What is an SoC, the "system on a chip" in your phone?

SoC stands for system on a chip: several functions an electronic device needs, such as the processor that computes (the CPU), the graphics processor that draws the screen (the GPU), the circuits that manage memory, wireless communication and the circuits that process camera pictures, all built on one chip. The most important chip in a phone, tablet or watch is usually an SoC. The CPU is just one part of it.

For what a chip is in the first place, start with "What is a chip?".

What's inside an SoC?

Think of an SoC as an office building: each floor is a department doing its own job, and they all work together. Every SoC is laid out differently, but one in a phone often has these blocks:

• Processor (CPU): runs the program step by step, calculating and making decisions. It directs the rest of the chip. It often has several cores, that is, several copies of the processor that can each work on their own.
• Graphics processor (GPU): draws the screen. It's very good at doing lots of identical small calculations at once, which games and smooth on-screen motion rely on.
• Circuits for AI: often called an NPU, made for AI jobs such as recognizing faces or speech.
• Image processing: often called an ISP, it turns the raw data from the camera into a photo.
• Wireless: the circuits that connect to the mobile network and other wireless signals.
• Memory control: manages reading and writing data in the memory outside the chip. The chip also holds a little very fast memory of its own, called cache.

There are also circuits that connect to things outside the chip, such as the screen and the charging port. All told, a phone's SoC now holds more than ten billion transistors (What is a transistor?).

How is an SoC different from a CPU?

In one line: the CPU is one department in the building; the SoC is the whole building.

In a traditional desktop computer, the CPU is one chip; memory, the graphics card and networking are separate chips spread over the circuit board, connected by wires on the board. A phone has no room for that many, so most of them go into one chip. That's why, when people talk about a phone's "processor", they usually mean the whole SoC, not just the CPU inside it.

The microcontrollers (MCUs) common in home appliances and remote controls also put a processor, a little memory and some supporting circuits on one chip. They're a lot like an SoC, only much smaller and simpler.

Why build it all as one chip?

• Small: several chips, plus the wires between them, shrink into one. A smaller circuit board is what lets a phone be thin and a watch fit on a wrist.
• Less power: a signal that leaves the chip has to pass through a pin and travel along a wire several centimeters long on the circuit board. That wire is much "heavier" than the wires inside a chip, and each time data is sent over it, it takes several times more power. Keep the signal inside the chip and that power is saved. Battery-powered things care about this most (where a chip's power goes: "Why do chips use power?").
• Fast: the paths inside a chip are short, and many more wires fit side by side, so data goes back and forth quickly.
• Cheaper in large numbers: one chip replaces several, so there are fewer parts to buy and solder; the more you sell, the less of the design cost each chip has to carry.

What's the catch?

A huge, hard design

More than ten billion transistors and several completely different kinds of circuit have to be packed together, work at the same time, and not disturb each other. So hardly any SoC has every block drawn from scratch: many blocks were designed and checked before and are reused. Just making sure the whole chip does the right thing takes up a big share of the time (what design involves: "VLSI design vs working in a fab: what's the difference?").

Slow and expensive

From the start of the design to the first chips, it often takes from several months to more than a year. Once the design is done, at tape-out, a full set of photomasks has to be made, one for each layer's pattern, and each newer generation costs more. If a mistake turns up after tape-out, the design has to be changed and the masks made again, which means waiting several more months.

One small bad block can sink the whole chip

Every wafer picks up a few defects too small to see. Whichever block of the SoC a defect lands on is broken, and since every function is on the same chip, one broken block often makes the whole chip useless. The bigger the chip, the more likely it is to be hit, and the lower the yield ("What does "yield" mean in chip news?" explains it with a tray of cookies).

There is one way around it: where a chip has several identical parts, such as several processor cores, a broken one can be switched off and the chip used as a lower-spec version. But a block the chip has only one of usually can't be saved that way.

Every block has to be made the same way

One chip can only be made with one generation of process. For the blocks that compute, the newest generation pays off: smaller, faster, lower power. But the blocks that talk to the outside world or handle analog signals barely shrink on the newest generation, and cost more there (why each new generation costs more: "The news says "2 nm". What exactly is 2 nm?").

Some things can't go in at all. A phone's DRAM and the flash memory that stores your photos are made very differently from logic chips, so they're usually separate chips, sometimes stacked right on top of the SoC ("How is memory different from a processor?"). The parts in a phone that amplify wireless signals are often made from a different semiconductor material altogether ("The news says "wide-bandgap semiconductors". What are SiC and GaN?").

How are SoC, SiP and chiplets different?

When an SoC gets too big or too expensive, or something won't fit in, there's another direction: instead of building everything on one chip, package separately made chips together. You'll hear about two ways:

• SiP (system in package): a chip and small parts such as resistors and capacitors are mounted on a tiny board and sealed into one small module ("What is SiP?").
• Chiplets: one big chip is split into several pieces, each made and tested on its own, then placed side by side or stacked and densely connected, so together they work like one big chip. Putting them together is the job of the advanced packaging in the news ("The news says "advanced packaging". What is it?").

	SoC (system on a chip)	SiP (system in package)	Chiplets
In one line	Every function on one chip	Chips and small parts packed into one small module	One big chip split into pieces, then packaged together
How each part is made	All in the same process	Each chip can be made the way that suits it	Each piece can use a different generation
Paths between parts	Shortest, inside the chip	Longer than inside a chip, shorter than on a board	A bit longer than inside a chip, but short with many wires
If one part is bad	The whole chip is often useless	The whole module is useless, so every chip is tested first	Every piece is tested first; only good ones are put together
Where you find it	The main chip of phones, tablets, watches	Small things like earbuds, watches, hearing aids	Very large chips such as AI chips

It isn't a choice of one. Many SiPs hold an SoC inside, with the small parts it needs around it; and chiplets are often a big chip that would have been made as one, split up and made in pieces.

What has an SoC inside?

Phones, tablets and smartwatches are the most common. In small machines like home routers, the main work is often done by one SoC too. What they have in common: they need to be small and low-power, and still do a lot.

In fab24hr

fab24hr's main story, The life of a chip, follows a fictional hearing-aid chip, "Glowmere": its pilot run on the R&D line, its wafers in the production fab, then the test-and-assembly house. The game doesn't say whether Glowmere is an SoC, or what blocks it's divided into. In chapter 11, "The Ball Lifted", Shu-Hui, the line leader, walks you down the assembly line and tells you: "Glowmere goes into SiP modules: a small board with the chip on it, plus a few resistors and capacitors."

Tap "Clock in", choose "A day in the fab", tap "Test-and-assembly house" on the "Your role today" card and swipe to the assembly process engineer. You get one shift by the time on your device; all four are on this assembly line, and the afternoon one is chapter 11.

The companies, the chip and the people in the game are fictional; what an SoC is and why it's built this way follow public, general sources. For other jargon, see Chip fab jargon in plain English.