High Frequency Power Metamorphic HEMT C. S. Whelan, K. Herrick, J. Laroche, K. W. Brown, F. Rose, Y. Zhang, P. Balas, R. E. Leoni III, W. E. Hoke, S. Lichwala, J. Kotce, T. E. Kazior Raytheon RF Components 362 Lowell St. Andover, MA 01810, cwhelan@raytheon.com Abstract - By tailoring the device’s material, geometry and processing, our device designers have fabricated a state-of-the-art high frequency Metamorphic HEMT device with a G max of 12 dB, a power density of 360 mW/mm, and PAEs exceeding 30% at 95 GHz. This device had been utilized to create a range of W-band amplifiers with excellent performance, including a 266 mW PA operating at 90 GHz. I. INTRODUCTION GaAs based metamorphic HEMT (MHEMT) technology has emerged as an attractive, low cost alternative to InP HEMTs. The strain-induced imperfections caused by high indium content layers on GaAs are eliminated in metamorphic devices by providing a properly grown lattice-grading buffer between the substrate and active device layers. With this limitation overcome, it is now possible to provide the superior performance of InP-based devices with the cost advantages of highly manufacturable 4-inch GaAs wafers that can easily be integrated on existing GaAs fabrication lines [1]-[20]. 0 2000 4000 6000 8000 10000 12000 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Indium Content Channel [In(x)Ga(1-x)As] 300K Mob. of Electrons (cm2/V.s) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Strained Delta Ec (eV) MHEMTs GaAs Devices InP Devices (cm 2 /V-s) Figure 1. As indium is added to the channel, both the mobility and well depth increase (when a strained InAlAs layer is used). We have fabricated metamorphic HEMT devices with channel indium contents (In x Ga 1-x As) ranging from 20% to 85% and measured such properties as band gap, mobilities, F t , NF, gain, PAE, power densities and on- and off-state breakdown voltages. Figure 1 shows that the measured mobility of channel electrons increases with increasing channel In content, due mainly to the reduction in electron effective mass and reduced intervalley scattering (increasing E L -E Gamma ). These improvements give rise to higher channel velocities, improving F t . By straining the Schottky layer (with higher Al contents than lattice matched to In x Ga 1-x As), one can achieve improve confinement further, reducing NF and improving maximum current density (Figure 1). This collection of data has formed the basis to optimize a metamorphic GaAs HEMT for nearly any frequency and application. We have successfully exploited the freedom of metamorphic device tailoring with our 4”, 60% indium, low noise device technology that is currently in production. This paper describes a new pursuit and the results obtained by optimizing the material and processing to address the need for low cost, high gain, power amplifiers at W-band. II. DEVICE AND PROCESSING The MHEMT devices are mesa etched for isolation using a sulfuric based etchant. A series of metals containing Au/Ge are evaporated and annealed to form an ohmic contact, with contact resistance of approximately 0.17 Ohm-mm. Following ohmic formation, first recess and gate etching are performed selectively using a self-aligned gate method. 0.15 micron Ti/Pt/Au gates with wide T-tops (to reduce gate resistance) are then evaporated. Finally, silicon nitride is used to passivate the devices. The devices are thinned to 50 microns before being plated with gold. The devices employ individual source vias to reduce source inductance and improve thermal dissipation from the channel of the device. The DC performance data for our new power MHEMT