Materials Science Forum Vols. 740-742

Abstract: 4H-SiC bipolar Darlington transistors (D-BJTs) for low voltage applications have been fabricated, simulated and characterized up to 300 °C, where they exhibit a current gain of 460. The influence on D-BJT current gain of relative current capability of driver and output BJTs has been investigated, and the collector resistance has been identified as the main limitation for the D-BJTs.

Abstract: Epoxy moulded power modules with a small footprint of 40 mm x 55 mm were fabricated with two switches, each consisting of six parallel 1200 V 50 A rated BJTs and Schottky diodes. The SiC-based power modules have very low on-resistance of 3.3 mΩ and a current gain of 80, both at room temperature. An inverter with specially designed drive circuits was constructed using the power modules and an efficiency of 98.5 % was shown for an output power of 12 kW.

Abstract: Non ion-implantation mesa etched 4H-SiC BJT with three-zone JTE of optimized lengths and doses (descending sequences) has been simulated. This design presents an efficient electric field distribution along the device. The device area has been optimized and considerably reduced. As a result of this comprehensive optimization, a high breakdown voltage and high current gain have been achieved; meanwhile the device area with a constant emitter and base contact area has been reduced by about 30%.

Abstract: In this paper, we report our recently developed 1 cm², 15 kV SiC p-GTO with an extremely low differential on-resistance (_RON,diff) of 4.08 mΩ•cm² at a high injection-current density (J_AK) of 600 ~ 710 A/cm². The 15 kV SiC p-GTO was built on a 120 μm, 2×10¹⁴/cm³ doped p-type SiC drift layer with a device active area of 0.521 cm². Forward conduction of the 15 kV SiC p-GTO was characterized at 20°C and 200°C. Over this temperature range, the R_ON,diff at J_AK of 600 ~ 710 A/cm² decreased from 4.08 mΩ•cm² at 20°C to 3.45 mΩ•cm² at J_AK of 600 ~ 680 A/cm² at 200°C. The gate to cathode blocking voltage (V_GK) was measured using a customized high-voltage test set-up. The leakage current at a V_GK of 15 kV were measured 0.25 µA and 0.41 µA at 20°C and 200°C respectively.

Abstract: A growing demand for smart and flexible photovoltaic power conversion and pulsed-power systems is leading to rapid development and commercialization of medium voltage 6.5 - 24 kV, wide-bang gap rectifiers and switches. Conventional silicon bipolar switches are limited to roughly 8 kV breakdown voltages and scaling up the voltage rating requires very thick wafers presenting significant manufacturing challenges. Very thick drift layers of silicon devices also translate into a very high minority carrier charge injected during forward conduction for an efficient conductivity modulation, hence leading to an extremely slow switching speed and poor efficiency. In this paper USCi presents the development of 6.5 kV 4H-SiC gate-turn-off thyristors (GTOs) with multiple floating guard-ring edge termination, and describes their application in an AC-link grid-tied solar inverter system.

Abstract: In this paper we highlight our latest results on high voltage SiC thyristors comprising an etched JTE. Compared to our previous design concepts, the thyristors described here are larger in size and have been investigated regarding pulsed power applications. Quasi-static on-state characteristics show that the devices withstand a repetitive current load of up to 16 A corresponding to a current density of 825 A/cm². Their switching behavior was evaluated up to 1000 V demonstrating characteristic waveforms at turn-on and gate turn-off. Moreover, pulsed current characteristics show that the typical device under test sustained a current pulse of 20 μs with a peak value of 200 A and 10 kA/cm², respectively.

Abstract: We report on switch-on of 12 kV, 1cm2 optically triggered 4H-SiC thyristor fabricated by CREE Inc., to Imax=270 А with current rise time of ~ 3 s. Temperature dependence of holding current Ih in this thyristor has been experimentally studied in the temperature range from 300 to 425 K. It is shown that measurements of Ih temperature dependence under condition of optical switch-on at small anode bias and large load resistance reveal the existence of a ”weak point” within the optical window. This point is characterized by a much smaller critical charge than that within the remaining part of the window.

Abstract: Two designs (A and B) of 4H-SiC thyristors for pulse power applications were implemented and characterized in this work. Both designs have the same layout and epi-layer stack except for the anode layers: thyristors with design A (baseline) had a thin (~0.5 um) anode while devices with design B (optimized) consisted of a heavily doped cap layer (~0.5 um, ~10¹⁹/cc) and ~1.5 um p-type layer with lower doping (~1018/cc). All devices were fabricated in 4” 4H-SiC sub^{Superscript text}strates (three wafers per each design) and were fully characterized at the wafer level including measurements of forward voltage, blocking voltage, leakage current, and holding current. It was shown that the mean value of the holding current in the thyristors with thin anode was significantly higher (0.7A) than that of the thyristors with thick anode (0.1A), while other parameters had practically the same values. The open circuit voltage decay (OCVD) method was used for measurements of the minority carrier lifetime in order to correlate it with the holding current. Impact of material properties and device design parameters on the holding current is discussed as well.

Abstract: This paper describes using of silicon carbide for micromechanical systems. Low stressed sensitive membrane signal converters, thin film transducers and piezoresistive sensors were formed based on silicon carbide films. The mechanical properties of silicon carbide films were determined.

Abstract: This paper presents an improved version and new results on a temperature sensor based on SiC Schottky Barrier Diode (SBD). SiC SBD structures of different areas were packaged in a metallic-glass case. The encapsulated sensor was electrically measured at several temperatures. A good linearity of the forward voltage measured at a constant current versus temperature dependence was obtained in the temperature range of 150-400°C where the sensor is meant to operate. Optical investigation, correlated with electrical measurements, prove the reliability of the sensor structure and of the package solutions at temperatures up to 400°C.