Status of Silicon Carbide Power Electronic Device R&D and Static/Dynamic Analysis (High-Voltage Probe Station)

Published: 2021-05-29 Views: 次

Analysis of R&D Status of Silicon Carbide Power Electronic Devices

　　Over the past fifteen to twenty years, the field of silicon carbide (SiC) power electronic devices has achieved remarkable progress. The performance metrics of the SiC devices developed far exceed those of current silicon-based devices, and some SiC devices have been successfully commercialized. They are beginning to replace silicon-based power electronic devices in several important energy sectors, initially demonstrating enormous potential. Continued advances in SiC power electronic devices will play a revolutionary role in driving the development of power electronics technology. With progress in SiC single-crystal and epitaxial material technologies, various types of SiC devices have been developed. SiC devices mainly include diodes and switches. SiC diodes mainly include Schottky barrier diodes and their novel structures, as well as PiN diodes. There are many types of SiC switches; representative ones include the metal-oxide-semiconductor field-effect transistor (MOSFET), the junction field-effect transistor (JFET), and the insulated-gate bipolar transistor (IGBT).

　　1. Commercialization of SiC Diodes

　　Among SiC power electronic devices, SiC diodes were the first to be commercialized. In 2001, Germany's Infineon was the first to launch SiC diode products, and manufacturers such as Cree and STMicroelectronics in the United States followed suit. In Japan, Rohm, New Japan Radio, and Renesas Electronics put SiC diodes into production. Many companies are developing Schottky barrier diodes (SBDs) and junction barrier Schottky (JBS) structure diodes. Currently, SiC diodes are available in voltage classes from 600 V to 1700 V and current ratings up to 50 A.

　　SiC Schottky diodes offer near-ideal dynamic performance. As a unipolar device, there is no charge storage during operation, so its reverse recovery current is caused solely by the junction capacitance of its depletion layer; its reverse recovery charge and reverse recovery loss are one to two orders of magnitude lower than those of Si ultrafast recovery diodes. More importantly, the turn-on loss of the matching switch can also be greatly reduced, thereby increasing the switching frequency of the circuit. In addition, it has virtually no forward recovery voltage, so it can conduct immediately with no turn-on delay, unlike bipolar devices. At room temperature, its forward conduction voltage drop is essentially the same as that of Si ultrafast recovery devices, but because the on-resistance of SiC Schottky diodes has a positive temperature coefficient, this facilitates paralleling multiple SiC Schottky diodes. When the single-chip area and current of the diode are limited, this can greatly increase the capacity of SiC Schottky diodes, making their use in higher-capacity applications possible. The largest-capacity SiC diode reported in the laboratory has reached 6500 V/1000 A. Because the development of SiC switches lags behind that of diodes, the more common practice today is to package SiC diodes together with Si IGBTs and MOSFETs in a single module to form high-power switch combinations. Currently, the SiC Schottky diodes from Cree, Microsemi, Infineon, and Rohm are used in variable-frequency or inverter devices to replace silicon-based fast recovery diodes, significantly improving operating frequency and overall system efficiency. Low- and medium-voltage SiC Schottky diodes have already had a significant impact in high-end communications switching power supplies and photovoltaic grid-connected inverters.

　　The development directions for SiC Schottky diodes are substrate thinning technology and the trench JBS structure. Substrate thinning can effectively reduce the on-resistance of low-voltage SiC Schottky diodes, enhance the device's surge current capability, and reduce thermal resistance. In September 2012, Infineon released its fifth-generation SiC SBD products, employing substrate thinning technology for the first time. In the SiC lattice, the depth of the ion-implanted p-well in the JBS structure is limited (<1 µm), and under reverse bias the shielding effect of the shallow p-n junction on the Schottky junction is not particularly pronounced; it only becomes evident when the spacing between adjacent p-wells is small, but this simultaneously narrows the forward conduction channel, significantly increasing the forward voltage drop. To solve this problem, the development direction for the new generation of SiC Schottky diodes is the trench JBS structure. Cree's new-generation SiC Schottky diodes employ both the trench JBS structure and substrate thinning technology; compared with conventional JBS diodes, both forward and reverse characteristics are improved, increasing current density (chip area reduced by 50%) as well as blocking voltage (increased by 150 V) and avalanche capability.

