Silicon carbide transistors are increasingly being used in high-voltage power converters due to their ability to meet the stringent requirements of these applications in terms of size, weight, and/or efficiency.
The superior material properties of silicon carbide (SiC) allows for the design of fast-switching unipolar devices instead of Insulated Gate Bipolar Transistor (IGBT) switches. As a result, solutions that were previously only possible in the low-voltage world with voltages of 600V and below are now also possible at higher voltages. The end result is increased efficiency, higher switching frequencies, less heat dissipation, and space savings—benefits that reduce overall system cost.
When it comes to reliable SiC devices, metal-oxide-semiconductor field-effect transistors (MOSFETs) are widely accepted. JFET structures initially appeared to be the ultimate solution for combining performance and reliability in a SiC transistor.
Trench-based SiC MOSFETs, on the other hand, have become feasible with the now-established 150mm wafer technology. This would solve the dilemma of double-diffused metal-oxide semiconductors (DMOS) structures having either high performance or high reliability.
Wide bandgap power devices, such as SiC driver and transistors or gallium nitride high electron mobility transistors (GaN HEMTs), are now common components in power electronics designers’ toolboxes. Why, though? What makes silicon carbide so interesting when compared to traditional silicon?
What makes SiC MOSFET components so appealing to design engineers that they include them in so many of their designs despite their higher cost when compared to silicon high-voltage devices? Let’s take a look at some of them.
Low Losses and a High-Breakdown Field Are Key
Engineers working on power conversion systems are constantly striving to reduce energy losses during the conversion process. Modern systems rely on technologies that switch solid-state transistors on and off in conjunction with passive elements.
Several factors influence the losses associated with the transistors used.
- First, design engineers must consider conducting phase losses. A classical resistance is used to define these in MOSFETs. It’s a fixed conduction loss determinator in the form of a knee voltage (Vce sat) and a differential resistance of the output characteristic in IGBTs. Losses during the blocking phase are typically ignored.
- Second, when switching, design engineers should keep in mind that there is always a transition phase between the ON and OFF states (Figure 1). The device capacitances define the majority of the associated losses. Due to the minority carrier dynamics in IGBTs, additional contributions are in place (turn-on peak, tail current).
Based on these considerations, you’d think that the device of choice would always be a MOSFET. However, at high voltages, the resistance of SiC MOSFETs becomes so high that the total loss balance is inferior to that of IGBTs, which can use charge modulation by minority carriers to reduce resistance in conduction mode.
Figure 1: A graphical comparison of the switching process and static I-V behavior is shown in the figure.
When wide bandgap semiconductors are considered, the situation changes. Figure 2 summarizes the key physical properties of SiC and GaN in comparison to silicon. The direct relationship between a semiconductor’s bandgap and critical electric field is significant. When compared to silicon, it is en times higher with SiC.
Figure 2: The image emphasizes critical physical properties of SiC and GaN versus silicon.
High-voltage component design is altered as a result of this feature. The impact is depicted in Figure 3 using a 5kV semiconductor device as an example. The moderate internal breakdown field of silicon forces semiconductor designers to use a relatively thick active zone. Furthermore, because only a few dopants can be incorporated in the active region, the series resistance is high (as indicated in Figure 1).
The active zone can be made much thinner with a 10 times higher breakdown field in SiC driver. Simultaneously, many more free carriers can be incorporated, resulting in significantly higher conductivity.
In the case of silicon carbide, it can be said that the transition between fast-switching unipolar devices like MOSFETs or Schottky diodes and slower bipolar structures like IGBTs and p-n diodes has now shifted to much higher blocking voltages.
Figure 3: SiC has higher blocking voltages than standard silicon.
Or, conversely, what was possible with silicon in the low-voltage range around 50V is now possible with SiC for 1200V devices.
WBG technology advancements and the superior material properties of silicon carbide allow these devices to operate with lower switching losses, faster switching, and a thinner active zone, resulting in designs with higher switching frequencies and better space savings. As a result, SiC MOSFETs are becoming the preferred option for power conversion applications over traditional silicon.