SiC Schottky Rectifiers Research Articles

(Invited) How to Achieve Low Thermal Resistance and High Electrothermal Ruggedness in Ga2O3 Devices?

Ultra-wide bandgap gallium oxide (Ga2O3) devices have recently emerged as promising candidates for power and RF electronics. The low thermal conductivity of Ga2O3 has arguably been the most serious concern for these devices. Despite many simulation studies, there still lacks an experimental report on the thermal resistance and electrothermal ruggedness of a large-area, packaged Ga2O3 device. Recently, our team for the first time demonstrated large-area Ga2O3 devices with different packaging configurations and measured the thermal resistance and surge current capabilities of these packaged Ga2O3 devices. This paper reviews the key results in our efforts. It is shown that, contrary to some popular belief, Ga2O3 devices with proper packaging can achieve high thermal performance in both short transients and the steady state. The double-side-packaged Ga2O3 Schottky rectifiers show a junction-to-case thermal resistance lower than that of the similarly-rated commercial SiC Schottky rectifiers. In addition, these Ga2O3 rectifiers can survive a higher peak surge current as compared to SiC rectifiers. The critical enabler for these excellent performances is the direct junction cooling with minimal heat going through the Ga2O3 chip. Our work proves the viability of Ga2O3 devices for high power applications and manifests the significance of packaging for their die-level thermal management.

Surge Current and Avalanche Robustness of Commercial 1200 V SiC Schottky Diodes

This paper investigates the 10 ms surge current (IFSM) and single-pulse avalanche energy (EAS) limits of 1200 V/10 A SiC Schottky rectifiers from several commercial vendors. GeneSiC’s 1200 V/10 A diode recorded maximum IFSM values of 162 A at TC=25°C and 135 A at TC=150°C, which were the highest amongst investigated devices. The diode sustains junction temperatures as high as 910°C during surge current operation, which results in eventual failure of the Aluminum based metallization.

SiC MPS Devices: One Step Closer to the Ideal Diode

We report on the development of a new generation of SiC Schottky rectifier devices employing a Molybdenum based barrier metal system and a new stripe cell design for field shielding and optimized area utilization. The Schottky barrier height is reduced and thus the conduction losses are decreased significantly. The balance between forward conduction and reverse leakage losses as well as the homogeneity and stability of the new barrier system are investigated carefully.

Silicon Carbide Junction Transistors and Schottky Rectifiers optimized for 250°C operation

ABSTRACTElectrical performance and reliability of SiC Junction Transistors (SJTs) and Schottky rectifiers are presented. The 650 V/50 A-rated SiC SJTs feature current gains (β) up to 110 at room-temperature, 70 at 250°C, and stable breakdown characteristics. Single current pulse measurements indicate an almost invariant β up to 800 A/cm2 at 175°C – a measure of the SOA boundary for pulsed current SJT operation. Lower than 5 mA/cm2 leakage currents are measured on the SJTs at the rated blocking voltage and at 250°C. 1200 V Schottky rectifiers designed for high-temperature operation display < 3 mA/cm2 leakage currents up to 250°C. A 10x reduction in leakage current and 23% reduction in junction capacitance are observed when compared to the nearest competitor. The high-temperature Schottky rectifiers and SJTs display stable breakdown voltages and on-state characteristics after long-term HTRB stressing. A significant improvement in current gain stability is achieved by fine-tuning the fabrication process.

1200 V SiC Schottky Rectifiers optimized for ≥ 250 °C operation with low junction capacitance

Electrical Characteristics of Industry's first commercially available 1200 V rated SiC Schottky rectifiers, specially designed for operation at ≥ 250 °C are presented. These high-temperature SiC rectifiers fabricated in 1, 5, and 20 A current ratings feature reverse leakage currents of < 3 mA/cm2 at 1200 V up to temperatures as high as 300 °C. GeneSiC's 1200 V/20A High Temperature Schottky (designated SHT) rectifier offers a 10x reduction in leakage current and a 23% reduction in junction capacitance when compared to its nearest SiC Schottky rectifier competitor. In addition, these SHT rectifiers demonstrate superior surge-current ratings, and temperature-independent switching capability up to their rated junction temperatures.

