The news says "wide-bandgap semiconductors". What are SiC and GaN?

The news often talks about "wide-bandgap semiconductors", "silicon carbide in electric cars" and "GaN fast chargers". It usually means two materials: silicon carbide (SiC) and gallium nitride (GaN). What they share is a "bandgap" much wider than silicon's. Chinese-language news calls them "third-generation semiconductors", but that "generation" is about the material chips are made from, not a newer generation of silicon chipmaking, and it has nothing to do with "2 nm".

Why "third generation"?

It's a grouping common in Chinese-language media, roughly in the order the materials came into wide use:

• First generation: silicon and germanium, each a single element. The vast majority of chips today are made of silicon.
• Second generation: compound semiconductors such as gallium arsenide and indium phosphide, made of two elements combined. They're often used in the parts of a phone that amplify radio signals, and in lasers for fiber-optic communication.
• Third generation: silicon carbide (silicon plus carbon) and gallium nitride (gallium plus nitrogen), also compounds, with an especially wide bandgap.

In English they are usually just called wide-bandgap semiconductors (WBG). And "third generation" doesn't mean the first two were replaced: all three are still in use.

What is a bandgap?

What is a semiconductor, and why is it called "semi"? explains it with hands: each silicon atom holds its neighbors with four hands, and an electron that's being held can't move. For an electron to let go and move freely (that is, to conduct), it needs a push: some energy. The smallest amount of energy an electron needs to "jump up" and start conducting is the bandgap.

Think of it as a step: an electron can only run around once it has jumped up the step, and the higher the step, the harder the jump.

The unit, eV, is the electronvolt: the energy one electron gets from a push of 1 volt.

Material	Bandgap (approx.)
Silicon	1.1 eV
Gallium arsenide	1.4 eV
Silicon carbide	3.2–3.3 eV
Gallium nitride	3.4 eV

The step for silicon carbide and gallium nitride is about three times as high as silicon's. That's what "wide bandgap" means.

What does a wide bandgap buy you?

It takes more heat

The hotter it gets, the harder the atoms jiggle, and they shake some electrons loose. Silicon's step is low: when it gets too hot, more electrons jump up on their own than were deliberately added by doping, and the switch can't shut off; it keeps leaking. Silicon carbide and gallium nitride have a high step, so they still switch normally at temperatures far above what silicon can handle.

It takes higher voltage

At high voltage, the electric field inside the material gets strong enough to rip electrons free. Those electrons knock more electrons loose, like an avalanche, and the device fails. A material with a high step needs a much stronger field before that happens: silicon carbide and gallium nitride withstand a field about ten times stronger than silicon does.

It switches faster and wastes less

Because it takes a field ten times stronger, the layer that holds back the voltage can be about a tenth as thick for the same voltage. A thinner layer means much less resistance when current flows, and the device can be smaller. A smaller device has less charge to fill and empty each time it switches (the small cup in What is a transistor?), so it switches faster and wastes less heat.

Why does fast switching matter?

Because these two materials are often used as switches that handle power (power devices). Power from a wall socket or an electric car's battery has to be converted before a phone or a motor can use it: a switch flips on and off a great many times a second, chopping the power into small slices that are then smoothed into the voltage that's needed. Every flip wastes a little energy as heat. A switch that wastes less saves energy and runs cooler; one that switches faster lets the coils and capacitors inside be smaller, so the charger shrinks too.

Where are SiC and GaN used?

They are good at different things:

• Silicon carbide: higher voltages and more power, and it carries heat away better than silicon. You'll find it in the part of an electric car that feeds power to the motor, in the car's on-board charger, in charging stations, and in the equipment that connects solar power to the grid.
• Gallium nitride: switching very fast, mostly where the voltage isn't as high (up to a few hundred volts or so). You'll find it in fast chargers for phones and laptops, in server power supplies, and in the parts that amplify radio signals in base stations and radar. Blue and white LEDs are made from materials in the gallium nitride family too.

So why not use them for everything?

• The crystals are hard to grow: a silicon ingot is pulled out of molten silicon while it spins (Why are wafers round?). Silicon carbide is mostly made another way: above 2,000 °C, powder is turned into vapor, which slowly settles onto a small seed crystal. It grows tens to hundreds of times slower than a silicon ingot is pulled, and the crystal is more likely to have defects.
• Hard to cut: silicon carbide is nearly as hard as diamond. It is hard to slice and hard to polish, and both take a lot of time.
• Small, expensive wafers: silicon carbide wafers are usually 150 mm (6 inches) across and moving to 200 mm (8 inches), smaller than 300 mm (12-inch) silicon wafers. Each wafer holds fewer devices, and each one costs much more.
• Gallium nitride usually grows on another wafer: large gallium nitride crystals are very hard to make, so a thin layer of it is usually grown on a silicon, silicon carbide or sapphire wafer. The atoms in the two materials are spaced differently, like stacking bricks of two different sizes, so defects form where they meet.
• Yield: wherever a defect lands, that device may fail to hold back the voltage, or may leak. Devices that carry large currents are often big, so they're more likely to hit a defect, and a high yield is hard to reach (What does "yield" mean in chip news?).

Also, chips like phone processors and memory, with billions of tiny switches, depend on making switches smaller and packing in more of them, so a wide bandgap doesn't help them; and for power chips at lower voltages, silicon is good enough. That's why most chips, including most power chips, are still made of silicon.

Is this the same thing as "2 nm"?

No. "2 nm" is the name of a generation of silicon chipmaking, and it's about how many switches fit in the same space (The news says "2 nm" explains it). "Third-generation semiconductors" means a different material, and it's about handling high voltage, large currents and heat.

Power switches have to let large currents through and hold back high voltages, so they are big to begin with and don't need the finest line width, just like the power chips in The news says "mature nodes". The two solve different problems; neither is more advanced than the other.

Next time you see it in the news

• "Wide-bandgap" or "third-generation semiconductors": usually silicon carbide and gallium nitride, two materials with a wide bandgap. Not a new generation of silicon chipmaking.
• "Electric cars switch to silicon carbide": the switches that handle power for the motor and for charging are made of silicon carbide, so less energy is wasted in conversion and the parts can be smaller.
• "GaN fast charger": the switches inside the charger are made of gallium nitride; they switch fast, so the charger can be smaller.
• "Ultra-wide-bandgap" or "fourth-generation semiconductors": materials with an even wider bandgap, such as gallium oxide and diamond, mostly still in research or at an early stage.

In fab24hr

The fab in the game makes silicon chips, with 25 silicon wafers, 30 cm (12 inches) across, in each box. There is no silicon carbide or gallium nitride fab in the game.

Much of the work is the same, though: silicon carbide devices are also made on wafers, with litho to print the patterns, etch, film growth, atoms shot in and high heat. The difference is in the conditions. Atoms added to silicon carbide barely spread on their own, so doping selected areas is done almost entirely by ion implantation, and afterwards the wafers are annealed at over 1,500 °C, far hotter than the 800 to 1,200 °C commonly used for silicon wafers in a furnace.

In "A day in the fab", on the "Your role today" card, tap "Production fab", swipe to the process engineer, then tap "Implant" or "Furnace" under the name: your job is the doping and heating of silicon wafers. The companies, people and tools in the game are fictional; this article is written from public, general sources, and where sources disagree on a number, it gives only the rough size. For other jargon, see Chip fab jargon in plain English.