Andrei Konstantinov

2.2 kV SiC BJTs with Low VCESAT Fast Switching and Short-Circuit Capability

This paper reports large active area (15 mm2) 4H-SiC BJTs with a low VCESAT=0.6 V at IC=20 A (JC=133 A/cm2) and an open-base breakdown voltage BVCEO=2.3 kV at T=25 °C. The corresponding room temperature specific on-resistance RSP-ON=4.5 mΩcm2 is to the authors knowledge the lowest reported value for a large area SiC BJT blocking more than 2 kV. The on-state and blocking characteristics were analyzed by device simulation and found to be in good agreement with measurements. Fast switching with VCE rise- and fall-times in the range of 20-30 ns was demonstrated for a 6 A 1200 V rated SiC BJT. It was concluded that high dynamic base currents are essential for fast switching to charge the BJT parasitic base-collector capacitance. In addition, 10 μs short-circuit capability with VCE=800 V was shown for the 1200 V BJT.

Operation of Silicon Carbide BJTs Free from Bipolar Degradation

The mechanisms of bipolar degradation in silicon carbide BJTs are investigated and identified. Bipolar degradation occurs as result of stacking fault (SF) growth within the low-doped collector region. A stacking fault blocks vertical current transport through the collector, driving the defective region into saturation. This results in considerable drop of emitter current gain if the BJT is run at a reasonably low collector-emitter bias. The base region does not play any significant role in bipolar degradation. Long-term stress tests have shown full stability of large-area high-power BJTs under minority carrier injection conditions provided the devices are fabricated using low Basal Plane Dislocation (BPD) material. However, an approximately 20% current gain compression is observed for the first 30-60 hours of burn-in under common emitter operation, which is related to instability of surface recombination in the passive base region.

Large Area 1200 V SiC BJTs with β>100 and ρON<3 mΩcm2

Large (4.3 mm2) area SiC BJTs were demonstrated with a current gain of 117 and a specific on-resistance of 2.8 mΩcm2. The open-base and open-emitter breakdown voltages are stable and with margin sufficient for 1200 V blocking. Fast and tail-current free switching behaviour was shown with rise- and fall-times in the range of 10-30 ns and the SiC BJTs were shown to be rugged in short-circuit and unclamped inductive switching.

1200 V SiC BJTs with Low VCESAT and High Temperature Capability

Vertical epitaxial NPN SiC BJTs for 1200 V rating were fabricated. Very low collector-emitter saturation voltages VCESAT=0.5 V at IC=6 A (JC=140 A/cm2) and T=25 °C and VCESAT=1.0 V at IC=6 A and T=250 °C were measured. The common emitter current gain at IC=6 A is 71 at T=25 °C and 32 at T=250 °C, respectively. A SPICE model was developed for the BJT including the parasitic capacitances of the internal pn junctions, as well as temperature dependence of the current gain and the collector series resistance. The IC-VCE characteristics of the BJT are in good agreement with the SPICE model between 25 °C and 250 °C. Fast switching measurements were performed showing a VCE voltage fall-time of 22 ns and a VCE voltage rise-time of 11 ns.

Silicon Carbide Schottky Rectifiers with Improved Avalanche Ruggedness

Silicon carbide Schottky-barrier diode (SBD) rectifiers have been manufactured with low on-state voltages, high surge currents and high avalanche ruggedness. Non-destructive unclamped inductive switching currents of 188 A (mean) are achieved for the 1200 V 15 A rectifier. Very tight distribution of maximum sustained UIS current is confirmed. We relate improved avalanche ruggedness to bulk avalanche breakdown and show the breakdown pattern of the new Schottky rectifier being the same type as that for the p-n diode.

Stability of Current Gain in SiC BJTs

In this work, large area SiC BJTs with good long-term stability in 1000 hrs DC stress tests are demonstrated. It is also illustrated how wafer scanning techniques can be used to reject BJT dies with basal plane dislocations, thereby eliminating the risk for bipolar degradation.

Pattern of Near-Uniform Avalanche Breakdown in Off-Oriented 4H SiC

A streaky emission pattern was observed for near-uniform avalanche breakdown in large-area high-voltage p-n diodes in off-oriented 4H SiC. The streaks originate from unilateral expansion of microplasmas in the direction of the C-axis inclination. The unilateral expansion of avalanche plasma is attributed to anisotropy of high-field carrier transport in 4H SiC. At a high current density, the plasma streaks coalesce to form continuous plasma region over the entire area of the high-power device.

1200 V, 3.3 mΩ SiC Bipolar Junction Transistor Power Modules

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.

Detection of crystal defects in high doped epitaxial layers and substrates by photoluminescence

In this work, the detection and characterization of various crystal defects in high doped silicon carbide by photoluminescence (PL) is explored. The detection of basal plane dislocations in high doped epitaxial buffer layers is demonstrated using the near ultraviolet (NUV) spectra. Several characteristic defects in high doped 150mm substrates like grain boundaries and screw dislocations are also detected and characterized using the NUV PL spectra. Further characterization using molten potassium hydroxide etching is presented.