Analysis of the development status of silicon carbide power electronic devices

Release date：2019-07-01   Views：2037

In the past 15 to 20 years, the field of silicon carbide power electronic devices has made remarkable achievements. The performance index of the silicon carbide devices developed is far higher than that of the current silicon-based devices, and some silicon carbide devices have been successfully industrialized. In some important energy fields, silicon-based power electronic devices are gradually replaced, and show their great potential. The continuous progress of silicon carbide power electronic devices will play a revolutionary role in promoting the development of power electronic technology. With the development of SiC single crystal and epitaxial material technology, various types of SiC devices have been developed. SiC devices mainly include diodes and switches. SiC diodes mainly include Schottky barrier diodes and their new structures and pin type diodes. There are many kinds of SiC switches, such as MOSFET, JFET and IGBT.

1. SiC diode industrialization

In SiC Power electronic devices, SiC diode is the first to realize industrialization. In 2001, Infineon company of Germany took the lead in launching SiC diode products, and American Cree and Italian French semiconductor manufacturers also followed suit to launch SiC diode products. In Japan, ROM, Nippon wireless and Renesas electronics have put into production SiC diodes. Many enterprises are developing Schottky barrier diode (SBD) and JBS structure diode. At present, there are products with voltage level of 600V ~ 1700V and current level of 50A.

SiC Schottky diodes can provide near ideal dynamic performance. As a single device, there is no charge storage in its working process, so its reverse recovery current is only caused by its depleted layer junction capacitance. Its reverse recovery charge and its reverse recovery loss are one to two orders of magnitude lower than that of Si ultrafast recovery diodes. More importantly, the turn-on loss of the matched switch tube can be greatly reduced, so the switching frequency of the circuit can be increased. In addition, it has almost no forward recovery voltage, so it can turn on immediately and there is no turn-on delay of bipolar devices. At room temperature, the normal conduction voltage drop of SiC Schottky diodes is basically the same as that of Si ultrafast recovery devices. However, due to the positive temperature coefficient of on resistance of SiC Schottky diodes, it is conducive to parallel connection of multiple SiC Schottky diodes. The capacity of SiC Schottky diodes can be greatly increased when the area and current of single chip are limited, which makes it possible to apply in larger capacity. At present, the maximum capacity of SiC diode reported in the laboratory has reached the level of 6500v / 1000A. Since the development of SiC switches lags behind that of diodes, it is more common to package SiC diodes, Si IGBT and MOSFET devices in one module to form high-power switch combination. At present, SiC Schottky diodes from Cree, MICROSEMI, Infineon and Rohm are used to replace silicon based fast recovery diodes in frequency conversion or inverter devices, which significantly improves the working frequency and overall efficiency. At present, medium and low voltage SiC Schottky diodes have great influence on high-end communication switching power supply and photovoltaic grid connected inverter.

The development direction of SiC Schottky diode is substrate thinning technology and trench JBS structure. Substrate thinning technology can effectively reduce the on resistance of low voltage SiC Schottky diodes, enhance the surge current capability of devices, and reduce the thermal resistance of devices. Infineon released the fifth generation of SiC SBD products in September 2012, using substrate thinning technology for the first time. In SiC lattice, the depth of ion implanted p-well in JBS structure is limited (< 1um). The shielding effect of shallow p-n junction on Schottky junction is not obvious under the condition of reverse bias. Only when the distance between adjacent p-wells is small can it be highlighted. However, the narrowing effect of forward conduction channel width leads to a significant increase of forward conduction voltage drop. In order to solve this problem, the development direction of the new generation SiC Schottky diode is trench JBS structure. Cree's new generation SiC Schottky diodes adopt trench JBS structure and substrate thinning technology. Compared with the traditional JBS diodes, the forward and reverse characteristics are improved, not only increasing the current density (chip area reduced by 50%), but also improving the blocking voltage (increasing 150V) and avalanche capability.

