Papers by Author: Per Åke Nilsson

Abstract: Silicon Carbide MESFETs for microwave frequencies were made using a field-plated buried gate approach. The devices were fabricated using passivation oxides with different interface trap densities. By using a passivation oxide with a reduced interface trap density, grown in a sodium containing ambient, it was possible to achieve a very high continous wave output power density of the device: 8 W/mm at 3 GHz and 1 dB compression.

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 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: In order to increase the output power and drain efficiency, MESFETs in SiC have been made with a double gate recess technique. Typical device characteristics of the MESFETs are drain currents of 380mA/mm, breakdown voltages of 80V and ft/fmax of 10/25 GHz respectively. These transistors exhibit power densities of 3W/mm@3GHz in class AB operation and drain efficiencies of 60%. Packaged devices with 3 mm gate periphery of this type, with via-hole grounding, gave power densities of 1.2 W/mm@6GHz at 50 V drain bias.

Abstract: Planar microwave Schottky diodes on 4H-SiC have been designed, processed and measured. Different Schottky metals were tested to study the influence on the microwave performance. A maximum extrinsic cut-off frequency of 30.8GHz was achieved for a Tungsten/SiC-Schottky diode. The diode geometry dependence on both the cut-off frequency and the breakdown voltage was investigated. The breakdown voltage was found to be linearly dependent on the anode-cathode distance.

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: 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.