What Is a Power Semiconductor? SiC, GaN and IGBTs Explained
A power semiconductor is a chip component that changes the form of electricity, or controls how much electricity goes to a motor. It handles jobs like converting between AC and DC, raising or lowering voltage, and setting motor speed. It shapes electricity by switching a large current on and off very fast, so little energy is wasted as heat.
What you'll learn
- What "converting power" means (AC and DC, voltage, motor speed)
- How fast on-off switching shapes electricity without throwing the extra away as heat
- What the main parts do: diodes, power MOSFETs and IGBTs
- How these parts differ from computing chips, and why SiC and GaN are in the news
Prerequisites: none. It helps to know that a transistor is "a switch controlled by electricity" ("What Is a Transistor?").
In everyday life: from phone chargers to bullet trains
A phone charger changes the electricity from the wall outlet into a form the phone can accept. A home outlet supplies alternating current, for example at 100 volts in Japan. The phone, however, takes direct current at a much lower voltage. The part at the heart of this conversion is a power semiconductor.
Direct current (DC) always flows in the same direction. It is what comes out of a battery. Alternating current (AC) changes direction back and forth 50 or 60 times a second. AC makes it easy to change the voltage and to send power over long distances. That is why power lines and wall outlets use AC.
Similar conversions happen all around us.
- Air conditioners, refrigerators and washing machines: many models first turn the AC from the outlet into DC. Then they rebuild it as AC at whatever speed they need, to change how fast the motor turns
- Electric cars: they turn the battery's DC into AC to drive the motor
- Trains: they turn the power taken from the overhead wire into AC for the motors
- Solar power: it turns the DC made by the panels into AC that a home can use
A device that turns DC into AC is called an inverter. It is the "inverter" in an "inverter air conditioner."
What "converting power" means
Voltage is how hard electricity is pushed. Current is how much electricity flows. The energy electricity carries each second is called power, and it equals voltage times current. So the same power can travel in different forms: "high voltage, small current" or "low voltage, large current."
When a power semiconductor "converts power," it changes this form to suit the device that uses it. Roughly, there are four kinds of conversion.
| Conversion | Name | Examples |
|---|---|---|
| AC → DC | Rectification | Chargers, power supplies in home appliances |
| DC → AC | Inverter | Driving motors, solar power |
| DC → DC at a different voltage | DC-DC converter | Inside computers, electric cars |
| AC → AC at a different frequency | (often goes through DC) | Air conditioners, trains |
Frequency is how many times a second AC goes back and forth. As in the last row of the table, AC is often turned into DC first and then rebuilt.
The figure below gives a rough picture of the conversions inside an electric car.
An electric car's battery holds DC at several hundred volts. Some cars, such as the 2019 Porsche Taycan, use around 800 volts. The inverter turns this DC into AC to drive the motor. Devices like the lights and the navigation system run on 12-volt DC, so the car also needs a conversion that lowers the voltage.
When the car brakes, the motor works as a generator. The inverter then works in reverse: it turns the AC that is produced into DC and sends it back to the battery. This is called regenerative braking.
How it works, part 1: don't throw extra power away as heat
Say you want to lower 12-volt DC to 5 volts. The simplest way is to put a part that resists current in the path. That part turns the extra 7 volts into heat. The current is the same at the input and the output, so at most 5/12, or about 42%, of the power is used. The rest becomes heat (our own calculation).
Many power supplies that use power semiconductors work differently. They switch the 12 volts on and off, very fast, over and over. If the switch is on for 5/12 of the time, the voltage averages out to 5 volts.
Electricity chopped into pieces like this cannot be used while it is still jagged. Parts that store electricity briefly and release it gradually, coils and capacitors, smooth it out. This way of setting the output by changing the share of on-time is called PWM (pulse-width modulation).
Why does this method make so little heat? The power lost in a switch is "the voltage across the switch" times "the current through it." When the switch is on, current flows, but there is almost no voltage across the switch. When it is off, there is voltage across it, but no current flows. Either way, the product is small.
The loss is not zero, though. Even when on, the switch still has a small resistance. This is called on-resistance, and the smaller it is, the lower the loss.
Another loss comes from the moment the switch changes between on and off. For that instant, voltage and current are both present. This is called switching loss. It grows with the number of switchings, so parts that can switch fast are in demand. Lost power turns into heat, so parts that handle a lot of power always need a way to get rid of heat.
