SiC Schottky Diodes Research Articles

Heavy Ion Induced Degradation in SiC Schottky Diodes: Incident Angle and Energy Deposition Dependence

Heavy-ion-induced degradation in the reverse leakage current of SiC Schottky power diodes exhibits a strong dependence on the ion angle of incidence. This effect is studied experimentally for several different bias voltages applied during heavy-ion exposure. In addition, TCAD simulations are used to give insight on the physical mechanisms involved.

Heavy Ion Induced Degradation in SiC Schottky Diodes: Bias and Energy Deposition Dependence

Experimental results on ion-induced leakage current increase in 4H-SiC Schottky power diodes are presented. Monte Carlo and TCAD simulations show that degradation is due to the synergy between applied bias and ion energy deposition. This degradation is possibly related to thermal spot annealing at the metal semiconductor interface. This thermal annealing leads to an inhomogeneity of the Schottky barrier that could be responsible for the increase leakage current as a function of fluence.

Prototyping and Characterization of 1.2KV SIC Schottky Diodes for TWTA Application: The Challenge to Meet the User Specification

Prototyping and Characterization of 1.2KV SIC Schottky Diodes for TWTA Application: The Challenge to Meet the User Specification

Study of Current-voltage Characteristic Using Deep Level Transient Spectroscopy Technique of Schottky Diode Made of SIC

Current spectroscopy was employed to investigate Silicon Carbide (SiC). DLTS standard setup was performed to study the current voltage measurement of respective schottky diode. The current voltage measurement of SiC schottky diode was carried out at different temperatures keeping the same biasing setting. From these measurements the behavior of the material is: The ideality factor of SiC at room temperature was found to be 1.9894 that was little bit improved i.e. ~ 1.7268 when temperature was increased ~400K. However values increased with decrease in temperature for the material. The higher value of ideality factor is attributed to high diffusion or tunneling current. The barrier height of SiC at room temperature was calculated as 0.995eV which remained nearly constant with increase in temperature. The change in the barrier height is related to the effective leakage current at high temperature. Reverse saturation current calculated for SiC at room temperature was 6.5706 ×10 -13 A. Its value increased up to 1.72×10 -9 A at 400K.

Study of Current-voltage Characteristic Using Deep Level Transient Spectroscopy Technique of Schottky Diode Made of SIC

Current spectroscopy was employed to investigate Silicon Carbide (SiC). DLTS standard setup was performed to study the current voltage measurement of respective schottky diode. The current voltage measurement of SiC schottky diode was carried out at different temperatures keeping the same biasing setting. From these measurements the behavior of the material is: The ideality factor of SiC at room temperature was found to be 1.9894 that was little bit improved i.e. ~ 1.7268 when temperature was increased ~400K. However values increased with decrease in temperature for the material. The higher value of ideality factor is attributed to high diffusion or tunneling current. The barrier height of SiC at room temperature was calculated as 0.995eV which remained nearly constant with increase in temperature. The change in the barrier height is related to the effective leakage current at high temperature. Reverse saturation current calculated for SiC at room temperature was 6.5706 ×10 -13 A. Its value increased up to 1.72×10 -9 A at 400K.

On the Design Process of a 6-kVA Quasi-Z-inverter Employing SiC Power Devices

This paper presents the design process of a 6-kVA quasi-Z-source inverter built with SiC power devices, in particular, employing SiC MOSFETs and SiC Schottky diodes. The main design target is to find the optimal parameters and a good agreement between the efficiency and power density of the converter. The performance of the system may be influenced not only by the switching frequency but also from the specific pulsewidth modulated (PWM) method or type of SiC MOSFET, and, therefore, various design cases are analyzed. At a final step, the 6 kVA/3 × 400 VAC inverter employing the 80 mΩ SiC MOSFETs and operating at 100 kHz with the minimum switching number method is chosen for investigation and a laboratory prototype is built. From experiments, the high performance of the designed system is confirmed. More specifically, it is shown that an efficiency above 95.6% (at 400 VDC, B = 1.9) and a power density higher than 2 kW/dm3 have been reached. Last but not least, the obtained results, which can be recognized as leading in the area of impedance source converters, show the great benefits gained by employing the new power semiconductor devices.

