Papers by Author: Herbert Zirath

Abstract: Abstract. The aim of this study is to compare DC characteristics of ‘as-grown’ and hydrogen (H)-intercalated epitaxial graphenes on SiC substrates [1,2]. Epitaxial graphene is grown on SiC at 1400-1600C, and H-intercalation is performed via in-situ introduction of Hydrogen during the graphitization process [6]. The fabrication processing steps used to define test structures are identical for the two materials. Results on the DC behaviour and uniformity issues with respect to both materials are reported. As-grown material behaves as a linear resistance, while H-intercalated demonstrates a non-linear characteristic. Hysteresis effects and time dependent behaviors are also observed in both materials. Extensive Hall measurements are performed on both materials with the aim of providing a qualitative understanding of material uniformity in both epi-graphenes.

Abstract: SiC MESFETs were scaled both laterally and vertically to optimize high frequency and high power performance. Two types of epi-stacks of SiC MESFETs were fabricated and measured. The first type has a doping of 3×1017 cm-3 in the channel and the second type has higher doping (5×1017 cm-3) in the channel. The higher doping allows the channel to be thinner for the same current density and therefore a reduction of the aspect ratio is possible. This could impede short channel effects. For the material with higher channel doping the maximum transconductance is 58 mS/mm. The maximum current gain frequency, fT, and maximum frequency of oscillation, fmax, is 9.8 GHz and 23.9 GHz, and 12.4 GHz and 28.2 GHz for the MESFET with lower doped channel and higher doping, respectively.

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.