Can advanced semiconductors reduce greenhouse gas emissions enough to play a role in the fight against climate change? The answer is yes. Such changes are actually being carried out in an orderly manner.
Beginning around 2001, the compound semiconductor GaN sparked a lighting revolution that, in some ways, was the fastest technological change in human history. According to a study by the International Energy Agency, in just two decades, the share of GaN-based light-emitting diodes in the global lighting market has grown from zero to more than 50%. Research firm Mordor Intelligence recently predicted that globally, LED lighting will reduce lighting electricity consumption by 30% to 40% over the next seven years. Globally, lighting accounts for about 20% of electricity consumption and 6% of CO2 emissions, according to the United Nations Environment Programme.
Each wafer contains hundreds of state-of-the-art power transistors
This revolution is far from over. Indeed, it is about to leapfrog to a higher level. The semiconductor technology gallium nitride (GaN) that has transformed the lighting industry is also part of a revolution in power electronics, which is gaining momentum. Because one of the compound semiconductors – silicon carbide (SiC) – has begun to replace silicon-based electronics in the huge and important field of power electronics.
GaN and SiC devices perform better and are more efficient than the silicon components they are replaced. There are hundreds of millions of such devices around the world, many of which run for hours a day, so the energy savings will be enormous. The rise of GaN and SiC power electronics will eventually have a greater positive impact on Earth's climate than GaN LEDs replacing incandescent lamps and other traditional lighting.
Almost everywhere AC must be converted to DC or DC to DC, less power is wasted. This conversion occurs with wall chargers for phones or laptops, larger chargers and inverters that power electric cars, and elsewhere. There will be similar savings as other silicon bases fall into the new semiconductors. Wireless base station amplifiers are one of the growing applications where these emerging semiconductors clearly have advantages. In efforts to mitigate climate change, eliminating wasted power consumption is a low-hanging fruit, and these semiconductors are how we harvest it.
This is a new example of a common pattern in the history of technology: two competing innovations that bear fruit at the same time. How will all this get out of it? In which applications will SiC dominate and where will GaN dominate? A close look at the comparative advantages of these two semiconductors can give us some solid clues.
Why electricity conversion is important in climate calculations
Before we get to the semiconductors themselves, let's first consider why we need them. First: power conversion is everywhere. It goes far beyond the small wall chargers that power our smartphones, tablets, laptops, and countless other gadgets.
Power conversion is the process of transforming electricity from a usable form to the form required for a product to perform its function. There is always some energy lost in this conversion, and since some of these products run continuously, a lot of energy savings can be achieved. Recall this: despite California's soaring economic output, the state's electricity consumption has been largely flat since 1980. One of the most important reasons why demand has remained flat is that the efficiency of refrigerators and air conditioners has increased significantly during this period. One of the most important factors in this improvement is the use of variable-speed drivers based on insulated-gate bipolar transistors (IGBTs) and other power electronics, resulting in a significant increase in efficiency.
GaN and silicon carbide: their competitive areas
In the high-voltage power transistor market, gallium nitride devices dominate applications below 400 volts or so, while silicon carbide now has an advantage in applications above 800 volts and above (the market above 2000 volts is relatively small). As GaN devices improve, the important battlefield landscape between 400 and 1,000V will change. For example, with the introduction of 1,200V GaN transistors (expected in 2025), EV inverters, the most important market, will join the fray.
SiC and GaN will significantly reduce emissions. According to an analysis of publicly available data by Transphorm, a GaN device company founded in 2007, GaN-based technologies alone could reduce greenhouse gas emissions by more than 1 billion tons in the United States and India by 2041. Data comes from the International Energy Agency, Statista, and other sources. The same analysis suggests energy savings of 1,400 terawatt hours, or 10 to 15 percent of the projected energy consumption of both countries that year.