Development and characterisation of pressed packaging solutions for high-temperature high-reliability SiC power modules

SiC is a wide bandgap semiconductor with better electrothermal properties than silicon, including higher temperature of operation, higher breakdown voltage, lower losses and the ability to switch at higher frequencies. However, the power cycling performance of SiC devices in traditional silicon packaging systems is in need of further investigation since initial studies have shown reduced reliability. These traditional packaging systems have been developed for silicon, a semiconductor with different electrothermal and thermomechanical properties from SiC, hence the stresses on the different components of the package will change. Pressure packages, a packaging alternative where the weak elements of the traditional systems like wirebonds are removed, have demonstrated enhanced reliability for silicon devices however, there has not been much investigation on the performance of SiC devices in press-pack assemblies. This will be important for high power applications where reliability is critical. In this paper, SiC Schottky diodes in pressure packages have been evaluated, including the electrothermal characterisation for different clamping forces and contact materials, the thermal impedance evaluation and initial thermal cycling studies, focusing on the use of aluminium graphite as contact material.

Examinations of Selected Thermal Properties of Packages of SiC Schottky Diodes

Abstract This paper describes the study of thermal properties of packages of silicon carbide Schottky diodes. In the paper the packaging process of Schottky diodes, the measuring method of thermal parameters, as well as the results of measurements are presented. The measured waveforms of transient thermal impedance of the examined diodes are compared with the waveforms of this parameter measured for commercially available Schottky diodes.

SiC detectors to monitor ionizing radiations emitted from nuclear events and plasmas

ABSTRACTSilicon Carbide (SiC) semiconductor detectors are increasingly employed in Nuclear Physics for their advantages with respect to traditional silicon (Si). Such detectors show an energy resolution, charge mobility, response velocity and detection efficiency similar to Si detectors. However, the higher band gap (3.26 eV), the lower leakage current (∼10 pA) maintained also at room temperature, the higher radiation hardness and the higher density with respect to Si represent some indisputable advantages characterizing such detectors. The devices can be employed at high temperatures, at high absorbed doses and in the case of high visible light intensities, for example, in plasma, for limited exposition times without damage. Generally SiC Schottky diodes are employed in reverse polarization with an active region depth of the order of 100 µm, purity below 1014 cm−3 and an active area lower than 1 cm2. Measurements in the regime of proportionality with the radiation energy released in the active region and measurements in time-of-flight configuration are employed for nuclear emission events produced at both low and high fluences. Alpha spectra demonstrated an energy resolution of about 1.3% at 5.8 MeV. Radiation emission from laser-generated plasma can be monitored in terms of detected photons, electrons and ions, using the laser pulse as a start signal and the radiation detection as a stop signal, enabling to measure the ion velocity by knowing the target-detector flight distance. SiC spectra acquired in the Messina University laboratories using radioactive ion sources and at the PALS laboratory facility in Prague (Czech Republic) are presented. A preliminary study of the use of SiC detectors, embedded in a water equivalent polymer, as a dosimeter is presented and discussed.

Criterion for the Stability Against Thermal Runaway During Blocking Operation and Its Application to SiC Diodes

Silicon-Schottky diodes are well known for their poor blocking behavior at high temperatures and high blocking voltages. This behavior is particularly problematic, because the high leakage currents [1] lead to self-heating, which increases the leakage further. Due to this fatal feedback loop, a thermal runaway [2] may occur. Silicon carbide (SiC) devices are supposed to show much lower leakage current levels and a better thermal stability. However, early SiC-Schottky diodes were affected by nonideal leakage current densities [3] – [5] and, therefore, seriously threatened by thermal runaway as well. In this paper, the blocking stability of commercial SiC-Schottky and SiC-p-i-n diodes is investigated. Due to different leakage mechanisms in SiC-Schottky and SiC-p-i-n diodes, both the device types show different voltage and temperature dependences of the leakage current. Nevertheless, the derived stability criterion (SC) is a reasonable approximation for both the diode types. In agreement with the SC, experimental results prove that thermal stability is reached during high-temperature operation and even under worst cooling conditions. Within the specified operating range, thermal runaway is, thus, no longer a limiting factor. However, the risk of a thermal runaway has to be reassessed when going to higher voltage, higher operating temperatures, and less cooling as promised by the SiC devices.

(Invited) Wide Band-Gap on Its Hard Way up - The Trouble Starts Just Outside the Chip

