Components for AlGaN/GaN Power Amplifiers
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Components for AlGaN/GaN Power Amplifiers B. Jacobs1, P. Wingelaar1, M. Kramer1, S. Falcone1, F. Karouta1 P. De Hek2, E. Suijker2, R. Van Dijk2 1 Eindhoven University of Technology, Department of Electrical Engineering, Opto-electronic devices group, PO box 513, NL-5600 MB, Eindhoven, The Netherlands 2 TNO Physics and Electronics Laboratory, 2509 JG, The Hague, The Netherlands ABSTRACT Ohmic contacts and Schottky contacts were made on an undoped AlGaN/GaN FET structure. Despite the high Al content (33%), we were still able to obtain a contact resistance of 0.3 Ωmm. Pulsed measurements showed the large effect of self-heating even for circular contacts with a radius of 50 µm. The behavior of the Ni/Au Schottky contacts is according to the charge control model; the reverse current and capacitance only scale with the area of the diode. Tests with polygon type diodes showed no dependence of the reverse current on the number of polygon corners. The reverse current decreased when the devices were aged at 400 0C for 30 hrs. Coplanar Waveguide discontinuities were realized on AlN substrates. A scalable lumped element model was derived from measurements for T-junctions, transmission lines, bends and crosses. INTRODUCTION The improvement in AlGaN/GaN FET material growth and epitaxial layer design has led to increasing power output and gain of HEMT devices and power amplifiers. Metalsemiconductor contacts play an important part in achieving higher performance of these devices. Ohmic contact resistance must decrease to allow higher gain and less power dissipation and the Schottky contacts should have a high breakdown voltage and low leakage. Both must have a good line definition to allow close spacing of the gate and source. In addition to the FET itself, one needs passive components to transfer the power from the transistor to the outside world. If one pursues a monolithic solution on sapphire or a flip-chip technique on a suitable carrier like AlN, one needs Coplanar Waveguide technology (CPW) because of the unavailability of via-holes. This technology is not so well developed as microstrip design so libraries need to be developed for elements like transmission lines, crosses, resistors etc. METAL-SEMICONDUCTOR CONTACTS In previous work [1] we reported on the optimization of the Ti/Al/Ni/Au metalization scheme for use as an ohmic contact to AlGaN/GaN FET structures. In the same work we also reported preliminary results on Ni/Au diodes, which suffered from high leakage currents and undesired C-V behavior. In this work we present a more detailed study on Ni/Au Schottky contacts made on an undoped FET structure on sapphire. The structure consisted of a 20 nm undoped Al.33Ga.67N donor layer on top of a 1 µm GaN buffer. Also, the optimized Ti/Al/Ni/Au ohmic contact was tried on this structure to compare with our previous work on doped structures.
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Ohmic contacts Ti/Al/Ni/Au (30/180/40/150 nm) ohmic contacts were made on the undoped structure mentioned above and on the Si-doped Al.25Ga.75N/GaN FET structure used in our pre
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