SiC Schottky Diodes Research Articles

Power Efficiency Improvement of Three-Phase Split-Output Inverter Using Magnetically Coupled Inductor Switching

The conventional three-phase split-output inverter (SOI) has been used for grid-connected applications because it does not require dead time and has no shoot-through problems. Recently, the conventional inverter uses the silicon carbide (SiC) schottky diodes for the freewheeling diodes because of its no reverse-recovery problem. Nevertheless, in a practical design, the SiC schottky diodes suffer from current overshoots and voltage oscillations. These overshoots and oscillations result in switching-power losses, decreasing the power efficiency of the inverter. To alleviate this drawback, we present a three-phase SOI using magnetically coupled inductor switching technique. The magnetically coupled inductor switching technique uses one auxiliary diode and coupled inductor for each switching leg in the three-phase SOI. By the operation of the coupled inductor, the main diode current is shifted to the auxiliary diode without the reverse-recovery process. The proposed inverter reduces switching-power losses by alleviating current overshoots and voltage oscillations of SiC schottky diodes. It achieves higher power efficiency than the conventional inverter. We discuss experimental results for a 1.0 kW prototype inverter to verify the performance of the proposed inverter.

6.5 kV Si/SiC Hybrid Power Module Technology

Substituting Si diodes with SiC Schottky diodes in Si insulated gate bipolar transistor (IGBT) modules is beneficial, as it can reduce power losses in electrical systems significantly. The fast switching nature of the SiC diode will allow Si IGBTs to operate at their full high-switching-speed potential, which at present is not possible because of the Si diodes. In this work, the electrical test results for Si-IGBT/4HSiC-Schottky hybrid substrates (hybrid SiC substrates) are presented. Comparisons of the 6.5 kV Si and hybrid SiC at room temperature and high temperature have shown that the switching losses in hybrid SiC substrates are low as compared to those in Si substrates but necessary steps are required to mitigate the ringing observed in the output waveforms.

Output Capacitance Loss Characterization of Silicon Carbide Schottky Diodes

In high-frequency (HF) and very-high-frequency (VHF) rectifiers, silicon carbide (SiC) Schottky diodes exhibit higher losses compared to what is reported in manufacturer-provided simulation models, with additional power loss stemming from energy dissipation during the charging and discharging of the junction output capacitance ( $C_{J}$ ). Because these losses are not included in manufacturer simulation models and have not been well-studied for commercially available SiC Schottky diodes, we have experimentally measured them using the Sawyer–Tower circuit. From these measurements, we compare the losses across manufacturers, current rating, generation, voltage rating, and packaging. We then demonstrate the performance of these devices on a 20-MHz class-DE rectifier and compare their power dissipation from the simulation and experimental measurements. Finally, by incorporating these losses in device power dissipation calculations in various rectifier topologies, we propose suggestions on device selection to optimize converter efficiencies.

Silicon carbide schottky diodes forward and reverse current properties upon fast electron radiation

This paper investigates on the reaction of 10 and 15MGy, 3MeV electron irradiation upon off-the-shelves (commercial) Silicon Carbide Schottky diodes from Infineon Technologies (model: IDH08SG60C) and STMicroelectronics (model: STPSC806). Such irradiation reduces the forward-bias current. The reduction is mainly due to the significant increase of the series resistance (i.e. Infineon: 1.45Ω at before irradiation → 121×103 Ω at 15MGy); STMicroelectronics: 1.44Ω at before irradiation → 2.1×109 Ω at 15MGy). This increase in series resistance gives 4.6 and 8.2 orders of magnitude reduction for the forward-bias current density of Infineon and STMicroelectronics respectively. It is also observed that the ideality factor and the saturation current of the diodes increases with increasing dose (i.e. ideality factor- Infineon: 1.01 at before irradiation → 1.05 at 15MGy; STMicroelectronics: 1.02 at before irradiation → 1.3 at 15MGy | saturation current- Infineon: 1.6×10-17A at before irradiation → 2.5×10-17A at 15MGy; STMicroelectronics: 2.4×10-15A at before irradiation → 8×10-15A at 15MGy). Reverse-bias leakage current density in model by Infineon increases by one order of magnitude after 15MGy irradiation, however, in model by STMicroelectronics decreases by one order of magnitude. Overall, for these particular samples studied, Infineon devices have shown to be better in quality and more radiation resistance toward electron irradiation in forward-bias operation while STMicroelectronics exhibit better characteristics in reverse-bias operation.

