Papers by Author: G. Gudjónsson

Abstract: We present new results on 4H-SiC RF power MOSFETs. By improvements in device layout we obtain better high frequency performance compared to the first generation of devices. An extrinsic transition frequency fT=11.4 GHz was achieved and fmax=11.2 GHz for a device with 0.5 µm nominal channel length. Functional devices with 0.3 µm nominal channel length were also made. These devices gave fT=15.1 GHz and fmax=19.5 GHz but they have lower breakdown voltages and therefore lower overall performance. The measured devices are double fingered with 0.8 mm total gate width.

Abstract: We demonstrate how sodium enhanced oxidation of Si face 4H-SiC results in removal of near-interface traps at the SiO2/4H-SiC interface. These detrimental traps have energy levels close to the SiC conduction band edge and are responsible for low electron inversion channel mobilities (1-10 cm2/Vs) in Si face 4H-SiC metal-oxide-semiconductor field effect transistors. The presence of sodium during oxidation increases the oxidation rate and suppresses formation of these nearinterface traps resulting in high inversion channel mobility of 150 cm2/Vs in such transistors. Sodium can be incorporated by using carrier boats made of sintered alumina during oxidation or by deliberate sodium contamination of the oxide during formation of the SiC/SiO2 interface.

Abstract: We report investigations of MOS and MOSFET devices using a gate oxide grown in the presence of sintered alumina. In contrast to conventionally grown dry or wet oxides these oxides contain orders of magnitude lower density of near-interface traps at the SiO2/SiC interface. The reduction of interface traps is correlated with enhanced oxidation rate. The absence of near-interface traps makes possible fabrication of Si face 4H-SiC MOSFETs with peak field effect mobility of about 150 cm2/Vs. A clear correlation is observed between the field effect mobility in n-channel MOSFETs and the density of interface states near the SiC conduction band edge in n-type MOS capacitors. Stable operation of such normally-off 4H-SiC MOSFET transistors is observed from room temperature up to 150°C with positive threshold voltage shift less than 1 V. A small decrease in current with temperature up to 150°C is related to a decrease in the field effect mobility due to phonon scattering. However, the gate oxides contain sodium, which originates from the sintered alumina, resulting in severe device instabilities during negative gate bias stressing.

Abstract: We have made a 4H-SiC RF power MOSFETs with cutoff frequency up to 12 GHz, delivering RF power of 1.9 W/mm at 3 GHz. The transistors withstand 200 V drain voltage, are normally-off, and show no gate lag, which is often encountered in SiC MESFETs. The measured devices have a single drain finger and a double gate finger and a total gate width of 0.8 mm. To our knowledge this is the first time that power densities above 1 W/mm at 3 GHz are reported for SiC MOSFETs.

Abstract: We report on fabrication and characterization of n-channel Si face 4H-SiC MOSFETs made using sublimation grown epitaxial material. Transistors made on this material exhibit record-high peak field effect mobility of 208 cm2/Vs while reference transistors made on a commercial epitaxial material grown by chemical vapor deposition (CVD) show field effect mobility of 125 cm2/Vs. The mobility enhancement is attributed to better surface morphology of the sublimation grown epitaxial layer.

Abstract: Lateral inversion channel metal-oxide-semiconductor field-effect transistors (MOSFETs) were manufactured on 6H-SiC and two gate oxidation recipes were compared. In one case the gate oxide was grown in N2O using quartz environment. The resulting peak field-effect mobility was µFE=43 cm2/Vs. In the other case the gate oxide was grown in oxygen using alumina environment and the resulting peak field-effect mobility was µFE=130 cm2/Vs. Oxidizing in an environment made from sintered alumina introduces contaminants into the oxide that effect the oxidation in several^ways. The oxidation rate is increased and the resulting SiC/SiO2 interface allows higher inversion channel mobility.

Abstract: We report investigations of Si face 4H-SiC MOSFETs with aluminum ion implanted gate channels. High quality SiO2/SiC interface is obtained both when the gate oxide is grown on p-type epitaxial material and when grown on ion implanted regions. A peak field effect mobility of 170 cm2/Vs is extracted from transistors with epitaxially grown channel region of doping 5x1015 cm-3. Transistors with implanted gate channels with aluminum concentration of 1x1017 cm-3 exhibit peak field effect mobility of 108 cm2/Vs, while the mobility is 62 cm2/Vs for aluminum concentration of 5x1017 cm-3. The mobility reduction with increasing acceptor density follows the same functional relationship as in n-channel Si MOSFETs.