X band GaN Based MMIC Power Amplifier With 36.5dBm P 1-dB for Space Applications Armagan Gurdal 1, 2 , Burak Alptug Yilmaz 1,2 , Omer Cengiz 1 , Ozlem Sen 1 , Ekmel Ozbay 1, 2 1 Nanotechnology Research Center, Bilkent University, 06800 Ankara, Turkey 2 Department of Electrical and Electronics Engineering, Bilkent University, 06800 Ankara, Turkey Author's e-mail: agurdal@bilkent.edu.tr, b_yilmaz@bilkent.edu.tr, omerc@bilkent.edu.tr, ozlemsen@bilkent.edu.tr, ozbay@bilkent.edu.tr Abstract— An X-Band Monolithic Microwave Integrated Circuit (MMIC) High Power Amplifier (HPA) with coplanar waveguide (CPW) based on AlGaN/GaN on SiC technology is presented in this paper. Coplanar waveguide technology (CPW) is chosen for the simplicity and reduced cost of fabrication since CPW process has no via. High Electron Mobility Transistors (HEMTs) are matched for the 8 GHz- 8.4GHz frequency band for maximum output power. The Amplifier has a small signal gain over 10 dB, output power of 36.5dBm at 1 dB gain compression point (P 1dB ), 40% power added efficiency (PAE) at (P 1dB ) in the desired frequency band (8 GHz-8.4 GHz) with V ds = 30V. Keywords— MMIC; Power Amplifier; GaN HEMTs; coplanar waveguide; AlGaN/GaN I. INTRODUCTION Due to highly demanded physical and electrical properties of gallium nitride (GaN), AlGaN/GaN has become a promising material choice for wideband power amplification applications. GaN based High Electron Mobility Transistors (HEMTs) offer far superior features such as high current density, high breakdown voltage, and high saturation velocity compared to gallium arsenide (GaAs) based HEMTs which are widely used in conventional devices for amplification purpose [1]-[2]. Its excellent thermal and chemical stability furthermore makes it ideal material for high power space applications in harsh environments. Moreover, GaN offers several key advantages such as higher efficiency in systems with limited prime energy, radiation hardness and higher linearity compared to classical semiconductors. Linearity, efficiency and radiation hardness is quite crucial specially for X-band data downlink of low earth orbit (LEO) satellites where QPSK, 8PSK, etc. modulation schemes are used. Features described above make GaN based power amplifiers to be the best solution for these applications. In this paper, X-band power amplifier designed, fabricated and summarized which is suitable for X-band LEO satellite downlink (8.0 GHz-8.4GHz). Presented amplifier is designed to be used with 8PSK modulation scheme and since spectral growth mask is an important constraint, the output power at 1dB gain compression (P 1dB ) is given instead of the saturated output power. The fabrication details of the process is given in section II, the measured performance of the active device cell with eight gate-fingers, each 125 µm wide (8x125 µm) is defined in section III and the MMIC design considerations, design phase and the simulated amplifier performance comparing with the measured amplifier performance are reported in section V. II. FABRICATION TECHNOLOGY X band power amplifier designed by using the GaN MMIC process by Bilkent University Nanotechnology Research Center (NANOTAM). AlGaN/GaN HEMT epitaxial structure was grown on a semi-insulating SiC substrate by metal organic chemical vapor deposition (MOCVD). The structure includes from bottom to top, an 15 nm-thick AlN nucleation layer, a 2 µm-thick undoped GaN buffer layer, an approximately 1.5 nm-thick AlN interlayer, a 20 nm-thick undoped Al 0,22 Ga 0,78 N layer and a 2 nm-thick GaN cap layer on the top of the structure. The Hall mobility was 1384 cm 2 V -1 s -1 whereas sheet carrier concentration was 1.51×10 13 cm -2 . Fabrication process started with mesa etching. Ohmic contacts formation was done by Ti/Al/Ni/Au metal stack with the thicknesses of 12, 120, 35 and 65 nm, respectively. Ohmic contact metals were deposited by e- beam evaporation method. After this step, electron beam lithography was used to define the gate region for -gate region, for -gate fabrication and these regions were deposited with Ni/Au metals. The gate structure of HEMTS is in the form of Г- shaped gate (Fig.1) in order to increase the breakdown voltage and output power while having reasonable gain. Next step in fabrication, the devices were passivated with a 300 nm-thick Si 3 N 4 layer Figure 1. Modelled view of HEMT structure and SEM image of fabricated Г-shaped gate