Silicon dominated the transistor world for decades but that is slowly changing. Compound semiconductors that are made of two or three materials have been developed, offering distinct advantages and superior properties than traditional silicon transistors.
Compound semiconductors, for example, gave us the LED: one type is composed of a mixture of gallium arsenide (GaAs) and gallium arsenide and phosphorus (GaAsP), while others are composed of phosphorus and indium.
While compound semiconductors are more difficult to manufacture and more expensive, they provide significant advantages over silicon. Compound semiconductors are meeting the stringent specifications of designers of new, more demanding applications such as automotive electrical systems and electric vehicles (EVs).
Power transistors made of gallium nitride and silicon carbide are two compound semiconductor devices that have emerged as viable solutions. These devices compete with silicon power LDMOS MOSFETs and superjunction MOSFETs, which have long lifetimes.
A gate driver optocoupler is commonly used to power and drive silicon-based semiconductors such as IGBTs and Power MOSFETs. This gate driver optocoupler provides galvanic insulation between the control circuits and the power semiconductors from high voltages. In some ways, GaN and SiC devices are similar, but they are not the same.
Wide-Bandgap Semiconductors
Compound semiconductors are also known as wide-bandgap devices. WBG compound semiconductors have higher bandgap energy and higher electron mobility, resulting in characteristics that are superior to silicon’s.
WBG compound semiconductor transistors have higher breakdown voltages and higher temperature tolerance. For high-voltage and high-power applications, these devices outperform silicon equivalents.
WBG transistors also switch faster and can operate at higher frequencies than silicon transistors. Lower on-resistance means they dissipate less power, promoting efficiency. This one-of-a-kind combination of properties makes these devices appealing for some of the most demanding circuits used in automotive applications, particularly hybrid electric vehicles (HEVs) and electric vehicles (EVs).
To meet the challenges of automotive electrical equipment, GaN and SiC transistors are becoming more widely available.
The following are the primary benefits of GaN and SiC devices:
- Devices for 650, 900, and 1,200 V offer high-voltage capability.
- Improved switching speed
- Increasing the operating temperature
- Lower conduction resistance, lower power dissipation, and increased efficiency
SiC Transistors
SiC transistors are e-mode MOSFETs by nature. These devices can switch at frequencies as high as 1 MHz with much higher voltage and current levels than silicon MOSFETs. The maximum drain-source voltage is around 1,800 V, with a current capability of up to 100 A.
Furthermore, SiC devices have a much lower on-resistance than silicon MOSFETs, making them more efficient in all switching power applications. One significant disadvantage is that they require a higher voltage for smart gate drivers than other MOSFETs, though this is changing as design technology advances.
To turn on the device with a low on-resistance, SiC devices require 18 to 20 V of voltage for smart gate drivers. For full conduction, standard Si MOSFETs require a gate voltage of less than 10 V.
GaN Transistors
GaN transistors carved out an early niche in the field of radio-frequency (RF) power. Because of the nature of the materials, a depletion-mode (d-mode) field-effect transistor was developed (FET).
D-mode FETs, also known as pseudomorphic high-electron–mobility transistors (pHEMTs), are naturally “on” devices; without a gate control input, a natural conduction channel exists. The channel conduction is controlled by gate input signals, which also turn the device on and off.
GaN devices are commonly used as power amplifiers in wireless equipment at frequencies up to 100 GHz. Cellular base station power amplifiers, military radar, satellite transmitters, and general RF amplification are just a few of the primary applications. They have, however, been incorporated into a variety of switch-mode power supply applications such as DC/DC converters, inverters, and battery chargers due to their high voltage (up to 1,000 V), high temperature, and fast switching.
WBG Transistor Competition
GaN and SiC devices compete with well-established semiconductors such as Si LDMOS MOSFETs, superjunction MOSFETs, and IGBTs. GaN and SiC transistors are gradually replacing these older devices in a variety of applications.
In many applications, SiC devices are replacing IGBTs, for example. SiC devices can switch at higher frequencies (100 kHz or higher as opposed to 20 kHz), reducing the size and cost of any inductors or transformers while increasing efficiency. SiC MOSFETS can also be driven using gate driver optocoupler.
Here are the highlights of the GaN versus SiC comparison:
- GaN switches at a faster rate than Si.
- SiC has a higher operating voltage than GaN.
- SiC necessitates a high gate drive voltage.
- GaN and SiC are gradually replacing superjunction MOSFETs. SiC appears to be the preferred material for on-board chargers (OBCs). This trend will undoubtedly continue as engineers discover and gain experience with newer devices.
Automotive Applications
Many power circuits and devices can be improved by using GaN and SiC in their design. Automotive electrical systems are one of the biggest beneficiaries. Modern HEVs and EVs are outfitted with equipment that can make use of these devices.
DC/DC converters, OBCs and motor drivers are some popular applications.
DC/DC Converters
These power circuits reduce the high battery voltage to a lower voltage that can then be used to power other electrical devices. Battery voltages can now reach 600 or 900 volts, which a DC/DC converter reduces to 48 or 12 volts, or both, for the operation of other electronic components.
A DC/DC converter on the high-voltage bus between the battery pack and the inverter can also be used in HEVs and EVs.
Traction Motor Driver
The traction motor is a high-output alternating current (AC) motor that drives the vehicle’s wheels. The driver is an inverter that converts the battery voltage into three-phase alternating current (AC), which powers the motor.
OBCs
Plug-in HEVs and EVs have an internal battery charger that connects to an alternating current power source. This enables at-home charging without the use of an external AC-to-DC charger.
Both GaN and SiC transistors offer the automotive electrical designer forgiving and easier designs, as well as superior performance, due to their high voltage, high current, and rapid switching.
