Materials Science Forum Vols. 821-823

Abstract: To improve the high resistance and low Breakdown Voltage (BV) of 4H-SiC SBD, the metal annealing process is usually used to to stabilize SBH. We confirmed that post metal annealing after the chip process also stabilizes SBH by the post annealing experiment of applying failure chips (4H-SiC Ti/Al SBD) that have a forward current (I_F) under 1 [A] with high resistance, because of the metal annealing process error. The result of experiments showed that the I_F increment and BV decrement are proportional to the applied temperatures over 450 °C, and the second additional post annealing shows a decrease of I_F and BV. Aluminum and Titanium transformation with post metal annealing made a decrease of SBH, so that the on-resistance is decreased and BV is decreased (in severe cases, the intense post annealing generates Aluminum spiking). From a result of this work, using a suitable post metal annealing, we can improve the I_F of SiC SBD with a high resistance failure from the metal process event.

Abstract: Characteristics of SiC MOSFETs and SBDs with 3.3 kV-class have been presented. Static Characteristics of the MOSFET showed a specific on-resistance of 14.2 mΩ cm². A breakdown voltage of 3850 V is obtained by using the dose optimized edge termination structure as we have previously reported [1]. At the same time, reverse leakage current of the 3.3 kV SiC SBDs can be suppressed by the JBS structure and the edge termination which is also used in the MOSFETs. By using the MOSFETs and SBDs, we have demonstrated the superior capability of the 3.3 kV 400 A full SiC 2 in 1 modules with a compatible case and terminal configurations to Si IGBT modules. Dynamic characteristics of the full SiC module in an inductive load switching exhibits superior turn-on and turn-off properties even at a high drain voltage of 1650 V, demonstrating the availability of high voltage SiC power systems.

Abstract: We fabricated trench Junction Barrier Schottky (JBS) diodes, and investigated the effect on the reduction of leakage current and the device yield. First, by calculating of electric field at the Schottky contact interface (Es), we found that the trench JBS structure can reduce Es one digit smaller than the planar JBS structure, setting 80^o < The bevel angle θ < 90^o. Then, 600 V / 50 A trench JBS diodes are developed and characterized. The leakage current of a trench JBS diode at 600V is 10^-2 times smaller than that of planar JBS diode by effectively reducing Es. This enables to reduce the number of low break down samples and raise the yield compared to the planar JBS structure.

Abstract: 4H-SiC Schottky Barrier Diodes (SBD) have been developed using p-type buried grids (BGs) formed by Al implantation. In order to reduce on-state resistance and improve forward conduction, the doping concentration of the channel region between the buried grids was increased. The fabricated diodes were encapsulated with TO-254 packages and electrically evaluated. Experimental forward and reverse characteristics were measured in the temperature range from 25 °C to 250 °C. On bare die level, the forward voltage drop was reduced from 5.36 V to 3.90 V at 20 A as the channel doping concentration was increased, which introduced a low channel resistance. By the encapsulation in TO-254 package, the forward voltage drop was decreased approximately 10% due to a lower contact resistance. The on-state resistance of the identical device measured on bare die and in TO-254 package increased with increasing temperature due to the decreased electron mobility in the drift region resulting in higher resistance. The incremental contact resistances of the bare dies were larger than in the packaged devices. One key issue associated with conventional Junction Barrier Schottky (JBS) diodes is a high leakage current at high temperature operation over 200 °C. The developed Buried Grid JBS (BG JBS) diode has significantly reduced leakage current due to a better field shielding at the Schottky contact. The leakage current of the packaged BG JBS diodes is compared to pure SBD and commercial JBS diodes.

Abstract: We compare the static electronic performance of the state-of-the-art Junction-Barrier-Schottky (JBS) rectifier (manufactured by ion-implantation) against the Trench-MOS Barrier Schottky (TMBS) rectifier (manufactured by trench-etching and subsequent oxidation). In our 2D numerical simulations, we have chosen identical specifications for the epitaxial drift-layer (3.3 kV application voltage, e.g. traction) and back-side device-design, while investigating the impact of the top-cell active area design on both rectifier IV-characteristics. To enable a meaningful comparison, we designed the depth d of the shields for the electric field E equally deep (p⁺ peak-plateau d_JBS equals trench-depth d_t), such that the peak E-field (close to avalanche breakdown) inside the drift-layer of the device is located at a comparable depth near the anode-side of the rectifier (see Fig. 2). By studying systematically the ratio between shield-to Schottky contact-length (w/s ratio), we found that the unipolar conduction state of the TMBS design is basically unaffected by enlarging the Schottky-contact length s, which is enterly different for the JBS-case.

