The majority of electronics power devices have traditionally been built using silicon, a semiconductor that can be treated almost entirely without leaving behind any faults. However, silicon’s theoretical performance has now almost totally been achieved, emphasizing some of this material’s limitations, such as its limited capacity to block voltage, its restricted ability to transfer heat, its limited efficiency, and its non-negligible conduction losses. When compared to silicon, wide bandgap semiconductors (WBG), such as silicon carbide (SiC) and gallium nitride (GaN), perform better due to their greater switching frequency, efficiency, operating temperature, and operating voltage.
The number of power-conversion stages in EVs and HEVs can result in cumulative power losses of up to 20% of the initially available power. When used to create voltage converters, power MOSFETs, and high efficiency Schottky diodes, WGB semiconductors significantly increase the efficiency of power conversion stages and are a viable alternative to silicon in these applications. WBG semiconductors enable significant advancements over silicon (Si) and gallium arsenide (GaAs), including increased power efficiency, smaller size, lighter weight, and lower total cost.
WBG properties
The energy gap between the top limit of the valence band and the lower limit of the conduction band is relatively big in WBG materials. Thermal or visual excitation can cause electrons to cross the bandgap and reach the conduction zone. Due to the bandgap, semiconductors can alternate between the interdiction (OFF) and conduction (ON) states depending on electrical characteristics that can be changed externally. Silicon carbide and gallium nitride, two WBG materials, have bandgaps (3.3 eV and 3.4 eV) that are much greater than those of silicon and gallium arsenide (1.12 eV and 1.4 eV).
Wide bandgap semiconductors have the potential to operate at higher temperatures, voltages, and frequencies, as well as having a stronger electric breakdown field. Wide bandgaps result in higher breakdown voltage and an associated larger breakdown electric field. WBG semiconductors like GaN and SiC offer considerable performance increases and enable operation with efficiency and reliability even in the harshest environments by surpassing the theoretical limitations provided by silicon.
The main advantages of WBG devices over silicon can be summed up as follows:
– Better high frequency performance
– Reduced on-resistance
– Higher breakdown voltage
– Stronger thermal conductivity
– Operation at higher temperatures
– Higher reliability
– Nearly zero reverse recovery time
Automotive applications of SiC and GaN
For implementing certain functionalities, SiC-based devices are an effective replacement for silicon-based ones. A vital part of the car is the main inverter. It manages the electric motor (synchronous, asynchronous, or brushless DC, depending on the type) and collects the energy released during regenerative breaking, returning it to the battery. The DC-DC converter in EVs and HEVs is responsible for converting high-voltage batteries into the 12V power system bus.
There are several different types of high-voltage batteries on the market today, with a variety of voltage levels and power classes (often between 1kW and 5kW). Depending on whether the regenerative circuit will allow monodirectional or bidirectional energy transfer, additional alternative components can be needed. Several auxiliary systems, including air conditioning, electronic power steering, a PTC heater, oil pumps, and cooling pumps, are powered by an auxiliary inverter/converter that draws energy from a high-voltage battery. Battery state is managed by the battery management system (BMS) during charging and discharging. In order to increase the battery lifetime, this process must be carried out carefully. Cell utilization must be optimized as battery ages, balancing their performance during charging and discharging. A crucial function is played by the on-board battery charger. Given that the same circuit must accommodate various voltage and current levels, this is an extra requirement for the designers. Future capabilities like power transmission in both directions (where the charger also feeds power from the automobile to the smart grid) must be supplied.
Electric motors with increasingly smaller sizes and better performance are needed for automotive applications. The classic silicon MOSFET and IGBT-based motor driver circuits struggle more and more to handle these kinds of demands. Theoretically, silicon technology has reached its limits. These limitations mostly relate to power density, breakdown voltage, and switching frequency, all of which have an impact on power losses. These restrictions mostly manifest as a sub-optimal degree of efficiency, with additional potential issues in operating at high temperatures and rapid switching rates. Consider a silicon-based power device which operates at a switching frequency of at least 40 kHz. With cascading effects on overall power losses, switching losses are greater than conduction losses under those circumstances.
A proper heat sink will be required to remove the excess heat produced. However, this solution is more expensive, and increases the weight and size of the device. In high voltage and high switching frequency motor control applications, HEMT (High Electron Mobility Transistor) devices based on gallium nitride (GaN) offer improved electrical properties and present themselves as a legitimate alternative to MOSFET and IGBT transistors.
While conduction losses for silicon and gallium nitride are nearly constant in both materials, the switching losses exhibit different behaviors. In bfact, the switching losses of a GaN HEMT transistor are much lower than those of a silicon MOSFET or IGBT as the switching frequency rises, and this difference is even more pronounced the higher the switching frequency.
Due to their superior qualities, such as low resistance, high thermal conductivity, high breakdown voltage, and high saturation velocity, silicon carbide and gallium nitride technologies are emerging as the most effective, compact, and lightweight solutions to meet the requirements of EVs and HEVs as MOSFETs and IGBTs approach their theoretical limits.
