Papers by Author: Mykola S. Boltovets

Abstract: Multi-diode broadband microwave signal modulators based on 4H-SiC p-i-n diodes were fabricated and fully characterized. The three-diode modulators are characterized by a transmission loss of 1-2 dB and isolation of 27-34 dB in the 2-7 GHz frequency range. Two-diode modulators were specially designed for high-temperature operation. These modulators are characterized by a transmission loss of 1.1-2.6 dB and isolation of 33-44.5 dB, in the 2-7 GHz frequency range at temperatures up to 300°C.

Abstract: 4H-SiC p-i-n diodes were designed, fabricated and characterized for use in microwave applications. The diodes exhibited a blocking voltage of 1100 V, a 100 mA differential resistance of 1-3 &, a capacitance below 0.5 pF at a punchthrough voltage of 100 V and a carrier effective lifetime between 15-27 ns. Single 4H-SiC p-i-n diode switches, operating in X-band, exhibited insertion loss 0.7 dB, isolation up to 25 dB and were able to handle microwave power up to 2.2 kW in pulsed mode of operation. The switching speed of the switches has not exceeded 20 ns.

Abstract: 4H-SiC p-i-n diodes were fabricated on epitaxial layers grown by Sublimation Epitaxy in Vacuum (SEV) and were evaluated for microwave power switching applications. Full electrical characterization (C-V, DC I-Vs, reverse recovery characteristics, low and high power microwave testing) has been performed. The results showed that SEV-grown SiC material is suitable for bipolar device fabrication. A doping higher than 1019 cm-3 for the p-type contact layer and lower than 1016 cm-3 for the n-type base layer is necessary to demonstrate microwave p-i-n diodes with similar performance as the ones fabricated on commercially available CVD-grown material.

Abstract: Forward current-voltage (I-V) characteristics and non-equilibrium carrier lifetime, τ were measured in 4H-SiC pin diodes (10-kV rated, 100 μm base width). The τ value was found to be 3.7 μs at room temperature by measurements of open circuit voltage decay. To the best of the authors' knowledge, the above lifetime value is the highest reported for 4H-SiC. The forward voltage drops were measured to be 3.44 V at current density of 100 A/cm2 and 5.45 V at 1000 A/cm2 showing a very deep modulation of the blocking base by injected carriers. Diodes operated well at elevated temperatures up to 400oC. No essential forward degradation was detected after 300- A×min current stress at 400oC.

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: Steady-state and transient characteristics of packaged 6-kV 4H-SiC junction diodes have been investigated in the temperature range Т = 300 – 773 К. Analysis of the forward current-voltage characteristics and reverse current recovery waveforms shows that the lifetimeτ of non-equilibrium carriers in the base of the diodes steadily increases with temperature across the entire temperature interval. The rise in τ and decrease in carrier mobilities and diffusion coefficients with increasing temperature nearly compensate each other as regards their effect on the differential resistance of the diode, Rd. As a result, Rd is virtually temperature independent. An appreciable modulation of the base resistance takes place at room temperature even at a relatively small current density j of 20 A/cm2. At T = 800 K and j = 20 A/cm2, a very deep level of the base modulation has been observed. The bulk reverse current is governed by carrier generation in the space-charge region via a trap with activation energy of 1.62 eV. The surface leakage current of packaged structures does not exceed 2×10-6 А at T = 773 K and a reverse bias of 300 V.

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: In this communication we present the results of study of new contact systems to GaN epitaxial layers grown on sapphire and n-type 6H-SiC monocrystals. The TiBx nanostructure phase has been used during manufacturing Ti – Al – TiBx – Au and TiBx contact systems. The n-GaN epitaxial layers of 1 µm thickness were grown on [0001] sapphire substrate by vapor-phase epitaxy. The n-type 6H-SiC monocrystals were grown by Lely method with the donor concentration of 2x1018 cm3. The layers of Ti, Al, TiBx and Au were deposited by magnetron sputtering followed by high-temperature annealing.

Abstract: The switching characteristics of 4Н-SiС p-i-n diodes with 6 µm long i-region were investigated in the 20÷500 °С temperature ranges. It is shown that the diode reverse current increases with temperature and does not exceed 10-7 А at temperature of 500 °С (UR = 100 V). The diode resistance rF at forward current of 40 mA decreases as temperature increases from 20 up to 500 °С. The effective minority charge carrier lifetime in the i-region (τр) was determined from the diode switching (from forward current to reverse voltage) characteristics; it was about 5 ns. As temperature increases from 20 up to 500 °С, τр increases by a factor of 3. We discuss the possibility of application of such diodes (i) in microwave switching facilities and (ii) as temperature sensors. A comparison is made between the parameters of 4Н-SiС p-i-n diodes and those of Si p-i-n diodes with comparable values of calculated blocking voltage.