The Role of Near-Interface Traps in Modulating the Barrier Height of SiC Schottky Diodes

The role of traps in the operation of Schottky barrier diodes is poorly understood. To explore this, SiC Schottky barrier diodes with a high density of near-interface traps were intentionally fabricated. By applying forward current stress, we demonstrate that the barrier height can be changed by changing the occupancy of the traps. The response time of these traps extends from seconds to hundreds of seconds. We also show that these traps can subsequently be emptied by thermal emission or by tunneling. The results are inconsistent with the existence of an interfacial oxide layer, which shows that the traps are distributed in both energy and lateral depth and, consequently, clarifies their role in creating the Schottky barrier.

Morphological and electrical properties of Nickel based Ohmic contacts formed by laser annealing process on n-type 4H-SiC

This work reports on the morphological and electrical properties of Ni-based back-side Ohmic contacts formed by laser annealing process for SiC power diodes. Nickel films, 100 nm thick, have been sputtered on the back-side of heavily doped 110 µm 4H-SiC thinned substrates after mechanical grinding. Then, to achieve Ohmic behavior, the metal films have been irradiated with an UV excimer laser with a wavelength of 310 nm, an energy density of 4.7 J/cm2 and pulse duration of 160 ns. The morphological and structural properties of the samples were analyzed by means of different techniques. Nanoscale electrical analyses by conductive Atomic Force Microscopy (C-AFM) allowed correlating the morphology of the annealed metal films with their local electrical properties. Ohmic behavior of the contacts fabricated by laser annealing have been investigated and compared with the standard Rapid Thermal Annealing (RTA) process. Finally, it was integrated in the fabrication of 650V SiC Schottky diodes.

A vertical optimization method for a simultaneous extraction of the five parameters characterizing the barrier height in the Mo/4H–SiC Schottky contact

The temperature dependence of the parameters related to the barrier height inhomogeneities for Mo/4H–SiC Schottky diode in 298–498 K temperature range has been investigated. Due to the barrier height inhomogeneities that prevail at the interface of the Schottky diode, a Gaussian distribution of the barrier height is assumed. We have extracted simultaneously, for every temperature, all the parameters characterizing the barrier height such as the mean barrier height $$ \bar{\phi }_{{{\text{B}}0}} $$ , the coefficients ρ2, ρ3 quantifying the deformation of the barrier height, the corresponding temperature T0 modeling the divergence of the ideality factor n from the unity, the standard deviation of the Gaussian distribution of the barrier σs and also the series resistance Rs using a vertical optimization process on the current without any graphical extraction about ρ2, ρ3, $$ \bar{\phi }_{{{\text{B}}0}} $$ , σs and Rs. The extracted parameters like ( $$ \bar{\phi }_{{{\text{B}}0}} $$ , ρ2, ρ3, σs, Rs) were found to be a temperature dependent. Moreover, an excellent agreement was obtained between the I–V–T plots calculated with the extracted parameters using a vertical optimization process with the experimental one.

