Silicon Carbide: The Future of Power Electronics

In today's modern world, the demand for power-based devices is on the rise due to an increase in applications like heating, robotics, electric/hybrid vehicles, electric power transmission, etc. Owing to these factors, the enhancement of high voltage and low loss power-based devices are highly crucial for innovative grid applications providing high performance and lower cost. With the invention of MOSFETs and IGBTs, silicon-based power-switching devices have enabled systems to achieve greater outputs with higher efficiency.

Recent research has shown Silicon Carbide (SiC) devices as a new emerging technology with multiple features lacking in traditional Silicon. SiC has a wider bandgap than Si, allowing for higher voltage blocking, and making it suitable for high-power and high-voltage applications. Moreover, SiC also has a  lower thermal resistance than Si, implying that it can dissipate heat more effectively with higher reliability

Features and Advantages of SiC over Si

The main advantage of SiC is its wide bandgap, which is three times larger than that of silicon. The wide bandgap of SiC means that it can block a larger amount of voltage than silicon, making it suitable for use in high-voltage power electronics. The high breakdown voltage of SiC makes it ideal for high-power applications, such as high-voltage power inverters and converters. In addition to its wide bandgap, SiC also has a low thermal resistance, which allows it to dissipate heat more effectively. This makes it ideal for high-temperature applications where heat management is a critical concern. The low thermal resistance of SiC can help reduce power losses and improve the efficiency of power systems.

Fig 1: Schematic diagram of the electric field distribution

The above figure illustrates the electric field distribution in a one-sided abrupt junction for SiC and Si at the same breakdown voltage. Considering the ten times higher breakdown of Sic when compared to Si, the width of voltage-blocking layers of SiC power devices is 1/10, and doping concentration should be increased by two orders of magnitude when referred to Si counterparts. Fast switching with minimum reverse recovery (also known as low reverse recovery charge) is an important feature of SiC power devices that has significant benefits in power electronics applications.

In traditional power electronics applications, bipolar power devices such as PiN diodes, IGBTs, bipolar junction transistors, and thyristors have been commonly used due to the ability to reduce high on-resistance through conductivity modulation through minority carrier injection. However, this bipolar design also results in minority carrier storage, leading to slow switching speed and large reverse recovery in the turn-off operation.

SiC unipolar devices, such as Schottky barrier diodes (SBDs) and FETs, offer a better solution for these applications. These devices have a low on-resistance even without conductivity modulation, and they exhibit fast switching speed with minimum reverse recovery, making them an ideal choice for medium- and high-voltage applications.

The below figure displays the major applications of unipolar and bipolar power devices for SiC and Si in terms of rated blocking voltage. Fig 2,  it is shown how SiC devices will replace Si devices in the range of 300 V to higher voltages.

Fig 2:  Applications of SiC and Si

Operation of SiC Devices

●  Material Development

SiC research and development initialized due to the need of having bulk and epitaxial growth technologies for long-term operations and economical benefits in terms of production. Unlike silicon, SiC does not melt at atmospheric pressure, and therefore must be grown through a sublimation method at extremely high temperatures above 2200°C. With improved control of the temperature gradient in the growth system and advancements in wafering technology, single crystalline SiC wafers with a four-inch diameter and acceptable quality are now readily available. The typical growth temperature is 1600°C and the growth rate is between 10 to 50 μm/h. The issue of high nitrogen contamination in the growth process has been resolved through the increase of the C/Si ratio in the gasses used or through the use of low-pressure chemical vapor deposition (CVD).

●  Process Development

The low diffusion constants of impurities in SiC make diffusion impractical for impurity doping. Therefore, ion implantation and in-situ doping during growth are the only techniques used in the fabrication of SiC devices. The most remarkable aspect of ion implantation into SiC is the post-implantation annealing at extremely high temperatures. The high temperatures required for post-implantation annealing make ion implantation the initial step in SiC device fabrication. Despite being subjected to high-temperature annealing, the implanted atoms experience very little diffusion.

Characteristics of various SiC power devices

● SiC Power diodes

Power diodes with Schottky contacts are designed to minimize the reverse leakage current and reduce the forward voltage drop, making them suitable for high-power applications. By forming a p-type region close to the edge of the Schottky contact, the field crowding effect is suppressed, leading to a lower resistance and a higher current-carrying capacity.

Fig 3: SiC vs. work function of metal n-type height formation.

Fig 3 shows how the barrier height is controllable from the ranges 1.0 eV to 2.2 eV. The ideal height for a 600- 1700 V Schottky Barrier Diode is approximately 1.1 -1.2 eV. These diodes are best suited for high-frequency power conversion devices like switching power supply devices.

● SiC Power switching devices

MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) are a type of power-switching device that is commonly used in various applications, including high-power conversion, motor control, and switching power supplies.

Fig 4: SiC power device specific on-resistance vs blocking voltage

Fig 4 depicts the SiC power device and its on-resistance vs blocking voltage. When it comes to a JFET, there is only one pn junction without any oxide reliability issue. In order to suppress the electric field penetration into the channel region, the architecture of the buried p-gate was essential. SiC power MOSFETs gave superior performances compared to Si, as to the high impedance levels due to the high breakdown limits of SiC. Moreover, trench MOSFETs are another power device with high-packing density cells. They have high channel mobility levels, which can be useful during high-frequency switching.

Conclusion

Silicon Carbide (SiC) based devices have shown a greater circuit resilience in terms of circuit operation for high-voltage, low-loss power devices. SiC as a material has great electrical characteristics as compared to its predecessor Silicon (Si) with a much higher efficiency rate for high power switching applications.

With further research and development, SiC power systems have the capability to be integrated into a variety of power converters/inverters at high temperatures due to their long-term reliability.