Abstract: Infineon’s 5^th Generation of 1200V SiC diodes uses a new compact chip design, realized by an optimized hexagonal merged-pn cell structure in the active area. This allows a higher n-doping in the epi layer due to improved E-field shielding resulting in a smaller differential resistance per chip area. Thanks to the merged-pn cell structure, depending on the diode ampere rating, a surge current capability now rated up to 14 times the nominal current ensures robust diode operation during surge current events in the application. The previous generations of 1200V SiC diodes could not make full use of the high breakdown field strength of the SiC material due to the instable avalanche which occurs at the edge termination only, and therefore, requiring a significant safety margin between rated voltage and breakdown voltage. Now the 5^th Generation is designed in a way that each cell contributes to the avalanche, enabling a much more avalanche rugged device.

Abstract: Electronic properties of radiation damage produced in 1700 V 4H-SiC MPS diodes by proton and carbon irradiation were investigated and compared. 4H-SiC epilayers, which formed the lowdoped N-base of MPS power diodes, were irradiated to identical depth with 670 keV protons and 9.6 MeV C⁴⁺ ions. Results show that irradiation with both projectiles produces strongly localized damage (deep levels) peaking at ion’s projected range. Compared to protons, heavier carbon ions introduce more defects with deeper levels in the SiC bandgap and more stable damage. Radiation damage act as electron traps and compensates donor doping of the epilayer and decreases electron mobility. Forward voltage drop of irradiated diodes then sharply increases when the peak concentration of introduced acceptor levels donor doping. The effect of both the proton and carbon irradiation can be simulated using a simple model accounting only for one dominant electron trap.

Abstract: This work shows thermal simulations of a package of 48/96 high-voltage (6.5 kV/1 kA) PiN diodes. A temperature dependent heat generation for a forward voltage of 3.6 V with a realistic heat generating volume in the diode of (2.7x2.7x0.01) mm3 was used. The thermal coupling of two diodes was determined to be less than 1 % for a distance between the diodes of 10 mm. The temperature distribution for the entire module has been studied for two different ceramic insulating materials, AlN and Al₂O₃, as well as for two numbers of diodes, 48 and 96. This led to a maximum temperature of 106 °C/118 °C (AlN, 48/96 diodes) and 123 °C/144 °C (Al₂O₃, 48/96 diodes). Assuming a constant applied voltage, a variance of ±0.5 V of the characteristic curve (forward voltage versus current) due to variations in the production process was considered fork single diodes. For a shift of +0.5 V for a single diode, the maximum temperature difference to the cooler temperature becomes approximately twice the original difference. Additionally, the operation under constant current (7.1 A, 10.2 A, 14.2 A) was studied including single diode failure. For single diode failure, the resulting change of the maximum temperature would be less than 3 %.

Abstract: Square shaped annular lateral p⁺–i–n⁺ diodes on high purity semi-insulating (HPSI) 4H-SiC are fabricated by Al⁺ and P⁺ ion implantation to obtain anode and cathode regions, respectively. All the diodes have the same size central anode surrounded by an intrinsic region, which is surrounded by an annular cathode. Anode area and annular cathode width are fixed for all diodes, only the lateral length of the intrinsic region is varied. Post implantation annealing is performed at 1950 °C for 10 min. Static forward and reverse characteristics are measured in the temperature range of 30 - 290 °C. For all diodes, the reverse current is below the instrument detection limit of 10^-14 A up to 100 °C at 200 V, the maximum reverse bias employed in this study. The reverse current increased up to low 10⁸ A for 200 V reverse bias at 290 °C. Forward currents overlap at the low voltage region once they exceed the instrument detection limit at ~1.6 V and 30 °C. The forward currents follow almost identical exponential trend at all measured temperatures while the diode series resistance increase with increasing anode-cathode distance and decreased with increasing temperature for the given intrinsic region lateral length.

Abstract: Simulations are presented of a lateral PiN power diode on a Si/SiC substrate for harsh environment, high temperature applications. Thermal simulations compare the Si/SiC solution to SOI, Si/SiO₂/SiC, bulk Si and SiC, showing that the Si/SiC architecture, with its thin Si film intimately formed on SiC, displays significant thermal advantages over any other Si solution, and is comparable to bulk SiC. Detailed electrical simulations show that in comparison to the same device in SOI, a Si/SiC PiN diode offers no deterioration of the on-state performance, improved self-heating effects at increased current and can potentially support higher breakdown voltages.