>1200 V, >50A SILICON CARBIDE SUPER JUNCTION TRANSISTOR

The electrical performance of GeneSiC's 1200 V/7 A SiC Super Junction Transistor (SJT) is compared with three best-in-class commercial Si IGBTs in this paper. Low leakage currents of < 100 μA at 325 °C operating temperature, switching transients < 15 ns at 250 °C, Common Source current gains of 63 and on-resistance as low as 220 mΩ were measured on the SiC SJTs. For switching 7 A, 800 V at 100 kHz, the SiC SJT+GeneSiC SiC Schottky rectifier as Free Wheeling Diode (FWD) achieved a total power loss reduction of about 64% when compared to the best all-Si IGBT+FWD configuration and a power loss reduction of about 47 %, when compared to the best Si IGBT + SiC Schottky FWD.

Active Devices for Power Electronics: SiC vs III-N Compounds – The Case of Schottky Rectifiers

The main rectifier device structures for power electronics based on SiC and on GaN are compared and the main issues for each structure are evaluated in terms of performance and manufacturability. The driving volume markets for power electronics devices correspond to the systems working on 127, 240 and 400 V energy supply networks, setting the device voltage handling to 300, 600, and 1200V respectively. We have limited the scope hereafter to the 600 V typical target, for which SiC Schottky rectifiers are now commercially available from at least 3 sources. The key physical properties for any semiconductor material used as the active layer of a unipolar device for power electronics are the breakdown field and carriers mobility. The bulk values are very similar for SiC and GaN. Two main other key issues are related to quality of the ohmic and Schottky contacts. For the ohmic contacts, adequate solutions have been found for both SiC and GaN. Surprisingly, on hetero-epitaxial GaN layers on sapphire despite of the very high crystal defects density ( ≥ 109cm-2 ), the ideality factor of the best Schottky contacts seems very promising. On the other hand, improving this ideality factor and the reverse leakage current for Schottky contacts on GaN layers grown on silicon substrate remains a fierce challenge. For the SiC Schottky rectifiers, cost and availability of the SiC substrates appear as the main residual limiting factors today. For GaN based rectifiers, although engineering device prototypes have already been published [1], there are both basic issues to be validated regarding reverse leakage current and reliability, and also difficult manufacturing issues to be solved in relation with device reliability, directly resulting from the nature of the possible substrates: mainly sapphire and silicon.

Advanced High-Voltage 4H-SiC Schottky Rectifiers

This paper presents advanced 4H-SiC high-voltage Schottky rectifiers with improved performance when compared to conventional 4H-SiC Schottky rectifiers. Two types of 4H-SiC junction barrier Schottky (JBS) rectifiers have been explored. These rectifiers offer Schottky-like ON-state and fast switching characteristics, while their OFF-state characteristics have a low leakage current similar to that of the PiN junction rectifier. Planar 4H-SiC JBS rectifiers, with more than 1-kV blocking capability and orders of magnitude lower reverse leakage current than that of conventional SiC Schottky rectifiers, have been demonstrated. In addition, a novel device structure, called lateral channel JBS rectifier, was designed and experimentally demonstrated in 4H-SiC with up to 1.5-kV blocking capability and pinlike reverse characteristics.

4H-SIC VERTICAL RESURF SCHOTTKY RECTIFIERS AND MOSFETS

This work presents a vertical RESURF structure, which utilizes 2-dimensional depletion in semiconductor to achieve high blocking capability. This approach is then implemented into both 4 H - SiC Schottky rectifiers and MOSFETs. Device characteristics are analyzed with numerical simulations and compared with both conventional Schottky rectifiers and Superjunction structure. Design-of-Experiment (DOE) is also used to optimize the trade-off between several design parameters.

Effect of Dopant Concentration on High Voltage 4H-SiC Schottky Diodes

AbstractPractical design of high-voltage SiC Schottky rectifiers requires the understanding of the influence of the epitaxial dopant concentration on the reverse and forward characteristics. This work analyzes the correlation between the dopant concentration and the I-V characteristics of Schottky diodes for a critical concentration range where the leakage current variations are more evident. The details of how high temperatures affect the properties of junctions have been carefully described to obtain further improvement in the future by proper device optimization. Dopant concentration of about 1.2 × 1016 cm-3 gives the best results in reverse characteristics without great losses in forward currents.