How it works, part 2: the main parts are diodes, power MOSFETs and IGBTs
Here are the three typical kinds of power semiconductor, greatly simplified.
Diode: a one-way valve
A diode lets current pass in one direction only. It has no control terminal. The direction of the voltage across it alone decides whether it passes or blocks current. Four diodes together can turn back-and-forth AC into a flow in one direction. This job is called rectification. Inside an inverter, diodes also give the motor current an escape path the instant a switch turns off.
Power MOSFET: a switch that changes state fast
The MOSFET introduced in "What Is a Transistor?" is a switch you turn on and off just by putting a voltage on an electrode called the gate. A power MOSFET is a MOSFET built to handle large currents and high voltages. Its strength is very fast switching. Silicon power MOSFETs are mainly used up to a few hundred volts, in chargers, computer power supplies and similar devices.
IGBT: a switch for large currents at high voltage
An IGBT is also controlled by the voltage on its gate, like a MOSFET. The difference is that it carries current with two kinds of particles: electrons and holes. A hole is a gap left where an electron is missing. When the IGBT is on, many of both particles flood in, and current passes more easily. So even a part built for high voltage can keep its on-state loss small.
The trade-off is that the current trails off a little when it turns off, so it switches more slowly than a MOSFET. Silicon IGBTs are available up to the 6,500-volt class. They are widely used in electric car inverters, trains, factory motors and air conditioners. "IGBT vs Power MOSFET" compares when to use each in a table.
How it works, part 3: an inverter changes motor speed
Many motors in air conditioners and electric cars turn in step with the frequency of the AC. If you can change the frequency, you can freely change how fast the motor turns. A common type of inverter switches six switches on and off in a set order. This builds AC with any frequency and size you want.
An air conditioner without an inverter runs its cooling unit (the compressor) at a fixed speed. It keeps the temperature by stopping and starting. With an inverter that changes the speed, the compressor can keep running slowly once the room is cool. Toshiba was the first in the world to mass-produce this kind of air conditioner: for businesses in 1980 and for homes in 1981. These were split units with separate indoor and outdoor parts. In 2020, the achievement was recognized as an IEEE Milestone by the IEEE, the professional society for electrical and electronic engineering.
In an electric car, the inverter changes the frequency and the size of the voltage to match how far the accelerator is pressed. This fine-tunes the motor's speed and force.
How they differ from computing chips: built to withstand, not to shrink
A computing chip such as a CPU packs billions of tiny transistors that run at a low voltage of about 1 volt. The smaller the transistors, the more fit on a chip and the better it performs ("What Does 2nm or 3nm Mean?").
Power semiconductors have a different goal. They must withstand high voltages, from tens to thousands of volts, and carry large currents with little loss. So their performance does not come from a race to shrink. It comes from better materials, structures and ways of removing heat.
To withstand a high voltage, a thick layer that conducts poorly takes up the voltage. The higher the voltage, the thicker this layer must be. In many power semiconductors, current flows vertically through this layer, from the top of the chip to the back. The whole area of the chip becomes a path for current, so it can carry a large current.
SiC and GaN: why new materials are getting attention
Most power semiconductors are made of silicon. In recent years, two other materials have often been in the news: SiC (silicon carbide) and GaN (gallium nitride).
The difference starts with the size of a "step" inside the material. An electron must climb this step before it can move freely. The step is called the band gap (see "What Is a Semiconductor?" for details). In SiC and GaN, the step is about three times as large as in silicon. Materials like these are called wide-bandgap semiconductors.
A material with a large step is hard to break down, even under a strong electric force. In a table in Toshiba's materials, SiC and GaN can withstand an electric field about 10 times as strong as silicon can. So they can hold back the same voltage with a much thinner layer. A thinner layer lets current pass more easily, so the on-state loss is smaller.
With a large step, electrons rarely jump it on their own when the material gets hot. So the device can still block current properly at high temperatures. SiC also conducts heat about three times as well as silicon, so heat escapes easily (GaN is about the same as silicon). Still, the highest temperature a product can actually run at also depends on its package and other parts.