Advantages of wide bandgap
Like ordinary transistors, power transistors can act as amplification devices or switches. An important example of the role of amplification is a wireless base station, which amplifies the signal for transmission to a smartphone. Around the world, the semiconductors used to make the transistors in these amplifiers are moving from silicon technology called laterally diffused metal-oxide semiconductors (LDMOS) to GaN. The new technology offers many advantages, including a 10% or more increase in energy efficiency depending on frequency. On the other hand, in power conversion applications, transistors act as switches rather than amplifiers. The standard technique is called pulse width modulation. For example, in common types of motor controllers, DC pulses are fed to a coil mounted on the motor rotor. These pulses create a magnetic field that interacts with the magnetic field of the motor's stator, causing the rotor to rotate. The speed of this rotation is controlled by changing the length of the pulses: the pattern of these pulses is a square wave, and the longer the pulse is "on" rather than "off", the greater the speed and torque provided by the motor. The power transistor completes the switching.
Pulse-width modulation is also used in switching power supplies, which is one of the most common examples of power conversion. Switching power supplies are the type of power to almost all personal computers, mobile devices, and appliances that run on direct current. Basically, the input AC voltage is converted to DC, which is then "chopped" into a high-frequency AC square wave. This chopping is done by a power transistor that generates a square wave by turning direct current on and off. A square wave is applied to a transformer, which changes the amplitude of the wave to produce the desired output voltage. To obtain a stable DC output, the voltage from the transformer is rectified and filtered.
The important point here is that the characteristics of the power transistor almost exclusively determine the ability of the circuit to perform pulse-width modulation, and therefore also the efficiency of the controller in regulating the voltage. The ideal power transistor completely blocks the current when it is in the off state, even when the applied voltage is high. This property is called high electrical breakdown field strength, and it indicates how much voltage the semiconductor can withstand. On the other hand, when it is on and on, this ideal transistor has very little resistance to the flow of current. This characteristic stems from the very high mobility of charges (electrons and holes) within the semiconductor lattice. Think of breakdown field strength and charge mobility as the yin and yang of power semiconductors.
Compared to the silicon semiconductors they replace, GaN and SiC are closer to this ideal state. First, consider the breakdown field strength. GaN and SiC are both wide-bandgap semiconductors. The band gap of a semiconductor is defined as the energy required for electrons in the semiconductor lattice to transition from the valence band to the conduction band, in electron volts. The electrons in the valence band participate in the bonding of atoms within the lattice, while the electrons in the conduction band can move freely in the lattice and conduct electricity.
In semiconductors with wide bandgaps, bonds between atoms are strong, so materials are often able to withstand relatively high voltages before the bonds break, and transistors are said to be damaged. Compared to GaN's 3.40 eV, silicon has a band gap of 1.12 electron volts. For the most common type of SiC, the band gap is 3.26 eV. [See table below, "Bandgap Menagerie"]
Operating speed and the ability to block high voltages are the two most important characteristics of power transistors. These two qualities, in turn, are determined by the key physical parameters of the semiconductor materials used to make transistors. The speed depends on the mobility and velocity of the charge in the semiconductor, while the voltage blocking depends on the band gap and electrical breakdown field of the material.
Now let's look at the mobility, which is measured in square centimeters per volt second (cm²/V·s). The higher the velocity at which the product of mobility and the electric field produces electrons, the greater the current carried for a given number of moving charges. For silicon, this number is 1,450; for SiC, it is about 950; For GaN, it's about 2,000. The exceptionally high value of GaN is why it can be used not only in power conversion applications, but also in microwave amplifiers. GaN transistors can amplify signals at frequencies up to 100 GHz – well above the 3 to 4 GHz that is typically considered the maximum value of silicon LDMOS. For reference, 5G has millimeter wave frequencies up to 52.6GHz. This highest 5G band is not yet widely available, however, frequencies up to 75GHz are being deployed in dish to dish communications, and researchers are now using frequencies up to 140GHz for indoor communications. The need for bandwidth is unsatisfactory.
