Materials Science Forum Vols. 527-529

Abstract: This paper investigates utilization of silicon carbide (SiC) Schottky power diodes as inverter Free Wheel Diodes (FWD) in a commercially available standard Econopak module also packaged with latest generation low-loss IGBT silicon. Static and switching characteristics of SiC diodes over standard module operating temperature 25 0C to 125 0C (298 0K - 398 0K) are measured. Module Turn-on, Turn-off and conduction losses vs. frequency are calculated and measured for three phase motor drive operation. Measurements are compared to standard modules using all Silicon (Si) IGBT- diode. System benefits justifying the increased SiC diode cost, such as EMI reduction, increased efficiency, reduced magnetic filter volume and reduced cooling requirements at higher allowable switching frequencies is investigated.

Abstract: We have carefully investigated a number of more than 120 selected chips fabricated on one wafer, by I-V measurements at two different precisely controlled temperatures and precision CV measurements at room temperature. From these measurements the net-doping concentration, the C-V (flat-band) barrier ΦCV, the ideality n, the apparent Richardson constant Aapp and the apparent I-V barrier Φapp have been extracted for each chip. An extremely unique C-V barrier was determined showing a relative standard deviation (sigma over mean) of only 0.086%. Moreover, the average ideality n was found to be as low as 1.028 exhibiting a relative standard deviation of only 0.35%. A clear linear correlation (ρ2 = 0.968) between ideality n and apparent I-V barrier was observed. The effective Richardson constant A** of 4H-SiC in 〈0001〉 directions could therefore be extracted to be most likely in the interval 70 Acm-2K-2 < A** < 80 Acm-2K-2.

Abstract: The electrothermal behavior of 4H-SiC 600 V Schottky diodes operated in forward mode is analyzed through numerical and analytically-based simulations. It is shown that the unexpected occurrence of voltage surges systematically detected in state-of-the-art devices is a thermally-induced effect due to the compound contribution of a) the negative temperature coefficient of the forward current at high voltages and b) the relatively high package-to-ambient thermal resistance. As a main result, it is demonstrated that the proposed approaches are suitable to accurately predict the value of a “critical” current density beyond which voltage surges may arise.

Abstract: Today silicon carbide (SiC) Schottky diodes are mainly used in the power factor control (PFC) unit of high end switched mode power supplies, due to their outstanding switching performance compared to Si pn diodes. In the case of the PFC it is required that the diodes are capable of handling surge currents up to several times the current of normal operation. The paper shows the surge current capability of a merged pn Schottky diode where the p-areas are optimized as efficient emitters. During normal operation the diode is behaving like a normal Schottky diode whereas during surge current condition the diode is behaving like a pn diode. For a sine half wave of 10 ms we achieved a non repetitive peak forward current capability of about 3700 A/cm2 which is about ten times rated current (for comparison: destructive current density of a standard Schottky diode ~ 1650 A/cm²). Additionally the device shows a stable avalanche and is able to withstand a single shot avalanche of 9.5 3s and 12.5 mJ.

Abstract: We theoretically and experimentally compare the performance of a new JBS rectifier structure, the Buried Channel JBS (BC-JBS) rectifier, with that of the Lateral Channel JBS (LC-JBS) rectifier with 1.5kV blocking capability in 4H-SiC. The BC-JBS rectifier employs buried p-type regions to create a vertical JFET region to reduce the surface electric field at Schottky contact during reverse blocking while the LC-JBS rectifier adds a lateral channel together with the vertical JFET region to protect the surface Schottky interface during high-voltage blocking conditions. The LC-JBS rectifier offers low reverse leakage current while the BC-JBS rectifier demonstrates lower specific on-resistance. The optimized LC-JBS rectifiers show low forward drop (<1.8V) with PiN-like reverse characteristics.

Abstract: Due to the significant achievements in SiC bulk material growth and in SiC device processing technology, this semiconductor has received a great interest for power devices, particularly for SiC high-voltage Schottky barrier rectifiers. The main difference to ultra fast Si pin diodes lies in the absence of reverse recovery charge in SiC SBDs. This paper reports on 4.5kV-8A SiC Schottky diodes / Si-IGBT modules. The Schottky termination design and the fabrication process gives a manufacturing yield of 40% for large area devices on standard starting material. Modules have been successfully assembled, containing Si-IGBTs and 4.5kV-SiC Schottky diodes and characterized in both static and dynamic regimes. The forward dc characteristics of the modules show an on-resistance of 33mohm.cm2 @ room temperatue (RT) and a very low reverse leakage current density (JR < 10 5A/cm2 @ 3.5kV). An experimental breakdown voltage higher than 4.7kV has been measured in the air on polyimide passivated devices. This value corresponds to a junction termination efficiency of at least 80% according to the epitaxial properties. These SiC SBDs are well suited for high voltage, medium current, high frequency switching aerospace applications, matching perfectly as freewheeling diodes with Si IGBTs.

Abstract: The defects formation in ion-irradiated 4H-SiC was investigated and correlated with the electrical properties of Schottky diodes. The diodes were irradiated with 1 MeV Si+-ions, at fluences ranging between 1×109cm-2 and 1.8×1013cm-2. After irradiation, the current-voltage characteristics of the diodes showed an increase of the leakage current with increasing ion fluence. The reverse I-V characteristics of the irradiated diodes monitored as a function of the temperature showed an Arrhenius dependence of the leakage, with an activation energy of 0.64 eV. Deep level transient spectroscopy (DLTS) allowed to demonstrate that the Z1/Z2 center of 4H-SiC is the dominant defect in the increase of the leakage current in the irradiated material.

Abstract: Silicon carbide (SiC) field plate terminated Schottky diodes using silicon dioxide (Si02) dielectric experience high electric field in the insulator and premature dielectric breakdown, attributed to the lower dielectric constant of the oxide. This problem can be addressed by using high-k dielectrics such as silicon nitride (Si3N4) that will reduce the field, increase the breakdown voltage and consequently improve the lifetime of the devices. While the advantages of single step field-plate terminated diodes are well-known, the breakdown voltage can be improved even further using a dual-step field-plate termination. Our 2D-numerical simulations using MEDICI have shown an improvement in breakdown voltages in excess of 25% compared to the traditional single-step field-plate terminated diodes.

Abstract: 4H-SiC floating junction Schottky barrier diodes (Super-SBDs) were fabricated. It was found that their properties are closest to the theoretical limitation, defined by the relationship between specific on-state resistance and breakdown voltage of 4H SiC-unipolar devices. They have a p-type floating layer designed as line-and-spacing. The specific on-state resistances of Super-SBDs with a few micrometers of spacing width were found to be nearly equal to those of conventional SBDs without p-type floating layer. The breakdown voltages of Super-SBDs were higher than those of conventional SBDs. Accordingly the properties of Super-SBDs have improved the trade-off between specific on-state resistance and breakdown voltage, and the highest value to date for Baliga’s Figure of Merit (BFOM) has been obtained.

Abstract: Scaling theory is applied in the design of power devices. The scaling law for power devices is presented. A new figure of merit (HFOM) is derived as an invariant of scale transformation, which is a function of avalanche breakdown field and regarded as a measure of the performance of a power device. The optimization of a SiC Schottky barrier diode with the floating junction structure (Super-SBD) has been performed using the HFOM as a measure of the performance. The performance of the optimized Super-SBD surpasses the performance limit of 4H-SiC devices with the conventional structure.