SiC MESFETs for High-Frequency Applications
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SiC MESFETs for
High-Frequency Applications
S. Sriram, A.Ward, J. Henning, and S.T. Allen Abstract Significant progress has been made in the development of SiC metal semiconductor field-effect transistors (MESFETs) and monolithic microwave integrated-circuit (MMIC) power amplifiers for high-frequency power applications. Three-inch-diameter high-purity semi-insulating 4H-SiC substrates have been used in this development, enabling high-volume fabrication with improved performance by minimizing surface- and substrate-related trapping issues previously observed in MESFETs. These devices exhibit excellent reliability characteristics, with mean time to failure in excess of 500 h at a junction temperature of 410C. A sampling of these devices has also been running for over 5000 h in an rf high-temperature operating-life test, with negligible changes in performance. High-power SiC MMIC amplifiers have also been demonstrated with excellent yield and repeatability. These MMIC amplifiers show power performance characteristics not previously available with conventional GaAs technology. These developments have led to the commercial availability of SiC rf power MESFETs and to the release of a foundry process for MMIC fabrication. Keywords: MESFETs, MMICs, reliability, rf power, silicon carbide, trapping.
Introduction Silicon carbide metal semiconductor fieldeffect transistors (MESFETs) offer significant advantages for next-generation commercial and military electronic systems due to their increased power density and high operating voltage. These characteristics enable higher-performance, lighter-weight, and wider-bandwidth systems than those using conventional Si or GaAs technology. Recent progress in this technology has been very rapid and includes the availability of 3-in.-diameter high-purity semi-insulating (HPSI) substrates and the development of a stable MESFET fabrication process with high device reliability suitable for practical use. The development of HPSI substrates has been important for eliminating trapping-related issues, such as hysteresis in drain–source current and frequency dependence of drain–source current, that were previously observed in SiC MESFETs.1,2 The larger-diameter substrates have led to lower device costs and have also accelerated the development of SiC monolithic microwave integrated circuits (MMICs), for which commercial foundry services are now
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available. The ability to fabricate complex, high-power SiC MMICs allows another degree of freedom for systems engineers in the development of next-generation radar, electronic warfare, and communications systems.
minimize capacitance due to contact with a substrate. The devices are fully passivated with 0.5 m of silicon nitride for environmental protection and reliability. The MESFET process is capable of very high uniformity. Figure 1 shows a histogram of the threshold voltage (Vth) of 6 mm MESFETs from a completed wafer. The standard deviation in the threshold voltage of 0.6 V illustrates the uniformity of the epitaxial layer and device fabr
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