These performance data are important, but they are not the only criteria for comparing GaN and SiC for any particular application. Other key factors include ease of use and cost of the equipment and its integrated systems. Taken together, these factors explain where and why each of these semiconductors is starting to replace silicon — and how their future competition might be out of the woods.
SiC is the leader in GaN in today's power conversion space
Cree (now Wolfspeed) introduced the first commercially viable SiC transistor superior to silicon in 2011. It can block 1,200 volts and has a fairly low resistance of 80 milliohms when conducting current. There are currently three different types of SiC transistors on the market. ROHM has a trench MOSFET (Metal Oxide Semiconductor Field Effect Transistor); DMOS (dual-diffusion MOS) by Infineon Technologies, ON Semiconductor Corp., STMicroelectronics, Wolfspeed, etc.; and Qorvo's vertical junction field-effect transistors.
One of the great advantages of SiC MOSFETs is their similarity to traditional silicon MOSFETs – even the same package. SiC MOSFETs work in much the same way as normal silicon MOSFETs. There are active, gate, and drain. When the device is turned on, electrons flow from a heavily doped n-type source through the lightly doped bulk region and are "discharged" through the conductive substrate. This similarity means that the learning curve for engineers to turn to SiC is small.
SiC has other advantages over GaN. SiC MOSFETs are essentially "fail-open" devices, meaning that if the control circuit fails for any reason, the transistor will stop conducting current. This is an important feature because it largely eliminates the possibility that a malfunction could lead to a short circuit and a fire or explosion. (However, the price paid for this feature is lower electron mobility, which increases the resistance when the device is turned on.) ）
But GaN is gaining new traction
GaN brings its own unique advantages. The semiconductor was first commercialized in the light-emitting diode and semiconductor laser markets in 2000. It is the first semiconductor capable of reliably emitting bright green, blue, violet, and ultraviolet light. But long before there was a commercial breakthrough in optoelectronics, researchers had demonstrated the promise of GaN in high-power electronics. GaN LEDs quickly became popular because they filled the gap of efficient lighting. But GaN for electronics must prove itself superior to existing technologies: in particular, Infineon's silicon CoolMOS transistors for power electronics, as well as silicon LDMOS and GaAs transistors for RF electronics.
The main advantage of GaN is its extremely high electron mobility. Current, the flow of charge, is equal to the concentration of charge multiplied by their speed. Therefore, you can get a high current due to high concentration or high speed or some combination of both. GaN transistors are unusual because most of the current flowing through the device is due to electron velocity rather than charge concentration. This means in practice that less charge must flow into the device to turn it on or off than Si or SiC. This, in turn, reduces the energy required per switching cycle and helps improve efficiency.
One of the two main types of GaN transistors is called an enhanced device. It uses a gate control circuit operating around 6 volts to control the main switching circuit, and when the control circuit is off, it can block voltages of 600 volts or higher. When the device is turned on (when 6V is applied to the gate), electrons flow from the drain to the source in a flat region called two-dimensional electron gas. In this region, electrons move easily – a factor that contributes to very high switching speeds – and are confined below the aluminum gallium nitride barrier. When the device is turned off, the area below the gate runs out of electrons, disconnecting the circuit below the gate and stopping the flow of current.
At the same time, GaN's high electron mobility allows switching speeds of up to 50 volts per nanosecond. This characteristic means that GaN-based power converters can operate efficiently at frequencies of hundreds of kilohertz, while power converters in silicon or SiC can operate at frequencies of about 100 kilohertz.
In general, high efficiency and high frequency make power converters based on GaN devices very small and light: high efficiency means smaller heat sinks, while operating at high frequencies means that inductors and capacitors can also be very small.
One disadvantage of GaN semiconductors is that they do not yet have reliable insulator technology. This complicates the design of fail-safe devices. In other words, if the control circuit fails, the fault opens.