2. Industrialization development of SiC JFET devices

Silicon carbide JFET has the characteristics of high input impedance, low noise and good linearity. It is one of the fast developing silicon carbide devices, and it is the first to realize commercialization. Compared with MOSFET devices, JFET devices have no reliability problems caused by gate oxide defects and low carrier mobility. At the same time, the unipolar operating characteristics make it maintain good high-frequency operation ability. In addition, JFET devices have better high temperature stability and reliability. The gate junction structure of SiC JFET devices makes the threshold voltage of JFETS usually negative, i.e. normally on devices. This is very disadvantageous to the application of power electronics and can not be compatible with current common drive circuits. Semisouth company and Rutgers University of the United States have developed enhanced devices with constant break state by introducing the device technology of trench injection or mesa groove structure (Ti vjfet). However, the enhanced devices are often formed at the expense of certain forward on resistance characteristics, so the normally on (depleted) JFET is easier to achieve higher power density and current capability, while the depleted JFET can realize the normal break state by cascading. The cascading method is realized by connecting a low voltage Si based MOSFET in series. The driving circuit of the cascaded JFET device is naturally compatible with the general silicon-based device driver circuit. The cascaded structure is very suitable to replace the original silicon IGBT devices in high voltage and high power applications, and directly avoids the compatibility problem of the drive circuit.

At present, silicon carbide JFET devices and to achieve a certain degree of industrialization, mainly by Infineon and siced products. The voltage level of the product is 1200V and 1700V, the maximum current level of single tube can reach 20a, and the current level of module can reach more than 100A. In 2011, the University of Tennessee reported a 50KW silicon carbide module. The module adopts 1200V / 25A SiC JFET in parallel, and the reverse parallel diode is SiC SBD. In 2011, global power electronics developed a SiC three-phase inverter made of SiC JFET under high temperature conditions. The peak power of the module is 50KW (the efficiency of the module under medium load level is 98.5% @ 10kHz and 10kW, which is higher than that of Si module. Rockwell adopted 600V / 5A in 2013 MOS enhanced JFET and silicon carbide diode are paralleled to make a three-phase electrode driving module with a current level of 25A. Compared with the current advanced IGBT and pin diode modules, the area of the module is reduced to 60% at the same power level (25a / 600V). The module aims to reduce on-state loss, switching loss and over-voltage and over-current in the power circuit.

3. The practical application of SiC MOSFET has made a breakthrough

Silicon carbide mosfe has always been the most attractive silicon carbide switch tube. It not only has ideal gate insulation characteristics, high-speed switching performance, low on resistance and high stability, but also its drive circuit is very simple, and it has the best compatibility with the existing power electronic devices (silicon power MOSFET and IGBT).

The long-term reliability of gate oxide layer and channel resistance are two main challenges for SiC MOSFET devices. In order to reduce the on loss and improve the reliability of the gate oxide layer, the research and development of reducing the on resistance and improving the reliability of the gate oxide layer has been carried out. One of the methods to reduce the on resistance is to increase the carrier mobility of the inversion channel and reduce the channel resistance. In order to improve the quality of gate oxide layer of silicon carbide MOSFET, reduce the concentration of surface defects, improve the number of carriers and mobility, one of the most common methods is to achieve nitrogen injection at the growth interface, also known as interface passivation, that is, after the gate oxide layer growth process is completed, high temperature annealing in nitrogen rich environment can be carried out, so as to improve the channel carrier mobility and reduce the channel channel channel Channel resistance to reduce conduction loss. The second way to reduce the on resistance is to use a trench gate structure with a trench just below the gate. At present, all the sicmosfets that have been put into production are "planar". In order to reduce the channel resistance, planar type cell is easy to cause the increase of JFET resistance, and there are some limitations in the reduction of on resistance. However, there is no JFET resistance in the groove type. Therefore, it is suitable to reduce the channel resistance and on resistance, but the surface problem of the side wall channel after trench etching has not been solved yet.