Characterization of non-uniform Ni/4H-SiC Schottky diodes for improved responsivity in high-temperature sensing

The development of high-temperature sensors (up to 450 °C), based on SiC Schottky diodes, which are able to work in harsh conditions, requires using high-barrier Schottky contacts, such as nickel silicide. It is well known that post metallization annealing, targeting silicide formation, leads to Schottky barrier height (SBH) non-uniformity on the contact surface. On the other hand, sensor performances are intimately tied to the value and stability of SBH. This paper discusses solutions for assessing temperature sensing performances of inhomogeneous SiC Schottky diodes, particularly targeting high-temperature, harsh industrial applications where conventional solutions are unreliable. Microphysical investigations and electrical measurements are carried out on Ni/4H-SiC Schottky diodes with varying levels of non-uniformity. Devices are characterized using SEM and EDX analyses, together with electrical parameter evaluation techniques. It is demonstrated that, despite highly inhomogeneous electrical behavior, SiC-Schottky diodes exhibit stable sensitivity (up to 2.33 mV/°C) with excellent linearity (R2 > 99.9%) over wide temperature (up to 450 °C) and bias ranges (at least 100 nA–10 µA).

Diode Reverse Recovery Process and Reduction of a Half-Wave Series Cockcroft–Walton Voltage Multiplier for High-Frequency High-Voltage Generator Applications

This paper investigates the diode reverse recovery process and reduction of a half-wave (HW) series Cockcroft–Walton (CW) voltage multiplier based on the steady-state analysis for high-frequency high-voltage (HV) generator applications. The diode reverse recovery process for a multistage voltage multiplier is analyzed after the introduction of steady-state operation. The diode reverse recovery problem is the bottleneck to further increase the circuit operation switching frequency for achieving high power density and short HV pulse rise and decay times. The diode reverse recovery problem is mainly caused by the diodes in the first-stage voltage multiplier. It is suggested that the most effective and economic way to alleviate the diode reverse recovery problem is by employing diodes without reverse recovery such as silicon carbide Schottky diodes in the first stage only. The silicon carbide Schottky diode without reverse recovery needs to be used only in the first stage of the voltage multiplier to effectively mitigate the reverse recovery problems at high frequency. The 300 kHz switching frequency three-stage voltage multiplier circuit hardware prototype experimental results finally validate the analysis. A technology demonstrator of a 300 kHz 8 kW 160 kV HV generator based on the proposed hybrid silicon carbide and silicon diode solution for the HW series CW voltage multiplier is provided finally.

Influence of deep defects on electrical properties of Ni/4H-SiC Schottky diode**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0400402).

In this paper, we investigate the influence of deep level defects on the electrical properties of Ni/4H–SiC Schottky diodes by analyzing device current–voltage (I–V) characteristics and deep-level transient spectra (DLTS). Two Schottky barrier heights (SBHs) with different temperature dependences are found in Ni/4H–SiC Schottky diode above room temperature. DLTS measurements further reveal that two kinds of defects Z1/2 and Ti(c)a are located near the interface between Ni and SiC with the energy levels of EC–0.67 eV and EC–0.16 eV respectively. The latter one as the ionized titanium acceptor residing at cubic Si lattice site is thought to be responsible for the low SBH in the localized region of the diode, and therefore inducing the high reverse leakage current of the diode. The experimental results indicate that the Ti(c)a defect has a strong influence on the electrical and thermal properties of the 4H–SiC Schottky diode.

High-Voltage Capacitive Nonlinear Transmission Lines for RF Generation Based on Silicon Carbide Schottky Diodes

Nonlinear transmission lines (NLTLs) have been studied over the past several years to generate high-power radio frequency signals. Their operation consists of a lumped line based on the nonlinear behavior of the LC section components, capacitors or inductors, as a function of the applied voltage or current, respectively. However, considering high-power signals, the application of these devices using nonlinear ceramic capacitors as the capacitive lumped lines are restricted to frequencies around 100 MHz since at high voltages, parasitic impedances in the line structure limit the maximum operating frequency. On the other hand, the use of variable capacitance diodes has enabled the operation of NLTLs at higher frequencies. This paper provides the results of a study of NLTLs for RF generation based on silicon carbide Schottky diodes. The principle of operation and its simplified theory are presented. Initially, the voltage dependence of the diode capacitance is modeled. Experimental and simulation results are compared. Frequency oscillations around 200 MHz were generated. The extraction of the radio frequency energy is realized, and the radiation of the electromagnetic wave is performed using double-ridged horn antennas. Finally, some parameters are analyzed, and all results discussed.