　　2. Commercialization Progress of SiC JFET Devices

　　Silicon carbide JFETs feature high input impedance, low noise, and good linearity. They are among the faster-developing SiC devices and were among the first to be commercialized. Compared with MOSFETs, JFETs do not suffer from reliability issues caused by gate oxide defects or the limitation of excessively low carrier mobility, and their unipolar operating characteristics enable them to maintain good high-frequency performance. In addition, JFETs offer better high-temperature stability and reliability. The junction-type structure of the SiC JFET gate means that the threshold voltage of a typical JFET is mostly negative—that is, it is a normally-on device—which is extremely unfavorable for power electronics applications and incompatible with currently common drive circuits. Semisouth (USA) and Rutgers University developed enhancement-mode devices with normally-off operation by introducing trench-implanted or mesa-trench structures (TI VJFET) into the device process. However, enhancement-mode devices are typically achieved at the expense of some forward on-resistance characteristics; therefore, normally-on (depletion-mode) JFETs more readily achieve higher power density and current capability, and depletion-mode JFET devices can achieve normally-off operation through cascode configuration. The cascode approach is realized by connecting a low-voltage Si-based MOSFET in series. The drive circuit of the cascoded JFET device is naturally compatible with common silicon-based device drive circuits. The cascode structure is well suited for replacing existing silicon IGBT devices in high-voltage, high-power applications and directly avoids drive-circuit compatibility issues.

　　At present, SiC JFET devices have achieved a certain degree of commercialization, mainly through products from Infineon and SiCED. Product voltage classes are 1200 V and 1700 V, with single-device current ratings up to 20 A and module current ratings above 100 A. In 2011, the University of Tennessee reported a 50 kW SiC module using paralleled 1200 V/25 A SiC JFETs with SiC SBDs as antiparallel diodes. In 2011, Global Power Electronics developed a high-temperature SiC three-phase inverter using SiC JFETs; the module had a peak power of 50 kW (with efficiency of 98.5% at 10 kHz and 10 kW under medium load, higher than that of Si modules). In 2013, Rockwell developed a three-phase motor drive module with a current rating of 25 A using 600 V/5 A MOS enhancement-mode JFETs in parallel with SiC diodes, and compared it with today's more advanced IGBT and PiN diode modules: at the same power rating (25 A/600 V), the area was reduced to 60%. The module is aimed at reducing conduction loss, switching loss, and overvoltage/overcurrent in the power loop.

　　3. Breakthroughs in the Practical Application of SiC MOSFET Devices

　　The silicon carbide MOSFET has always been the most closely watched SiC switch. It offers ideal gate insulation characteristics, high-speed switching performance, low on-resistance, and high stability, and its drive circuit is very simple; among SiC devices, it has the best compatibility with the drive circuits of existing power electronic devices (silicon power MOSFETs and IGBTs).

　　SiC MOSFET devices have long faced two main challenges: the long-term reliability of the gate oxide layer and the channel resistance problem. A large channel resistance leads to high conduction loss, and R&D has been ongoing to reduce on-resistance and improve gate oxide reliability in order to reduce conduction loss. One way to reduce on-resistance is to increase the carrier mobility of the inversion channel, thereby reducing channel resistance. To improve the quality of the SiC MOSFET gate oxide layer, reduce surface defect density, and increase carrier density and mobility, one of the most common methods is nitrogen implantation at the growth interface, also known as interface passivation—that is, after the gate oxide growth is complete, high-temperature annealing is performed in a nitrogen-rich environment. This increases channel carrier mobility, thereby reducing channel resistance and conduction loss. A second way to reduce on-resistance is to adopt a trench-gate structure with a trench dug directly beneath the gate. Currently, all SiC MOSFETs in production are planar. When planar devices are scaled down to reduce channel resistance, they tend to suffer from increased JFET resistance, which limits how far on-resistance can be reduced. The trench structure, by construction, has no JFET resistance. It is therefore suitable for reducing channel resistance and on-resistance, but Si trench MOSFETs have not yet solved the surface problems of the sidewall channel after trench etching.

