What is a power semiconductor? The chips that handle high voltage and big currents

A power semiconductor, also called a power device, is a semiconductor device built to switch and convert large currents and high voltages. It turns electricity from one form into another: the AC from a wall socket into the DC your phone needs, the power in an electric car's battery into the motor, the output of solar panels onto the grid. "Power" here means it handles a lot of electricity, not that it computes anything.

How is it different from an ordinary chip?

A chip like a phone's processor holds billions of tiny switches (What is a transistor?). Wired together, they decide whether each line's voltage is high (1) or low (0), and that is how they compute (What are logic gates?). What they want is switches that are many, small and fast.

A power device is often one big switch on its own. It doesn't compute. It has three jobs:

• Block high voltage: when it's off, the two ends may differ by tens of volts up to thousands of volts, and it must not break down.
• Carry a large current: when it's on, anything from a few amps to hundreds of amps may flow through it.
• Waste little as heat: it handles so much electricity that losing even a small share means a lot of heat.

A control chip next to it decides when it turns on and off. At low voltage and small currents, the switch and the control circuit can sit on the same chip, and many power management chips are built that way (Analog vs digital chips: what's the difference?).

How many times is power converted between the socket and your phone?

1. The socket: AC, with the current going back and forth fifty or sixty times a second, at about 100 to a little over 200 volts.
2. Rectifying: diodes let current flow only one way, turning the back-and-forth current into DC that flows one way.
3. Chopping and stepping down: a power MOSFET switches on and off tens of thousands to hundreds of thousands of times a second, chopping the DC into short slices that pass through a tiny transformer and come out at a few volts, then get smoothed out. Why it switches so fast is covered in The news says "wide-bandgap semiconductors". What are SiC and GaN?.
4. Inside the phone: the battery is around 3 to 4 volts, and the processor needs about 1 volt, so the voltage is stepped down again.

From rectifying on, power devices are switching on and off at every step. An electric car works the other way round: the battery is DC, the motor needs AC, and a set of switches called an inverter "chops" the DC into AC, setting the motor's speed as it does.

Diodes, MOSFETs, IGBTs: what does each one do?

Diodes: current goes one way only

A diode has just two ends and needs no one to tell it what to do: push current in from one end and it conducts, push from the other and it blocks. That makes it the usual choice for rectifying. It also often sits next to the two kinds of switch below: when the switch turns off, it gives the current that can't stop at once in a motor or coil somewhere to go, so the voltage doesn't spike too high.

Power MOSFETs: the fastest switchers

It works on the same principle as the transistors in a logic chip: voltage on the gate turns it on, and taking the voltage away turns it off. The difference is that it's made big, and the current often flows straight down from the front of the chip to the back. It switches fast and takes little effort to control, so it's most common in chargers and power supplies, anywhere below a few hundred volts. But the higher the voltage it must block, the thicker the voltage-blocking layer has to be, and the higher its resistance when it's on (What is epitaxy?). At very high voltages it stops paying off.

IGBTs: high voltage, big currents

IGBT stands for insulated-gate bipolar transistor. Think of it as a power MOSFET with one extra layer on the back. It's controlled the same way, by the voltage on the gate. The difference is that when it's on, that back layer pushes an extra crowd of charged particles into the voltage-blocking layer, which suddenly makes that layer conduct well. So at a few hundred to a few thousand volts and large currents, it wastes less while it's on, and it's common in electric cars, trains, big factory motors, and solar and wind power equipment. The price: when it turns off, those particles take a moment to clear out, so it switches more slowly, usually thousands to tens of thousands of times a second.

Where are power devices used?

• Chargers and power supplies for computers and servers: AC to DC, then a lower voltage.
• Electric cars: the inverter that runs the motor, the on-board charger, charging stations.
• Solar and wind power: solar panels produce DC, which must become AC before it can go onto the grid.
• Motors and appliances: air conditioners, fridges, washing machines, and the pumps and fans in factories often use variable-speed drives to run only as fast as needed, and that takes a set of power devices.

Where is power lost, and why does efficiency matter so much?

A power device is not a perfect switch. It loses energy at two moments:

• While it's on: even when it conducts, there's a little resistance. For a MOSFET, twice the current means roughly four times the heat.
• At the moment it switches: going from on to off or off to on, there's a brief stretch when neither the voltage nor the current is zero, so a little energy is lost every time. The more often it switches, the more that adds up.

Efficiency is the share of the power going in that comes out the other end; the rest becomes heat. Electricity is converted several times between the power plant and your hand, losing a bit each time. Each small gain in efficiency saves a lot of power, and the cooling can be smaller too.

Why are they often made on older processes, and made bigger?

Carrying a large current takes enough area for the current to flow through, the way a big flow needs a wide pipe. Blocking a high voltage takes a thick enough voltage-blocking layer. Both pull the opposite way from "make the switch smaller". So power devices don't compete on the finest line width; what matters is how well the voltage-blocking layer is grown and how good the contact on the back is. Many are made on lines that have been running for years (The news says "mature nodes").

Why is most of it silicon, and why are silicon carbide and gallium nitride coming in?

Silicon is cheap and the most proven, and most power devices are still silicon. Where the voltage is higher or the switching faster, silicon carbide and gallium nitride are coming in: they withstand a much stronger electric field, so the voltage-blocking layer can be much thinner. Silicon carbide is used more where the voltage and power are higher; gallium nitride more where switching has to be very fast, as in fast chargers and server power supplies. Why they handle high voltage, and why they haven't replaced silicon: see The news says "wide-bandgap semiconductors". What are SiC and GaN?.

Why do packaging and cooling matter so much?

That small share of wasted energy all comes out of one small piece of silicon, and if it gets too hot, the device fails or wears out sooner. So power device packaging (What is chip packaging?) pays special attention to heat and big currents:

• Die attach often uses metal solder, so the heat can flow out from under the chip.
• For big currents, wire bonding uses thicker wire, or the chip is connected with a metal strip instead.
• High-power ones often have several chips soldered onto a ceramic board that insulates electrically but conducts heat well, packaged as a "power module" with a heat sink below it; in electric cars they are often liquid-cooled.

Every time it switches on and off and heats up and cools down, the layers expand and shrink by different amounts, slowly pulling the solder joints and wires apart. That's why these parts get special reliability testing (What is reliability testing?).

In fab24hr

The game has no power semiconductors, and its fab doesn't make power devices. But you can see where the fab's power comes from: tap "Clock in", choose "A day in the fab", then on the "Your role today" card tap "Production fab", swipe to Facilities and tap "Power" under the name (marked Beta). You're the Power engineer, looking after the power from the substation out to every tool, plus the uninterruptible power supply and emergency generator for a power failure.

The substation's transformers use coils to raise or lower the voltage of AC, without power semiconductors. An uninterruptible power supply is different: its batteries store DC, which has to be turned back into AC before it goes out, and that is usually done with power devices. The game doesn't show its inside. On the late-night shift, the control room calls to say the front-end bay is asking whether it's the power, and you check the voltage record before you answer. The fab, people and tool numbers in the game are fictional; what a power semiconductor is comes from public, general sources, and voltages and currents are given only as orders of magnitude. For other jargon, see Chip fab jargon in plain English.