Extended T-type Inverter

Abstract This paper presents a new concept for a power electronic converter - the extended T-type (eT) inverter, which is a combination of a three-phase inverter and a three-level direct current (dc)/dc converter. The novel converter shows better performance than a comparable system composed of two converters: a T-type inverter and a boost converter. At first, the three-level dc/dc converter is able to boost the input voltage but also affects the neutral point potential. The operation principles of the eT inverter are explained and a simulation study of the SiC-based 6 kVA system is presented in this paper. Presented results show a serious reduction of the DC-link capacitors and the input inductor. Furthermore, suitable SiC power semiconductor devices are selected and power losses are estimated using Saber software in reference to a comparative T-type inverter. According to the simulations, the 50 kHz/6 kVA inverter feed from the low voltage (250 V) shows <2.5% of power losses in the suggested SiC metal oxide-semiconductor field-effect transistors (MOSFETs) and Schottky diodes. Finally, a 6 kVA laboratory model was designed, built and tested. Conducted measurements show that despite low capacitance (2 × 30 μF/450 V), the neutral point potential is balanced, and the observed efficiency of the inverter is around 96%.

Impact of design and process variation on the fabrication of SiC diodes

We have studied the influence of design and process variations on the electrical performance of SiC Schottky diodes. On the design side, two design variations are used in the active cell of the diode (segment design and stripe design). In addition, there are two more design variations employed for the edge termination layout of the diodes, namely, field limiting ring (FLR) and junction termination extension (JTE). On the process side, some diodes have gone through an N2O annealing step. The segment design resulted in a lower forward voltage drop (VF) in the diodes and the FLR design turned out to be a better choice for blocking voltages, in the reverse bias. Also, N2O annealing has shown a detrimental effect on the diodes’ blocking performance, which have JTE as their termination design. It degrades the blocking capability of the diodes significantly.

Operating principles and practical design aspects of all SiC DC/AC/DC converter for MPPT in grid-connected PV supplies

A 20 kW, 20 kHz high frequency (HF) link maximum power point tracking (MPPT) converter for a grid-connected PV supply, based on all silicon carbide (SiC) power semiconductors, is presented. In the developed converter, SiC power MOSFETs are used in the low-voltage PV panel side and SiC Schottky diodes on the high-voltage DC output, in order to maximize the power conversion efficiency and the power density. Operating principles of the resulting dual H-bridge MPPT converter and the practical aspects of the converter design and its circuit layout, are described in detail. The implemented converter performance is compared with that of a classical Si-IGBT and hybrid-IGBT based MPPT converter in terms of efficiency. This configuration can compete with the non-isolated MPPT converter topologies, such as the boost converter commonly used in grid-connected PV systems, since it allows the possibility of using a conventional two-level, three-phase grid-connected inverter. This is due to the enhanced common-mode EMI performance as compared to non-isolated MPPT topologies, resulting in a competitive high efficiency PV converter design with galvanic isolation. It has been shown that the converter size can be shrinked up to a power density of 1.6 kW/lt, with a DC-DC converter full-load efficiency of 98%. The resulting compact and highly efficient SiC power MOSFET based HF link MPPT converter is suggested to be a part of grid-connected, multi-string PV supplies with simple inverter topologies in the future.

3-D Wire Bondless Switching Cell Using Flip-Chip-Bonded Silicon Carbide Power Devices

This paper presents a three-dimensional (3-D) wire bondless power module using silicon carbide (SiC) power devices. Commercially available SiC power devices are designed for wire bonding. Wire bonds have an inherent parasitic inductance that limits high-frequency switching. This results in an underutilization of the full potential of SiC power devices, which have very low switching losses at high frequencies. Wire-bonded power modules run into a performance ceiling when it comes to ultrafast switching. This paper strives to provide a solution to this issue, which involves reconfiguring a commercially available bare die SiC power device into a flip-chip-capable device. A wire bondless SiC Schottky diode package was demonstrated and its performance was contrasted with a conventional wire-bonded package. A 24% reduction in the ON-state resistance was observed in the wire bondless package. As a next step, wire bondless SiC MOSFET packages were developed and tested in a half-bridge configuration in a highly integrated 3-D arrangement. This approach departs from the conventional concept of a power module—demonstrating a direct-bonded-copper-less and baseplate-less half-bridge switching cell. Double-pulse tests conducted on the cell showed >3× reduction in the parasitic inductance of the 3-D cell as compared with a conventional wire-bonded module.

