Differences between Gallium Nitride (GaN) and Silicon Carbide (SiC)

Differences between Gallium Nitride (GaN) and Silicon Carbide (SiC)

For decades, silicon has dominated the world of transistors. However, this situation is gradually changing. Compound semiconductors, composed of two or three materials, have been developed, offering unique advantages and superior 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 challenge is that compound semiconductors are more difficult to manufacture and more expensive. However, compared to silicon, they offer significant advantages. Newer, more demanding applications, such as automotive electrical systems and electric vehicles (EVs), are finding that compound semiconductors can better meet their stringent specifications.

Gallium Nitride (GaN) and Silicon Carbide (SiC) power transistors are two types of compound semiconductor devices that have emerged as solutions. These devices compete with long-life silicon power lateral diffused metal oxide semiconductor (LDMOS) MOSFETs and super-junction MOSFETs. GaN and SiC devices are similar in some aspects but also have significant differences. This article compares the two and provides some examples to help you make a decision for your next design.

Figure 1 illustrates the relationship between the power capability and switching frequency of popular high-voltage, high-current transistors and other devices, along with their main applications.

Wide Bandgap Semiconductors

Compound semiconductors are known as wide bandgap (WBG) devices. Without delving into lattice structures, energy levels, and other complex semiconductor physics, we can simply say that the WBG definition is a model that attempts to describe how current (electrons) flows in compound semiconductors.

WBG compound semiconductors have higher electron mobility and higher bandgap energy, translating into superior characteristics compared to silicon. Transistors made from WBG compound semiconductors have higher breakdown voltages and tolerance to high temperatures. These devices are more advantageous than silicon in high-voltage and high-power applications.

Figure 2. A dual-die dual field-effect transistor (FET) cascode circuit converts a GaN transistor into a normally-off device, achieving the standard enhancement mode operation in high-power switching circuits.

Compared to silicon, WBG transistors also 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 characteristics makes these devices attractive for some of the most demanding circuits in automotive applications, particularly 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 speeds.

Higher operating temperatures.

Lower on-state resistance, minimizing power dissipation and enhancing energy efficiency.

GaN Transistors

In the radio frequency (RF) power domain, GaN transistors have found early commercial opportunities. The nature of the material has enabled the development of depletion-mode field-effect transistors (FETs). Depletion-mode (or D-mode) FETs, known as pseudomorphic high electron mobility transistors (pHEMTs), are naturally "on" devices; without a gate control input, there is a natural conduction channel. The gate input signal controls the conduction of the channel and turns the device on and off.

Since enhancement-mode (or E-mode) devices, which are typically "off" in switching applications, are preferred, this has led to the development of E-mode GaN devices. Initially, this was achieved through cascode configurations of two FET devices (Figure 2). Now, standard E-mode GaN devices are available. They can switch at frequencies up to 10 MHz with power levels up to tens of kilowatts.

GaN devices are widely used in wireless devices as power amplifiers at frequencies up to 100 GHz. Some major use cases include cellular base station power amplifiers, military radars, satellite transmitters, and general-purpose RF amplifiers. However, due to their high voltage (up to 1,000 V), high temperature, and fast switching capabilities, they are also incorporated into various switching power supply applications, such as DC-DC converters, inverters, and battery chargers.

SiC Transistors

SiC transistors are naturally E-mode MOSFETs. These devices can switch at frequencies up to 1 MHz with voltage and current levels much higher than silicon MOSFETs. The maximum drain-source voltage can reach approximately 1,800 V, and the current capability is 100 amperes. Additionally, SiC devices have much lower on-resistance than silicon MOSFETs, resulting in higher energy efficiency in all switching 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 significant issue.

SiC devices require 18 to 20 volts of gate voltage to drive and turn on a device with low on-resistance. Standard silicon MOSFETs require less than 10 volts of gate voltage to fully turn on. Furthermore, SiC devices require a -3 to -5 V gate drive to switch to the off state. However, dedicated gate drive ICs have been developed to meet this need. SiC MOSFETs are typically more expensive than other alternatives, but their high-voltage, high-current capabilities make them well-suited for automotive power circuits.

WBG Transistor Competition

GaN and SiC devices both compete with other established semiconductors, particularly silicon LDMOS MOSFETs, super-junction MOSFETs, and IGBTs. In many applications, these older devices are gradually being replaced by GaN and SiC transistors.

For example, in many applications, IGBTs are being replaced by SiC devices. SiC devices can switch at higher frequencies (100 kHz+ vs. 20 kHz), allowing for a reduction in the size and cost of any inductors or transformers while improving energy efficiency. Additionally, SiC can handle larger currents than GaN.

To summarize the comparison between GaN and SiC, here are the key points:

GaN has faster switching speeds than silicon.

SiC has higher operating voltages than GaN.

SiC requires high gate drive voltages.

Super-junction MOSFETs are gradually being replaced by GaN and SiC. SiC seems to be the favorite for on- board chargers (OBCs). As engineers discover newer devices and gain experience using them, this trend will undoubtedly continue.

Automotive Applications

Many power circuits and devices can be designed and improved using GaN and SiC. One of the biggest beneficiaries is automotive electrical systems. Modern hybrid and electric vehicles contain equipment that can utilize these devices. Some popular applications include OBCs, DC-DC converters, motor drives, and LiDAR.Figure 3 identifies the main subsystems in electric vehicles (EVs) that require high-power switching transistors.

Figure 3. Wide Bandgap (WBG) On-Board Chargers (OBCs) for Hybrid and Electric Vehicles. The AC input undergoes rectification, power factor correction (PFC), and then DC-DC conversion (with one output for charging the high-voltage battery and another for the low-voltage battery).

DC-DC Converter

This is a power circuit that converts high battery voltage to a lower voltage to power other electrical equipment. Currently, battery voltages can range up to 600V or 900V. The DC-DC converter reduces this to 48V or 12V, or simultaneously to both 48V and 12V, for the operation of other electronic components (Figure 3). In hybrid electric vehicles (HEVs) and electric vehicles (EVs), the DC-DC converter can also be used for the high-voltage bus between the battery pack and the inverter.

On-Board Chargers (OBCs)

Plug-in hybrid and electric vehicles contain an internal battery charger that can be connected to an AC power source. This allows for charging at home without the need for an external AC-DC charger.(Figure 4).

Main Drive Motor Drives

The main drive motor is a high-output AC motor that drives the vehicle's wheels. The drive is an inverter that converts battery voltage to three-phase AC power to run the motor.

LiDAR

LiDAR refers to a technology that combines light and radar methods to detect and identify surrounding objects. It uses pulsed infrared lasers to scan a 360-degree area and detects reflected light. This information is converted into detailed three-dimensional 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, particularly autonomous driving, to improve the recognition of nearby objects. LiDAR devices operate within a DC voltage range of 12-24 volts, which is derived 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. IGBTs used in the high-voltage bridge are gradually being replaced by SiC MOSFETs.

Due to the high voltage, high current, and fast switching characteristics of GaN and SiC transistors, they offer automotive electrical designers flexibility and simpler designs with superior performance.