Papers by Author: Hans Hjelmgren

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 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.