Materials Science Forum Vols. 600-603

Abstract: The turnoff mechanism of SiC buried gate static induction transistors (SiC-BGSITs) were analyzed by three dimensional device simulation. A current crowding occurs in the portion near the channel center away from the gate contact during the initial phase of the turnoff operation, which is resulted from a non-uniform potential distribution through the gate finger with the displacement current flowing there. This increases the turnoff delay time. The impact of source length on the turnoff performance was made clear.

Abstract: This paper reports on the fabrication and testing of 6H-SiC junction field effect transistors (JFETs) and a simple differential amplifier integrated circuit that have demonstrated 2000 hours of electrical operation at 500 °C without degradation. The high-temperature ohmic contacts, dielectric passivation, and packaging technology that enabled such 500 °C durability are briefly described. Key JFET parameters of threshold voltage, on-state resistance, transconductance, and on-state current, as well as the gain of the differential amplifier integrated circuit, exhibited less than 7% change over the first 2000 hours of 500 °C operational testing.

Abstract: This paper presents silicon carbide sensor interface circuits and techniques for MEMSbased sensors operating in harsh environments. More specifically, differential amplifiers were constructed using integrated, depletion-mode, n-channel, 6H-SiC JFETs and off-chip passive components. A three-stage voltage amplifier has a differential voltage gain of ~50 dB and a gainbandwidth of ~200 kHz at 450oC, as limited by test parasitics. Such an amplifier could be used to amplify the signals produced by a piezoresistive Wheatstone bridge sensor, for example. Design considerations for 6H-SiC JFET transimpedance amplifiers appropriate for capacitance sensing and for frequency readout from a micromechanical resonator are also presented.

Abstract: SiC Lateral Trench JFET (LTJFET) technology is demonstrated as a promising candidate for use in high-temperature wireless telemetry systems. 4H-SiC LTJFETs were designed, fabricated and characterized for DC, and small-signal AC and RF performance at different case temperatures. Four-fold drain current reduction was observed at 460°C as compared to RT measurements. The measured threshold voltage shift was less than 2.3 mV/°C from 21°C to 460°C. A simple common source amplifier built using a fabricated device demonstrated stable small-signal AC performance after 100 hrs of operation at 450°C. Small-signal RF measurements were carried out on the packaged devices at different temperatures. GMax above 8 dB was measured over the L-band frequency range at RT. The average degradation of small-signal power gain measured at f=250 MHz did not exceed 0.0125 dB/ °C over the temperature ranging from 21°C to 365°C.

Abstract: A series of high voltage (HV) and low voltage (LV) lateral JFETs are successfully developed in 4H-SiC based on the vertical channel LJFET (VC-LJFET) device platform. Both room temperature and 300 oC characterizations are presented. The HV JFET shows a specific-on resistance of 12.8 mΩ·cm2 and is capable of conducting current larger than 3 A at room temperature. A threshold voltage drop of about 0.5 V for HV and LV JFETs is observed when temperature varies from room temperature to 300 oC. The measured increase of specific-on resistance with temperature due to a reduction of electron mobility agrees with the numerical prediction. The first demonstration of SiC power integrated circuits (PIC) is also reported, which shows 5 MHz switching at VDS of 200 V and on-state current of 0.4 A.

Abstract: 400V/2.5A 4H-SiC JFETs having a reduced surface field (RESURF) structure were fabricated. Measurements on the static and switching characteristics were carried out. The on-resistance was 0.86 W. The turn-on time (ton) and the turn-off time (toff) were 8ns and 10 ns, respectively. The fabricated JFETs showed low on-resistance and fast switching characteristics. 4H-SiC RESURF-type JFETs, which is a sort of lateral transistor, are preferable to a module configuration of switching devices. Moreover, they are promising for application to DC power supplies with higher efficiency and smaller size owing to their low on-resistance and fast switching characteristics.

Abstract: This paper reports fabrication and electrical characterization of 6H-SiC n-channel, depletion-mode, junction-field-effect transistors (JFETs) for use in high-temperature analog integrated circuits for sensing and control in propulsion, power systems, and geothermal exploration. Electrical characteristics of the resulting JFET devices have been measured across the wafer as a function of temperature, from room temperature to 450oC. The results indicate that the JFETs are suitable for high-gain amplifiers in high-temperature sensor signal processing circuits.

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 fabricated a 0.5-μm-gate MESFET on a bulk 4H-SiC semi-insulating substrate using ion implantation for the channel and contact regions. Our device design used a thin, highly doped channel layer, which was implanted at single energy to improve the device’s RF characteristics. The electrical characteristics of the ion-implanted MESFET annealed at 1700°C were better than those of the ion-implanted MESFET annealed at 1300°C. The fabricated ion-implanted MESFET has a maximum transconductance of 32.8 mS/mm and an fT/fmax of 9.1/26.2 GHz. The saturated output power was 26.2 dBm (2.1 W/mm) at 2 GHz. These values were the same as those of the conventional epitaxial MESFET with a recessed gate.

Abstract: Confocal μ-Raman was used to measure the operating temperatures in SiC MESFETS, AlGaN/GaN/SiC HEMT’s and 4H-SiC PiN diodes. Temperatures obtained from thermal imaging of the MESFETS compared well with those measured from Raman scattering. Operating temperatures were also obtained for large area PiN diode and it was shown that a single point at the center of the device can be used to measure the average temperature.