There are two options for achieving this NC feature. One approach is to equip the transistor with a gate that, when no voltage is applied to the gate, removes the charge in the channel and conducts current only when a positive voltage is applied to that gate. These are called enhanced mode devices. For example, they are provided by EPC, GaN Systems, Infineon, Innoscience, and Navitas. [See illustration, "Enhanced-Mode GaN Transistor"]
Another option is called a common-source common gate solution. It uses a separate, low-loss silicon FET to provide fail-safe functionality for GaN transistors. Power Integrations, Texas Instruments, and Transphorm use this common-source common gate solution. [See illustration, "Cascaded Depletion GaN Transistors"]
For safety, when the control circuit of a power transistor fails, it must fail into an open circuit state with no current flowing. This is a challenge for GaN devices because they lack gate insulator materials that are reliable in both high-voltage blocking and current-carrying states. A solution called cascade depletion mode uses a low-voltage signal on a silicon field-effect transistor (FET) to control a much larger voltage on a gallium nitride high electron mobility transistor [top right]. If the control circuit fails, the voltage across the FET gate drops to zero and the current flow stops [top left]. As the FET no longer conducts current, the GaN transistor also stops conducting because there is no longer a closed circuit between the drain and source of the combined device.
No semiconductor comparison is complete without considering cost. A rough rule of thumb is that smaller die size means lower cost, and die size is the physical area of the integrated circuit that contains the device.
SiC devices now typically have smaller chips than GaN devices. However, SiC has higher substrate and manufacturing costs than GaN, and in general, the final device cost for 5 kW and higher power applications is now similar. However, future trends may favor GaN. This belief of mine is based on the relative simplicity of GaN devices, which means that the production cost is low enough to overcome larger die sizes.
That said, for GaN to be suitable for many high-power applications that also require high voltages, it must have cost-effective, high-performance devices rated at 1,200V. After all, there are already available SiC transistors at this voltage. Currently, the closest commercial GaN transistors are rated at 900V. Recently, Transphorm also demonstrated a 1,200-V device fabricated on a sapphire substrate with electrical and thermal performance comparable to SiC devices.
Research firm Omdia's forecast for 1,200-V SiC MOSFETs puts the price at 16 cents per amp in 2025. The author estimates that the price of the first generation 1,200-V GaN transistors in 2025 will be lower than their SiC counterparts due to the lower cost of GaN substrates. Of course, this is just my opinion; We all know exactly what this will change in a few years.
GaN vs. SiC competition
With these comparative advantages and weaknesses in mind, let's consider each application one by one and shed light on how things might play out.
Electric vehicle inverters and converters
Tesla's adoption of SiC for its Model 3's on-board or traction inverter in 2017 was an early major victory for the semiconductor. In electric vehicles, traction inverters convert the direct current of the battery into alternating current of the motor. The inverter also controls the speed of the motor by changing the frequency of the alternating current. Today, Mercedes-Benz and Lucid Motors are also using SiC in their inverters, and other electric car manufacturers are also planning to use SiC in upcoming models, according to news reports. SiC devices are supplied by Infineon, OnSemi, Rohm, Wolfspeed and others. The power range of EV traction inverters typically ranges from about 35kW to 100kW for small EVs to about 400kW for large vehicles.
However, it is too early to call this race SiC. As I pointed out, to break into this market, GaN suppliers must offer 1,200-V devices. Electric vehicle electrical systems now typically operate only at 400 volts, but the Porsche Taycan has an 800-volt system, as do electric vehicles from Audi, Hyundai and Kia. Other automakers are expected to follow suit in the coming years. (Lucid Air has a 900-V system.) I hope to see the first commercial 1,200-V GaN transistor in 2025. These devices will be used not only in vehicles but also in high-speed public EV chargers.
The higher switching speeds that GaN is likely to achieve will be a strong advantage for EV inverters because these switches incorporate so-called hard-switching technology. Here, the way to improve performance is to switch from on to off very quickly to minimize the time the device holds high voltage and passes through high current.