Cree and Rohm of Japan have been able to provide industry-leading silicon carbide MOSFET devices. Silicon carbide MOSFET devices have been applied in the development of a 2.7mva solid-state power substation in the United States, which may be used in the distribution system of the next generation aircraft carrier CVN-21. The traditional low-frequency (60Hz) transformer can be transformed into a high-frequency (20kHz) solid-state power substation by using all SiC Power module. It is estimated that the weight of the transformer will be reduced from 6 tons to 1.7 tons, and the volume will be reduced from 10 cubic meters to 2.7 cubic meters, which will greatly improve the performance of the ship system. In 2012, Mitsubishi Motor of Japan developed a 11kw inverter by using silicon carbide MOSFET and Schottky diode. Compared with the inverter based on silicon device, it can reduce energy consumption by 70% and output power is 10W / cm3. Mitsubishi Electric Co., Ltd. of Japan reported a forced air-cooled three-phase 400V output all SiC inverter, which uses silicon carbide JFET and silicon carbide Schottky barrier diode. The power density of this device reaches 50KVA / L, which is much higher than the traditional silicon-based device. In March 2013, Cree released the second generation SiC MOSFET. Compared with the first generation products, the cost is reduced by reducing the chip area. For example, the chip area of the second generation is about 40% smaller than that of the first generation.

4. SiC IGBT devices

Due to the restriction of process technology, silicon carbide IGBT started late, and high voltage silicon carbide IGBT faces two challenges: the first challenge is the same as silicon carbide MOSFET devices, the reliability and low electron mobility caused by channel defects; the second challenge is that n-type IGBT needs p-type substrate, and the resistivity of p-type substrate is 50 times higher than that of n-type substrate. Therefore, the first IGBT made in 1999 used p-type substrate. After years of research and development, the resistance problem of p-type substrate has been gradually overcome. In 2008, a 13kv N-channel silicon carbide IGBT device was reported, with a specific on resistance of 22m Ω× cm2. The results show that when the current density of fet-15kv at room temperature is higher than that of fet-15kv, it can obtain a higher power consumption at the current density of less than 15kV. When the junction temperature is 127 ° C, IGBT can conduct higher current density than MOSFET when the power density is above 50 W / cm2. In the same year, the team also reported that the p-channel SiC IGBT with a blocking voltage of 12 kV had a conduction specific resistance reduced to 14 m Ω × cm2, reflecting the obvious conductivity modulation ability.

Silicon carbide IGBT devices have the advantages of high voltage applications over 10kV. In this field, silicon carbide MOSFET will face the problem of high on state resistance, but in the application below 10kV, the advantage of silicon carbide IGBT over silicon carbide MOSFET is not very obvious. In the application field above 15 kV, silicon carbide IGBT combines the characteristics of low power consumption and fast switching speed. Compared with silicon carbide MOSFET, silicon based IGBT, thyristor and other devices, SiC IGBT has significant technical advantages, especially suitable for high voltage power system applications. High temperature and high pressure silicon carbide IGBT devices will have a significant impact on high power applications, especially in power systems. It can be predicted that high-voltage silicon carbide IGBT device and pin diode device will become the core device of power electronic technology in the next generation smart grid technology.

