Materials Science Forum Vols. 527-529

Abstract: The charge generated in 6H-SiC n+p diodes by oxygen (O) ion irradiation at energies between 6 and 15 MeV was evaluated using the Transient Ion Beam Induced Current (TIBIC). The signal peak of the transient current increases, and the fall-time decreases with increasing applied reverse bias. The value of collected charge increases with increasing applied reverse bias, and the saturation of the collected charge was observed in high reverse bias regions (e.g. above 70 V in the case of 12MeV O-irradiation). The charge generated in the deeper region than the depletion layer is collected due to the "funneling effect". Almost all charge generated in n+p SiC diodes by O-irradiation between 6 and 15 MeV is collected when the length of the depletion layer becomes longer than the projection range of ions.

Abstract: A betavoltaic cell in 4H SiC is demonstrated. An abrupt p-n diode structure was used to collect the charge from a 1mCi Ni-63 source. An open circuit voltage of 0.95V and a short circuit current density of 8.8 nA/cm2 were measured in a single p-n junction. An efficiency of 3.7% was obtained. A simple photovoltaic type model was used to explain the results. Good correspondence with the model was obtained. Fill factor and backscattering effects were included. Efficiency was limited by edge recombination and poor fill factor.

Abstract: Forward voltage (VF) drift, in which a 4H-SiC PiN diode suffers from an irreversible increase in VF under forward current flow, continues to inhibit commercialization of 4H-SiC PiN diodes. We present our latest efforts at fabricating high blocking voltage (6 kV), high current (up to 50 A) 4H-SiC PiN diodes with the best combination of reverse leakage current (IR), forward voltage at rated current (VF), and VF drift yields. We have achieved greater than 60% total die yield onwafer for 50 A diodes with a chip size greater than 0.7 cm2. A comparison of the temperature dependent conduction and switching characteristics between a 50 A/6 kV 4H-SiC PiN diode and a commercially available 60 A/4.5 kV Si PiN diode is also presented.

Abstract: Forward voltage degradation has been reduced by fabricating diodes on the (000-1)C-face. The reverse recovery characteristics of the 4H-SiC pin diode on the (000-1)C-face have been investigated. The pin diode on the C-face has superior potential to that on the Si-face among all parameters of the reverse recovery characteristics. The pin diode on the Si-face after conducting a current stress test tends to exhibit a fast turn-off as compared with that before conducting the stress test. On the C-face, however, there is little difference in reverse recovery characteristics between before and after conducting the current stress test.

Abstract: The on-state and switching performance of high voltage 4H-SiC junction rectifiers are compared using numerical simulations and experimental characterization. Epitaxial and implanted anode PiN diodes as well as novel, advanced rectifiers have been fabricated in 4H-SiC using 110μm thick drift layers. The relatively low forward voltage drop of these epi-anode diodes (4.2V @ 100A/cm2) indicates moderate conductivity modulation, while the superior switching performance of the “MPS-like” rectifiers is demonstrated with reverse recovery characteristics at various temperatures and forward current densities.

Abstract: 4H-SiC PiN rectifiers with implanted anode and single-zone JTE were fabricated using AlN capped anneal. The surface damage during the high temperature activation anneal is significantly reduced by using AlN capped anneal. The forward drop of the PiN rectifiers at 100A/cm2 is 3.0V while the leakage current is less than 10-7A/cm2 up to 90% breakdown voltage at room temperature. With 6μm thick and 2×1016cm-3 doped drift layer, the PiN rectifiers can achieve near ideal breakdown voltage up to 1050V. Hole impact ionization rate was extracted and compared with previously reported results.

Abstract: We present the design and fabrication of the first high-voltage 4H-SiC RF power limiter. First, Schottky and PiN diodes are compared for power limiter applications by numerical simulations. Small-signal S-parameters and large-signal operation are then simulated based on our device design. The fabrication is based on a SiC PiN diode with at least 900V blocking capability and packaged into a 50
microstrip transmission line fixture. Small signal insertion and return losses are measured in the frequency band from 100MHz to 3GHz after packaging and agree well with our simulation. The limiter has an insertion loss of only 0.6dB at 1GHz. This is believed to be the first 4H-SiC high-voltage RF power limiter, as well as one of the best SiC limiters in terms of low small-signal transmission loss up to gigahertz frequencies.

Abstract: The packaged microwave 4H SiC pin diode chips (with i-region length of 6 μm, mesa diameter of 80 μm and blocking voltage of 1000 V) were investigated. We studied the parameters of diode I−V curve (in particular, the diode resistance RS at forward current) and the processes of diode switching from forward current of 50 mA to reverse voltage of 15 V, as well as C−V curves, in the 20−700 °C temperature range. At a voltage of 300 V, the diode reverse current was 10 (180) μA when temperature was 600 (700) °C. At a forward current of 40 mA, the diode resistance first decreases smoothly as temperature is increased from 20 up to 300 °C, and then grows up. As temperature is increased from 20 up to 700 °C, the effective lifetime τeff grows from 7 up to 50 ns, while the diode capacitance (in the 0−40 V reverse voltage range) grows smoothly as temperature is increased from 20 up to 400 °C.

Abstract: The results of mathematical simulation, development and investigation of a modulator with 4H SiC pin diodes are presented. We simulated the effect of bias modes on isolation and transmission between the modulator input and output in the 1−20 GHz frequency range, for pin diodes with 6 μm long i-region. It was calculated that the isolation in a modulator with three diodes may run into –45 dB, the transmission losses being no more than 2 dB. The modulator was made as an integrated circuit (IC) on the basis of nonsymmetrical strip lines (characteristic impedance of 50 Ω) incorporating chips of high-voltage 4H SiC pin diodes with iregion 6 μm long, mesa diameter of 60 μm and calculated avalanche breakdown voltage of 1000 V. We studied the experimental parameters of this modulator as a function of forward current and reverse voltage in the 2.4−12 GHz frequency range, as well as the microwave signal switching behavior. It was determined that the modulator is characterized by transmission losses of 1.0−2.0 dB and isolation of 27−34 dB (in the 2.4−7 GHz frequency range). The formation of microwave pulses with leading (trailing) edge of 22 (29) ns was also observed.

Abstract: The temperature-dependences of ionized dopant concentration at different doping levels are generalized and the preconditions for thermal instabilities due to self-heating are studied. The nonisothermal simulations of forward-biased SiC structures over a wide temperature range are performed by using the drift-diffusion 1D-simulator DYNAMIT. Results show that the incomplete doping ionization will be an important effect if impurity activation energies exceed 0.1, 0.2 and 0.3 eV for doping levels 1019, 1018 and 1017cm−3, respectively. For appearance of S-shaped selfheated I/V curves the respective values must exceed 0.2, 0.3 and 0.4 eV. Strong influence of incomplete dopant ionization on forward I/V curves of realistic 4H-SiC and 6H-SiC p-i-n structures is predicted by simulations. At that the dominating role of the thick substrate layer is shown.