In the foreseeable future, we will see the revolutionary impact of SiC on the power electronics industry. When SiC-MOSFET is used in solar, UPS, industrial and other applications, it can make the inverter more efficient, more output power, smaller system size, simpler cooling system (smaller radiator or air convection cooling) ).
In the 1940s and 1950s, the first generation of semiconductor materials represented by silicon (Si) and germanium (Ge) laid the foundation for the microelectronics industry. After decades of development, the preparation and process of silicon materials are becoming more and more perfect. The design and development of Si-based devices have also undergone many iterations and optimizations. They are gradually approaching the limit of silicon materials, and the potential for improving the performance of Si-based devices is increasing. small.
Modern electronic technology puts forward new requirements for semiconductor materials such as high temperature, high power, high voltage, high frequency and radiation resistance, and the third generation semiconductor material SiC with wide band gap has excellent switching performance, temperature stability and low electromagnetic interference (EMI), Ideal for next-generation power conversion applications, such as solar inverters, power supplies, electric vehicles, and industrial power.
The research and development of SiC power devices began in the 1970s. The quality and manufacturing process of SiC crystals were greatly improved in the 1980s. With the successful application of high-quality 6H-SiC and 4H-SiC epitaxial layer growth technologies in the 1990s, various SiC power devices And development has entered a period of rapid development.
SiC is a compound semiconductor material composed of silicon and carbon. The different combination of C atoms and Si atoms makes SiC possess a variety of lattice structures, such as 4H, 6H, 3C and so on. 4H-SiC is often used as a power device because of its high carrier mobility and high current density. The following table compares the physical properties of 4H-SiC and Si. We can clearly see that the width of the forbidden band of 4H-SiC is 3 times that of Si, the breakdown field strength is 10 times that of Si, the drift rate is 2 times that of Si, and the thermal conductivity is 2.5 times that of Si.
Figure 1: Comparison of physical parameters of Si, 4H-SiC and SiC
How do these excellent characteristics bring about changes in power devices? We next analyze in detail in three aspects.
Breakdown voltage and on-state resistance
Breakdown voltage is an important indicator of power devices. The forward voltage withstand capability of the power switching device is related to the length and resistivity of the drift region, and the on-state resistance of the unipolar power switching device is directly determined by the length and resistivity of the drift region, which is cubic with the electric field strength of the manufacturing material. Inversely proportional. Because 4H-SiC has 10 times the breakdown electric field strength of Si, SiC-based power devices allow the use of a thinner drift region to maintain a higher blocking voltage, thereby significantly reducing the forward voltage drop and conduction losses. As can be seen from the figure below, if you want to obtain a 5000V withstand voltage, use a substrate material with a doping of 2.5 * 1013 / cm3, the Si-based power device needs a drift layer thickness of 0.5mm, and the resistance per unit area is 10Ωcm2; The drift layer of 2.0e15 / cm3 requires a thickness of only 0.05mm, and the resistance per unit area is only 0.02Ωcm2.
Figure 2: Comparison of drift thickness and on-state resistance of Si and SiC power devices at the same voltage
On-off level
Using SiC instead of Si not only greatly reduces the on-state specific resistance, but also greatly reduces the dynamic loss. This is because the breakdown electric field strength of silicon carbide is 10 times that of silicon, and the electron saturation drift speed is also 2 times that of silicon, which is more conducive to increasing the operating frequency of the device. Traditional silicon-based high-frequency power devices, such as MOSFETs and Schottky diodes, have a higher voltage resistance while the forward voltage drop will also increase exponentially. Therefore, they are not suitable for high-voltage applications. The current common MOSFET voltage resistance is below 900V . Therefore, currently Si-IGBTs are mainly used in the high-voltage field, but IGBTs are bipolar devices, and there is a tail current during turn-off, which causes relatively large turn-off losses. SiC MOSFETs can withstand fairly high blocking voltages, and because they are unipolar devices, there is no tail current. The emergence of SiC will expand the application of MOSFETs and Schottky diodes to higher voltage levels. The on-resistance per unit area of ​​SiC is very low. Compared with Si devices of comparable power levels, the chip size of SiC devices can be greatly reduced, so the parasitic capacitance is lower, making the device easier to drive and the switching speed is faster. Because of the high-frequency operating characteristics of SiC devices, smaller transformers can be used in the system, thereby reducing switching losses and improving efficiency, and greatly reducing the size of the system.
Thermal characteristics
The forbidden band width of SiC is 3.23ev, and the corresponding intrinsic temperature can be as high as 800 degrees Celsius. If the temperature bottleneck of materials and packaging can be broken, the working temperature of power devices will be raised to a whole new level. SiC material has a thermal conductivity of 3.7W / cm / K, while the thermal conductivity of silicon material is only 1.5W / cm / K. Higher thermal conductivity can bring a significant increase in power density, while the design of the heat dissipation system Simpler, or directly using natural cooling.
Challenges and prospects
Although SiC has excellent performance, there are still some challenges from the widespread application. For example, the charge density of the SiC-SiO2 interface is much higher than that of Si-SiO2. Due to this effect, the channel electronic equivalent mobility of the SiC-MOSFET is as low as only 1-7cm2 / Vs, making the channel resistance much greater than the drift region resistance, which becomes the decision The main component of the device's on-state specific resistance. In order to obtain a reasonable on-state resistance, generally driving a SiC-MOSFET will select a higher gate voltage, and using a higher gate voltage will increase the electrical stress of the gate oxide layer, thereby adversely affecting the long-term reliability of the device .
In order to solve these problems, on the one hand, advances in SiC substrate processing, epitaxial growth, and manufacturing processes will greatly reduce the defect density; on the other hand, improvements in device structure will also help reduce the gate drive voltage and extend the device life, such as The trench gate structure used by Infineon CoolSiC MOSFETs forms a conductive channel on the C-plane of the SiC crystal. On this crystal face, there are fewer defects and the interface charge density is lower, thus allowing higher electron mobility, so that the device can use a driving voltage equivalent to silicon-based IGBT and MOSFET, about 15V.
Figure 3: SiC DMOS and SiC Trench MOSFET
In summary, with the characteristics of wide band gap, high breakdown electric field and large thermal conductivity, we will see the revolutionary impact of SiC on the power electronics industry in the foreseeable future. When SiC-MOSFET is used in solar, UPS, industrial and other applications, it can make the inverter more efficient, more output power, smaller system size, simpler cooling system (smaller radiator or air convection cooling) ).
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