5. SiC Power Module

Silicon carbide power module is the key development direction of global power electronic device enterprises. Silicon carbide power module has been applied in some high-end fields, including high-power density power conversion, high-performance motor drive, etc., and has broad application prospects and market potential. In the field of silicon carbide power module, the hybrid power module based on silicon carbide power diode and silicon-based IGBT was first developed. The first commercial high-power module using silicon carbide diodes and silicon-based IGBT is Infineon's primepack product. With the development of SiC devices, all SiC power modules have been developed. Cree company of the United States reported a silicon carbide MOSFET chip with blocking voltage of 10kV and current of 20a, and can obtain the current transmission capacity of 100A through parallel modules. In 2009, Cree company and Powerex company developed a dual switch 1200V, 100A silicon carbide power module. The module is composed of high voltage and high current resistant silicon carbide MOSFET devices and silicon carbide Schottky diodes. In 2011, U.S. Army Research Laboratory developed a 1200V / 800A bidirectional power module using 20 80A SiC MOSFETs and 20 50A SiC Schottky diodes. The experimental results show that the power loss is reduced by at least 40%, and the SiC module can work at the frequency doubling state of Si module under the same output current level. The module is expected to be used in the field of electric vehicles. In 2012, Fuji Electric Co., Ltd. of Japan developed a 1200V / 100A SiC Power Module Based on SiC MOSFET. Compared with the traditional aluminum wire bonding module, the module has lower internal inductance and lower loss. Compared with the traditional IGBT module with the same power, the module has a more compact structure, about 1 / 2 of the original size. In 2012, Roma company of Japan began to launch all SiC power modules. In 2013, Cree company of the United States and Mitsubishi company of Japan also launched 1200V / 100A all SiC Power module. These all SiC Power Modules combine silicon carbide MOSFET devices and Schottky diodes. They can replace the original silicon-based IGBT modules with rated current of 200-400a by using the characteristics of high-speed switch and low loss. Due to the improvement of device heat dissipation, the volume of the device is reduced by half, and the calorific value is small. The cooling device can be reduced to realize the miniaturization of the device. At the same time, the power loss during power conversion can be reduced by more than 85%, which greatly reduces the power loss of industrial equipment. The excellent characteristics of all SiC MOSFET (or JFET) module make it have great potential to replace silicon-based IGBT in applications below 10kV. The speed and range of replacement will depend on the maturity speed of SiC materials and device technology and the speed of cost reduction.

6. Summary

Silicon carbide power electronic devices will play more and more advantages in improving the efficiency of power utilization and realizing the miniaturization of power electronic devices. Silicon carbide power electronic devices can improve the efficiency of power utilization and reduce the power loss, because compared with silicon devices, silicon carbide devices have advantages in reducing the on resistance and switching loss. For example, in the inverter circuit composed of diode and switch tube, if the diode material is replaced by silicon carbide, the power loss of inverter can be reduced by about 15-30%. If the switch tube material is also replaced by SiC, the power loss can be reduced by more than half. The power electronic devices made of SiC have three characteristics that can make the power converter miniaturized: higher switching speed, lower loss and higher working temperature. Silicon carbide devices can switch at several times the speed of silicon devices. The higher the switching frequency is, the easier it is to realize miniaturization of energy storage and filtering components such as inductors and capacitors; the lower the power loss, the lower the calorific value, so the miniaturization of power converter can be realized; In terms of junction temperature, silicon devices reach the limit at 200 ° C, while silicon carbide devices can work at higher junction temperature and ambient temperature, which can reduce or eliminate the cooling mechanism of power converter.

With the technical progress of silicon carbide power electronic devices, compared with silicon devices, silicon carbide devices have not only great performance advantages, but also advantages in system cost. According to Cree's evaluation, the total cost of boost converter can be reduced by using the second generation SiC MOSFET and SiC diode compared with using silicon IGBT and silicon diode. Specifically, by increasing the switching frequency to reduce the inductor cost, the total cost can be reduced to a lower level than when using Si power components. Taking a 10kW step-up converter as an example, according to Cree's estimation, if the Si power element is used and the switch is operated at 20kHz, the cost is 181.4 US dollars, while if the SiC power element is used and driven at 60KHZ and 100kHz, the cost will be reduced to 170 US dollars and 163 us dollars respectively. The use of SiC power components is expected to reduce the total cost of power converters.

In many application fields of power electronic devices, such as power transmission system, distribution system, electric locomotive, hybrid electric vehicle, various industrial motors, photovoltaic inverter, wind power grid connected inverter, air conditioning and other white household appliances, servers and personal computers, silicon carbide devices will gradually show their advantages in performance and system cost reduction. As the main direction of the next generation of power electronic devices, silicon carbide power electronic devices will bring important technological innovation to power electronics, and promote the development of power electronics in the next 20 to 30 years.

Last：Application prospect of silicon carbide devices in electric vehicles

Next：Supply chain analysis of wireless charging transmitter for wafer shortage