Molecular Dynamics Simulations of Heavy Ion Induced Defects in SiC Schottky Diodes

Heavy ion irradiation increases the leakage current in reverse-biased SiC Schottky diodes. This letter demonstrates, via molecular dynamics simulations, that a combination of bias and ion-deposited energy is required to produce the degradation.

A Physics-Based Compact Model of SiC Junction Barrier Schottky Diode for Circuit Simulation

A physics-based compact model of silicon carbide (SiC) junction barrier Schottky diode for circuit simulation is developed in this paper. The semiconductor physics mechanism in SiC, such as temperature-dependent mobility and incomplete ionization, are considered. The detailed device parameters, including drift region concentration, drift thickness, active area, and Schottky barrier height, are modeled. Then, a datasheet-oriented parameter extraction procedure for the device parameters is presented for three kinds of commercial SiC Schottky diodes with junction barrier structure. The model is implemented in the circuit simulator PSpice. In order to verify this model, the Technology Computer-Aided Design Sentaurus simulation is conducted with the device parameters and physical models, which shows a good agreement with the PSpice simulation.

Electrical Characteristics of Solder-Free SiC Die/Metal Foil/AlN Plate Junctions Fabricated Using Surface Activated Bonding

In conventional power device modules, Si-based dies are attached to metal layers on ceramic plates by using solders. Given that power devices made of widegap materials such as SiC and GaN are going to be launched into market, packaging technologies that exploit their superiority are strongly sought for. Solder-free modules, i.e., modules with dies directly bonded to metal layers on ceramic plates are likely to be ideal since fracture in the solder layer is one of the essential factors that limit the thermal tolerance of conventional power modules. In this work, we fabricated an SiC die/metal/ceramic plate junction by using surface activated bonding (SAB) and examined its current-voltage (I-V) characteristics at elevated temperatures. As ceramics AlN was used since the thermal expansion coefficient of AlN is close to that of SiC. The metal layer on AlN plate was made of an Al foil.We prepared a 635-μm-thick AlN ceramic plate and a 30-μm-thick Al foil. The average roughness (Ra) of the surface in AlN plate was ∼0.5 μm. Ra of the Al foil was 0.06 and 0.19 μm in its mirror and matte surfaces, respectively. The matte surface of Al foil was directly bonded to the AlN plate through a surface activation process using an Ar fast atom beam. In a preparatory study we fabricated an AlN/Al foil/AlN junction by bonding AlN plates to the both sides of an Al foil. We observed the cross section of AlN/Al foil/AlN junction by using scanning electron microscopy. We found that the AlN plate and Al foil were bonded to each other without gaps at the both AlN/Al interfaces although Ra of AlN plates and Al foils was much larger than typical Ra of semiconductor substrates that were bonded using SAB (≤~1 nm). We prepared an SiC epi substrate by growing an n+-SiC buffer layer and an n--SiC epi layer on an n+-4H-SiC (0001) substrate. We formed an ohmic contact on the backside of the epi substrate by evaporating an Al/Ni/Au and annealing at 1000 °C. Then we made a 300-μm-ϕ Ni/Au Schottky contact on the top by evaporation and lift-off. An Al layer was evaporated on the top of ohmic contact so that an SiC Schottky diode die was fabricated. Then the Al surface of ohmic contact of SiC die was bonded to the mirror surface of Al foil on AlN plate using SAB. Note that no intermediate layers such as solder were inserted at bonding interfaces.We measured the I-V characteristics between the Schottky contact of SiC die and the surface of Al foil at temperatures between room temperature and 300 °C. A reverse-bias current was ~10-7 A/cm2 at a reverse-bias voltage up to 6 V for temperatures lower than 250 °C. A larger reverse-bias current (~10-6 A/cm2) was observed in measurement at 300 °C. In the forward characteristics, lower turn-on voltages and larger series resistances were observed for higher temperatures. It is notable that these features are typically observed for Schottky diodes. In addition, no change was observed in appearance of the SiC die/Al foil/AlN junction even after the measurement at 300 ℃. Although the bonding strength in Al foil/AlN interfaces is still unclarified, these results suggest that SAB technologies could be useful for realizing solder-free modules of power devices.Acknowledgment—The Al foil employed in the work was supplied from Toyo Aluminium K.K. Figure 1