Since the market introduction of the first commercial SiC-Schottky diode in 2001 wide band-gap (WBG) semiconductor devices made significant progress. In the meantime, SiC-MOSEFTs are commercially available from several manufacturers and also GaN-HEMTs appeared on the market. Devices from both materials are available in a growing variety of packages and the components conquer more and more applications and market shares.The electrical performance of those WBG-devices is a quantum leap compared to their silicon counter parts. The specific on-resistance is at least an order of magnitude lower than in silicon and the switching speed can be so high, that the switching process is finished within nanoseconds and the associated losses are orders of magnitude lower than in silicon. Due to the lack of electron-hole-plasma the safe operating area is extremely wide, there is no risk of filamentation and other instabilities, and diodes do not show a forward recovery over-voltage [1]. On top, the devices can be operated at much higher temperatures than silicon ones. Thus, WBG-devices seem to approach the ideal switch.However, this is only true for the semiconductor chip. Of course, there are a variety of technical challenges not to mention the significantly higher cost. SiC-MOSFETs show low channel mobility and low threshold voltage with a strange temperature behaviour pointing to interfaces far from ideal, while GaN-HEMTs show the current collapse and bulk leakage requiring significant voltage derating for stable operation.But the real trouble starts just outside the chip with the package. The packaging is a limiting factor with respect to parasitics preventing fast switching. Due to the high di/dt the smallest stray inductance has a large impact and can cause massive overvoltages as well as the high dv/dt leads to large currents even through small stray capacitances. In contrast to IGBTs, this cannot be compensated anymore by smart control circuitry because the switching event is over before the circuitry can respond [2]. And it is really difficult to measure such steep transients without introducing much additional stray inductance [3].The packaging is also a limiting factor with respect to materials preventing high temperature operation beyond 200°C. Either the packaging materials are not at all suited for higher temperatures or they cannot cope with the higher temperature-swings leading to much faster degradation of joints. Furthermore, Schottky-diodes show a rather high leakage current and might be subjected to thermal runaway at elevated temperatures [4].The package is also a limiting factor with respect to long term stability because of the extremely high fields on the semiconductor surface as well as in the packaging materials. Furthermore, the usual packages are not at all hermetic, so that moisture can intrude and degrades the blocking capability over time [5].On top, the passive components in the circuitry might not cope with the high switching speeds or the high frequencies. When going to frequencies beyond 100 kHz the losses in the inductive components become significant and in some applications even the limiting factor.After all, the WBG-chips might approach the ideal switch but the advantages cannot be fully exploited yet. Significant effort is still required to gain confidence in WBG-devices’ reliability and to make them attractive for a wider range of applications, consequently profiting from the economy of scale. Only this way, WBG-devices could step out of the high end niche and could really challenge silicon, which is still a powerful competitor. And even (theoretically) better WBG-materials like Ga2O3 are at the horizon.[1] S. Rugen et al., “Investigation of the Turn-on Behaviour of Silicon pin-Diodes and SiC-MPS-Diodes and its Impact on the Anti-parallel IGBT”, accepted for publication at EPE 2016[2] C. Bödeker et al., “Investigation of an overvoltage protection for fast switching silicon carbide transistors”, IET-PEL, volume 8, issue 12, pp. 2336–2342, 2015[3] M. Adelmund et al., “Optimisation of Shunt Resistors for Fast Transients”, accepted for publication at PCIM 2016[4] C. Bödeker et al., “Criterion for the Stability against Thermal Runaway during Blocking Operation and its Application to SiC-Diodes”, IEEE-JSTPE, already available in IEEEXplore, issue tbd., 2016[5] N. Kaminski, “Progress in Silicon Based Power Electronics and its Impact on Wide Band-gap Devices - Why use SiC and GaN if Silicon can do the job?”, JST’s International Forum on Power Electronics of Advanced Wide Bandgap Semiconductors, Kyoto, 2015

High Efficiency DC/AC/DC Converter Based on Synchronous Rectifier for Proton Exchange Membrane Fuel Cells

AbstractThis paper deals with the efficiency improvement of a DC/AC/DC converter used for proton exchange membrane fuel cell (PEMFC) coupling to an electrical vehicle DC bus. The power converter is based on a MOSFET inverter stage, a high frequency planar technology transformer stage and a full bridge diode rectifier output stage. The diode rectifier is a source of losses due to the diode forward voltage, which may be reduced by implementation of synchronous rectifier technology, especially for high output current. Mosfets with low drain‐to‐source on‐resistance are coupled to the diodes to reduce the losses. Control principle of this converter is presented with losses modeling and efficiency study in order to highlight the advantages of the synchronous rectification. Efficiency improvement is quantified from a comparison study with technological choices including silicon carbide (SiC) MOSFET and Schottky diodes. This study points out that the synchronous rectification becomes really relevant for an actual scale‐1 application and not for our reduced scale laboratory power converter. Finally, experimental validation of the control is presented with our laboratory test bench. The realization of a scale‐1 converter is under study to experimentally validate the advantages of the proposed synchronous rectification.