SiC: for high voltage and high power
SiC is mainly used for MOSFETs and diodes in the 600-volt class and above. Its current does not trail off at turn-off the way a silicon IGBT's does, so switching loss is lower too. SiC is starting to replace silicon IGBTs in electric car inverters, fast chargers and solar power converters.
In 2023, Denso developed its first inverter using SiC, and it went into the Lexus RZ electric car. According to Denso, the loss was less than half that of silicon in tests under specific conditions. Japan's N700S Shinkansen bullet train, in commercial service since 2020, also uses SiC in its drive system. This made the equipment under the floor smaller and lighter.
The weak point is price. SiC does not become a liquid at normal pressure. So, unlike silicon, its crystal cannot be pulled from a melt. In the main method, the raw material is turned into a gas at over 2,000 °C. The gas then settles bit by bit on a seed crystal, which grows. This takes a lot of time and effort (according to Mitsubishi Electric's explainer).
GaN: very fast switching for smaller devices
Most GaN power semiconductors are made by growing a thin GaN film on a silicon wafer. They are mainly used up to a few hundred volts, and they switch very fast.
Switching more often lets designers shrink the coils that smooth the jagged output, and the transformers that change voltage. Each switching cycle only has to store and release a smaller amount of energy. This is why GaN is used in small chargers that fit in your palm. It is also used in power supplies for data center servers.
GaN is also the material of blue LEDs. Isamu Akasaki, Hiroshi Amano and Shuji Nakamura, who invented efficient blue LEDs, received the 2014 Nobel Prize in Physics.
Silicon is still widely used, because it can be made cheaply in large volumes. The three materials are chosen by voltage, speed and price. "SiC vs GaN" and "Si vs SiC" compare them in tables.
Japan and power semiconductors
Japan is home to many companies that develop and make power semiconductors. In December 2023, Japan's Ministry of Economy, Trade and Industry (METI) approved a joint plan by ROHM and a Toshiba group company to expand production. ROHM will focus its investment on SiC, and the Toshiba side on silicon power semiconductors. Each will use the other's production capacity. Government support is up to 129.4 billion yen.
Common myths
Myth: Like CPUs, power semiconductors perform better the more they shrink, to "X nm"
In fact, they need thick layers and large areas to withstand high voltages. Their performance is set by materials, structure and heat removal. News stories call both of them "semiconductors," so people tend to judge them by the same yardstick.
Myth: SiC and GaN will eventually replace all silicon power semiconductors
In fact, each material is chosen for the job. SiC has low loss but costs more, and GaN is mostly used up to a few hundred volts. Silicon MOSFETs and IGBTs are cheap to make, so many devices still use them. News about new materials stands out, which makes it look as if the switch will happen all at once.
Myth: The "inverter" in an inverter air conditioner is the same as the circuit that flips 0 and 1
In fact, they share a name but are different things. A logic circuit that flips 0 to 1 and 1 to 0 (a NOT gate) is also called an inverter. In power electronics, turning DC into AC is called inversion, and a device that does it is an inverter. Both names come from the idea of "turning over," so they are easy to mix up.
By the numbers
| Item | Number | Note |
|---|---|---|
| Efficiency when 12 V is lowered to 5 V by turning the rest into heat | About 42% at most | The rest becomes heat (our own calculation) |
| Electric car battery | Several hundred V | Some cars use around 800 V |
| Silicon IGBT products | Up to the 6,500 V class | High-voltage parts go into trains and similar uses |
| Step size (band gap) | About 3× silicon | SiC about 3.3 eV, GaN about 3.4 eV, Si about 1.1 eV |
| Electric field it can withstand | About 10× silicon | The same voltage can be held back by a thinner layer |
| Loss in Denso's SiC inverter | Less than half | 2023, in tests under specific conditions |
A difference in efficiency is also a difference in heat. Less wasted heat means smaller cooling parts, and the whole device can be smaller.
Learn more
- Compare in a table: Converter vs Inverter, LDO vs Switching Regulator (throwing power away as heat versus switching), PN Junction Diode vs Schottky Diode
- Terms for the next step: DC-DC converter, gate driver, thermal resistance, power semiconductor device (detailed explanation)
- Try the numbers: the DC-DC converter calculator shows how the share of on-time relates to the output voltage
- Back to the basics: "What Is a Semiconductor?", "What Is a Transistor?"