Reliability of SiC Schottky Diodes with Mo2C Electrode

High current density electrical test was performed on SiC Schottky diodes. The shift in the Schottky barrier height was suppressed by adopting Mo2C as an electrode. Physical analysis revealed an epitaxial growth of Mo2C on SiC surface, which may be the origin of the stability. Introduction SiC Schottky barrier diodes (SBD) are used for high efficiency power conversion circuit owing to its high breakdown voltage, low ON-resistance and fast operation speed. However, pure metal and SiC surface reacts during high temperature annealing, forming an inhomogeneous interface [1]. Therefore, there is a concern in the diode characteristics change with time. We have shown diode characteristics with high temperature stability by using Mo2C as Schottky electrodes [2]. In this report, electric characteristics of SiC-SBD with Mo or Mo2C electrode when this devices is applied current of high current density and TEM analysis of Mo2C/SiC interface with high temperature stability are reported. Experiments A SiC wafer used in this study is an epitaxially grown 4°-off n-type 4H-SiC (0001) with a donor concentration of 1015cm-3 on a SiC substrate. Mo or Mo2C electrodes were sputter-deposited and patterned by reactive ion etching. The Mo and Mo2C samples were annealed in 500 or 850oC, respectively. Diode characteristics were measured before and after flowing a current density of 100A/cm2 for 500 s. Results and discussions The diode characteristics before and after the electrical current test are shown in fig. 1. Although increase in the Schottky barrier height was observed in both diodes, the amount of shift was suppressed with the Mo2C diode. This fact suggests that local interface reaction due to inhomogeneous current flow was suppressed. Moreover, only a slight degradation in the ideality factor was suppressed with the Mo2C electrode. Transmission electron microscope image of the diode, shown in fig. 2, revealed an epitaxial growth of a Mo2C layer on the SiC surface. The reliable diode characteristics might be originated from the sharp and uniform interface. Conclusion An electrical current test was performed on SiC Schottky diodes with Mo2C electrodes. A stable Schottky barrier height with a robust ideality factor was observed. TEM analysis revealed an epitaxial growth of Mo2C on the SiC surface with a sharp and uniform interface.

Experimental Demonstration and Analysis of a 1.35-kV 0.92-m $\Omega \cdot \text {cm}^{2}$ SiC Superjunction Schottky Diode

This paper presents the fabrication, experimental analysis and electrical characterization of the first functional SiC superjunction (SJ) device. A trench-etching-and-sidewall-implant method has been developed to implement the SJ principle on a SiC Schottky diode. Several key process steps, including deep trench etching, ion implantation, and high-temperature annealing, are found to have noticeable influences on the device performances. The corresponding influences are studied by both simulation-aided theoretical analysis and experimental measurements. The highest measured cell blocking voltage was 1350 V, which achieves 95% of the simulated blocking voltage for the perfectly charge-balanced SJ structure. The measured device specific on-resistance was 0.92 $\text{m}\Omega \cdot \text {cm}^{2}$ . The SJ drift region specific on-resistance as low as 0.32 $\text{m}\Omega \cdot \text {cm}^{2}$ was obtained after subtracting the substrate resistance. This result successfully breaks the SiC 1-D unipolar limit.