In addition to inverters, electric vehicles are often equipped with on-board chargers that use wall (mains) current to charge the vehicle by converting alternating current to direct current. Here, again, GaN is very attractive for the same reasons that make it ideal for inverters.
Grid applications
Ultra-high voltage power conversion for devices rated at 3kV or higher will remain the domain of SiC for at least the next decade. These applications include systems that help stabilize power grids, convert alternating current to direct current and back again at transmission level voltages, and other uses.
Chargers for phones, tablets, and laptops
Starting in 2019, companies such as GaN Systems, Innoscience, Navitas, Power Integrations, and Transphorm began selling GaN-based wall chargers.
GaN's high switching speed and its generally low cost make it a dominant player in low-power markets (25 to 500W), where these factors along with small size and robust supply chains are critical. These early GaN power converters had switching frequencies up to 300kHz and efficiencies of over 92%. They set a power density record, with numbers as high as 30W per cubic inch (1.83W/cmm³) — about twice the density of the silicon-based charger they're replacing.
An automated probe system applies high voltage to stress test the power transistors on the wafer. The automated system tests each of approximately 500 dies in minutes.
Solar microinverter
In recent years, solar power has been successful in both grid-scale and distributed (home) applications. For each installation, an inverter is required to convert the direct current of the solar panels into alternating current, power the home or release the electrical energy to the grid. Today, grid-scale PV inverters are the domain of silicon IGBTs and SiC MOSFETs. But GaN will begin to make inroads into the distributed solar market, especially.
Traditionally, in these distributed installations, all solar panels have an inverter box. But more and more installers prefer systems in which each panel has a separate microinverter and alternating current is combined before powering the house or powering the grid. Such a setup means that the system can monitor the operation of each panel to optimize the performance of the entire array.
Microinverters, or traditional inverter systems, are critical to the modern data center. Together with the battery, they create an uninterrupted power supply to prevent blackouts. In addition, all data centers use power factor correction circuits to adjust the AC waveform of the power supply to improve efficiency and eliminate features that can damage equipment. For these, GaN offers a low-loss and economical solution that is slowly replacing silicon.
5G and 6G base stations
GaN's superior speed and high power density will allow it to win and eventually dominate applications in the microwave space, especially 5G and 6G wireless as well as commercial and military radar. The main competition here is silicon LDMOS device arrays, which are cheaper but lower performance. In fact, GaN has no real competitors at frequencies of 4GHz and above.
For 5G and 6G wireless, the key parameter is bandwidth, as it determines how much information the hardware can efficiently transmit. The next generation of 5G systems will boast nearly 1GHz of bandwidth for ultra-fast video and other applications.
Microwave communication systems using silicon-on-insulator technology offer a 5G+ solution using high-frequency silicon devices, where the low output power of each device is overcome by a large number of arrays. GaN and silicon will coexist in this space for some time. The winner for a particular application will depend on the trade-off between system architecture, cost, and performance.
radar
The U.S. military is deploying a number of ground-based radar systems based on GaN electronics. These include the ground/air mission-oriented radar and active electronically scanned array radar built by Northrup-Grumman for the U.S. Marine Corps. Raytheon's SPY6 radar was delivered to the U.S. Navy and conducted its first sea tests in December 2022. The system significantly expands the range and sensitivity of the shipborne radar.
The battle for the wide bandgap has only just begun
Today, SiC dominates EV inverters and is often used in places where voltage blocking and power handling are critical and frequencies are low. GaN is the technology of choice where high-frequency performance is critical, such as 5G and 6G base stations, as well as radar and high-frequency power conversion applications such as wall plug adapters, microinverters, and power supplies.
But the tug-of-war between GaN and SiC has only just begun. Regardless of the competition, one application after another, one market after another, we can safely say that the global environment will be the winner. As this new cycle of technological renewal and rejuvenation moves forward inexorably, billions of tons of greenhouse gas emissions will be avoided in the coming years.