　　Cree (USA) and Rohm (Japan) are already able to supply industry-leading SiC MOSFET devices. The United States has applied SiC MOSFET devices to develop a 2.7 MVA solid-state power substation, which may be used in the power distribution system of the U.S. Navy's next-generation aircraft carrier, CVN-21. Using all-SiC power modules, conventional low-frequency (60 Hz) transformers can be converted into high-frequency (20 kHz) solid-state power substations, which are expected to reduce transformer weight from 6 tons to 1.7 tons and volume from 10 cubic meters to 2.7 cubic meters, greatly improving shipboard system performance. In 2012, Mitsubishi Electric of Japan developed an 11 kW inverter using SiC MOSFETs and Schottky diodes; it reduced energy loss by 70% compared with inverters built on silicon devices, with an output power of 10 W/cm³. Mitsubishi Electric also reported a forced-air-cooled, three-phase 400 V output all-SiC inverter using SiC JFETs and SiC Schottky barrier diodes; the power density of this unit reached 50 kVA/liter, far higher than conventional silicon-based units. In March 2013, Cree (USA) released its second-generation SiC MOSFET. Compared with first-generation products, costs were reduced by measures such as shrinking chip area. For example, for the 1.2 kV class, the second-generation chip area is about 40% smaller than the first generation.

　　4. SiC IGBT Devices

　　Constrained by process technology, silicon carbide IGBTs got off to a relatively late start. High-voltage SiC IGBTs face two challenges: the first is the same as for SiC MOSFET devices—reliability issues caused by channel defects and low electron mobility; the second is that N-type IGBTs require a P-type substrate, and the resistivity of P-type substrates is 50 times higher than that of N-type substrates. Therefore, the first IGBT, fabricated in 1999, used a P-type substrate. After years of R&D, the resistivity problem of P-type substrates was gradually overcome, and in 2008 a 13 kV N-channel SiC IGBT device was reported, with a specific on-resistance of 22 mΩ·cm². A report compared the forward conduction capability of 15 kV N-IGBTs and MOSFETs; the results showed that at room-temperature junction temperature and chip power density below 200 W/cm², the MOSFET can achieve higher current density, while at higher power densities the IGBT can achieve higher current density. At a junction temperature of 127 °C, the IGBT can conduct higher current density than the MOSFET at power densities above 50 W/cm². In the same year, the team also reported a P-channel SiC IGBT with a blocking voltage of 12 kV and a specific on-resistance reduced to 14 mΩ·cm², demonstrating pronounced conductivity modulation capability.

　　The advantageous application range for SiC IGBT devices is the high-voltage field above 10 kV. In this field, SiC MOSFET devices face the problem of excessively high on-resistance, but in applications below 10 kV, the advantage of SiC IGBTs over SiC MOSFETs is not very pronounced. In applications above 15 kV, SiC IGBTs combine low power loss and fast switching speed, offering significant technical advantages over SiC MOSFETs and silicon-based IGBTs, thyristors, and other devices, making them particularly suitable for high-voltage power system applications. New high-temperature, high-voltage SiC IGBT devices will have a major impact on high-power applications, especially in power systems. It is foreseeable that high-voltage SiC IGBT devices, together with PiN diodes, will become the core devices of power electronics technology in next-generation smart grid technology.

