Silicon-on-insulator (SOI)

Silicon-on-insulator (SOI) is a semiconductor wafer that adds a thin buried oxide insulating layer beneath the active silicon. This isolation reduces leakage and parasitic capacitance, making transistors faster and more power-efficient. SOI underpins RF chips for 5G, FD-SOI processors, and the silicon photonics now wiring AI data centers.

What is silicon-on-insulator?

Silicon-on-insulator (SOI) is a type of semiconductor wafer used to build chips. Instead of building transistors directly on a solid block of silicon ("bulk" silicon), an SOI wafer has three layers: a thin top layer of high-quality silicon where the transistors live, a **buried oxide layer** (usually silicon dioxide, often called the BOX) that acts as an electrical insulator, and a thick silicon base that provides mechanical support. That buried insulator is the whole point — it electrically isolates each transistor from the substrate underneath and from its neighbors.

How it works and why it helps

In ordinary bulk silicon, transistors leak small currents into the shared substrate and pick up *parasitic capacitance* — stray charge storage that slows switching and wastes power. The buried oxide in SOI cuts those leakage paths. The result, per industry sources, can be roughly 30% faster switching and up to about 3x lower dynamic power compared to bulk silicon at a similar node, plus immunity to latch-up (a destructive short-circuit failure mode). Less leakage also means SOI chips can run at lower voltages, which is exactly what battery-powered and dense, hot data-center hardware needs.

The main flavors of SOI

SOI isn't one thing. **RF-SOI** is tuned for radio-frequency switches and antenna tuners — almost every modern smartphone uses RF-SOI to route 4G/5G signals. **FD-SOI** (fully depleted SOI) uses an ultra-thin top silicon film and is a CMOS logic platform: foundries like GlobalFoundries (22FDX/18FDX), STMicroelectronics, and Samsung use it for low-power processors, IoT, automotive radar, and edge-AI chips. **Power SOI** serves automotive and industrial drivers. And **photonic SOI** — a thick silicon layer on oxide — is the substrate for silicon photonics, where the oxide confines light inside silicon waveguides.

Why SOI matters for AI and data centers

AI clusters are running into a wiring problem: as bandwidth per chip climbs past 100 Tb/s, copper links can't keep up over distance without huge power and heat. The industry's answer is **silicon photonics and co-packaged optics (CPO)** — moving data with light right next to the processor. Those optical circuits (waveguides, modulators) are typically built on SOI wafers, because the buried oxide is what keeps the light trapped in the silicon. Analysts at Yole and others expect optical interconnect components to grow sharply through the late 2020s, with CPO adoption ramping toward 2028-2030 as NVIDIA, Broadcom and others ship optical-enabled switches.

Where SOI sits in the supply chain

SOI is upstream raw material, not a finished chip. **Wafer makers** produce the engineered SOI substrate; **foundries** (GlobalFoundries, STMicro, Samsung, TSMC) print transistors or photonic circuits on it; **chip designers and OEMs** then package it into phones, cars, and data-center gear. The dominant SOI wafer maker is France's **Soitec**, whose patented Smart Cut process is used to make most of the world's SOI wafers; its longtime partner **Shin-Etsu Handotai** of Japan is the other major licensed producer. Soitec serves three markets it labels Mobile, Automotive & Industrial, and Edge & Cloud AI.

Key players and tickers

**Soitec (Euronext Paris: SOI)** is the pure-play leader in SOI wafers and the most direct way to track the technology. **Corning (NYSE: GLW)** is adjacent rather than an SOI wafer maker — but it is central to the same AI-optics wave through optical fiber, glass and silicon-photonics components, with multibillion-dollar 2025-2026 deals tied to NVIDIA and Meta. Other names in the broader ecosystem include GlobalFoundries and STMicroelectronics on the foundry side. As always, this is context, not investment advice — the underlying technology, not any single stock, is the story.

Frequently asked

Is SOI better than regular (bulk) silicon?

For specific jobs, yes — SOI gives lower leakage, faster switching, and better power efficiency, which is why it dominates smartphone RF front-ends and is strong in low-power and automotive logic. But SOI wafers cost more, and bulk silicon still wins on price for high-volume mainstream logic, so the two coexist.

What is the difference between FD-SOI and RF-SOI?

RF-SOI is optimized for radio-frequency switches and tuners (think 5G antennas), while FD-SOI is a fully-depleted CMOS logic platform for energy-efficient processors, IoT, and edge-AI chips. Both use a buried oxide, but FD-SOI uses a much thinner top silicon film and adds back-biasing to tune performance.

What does 'SOI' mean as a stock ticker?

On Euronext Paris, SOI is the ticker for Soitec, the world's leading SOI-wafer manufacturer. Note that 'SOI' on other exchanges can refer to unrelated companies, so always confirm the exchange and ISIN (Soitec is FR0013227113).

How does SOI connect to AI data centers?

Silicon photonics — the optical interconnects increasingly used to move data between AI chips — are built on photonic SOI wafers, where the buried oxide confines light inside silicon waveguides. As AI bandwidth outgrows copper, demand for these SOI-based optical components is rising.

Who invented and makes most SOI wafers?

France's Soitec commercialized the Smart Cut wafer-bonding process that made high-volume SOI viable; today its technology is used to make nearly all SOI wafers sold worldwide, with Japan's Shin-Etsu Handotai as the other main licensed producer.

Related companies

$SOI $GLW