hardware//electronics//semiconductor//CMOS

CMOS is the way almost every chip is built, pairing two complementary kinds of transistor on one piece of silicon, and it is the process behind processors, memories, camera sensors and the neural probes that carry their own amplifiers and converters. The name says what it is made of: Complementary Metal-Oxide-Semiconductor.

CMOS is the way almost every chip is built, pairing two complementary kinds of transistor on one piece of silicon, and it is the process behind processors, memories, camera sensors and the neural probes that carry their own amplifiers and converters. The name says what it is made of: Complementary Metal-Oxide-Semiconductor.

The two kinds, nMOS and pMOS, conduct for opposite gate voltages. Wired in pairs, one is off whenever the other is on, so a logic gate at rest has no path from the supply to ground and draws almost no current; that is what lets billions of transistors share one die without melting it. A CMOS chip is made in a fab layer by layer on a silicon wafer: the transistors first (the front end), then a stack of metal layers separated by insulators that wire them together (the back end), each layer printed by photolithography, a projection of light through a mask onto a light-sensitive coating. The process node (130 nm, 28 nm, 3 nm) names the generation of that printing, and the newest nodes need the extreme-ultraviolet machines made by ASML, while older ones print with deep-ultraviolet light.

A mature node is chosen when the analog circuits, the cost or the volume matter more than the transistor count.

Amplifiers gain little from smaller transistors, masks for a new node cost millions, and a few thousand chips a year never repay them.

A camera sensor is CMOS reading a grid. Each pixel turns light into a charge, switches connect rows and columns to shared amplifiers and converters on the same die, and the chip sends out numbers; a neural probe such as Neuropixels does the same with electrodes in place of pixels, which is where its name comes from.

Silicon-on-insulator (SOI) puts the transistors on a buried insulating layer. Less charge leaks into the substrate and neighbouring devices interfere less, which helps low-power and mixed analog and digital designs; Neuropixels is made in a 130 nm SOI process with six metal layers.

Extra steps can be added to a CMOS line. A process owned by a research fab can take materials or etches a commercial foundry would not run, which is how electrodes of titanium nitride and a shank carved by deep etching were added on top of the standard layers.