For decades, silicon has dominated the world of transistors. But this situation is gradually changing. Compound semiconductors composed of two or three materials have been developed, providing unique advantages and outstanding characteristics. For example, with compound semiconductors, we have developed light-emitting diodes (LEDs). One type is composed of gallium arsenide (GaAs) and gallium arsenide phosphide (GaAsP). Others use indium and phosphorus.
The problem is that compound semiconductors are more difficult to manufacture and more expensive. However, compared to silicon, they have significant advantages. New and higher demand applications, such as automotive electrical systems and electric vehicles (EVs), are discovering that compound semiconductors can better meet their strict specification requirements.
Gallium nitride (GaN) and silicon carbide (SiC) power transistors, two types of compound semiconductor devices, have emerged as solutions. These devices compete with long-life silicon power lateral diffusion metal oxide semiconductor (LDMOS) MOSFETs and super junction MOSFETs. GaN and SiC devices are similar in some aspects, but there are also significant differences. This article compares the two and provides some examples to help you make decisions for your next design.
Figure 1 shows the relationship between power capability and switching frequency of popular high-voltage, high current transistors and other devices, as well as their main applications.
Wide bandgap semiconductor
Compound semiconductors are known as wide bandgap (WBG) devices. If we do not comment on lattice structure, energy levels, and other headache inducing semiconductor physics, we will only say that the definition of WBG is a model that attempts to describe how current (electrons) flows in compound semiconductors.
WBG compound semiconductors have high electron mobility and bandgap energy, which are converted into properties superior to silicon. Transistors made of WBG compound semiconductors have higher breakdown voltage and high temperature tolerance. These devices have advantages over silicon in high-voltage and high-power applications.
Figure 2. The dual die dual field effect transistor (FET) cascade circuit converts GaN transistors into normally off devices, achieving the standard enhanced working mode in high-power switching circuits
Compared to silicon, WBG transistors have faster switching speeds and can operate at higher frequencies. Lower "on" resistance means they dissipate less power, thereby improving energy efficiency. This unique combination of features makes these devices attractive for some of the most demanding circuits in automotive applications, especially hybrid and electric vehicles.
GaN and SiC transistors are becoming readily available to meet the challenges of automotive electrical equipment. The main selling points of GaN and SiC devices are these advantages:
High voltage capability, with devices available at 650 V, 900 V, and 1200 V.
Faster switching speed.
Higher working temperature.
Lower on resistance, minimal power dissipation, and higher energy efficiency.
GaN transistor
GaN transistors have been found to have early business opportunities in the field of radio frequency (RF) power. The essence of this material enables the development of depletion type field-effect transistors (FETs). Depletion type (or D-type) FET, also known as pseudostate high electron mobility transistor (pHEMT), is a naturally "conducting" device; Due to the lack of gate control input, there exists a natural conduction channel. The gate input signal controls the conduction of the channel and turns on and off the device.
Due to the preference for enhanced (or E-type) devices that are typically "turned off" in switch applications, this has led to the development of E-type GaN devices. Firstly, there is a cascade of two FET devices (Figure 2). Now, standard e-type GaN devices have been developed. They can switch at frequencies up to 10 megahertz and power up to tens of kilowatts.
GaN devices are widely used in wireless devices as power amplifiers with frequencies up to 100 GHz. Some of the main use cases are cellular base station power amplifiers, military radars, satellite transmitters, and general-purpose RF amplifiers. However, due to high voltage (up to 1000 V), high temperature, and fast switching, they have also been incorporated into various switch power applications such as DC-DC converters, inverters, and battery chargers.
SiC transistor
SiC transistor is a natural E-type MOSFET. These devices can switch at frequencies up to 1 MHz, with much higher voltage and current levels than silicon MOSFETs. The maximum leakage source voltage is up to about 1800 V, and the current capacity is 100 amperes. In addition, the on resistance of SiC devices is much lower than that of silicon MOSFETs, resulting in higher energy efficiency in all switch mode power supply applications (SMPS designs). A key drawback is that they require higher gate drive voltages than other MOSFETs, but with design improvements, this is no longer a disadvantage.
SiC devices require a gate voltage of 18 to 20 volts to drive and conduct devices with low on resistance. A standard Si MOSFET only requires a gate voltage of less than 10 volts to fully conduct. In addition, SiC devices require a gate driver of -3 to -5 V to switch to the off state. However, dedicated gate driver ICs have been developed to meet this need. SiC MOSFETs are typically more expensive than other alternatives, but their high voltage and high current capabilities make them ideal for use in automotive power circuits.
Competition of WBG transistors
GaN and SiC devices compete with other mature semiconductors, especially silicon LDMOS MOSFETs, super junction MOSFETs, and IGBTs. In many applications, these old devices are gradually being replaced by GaN and SiC transistors.
For example, in many applications, IGBT is being replaced by SiC devices. SiC devices can switch at higher frequencies (100 kHz+and 20 kHz), allowing for a reduction in the size and cost of any inductor or transformer while improving energy efficiency. In addition, SiC can handle larger currents than GaN.
To summarize the comparison between GaN and SiC, the following are the key points:
The switching speed of GaN is faster than that of Si.
SiC has a higher operating voltage than GaN.
SiC requires a high gate driving voltage.
Super junction MOSFETs are gradually being replaced by GaN and SiC. SiC seems to be the favorite of car chargers (OBC). As engineers discover newer devices and gain usage experience, this trend will undoubtedly continue.
Automotive applications
Many power circuits and devices can be improved by designing with GaN and SiC. One of the biggest beneficiaries is the automotive electrical system. Modern hybrid and pure electric vehicles contain equipment that can use these components. Some popular applications include OBC, DC-DC converters, motor drivers, and LiDAR. Figure 3 indicates the main subsystems in electric vehicles that require high-power switching transistors.
Figure 3. WBG on-board charger (OBC) for hybrid and electric vehicles. The AC input undergoes rectification, power factor correction (PFC), and then DC-DC conversion (one output is used to charge the high-voltage battery, and the other is used to charge the low-voltage battery).
DC-DC converter. This is a power circuit that converts high battery voltage into lower voltage to operate other electrical equipment. The current voltage range of batteries is as high as 600 volts or 900 volts. The DC-DC converter reduces it to 48 volts or 12 volts, or both, for the operation of other electronic components (Figure 3). In hybrid electric vehicles and electric vehicles (HEVEVs), DC-DC can also be used for the high-voltage bus between the battery pack and the inverter.
Car chargers (OBCs). Plug in HEVEVs and EVs include an internal battery charger that can be connected to an AC power source. This allows charging at home without the need for an external AC-DC charger (Figure 4).
Main drive motor driver. The main drive motor is a high output AC motor that drives the wheels of the vehicle. The driver is an inverter that converts battery voltage into three-phase AC power to operate the motor.
LiDAR. LiDAR refers to a technology that combines light and radar methods to detect and identify surrounding objects. It uses pulsed infrared laser to scan a 360 degree area and detect reflected light. These pieces of information are transformed into detailed 3D images within a range of approximately 300 meters, with a resolution of a few centimeters. Its high resolution makes it an ideal sensor for vehicles, especially autonomous driving, to improve its ability to recognize nearby objects. The LiDAR device operates within a DC voltage range of 12-24 volts, which comes from a DC-DC converter.
Figure 4. A typical DC-DC converter is used to convert high battery voltage to 12 volts and/or 48 volts. The IGBT used in high-voltage bridges is gradually being replaced by SiC MOSFETs.
Due to their high voltage, high current, and fast switching characteristics, GaN and SiC transistors provide automotive electrical designers with flexible and simpler designs as well as excellent performance.
