Inductor

An inductor is a two-terminal component that stores electrical energy in a magnetic field, built by shaping a conductor so that current flowing through it creates a magnetic field linked to its own circuit. Any change in that current induces a voltage that opposes the change — the inductance, measured in henries, describes how strongly a given rate of current change generates that opposing voltage. Outside a chip, inductors are typically discrete wound-wire components; inside a chip, they're built as spiral or loop traces in the top metal layers, because a coil needs real physical area to generate meaningful magnetic field strength, unlike a resistor or capacitor that shrinks cleanly with geometry. How an inductor actually gets built and used, from raw structure to a finished circuit role, looks like this:

A few things are worth knowing about why an inductor is the odd one out among passive components, and why it's so much harder to put on a chip:

An inductor's value grows with loop area and the square of the number of turns, which is exactly why it doesn't shrink the way a resistor or capacitor does when a process node gets smaller. A resistor's value comes from a length-to-width ratio and a capacitor's from plate area over spacing, both of which can shrink in proportion as transistors shrink. An inductor's magnetic field strength depends on physical loop area and how many times a conductor wraps around that area, so shrinking the geometry directly shrinks the inductance — there's no equivalent trick of thinning a dielectric to claw the value back. This is the core reason on-chip inductors are large, inefficient component by chip standards, occupying far more area per unit of electrical function than a transistor, resistor, or capacitor ever would.

An inductor resists sudden current changes but passes a steady current freely — the exact mirror image of a capacitor, which resists voltage changes but passes current freely as voltage changes. This complementary behavior is why the two are paired in nearly every power and filtering circuit: a capacitor smooths voltage, an inductor smooths current, and switching power converters use both together to turn a chopped, switching voltage into a clean, steady output. The energy an inductor stores doesn't disappear when current stops instantly either — it has to go somewhere, which is why an inductor driven by a switch that opens abruptly produces a voltage spike, a behavior that circuit designers either harness deliberately or have to suppress with a protective diode.

A switching voltage regulator is the single most common reason a real inductor shows up on a modern board, and its job is converting one steady voltage into another without wasting the difference as heat. A linear regulator drops excess voltage as heat, which is simple but wastes power proportional to the voltage difference; a switching regulator instead turns a transistor rapidly on and off, and an inductor smooths that chopped square wave into a steady output current at a different voltage, storing energy during the on phase and releasing it during the off phase. Because efficiency matters enormously in battery-powered devices and in powering billions of transistors on a modern AI accelerator, switching regulators with inductors — not linear regulators — are what actually deliver power to a high-performance chip's silicon.

RF circuits use inductors specifically for the one property neither a resistor nor a capacitor alone can provide: a reactance that pushes against a capacitor's reactance to create a sharp resonance at a chosen frequency. An inductor and a capacitor wired together form a resonant circuit that strongly favors one frequency and rejects others, which is the basic mechanism behind radio tuning, oscillators, and the matching networks that connect a chip's RF transistors to an antenna efficiently. Because on-chip inductors are large and lossy compared to their discrete counterparts, RF designers constantly trade off between integrating an inductor on-die for compactness and using an external discrete inductor for better performance — a tradeoff that resistors and capacitors rarely force to this degree.

Read an inductor as stored magnetic energy with a scaling problem, not a shrinking component: unlike a resistor or capacitor, its value is tied directly to physical loop area and turn count rather than a ratio that gets smaller for free as a process node advances, which is why it remains the largest, most reluctantly integrated passive component on any chip, and why its role — smoothing current in a power converter or forming resonance in an RF tank — is usually worth that area cost only when nothing else can do the job.