　　5. SiC Power Modules

　　SiC power modules are currently a key development focus for major global power electronic device companies. SiC power modules have already achieved initial application in some high-end fields, including high-power-density power conversion and high-performance motor drives, and they have broad application prospects and market potential. In the field of SiC power modules, the first to be developed were hybrid power modules based on SiC power diodes and silicon-based IGBTs. The first commercially available high-power module using SiC diodes and silicon-based IGBTs was Infineon's PrimePACK product. With advances in SiC devices, all-SiC power modules have been continuously developed. Cree (USA) reported SiC MOSFET chips with a blocking voltage of 10 kV and a current of 20 A, and a current transfer capability of 100 A can be obtained by paralleling modules. In 2009, Cree and Powerex (USA) developed a dual-switch 1200 V, 100 A SiC power module composed of high-voltage, high-current SiC MOSFET devices and SiC Schottky diodes. In 2011, the U.S. Army Research Laboratory developed a 1200 V/800 A bidirectional power module using twenty 80 A SiC MOSFETs and twenty 50 A SiC Schottky diodes. The module was used as a full-bridge inverter and compared experimentally with Si devices; the results showed that power loss was reduced by at least 40%, and at the same output current rating the SiC module could operate at four times the switching frequency of the Si module. The module is expected to be used in electric vehicles. In 2012, Fuji Electric of Japan developed a 1200 V/100 A SiC power module based on SiC MOSFETs. The module uses a new wire-bond-free design and a silicon nitride ceramic substrate, can operate at temperatures up to 200 °C, and its flip-chip-like press-pack design gives it lower internal inductance than conventional aluminum wire-bonded modules, along with lower loss and a more compact structure—about half the size of a conventional IGBT module of the same power rating. In 2012, Rohm of Japan began launching all-SiC power modules, and in 2013 Cree (USA) and Mitsubishi (Japan) also launched 1200 V/100 A all-SiC modules. These all-SiC power modules combine SiC MOSFET devices and Schottky diodes; leveraging high-speed switching and low loss, they can replace conventional silicon-based IGBT modules rated at 200–400 A. Because of improved heat dissipation, the unit volume is halved, and lower heat generation allows the cooling system to be downsized, enabling miniaturization of the unit while cutting power conversion loss by more than 85%, greatly reducing the power loss of industrial equipment. The excellent characteristics of all-SiC MOSFET (or JFET) modules give them enormous potential to replace silicon-based IGBTs in applications below 10 kV; the speed and scope of replacement will depend on how quickly SiC material and device technologies mature and how quickly costs fall.

　　6. Summary

　　SiC power electronic devices will play an increasingly large role in improving energy efficiency and miniaturizing power electronic equipment. SiC power electronic devices can improve energy efficiency and reduce energy losses because, compared with silicon devices, SiC devices have advantages in reducing on-resistance and switching loss. For example, in an inverter circuit composed of diodes and switches, simply changing the diode material from silicon to silicon carbide can reduce the inverter's energy loss by about 15–30%; if the switch material is also changed to SiC, energy loss can be reduced by more than half. Power electronic devices made of silicon carbide have three characteristics that enable miniaturization of power converters: higher switching speed, lower loss, and higher operating temperature. SiC devices can switch several times faster than silicon devices. The higher the switching frequency, the easier it is to miniaturize energy-storage and filter components such as inductors and capacitors; lower energy loss means correspondingly less heat generation, enabling miniaturization of power converters; and in terms of junction temperature, silicon devices reach their limit at 200 °C, whereas SiC devices can operate at higher junction and ambient temperatures, allowing the cooling mechanism of power converters to be reduced or eliminated.

　　With technological advances in SiC power electronic devices, SiC devices now offer not only a huge performance advantage over silicon devices but also increasingly evident system cost advantages. According to an assessment by Cree (USA), compared with using silicon IGBTs and silicon diodes, using the company's second-generation SiC MOSFETs and SiC diodes can reduce the total cost of a boost converter. Specifically, by raising the switching frequency to shrink the inductor and reduce inductor cost, the total cost can be compressed to a level lower than when using Si power components. Taking a 10 kW-class boost converter as an example, according to Cree's estimates, if Si power components are used and switched at 20 kHz, the required cost is $181.4, whereas with SiC power components driven at 60 kHz and 100 kHz, the cost drops to $170 and $163, respectively. The use of SiC power components is expected to reduce the total cost of power converters.

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