Charge Transport Mechanisms in Heavy-Ion Driven Leakage Current in Silicon Carbide Schottky Power Diodes

Under heavy-ion exposure at sufficiently high reverse-bias voltages, silicon carbide (SiC) Schottky diodes are observed to exhibit gradual increases in leakage current with increasing ion fluence. Heavy-ion exposure alters the overall reverse current–voltage characteristics of these diodes, leaving the forward characteristics practically unchanged. This paper discusses the charge transport mechanisms in the heavy-ion damaged SiC Schottky diodes. A macro model, describing the reverse current–voltage characteristics in the degraded SiC Schottky diodes is proposed.

An Investigation of SiC Schottky Contact Barrier Inhomogeneity for Temperature Sensing Applications

This paper proposes a method of characterizing silicon carbide Schottky diodes with inhomogeneous contacts in temperature sensing applications. Using the energy activation technique, temperature intervals where the effective barrier height is constant are determined. Unlike the conventional barrier which increases with temperature for inhomogeneous diodes, the effective barrier has physical meaning and can be used for sensor performance evaluation. The utility of effective barrier analysis is confirmed on fabricated Ni/4H-SiC Schottky diodes with different annealing conditions and different degrees of barrier non-uniformity. The good agreement between calculated and experimental data proves the suitable behavior of inhomogeneous diodes as sensors for different temperature ranges.

High Voltage Diffusion-Welded Stacks on the Basis of SiC Schottky Diodes

In the present work we have considered the prototype of the high-voltage diode stack made on the basis of commercial SiC Schottky diodes. Implementation of vertical integration for four diode chips yielded stack with the reverse current of 25 μA under reverse voltage of 6 kV. The capacitance of the stack at zero bias is reduced more than three times in comparison with initial diodes. Reverse recovery time of the stack was 8.0 ns. This paper proposes a convenient analytical approach to the estimation of parameters of modular compositions with vertical architecture.

Performance Evaluation of Split Output Converters With SiC MOSFETs and SiC Schottky Diodes

The adoption of silicon carbide (SiC) MOSFETs and SiC Schottky diodes in power converters promises a further improvement of the attainable power density and system efficiency, while it is restricted by several issues caused by the ultrafast switching, such as phase-leg shoot-through (“crosstalk” effect), high turn-on losses, electromagnetic interference (EMI), etc. This paper presents a split output converter, which can overcome the limitations of the standard two-level voltage source converters when employing the fast-switching SiC devices. A mathematical model of the split output converter has been proposed to reveal how the split inductors can mitigate the crosstalk effect caused by the high switching speed. The improved switching performance (e.g., lower turn-on losses) and EMI benefit have been demonstrated experimentally. The current freewheeling problem, the current pulses and voltage spikes of the split inductors, and the disappeared synchronous rectification are explained in detail both experimentally and analytically. The results show that the split output converter can have lower power device losses compared with the standard two-level converter at high switching frequencies. However, the extra losses in the split inductors may impair the efficiency of the split output converter, which is verified by experiments in the continuous operating mode. A 95.91% efficiency has been achieved by the split output converter at the switching frequency of 100 kHz with suppressed crosstalk, lower turn-on losses, and reduced EMI.

Reliability modeling of Sn–Ag transient liquid phase die-bonds for high-power SiC devices

In this work, a novel foil-based transient liquid phase bonding process has been used to mount the SiC Schottky diodes. The Sn–Ag TLP interlayer material was produced in the form of preforms of multilayer foils, using electrochemical deposition. The foils were designed to keep the overall composition of Ag and Sn about 80% and 20% respectively. The optimized TLP bonding process parameters were used during the assembly process. The die-attachment characterizations revealed that resulting intermetallic compounds (Ag3Sn and ζ) have melting point beyond 480°C. The die-attachment produced low bending stresses, while heated from 30°C to 400°C. The reliability of Sn–Ag TLP bonded samples was studied during passive temperature cycling and during active power cycling. During power cycling, the crack rates were determined by measuring the crack lengths of the TLP bonded joints after failure. The failure criteria were set to be an increase of diode's forward voltage by 10% since the start of the power cycling tests. The thermo-mechanical simulations were performed to determine the damage parameter i.e. strain range amplitude ∆εp. Based on mechanical characterization of the TLP bonded layers, a plastic material model was used. The crack propagation rates were modeled using Paris' Law. Based on comparisons with state-of-the-art silver sintering technique, it can be stated that the TLP bonding is a promising die-attachment technique and its power cycling reliability is similar to silver sintering.

