Papers by Author: Sigo Scharnholz

Abstract: This paper presents for the first time a comparison between experimental measurements of Optical Beam Induced Current (OBIC) and finite element simulations on high-voltage bipolar diodes. Two peripheral protection structures were chosen: a simple MESA protection and a MESA + JTE combination. Comparable experimental and simulated results were obtained in both cases.

Abstract: 4H-SiC thyristors are of particular interest in pulsed power applications due to their ability to block high voltages and transfer high current densities along with fast switching times. Here, we present a paper that demonstrates both (i) the impact of the design parameters on the blocking characteristics based on simulations taking into account the anisotropy of 4H-SiC and (ii) a critical comparison to real devices having equivalent epitaxial structural design. Simulations and measurements show that an etched junction termination extension (JTE) is suitable to design high-voltage SiC thyristors. Concerning breakdown voltage, the real devices data agree to simulations for junction termination extension thickness in the relevant region. Besides the actual JTE thickness and doping concentration, the presence of a relatively thin field-stop layer might explain the discrepancy between experiment and simulation.

Abstract: In this paper, the static and dynamic characterization of a High Voltage (10kV) 4H-SiC Bipolar Junction Transistor (BJT) is presented. Using a high-voltage source in vacuum conditions, a breakdown voltage of 11 kV was measured. Results showed that both large and small BJTs exhibit similar on-state resistance per unit area and collector current density of 55 A.cm^-2. The current gain increases with a decrease in temperature, indicating reduced charge carrier recombination at lower thermal energies. Also, BJT have been characterized in switching mode at 1 kV. The study concludes that 4H-SiC BJT demonstrates promising electrical performance for high-efficiency applications in harsh environments.

Abstract: In this paper, a first demonstration of the optical triggering of a 10 kV 4H-SiC Bipolar Junction Transistor is reported. A laser emitting UV (349 nm) has been used for the generation electron-hole pairs within the device. A current density of about 20 A.cm^-2 has been obtained. This low value in comparison with 100 A.cm^-2 for “conventional” BJT is due to the narrow pulse width (5 ns). The current waveform shows the effect of the carrier lifetime in the base and collector regions. From these measurements, we have extracted the I_C (V_CE) characteristics for different laser optical power and the switch-on time which is about 1 µs.

Abstract: This paper presents experimental 1.2 kV, 10 A SiC thyristors with different amplifying gate design. In contrast to comparative devices (with simple gate) the amplifying gate thyristors show a characteristic snap-back and a higher gate current to trigger. Their gate-anode I-V characteristics comply with the underlying design constraint, regarding the resistances of pilot and main thyristor: (R_P > R_M). Moreover, the turn-on waveforms of well-designed amplifying gate thyristors reveal peak-shaped inversions in the gate current and voltage transients, providing clear evidence of the successive triggering of pilot and main thyristor.

Abstract: Optical Beam Induced Current (OBIC) measurements are performed on 4H-SiC avalanche diodes with a very thin and a highly doped active region. A pulsed green laser, with a wavelength of 532 nm, illuminates a reverse biased diode leading to generate electron-hole pairs in the space charge region. Comparison between the 4H-SiC bandgap and the incident photon energy shows that single photon absorption process can be neglected and two-photon absorption process dominates in this case. Ionization rates are then extracted from multiplication curve in a high electric field range (3 to 5 MV.cm^–1). Results are in good agreement with previous ones obtained on the same diodes using single photon absorption process.

Abstract: The behavior of 4H-SiC power devices in severe environment with varying temperature is a key characteristic indicating their reliability. This paper shows the dependence of the ionization rates of 4H-SiC with respect to temperature. Optical Beam Induced Current (OBIC) measurements have been performed on PN junctions to determine the multiplication coefficient for temperature varying between 100 and 450K. That allows extracting the ionization rates by fitting the curves of multiplication coefficient.

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: Optical Beam Induced Current (OBIC) measurements have been performed on 4H-SiC avalanche diodes with very thin and highly doped avalanche region. The light source used in this study is an Ar-laser with a wavelength of 351 nm which results in a mixed carrier injection. From these measurements, impact ionization coefficients for 4H-SiC have been extracted in the electric field range from 3 to 4.8 MV/cm. In combination with ionization coefficients in our previous paper extracted from diodes with lowly doped avalanche region, we propose a set of parameters of impact ionization coefficients for 4H-SiC, applicable to a wide electric field range.

Abstract: 4H-SiC vertical bipolar power diodes have been fabricated with bilayer metallic anode contact based on an Al-Ti-Ni ohmic contact and a thick Al over-metallization. An optical window of 100 × 100 μm2 has been created through the anode contact with a SIMS Cameca IMS 4F equipment using Cs+ primary ions at 10 kV and with a beam spot size of 100 nm. The current/voltage characteristics of the diodes show that the SIMS process does not induce an increase of the leakage currents in forward nor in reverse bias. OBIC UV photogeneration occurs under the optical window and not under the contact metal.