DESIGN OF HIGH VOLTAGE 4H-SiC SUPERJUNCTION SCHOTTKY RECTIFIERS

This paper presents a novel Schottky rectifier structure based on the superjunction approach, which is utilizes 2- and 3-D field shaping to increase the avalanche breakdown voltage. Device forward and blocking characteristics are analyzed with numerical simulations and compared with conventional 4 H - SiC Schottky rectifiers. Optimal design tradeoffs between breakdown voltage and specific on-resistance are obtained for high voltage 4 H - SiC superjunction Schottky rectifiers. The results show that the new structure can provide a 20 × lower R on,sp than conventional Schottky rectifier for 6kV device. In addition, device termination and possible fabrication steps for the superjunction devices are also presented.

HIGH-VOLTAGE DIAMOND SCHOTTKY RECTIFIERS

In this paper, we simulate and fabricate diamond schottky rectifiers. The growth rate of pure diamond single crystal epitaxial is from 0.5 up to 100μm/hr with boron doping concentration around 1 × 1014 cm -3 to 1 × 1016 cm -3. A "liftoff" technology is used to provide the wafer. Theoretical calculation indicates that the diamond shottky rectifier has a significant lower voltage drop than SiC schottky rectifier and comparable with SiC PiN diode with the blocking voltage higher than 10 kV. A maximum 50 kHz operating frequency at switching voltage 25 kV is shown based on thermal limit. Vertical structure devices with 70μm epi layer achieve 18 A/cm2 at 250°C at 7 V forward drop as shown with a breakdown voltage of only 600 V. A breakdown voltage of 8 kV at 100μm distance is recorded for lateral structure devices without ohmic contact (back to back Schottky diodes), 12.4 kV at 300μm distance.

Comparison of stability of WSiX/SiC and Ni/SiC Schottky rectifiers to high dose gamma-ray irradiation

SiC Schottky rectifiers with moderate breakdown voltages of ∼450 V and with either WSiX or Ni rectifying contacts were irradiated with Co-60 γ-rays to doses up to ∼315 Mrad. The Ni/SiC rectifiers show severe reaction of the contact after irradiation at the highest dose, badly degrading the forward current characteristics and increasing the on-state resistance by up to a factor of 6 after irradiation. By sharp contrast, the WSiX/SiC devices show little deterioration of the contact with the same conditions and changes in on-state resistance of <20%. The WSiX contacts appear promising for applications requiring improved contact stability.

High Dose Gamma-Ray Irradiation of SiC Schottky Rectifiers

SiC Schottky rectifiers were irradiated with Co-60 γ-rays at doses up to approximately 600 Mrad. The reverse breakdown voltage (V B ) was reduced from 550 to 330 V for all these doses. The forward current-voltage characteristics were significantly degraded by the irradiation, even at the lowest doses (300 Mrad). The forward current after irradiation was reduced by more than three orders of magnitude at 1.5 V bias and the Ni Schottky metal showed major changes in morphology whose extent correlated with the γ-dose. The SiC appears to be stable to very high doses and the observed changes in device performance are due to metal contact failures. The power figure-of-merit V 2 B/R o n , where R o n is the on-state resistance, degraded from 4560 kW cm - 2 in the unirradiated rectifiers to 11 kW cm - 2 after a dose of 600 Mrad.

Static and dynamic characterization of large-area high-current-density SiC Schottky diodes

The static and dynamic characteristics of large-area, high-voltage 4H-SiC Schottky barrier diodes are presented. With a breakdown voltage greater than 1200 V and a forward current in excess of 6 A at 2 V forward bias, these devices enable for the first time the evaluation of SiC Schottky diodes in practical switching circuits. These diodes were inserted into standard test circuits and compared to commercially available silicon devices, the results of which are reported here. Substituting SiC Schottky diodes in place of comparably rated silicon PIN diodes reduced the switching losses by a factor of four, and virtually eliminated the reverse recovery transient. These results are even more dramatic at elevated temperatures. While the switching loss in silicon diodes increases dramatically with temperature, the SiC devices remain essentially unchanged. The data presented here clearly demonstrates the distinct advantages offered by SiC Schottky rectifiers, and their emerging potential to replace silicon PIN diodes in power switching applications.