Temperature and Switching Rate Dependence of Crosstalk in Si-IGBT and SiC Power Modules

The temperature and $dV/dt$ dependence of crosstalk has been analyzed for Si-IGBT and SiC-MOSFET power modules. Due to a smaller Miller capacitance resulting from a smaller die area, the SiC module exhibits smaller shoot-through currents compared with similarly rated Si-IGBT modules in spite of switching with a higher $dV/dt$ and with a lower threshold voltage. However, due to high voltage overshoots and ringing from the SiC Schottky diode, SiC modules exhibit higher shoot-through energy density and induce voltage oscillations in the dc link. Measurements show that the shoot-through current exhibits a positive temperature coefficient for both technologies, the magnitude of which is higher for the Si-IGBT, i.e., the shoot-through current and energy show better temperature stability in the SiC power module. The effectiveness of common techniques of mitigating shoot-through, including bipolar gate drives, multiple gate resistance switching paths, and external gate–source and snubber capacitors, has been evaluated for both technologies at different temperatures and switching rates. The results show that solutions are less effective for SiC-MOSFETs because of lower threshold voltages and smaller margins for negative gate bias on the SiC-MOSFET gate. Models for evaluating the parasitic voltage have also been developed for diagnostic and predictive purposes. These results are important for converter designers seeking to use SiC technology.

-ray detector based on n-type 4H-SiC Schottky barrier diode

Silicon carbide (SiC) is a wide band-gap, high-temperature-resistant, and radiation-resistant semiconducting material, which can be used as a radiation detector material in harsh environments such as high radiation background and high temperatures. Schottky barrier diode radiation detectors are fabricated using 100 upm-thick n-type 4H-SiC epitaxial layers for low energy -ray detection. The spectrum responses of 4H-SiC Schottky barrier detectors are investigated by irradiation of -ray from 241Am source. Schottky diodes are prepared by magnetron-sputtering 100 nm-thick nickel on epitaxial surface (Si face) to obtain Schottky contact and Ni/Au on substrate surface (C face) to obtain Ohmic back contact, respectively. Room temperature current-voltage (I-V) and capacitance-voltage (C-V) curves are measured to study the properties of Schottky diodes. Ohmic characteristic measurement shows that the Ohmic contact is formed after annealing in a temperature range of 900-1050℃, and the lowest specific contact resistivity of 2.5510-5 cm2 is obtained after annealing at 1050℃. The forward I-V curve reveals that the Schottky barrier height and the ideality factor are 1.617 eV and 1.127, respectively, indicating that the main current transportation process is the thermal electron emission. From the C-V curve, besides the net dopant concentration being inferred to be 2.9031014 cm-3, the profile of the free carrier concentration in epitaxial layer is also studied. A comparision of the reverse I-V curves of SiC Schottky diodes with different epitaxial layer thickness shows that the diode with 100 upm-thick epitaxial layer has a constant reverse leakage current when the bias voltage is less than 400 V, showing good rectification characteristics. By applying a reverse bias of 500 V, the diode has a leakage current of 2.11 nA, exhibiting a relatively high breakdown voltage. The depletion layer width of SiC detector is calculated to be 94.4 m at 500 V, indicating that the epitaxial layer is almost fully depleted. The signal of SiC detector through preamplifier displays a relatively low amplitude pulse (15 mV). A typical -ray spectrum response from SiC detector shows 9.49% (5.65 keV) energy resolution for 59.5 keV with a reverse bias of 300 V. The potential causes of poor count rate and energy resolution of fabricated detectors are analyzed in this article. The lower count rate is mainly caused by the narrow depletion layer, resulting in fewer photons deposited in sensitive region which can generate carriers. The poor energy resolution of SiC detector can be attributed to the electronic noise of read-out circuit, the pre-match amplifier circuit for detector needs to be improved, in addition, the extra defects existing in detector caused by increasing thickness of epitaxial layer can also deteriorate the detector performance.

Influences of ICP etching damages on the electronic properties of metal field plate 4H-SiC Schottky diodes**Project supported by the Suzhou Research Fund (No. BY2011129) and the State Grid Corporation of China Research Fund (No. 525500140003).

Inductively coupled plasma (ICP) etching of 4H-SiC using SF6/O2 gas mixture was studied systematically and the effect of etching was examined by metal field plate SiC Schottky diodes (SBDs). It was found that the etch rate as well as SiC surface morphology were related with ICP power, RF power, pressure, the flow of SF6 and O2. Etching damages (the cone-in-pits and pits) generated at high chuck self-bias were observed, and they were thought to be caused by SiC defects. The degradation of both the reverse and forward I–V performances of SiC SBDs was ascribed to the cone-in-pits and pits. Moreover, the absolute value of forward current is even less than the reverse counterpart in the absolute value voltage range of 0–50 V for SiC SBDs with etching damages.