laboratory, Department of Micro- electronics and Nanoscience—MC2, Chalmers University of Technology, SE 412 96 Göteborg, Sweden (e-mail: jan.grahn@chalmers.se). 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.2008.2006798 technology. This makes the InAs/AlSb HEMT potentially supe- rior for low-noise and high-frequency applications demanding ultra-low-power consumption. Promising results have recently been reported when implementing this technology in low-noise amplifiers [1]. Such circuit designs require accurate small-signal modeling of the InAs/AlSbHEMT. However, compared to the InP HEMT technology, the InAs/AlSb HEMT suffers from an elevated gate-leakage current . This is both due to the narrow bandgap as well as the type-II staggered energy-band diagram alignment of the InAs/AlSb heterostructure. In order to accu- rately model both RF and noise behavior of InAs/AlSb HEMTs, the relatively high must be considered in the small-signal model (SSM). In the literature, the conventional field-effect transistor (FET) SSM is often used when modeling InAs/AlSb HEMTs [2], [3]. However, the high appears as a shunting effect of and in the SSM. The shunting effect of has previously been briefly addressed for this technology [4]. At high , the mod- eling of impact ionization in the SSM [5] has to be modified as well. Furthermore, device noise characterization and noise modeling for InAs/AlSb HEMTs are not frequently reported in the literature; Minimum noise figure measurements have been presented in [6] and [7]. Further device modeling based on accu- rate electrical characterization is needed for assessment of the InAs/AlSb HEMT technology and its potential for low-power and low-noise applications. In this paper, electrical characterization and small-signal modeling of narrow-bandgap InAs/AlSb HEMTs with respect to -parameters and noise are discussed. The device noise has been characterized at a low drain-to-source voltage of 0.2 V by 50 noise measurements followed by noise modeling. II. HEMT STRUCTURE AND FABRICATION The InAs/AlSb HEMT epitaxial structure was grown by molecular beam epitaxy (MBE) on semi-insulating InP sub- strate. In Table I, the HEMT epitaxial structure is given. The InAs channel was grown on metamorphic buffer layers fol- lowed by an AlSb Schottky layer. An InAlAs protection layer was added on top of the AlSb Schottky layer and the structure was finalized by a highly doped InAs contact layer. Details on X-ray diffraction and Hall measurements have been reported elsewhere [8], [9]. The devices were fabricated by photolithog- raphy, electron-beam lithography (EBL) and a combination of wet and dry etching techniques. Mesa isolation was obtained by dry etching [8]. Pd/Pt/Au ohmic contacts were subsequently 0018-9480/$25.00 © 2008 IEEE