High breakdown voltage Au/Pt/GaN Schottky diodes

Au/Pt/GaN Schottky diode rectifiers were fabricated with reverse breakdown voltage (VRB) up to 550 V on vertically depleting structures and >2000 V on lateral devices. The figure-of-merit (VRB)2/RON, where RON is the on-state resistance, had values between 4.2 and 4.8 MW cm−2. The reverse leakage currents and forward on-voltages were still somewhat higher than the theoretical minimum values, but were comparable to SiC Schottky rectifiers reported in the literature. These devices show promise for use in ultrahigh-power switches.

Electrical characterization of inhomogeneous Ti/4H–SiC Schottky contacts

Forward I( V) measurements of Titanium/4H–SiC Schottky rectifiers are presented in a large temperature range. Significant dispersion is observed on the room temperature characteristics. Some of the rectifiers show good agreement with the thermionic model. Other samples present excess current at low voltage level. For these rectifiers we have been able to fit the experimental characteristics assuming the coexistence of low and high barrier height areas. In this model, the forward current results from the addition of the thermionic currents provided by the two parallel diodes. Forward I( V) measurements as a function of temperature have been performed from 100 to 600 K. On a given diode, a transition between one barrier and two barriers behaviours can be evidenced as temperature changes. Variations in the calculated barrier height and ideality factor are therefore evidenced. It is shown that the excess current at low voltage depends on the ratio of the low Schottky barrier region over the total diode area. Material inhomogeneity is one possible explanation for the existence of inhomogeneous barrier height on the Ti/4H–SiC contact.

Design considerations and experimental analysis of high-voltage SiC Schottky barrier rectifiers

Practical design of high-voltage SiC Schottky rectifiers requires an understanding of the device physics that affect the key performance parameters. Forward characteristics of SiC Schottky rectifiers follow thermionic emission theory and are relatively well understood. However, the reverse characteristics are not well understood and have not been experimentally investigated in-depth. In this paper we report the analysis and experimental results of both the forward and reverse characteristics of high-voltage SiC Schottky rectifiers. Ti and Ni Schottky rectifiers with boron implant edge termination were fabricated on n-type 4H SiC samples. Ni Schottky rectifiers fabricated on a 13-/spl mu/m thick 3.5/spl times/10/sup 15/ cm/sup -3/ epilayer have a current density of 100 A/cm/sup 2/ at approximately 2 V forward bias and a reverse leakage current density of less than 0.1 A/cm/sup 2/ at a reverse bias of 1720 V. The reverse leakage current is observed to depend on device area, Schottky barrier height, electric field at the metal-semiconductor interface, and temperature (a decreasing temperature dependence with increasing reverse bias). In addition. the reverse leakage current magnitude is larger and the electric field dependence is stronger than predicted by thermionic emission and image-force barrier height lowering. This suggests the reverse leakage current is due to a combination of thermionic field emission and field emission.

Comparison of 6H-SiC, 3C-SiC, and Si for power devices

The drift region properties of 6H- and 3C-SiC-based Schottky rectifiers and power MOSFETs that result in breakdown voltages from 50 to 5000 V are defined. Using these values, the output characteristics of the devices are calculated and compared with those of Si devices. It is found that due to very low drift region resistance, 5000-V SiC Schottky rectifiers and power MOSFETs can deliver on-state current density of 100 A/cm/sup 2/ at room temperature with a forward drop of only 3.85 and 2.95 V, respectively. Both devices are expected to have excellent switching characteristics and ruggedness due to the absence of minority-carrier injection. A thermal analysis shows that 5000-V, 6H-, and 3C-SiC MOSFETs and Schottky rectifiers would be approximately 20 and 18 times smaller than corresponding Si devices, and that operation at higher temperatures and at higher breakdown voltages than conventional Si devices is possible. Also, a significant reduction in the die size is expected. >