resistor
A resistor is a two-terminal component that opposes the flow of electric current, converting electrical energy into heat in direct proportion to how much current passes through it. The relationship is Ohm's law: voltage across a resistor equals current through it times its resistance, measured in ohms. Outside a chip, resistors are discrete parts on a circuit board; inside a chip, they're built directly into the silicon as thin strips of doped polysilicon, diffused silicon, or deposited metal film, sized by geometry rather than picked off a shelf. How a resistor actually gets built and used, from raw material to a finished circuit role, looks like this:
A few things are worth knowing about why a resistor's value is set by geometry and material, not by a single magic ingredient:
A resistor's value comes from a simple multiplication: sheet resistance times length divided by width, which is why chip designers draw resistors as shapes, not numbers. Every resistive material has a sheet resistance — how much a single square patch of it resists current, measured in ohms per square — set entirely by the material and how it was deposited or doped. Doubling the length of a strip doubles its resistance, because current has twice as far to fight through; doubling the width halves it, because there's twice as much cross-section for current to flow through. A designer who needs a precise resistance value draws a strip with the right length-to-width ratio for the sheet resistance available on that process, which is why resistor layout on a chip looks like simple rectangles rather than a discrete component symbol.
A resistor dissipates power as heat, and that power scales with the square of the current, not linearly — which is why current-limiting and power budgets matter more than resistance value alone. Power equals current squared times resistance, so doubling the current through a fixed resistor quadruples the heat it has to dissipate, not just doubles it. On a chip, this matters directly: an on-chip resistor carrying too much current for its size overheats, drifts in value as temperature rises, and can eventually fail — which is why designers size resistor geometry not just for the target resistance value but for the worst-case power it will actually dissipate in operation.
Real resistors don't hold their rated value exactly, and the amount they can drift — tolerance — is itself a designed and specified property, not an afterthought. A resistor specified at a given value with a given tolerance, say plus or minus 1%, is guaranteed to fall within that range at manufacture, but its value also shifts with temperature, described by a temperature coefficient of resistance. Thin-film metal resistors are chosen specifically because they hold tighter tolerance and a lower temperature coefficient than polysilicon or diffused resistors, which is why precision circuits like data converters and voltage references lean on thin-film resistors even though they cost more die area and process complexity to build.
The same physics shows up in four different circuit roles, and recognizing which role a resistor is playing explains why its value was chosen. A pull-up or pull-down resistor holds a digital input at a known logic level when nothing else is actively driving it, so its value is chosen to be large enough to not waste power but small enough to firmly set the level against noise. A voltage divider uses two resistors in series to produce a fraction of an input voltage, with the ratio between the two values — not their absolute size — setting the output fraction. A current-limiting resistor in series with a component like an LED protects it from drawing more current than it can survive, sized directly from Ohm's law given the supply voltage and the component's rated current. And a termination resistor at the end of a high-speed signal line absorbs the signal instead of letting it reflect back and corrupt the next bit, with its value matched to the transmission line's characteristic impedance rather than picked arbitrarily.
Read a resistor as a shape with a job, not a fixed component: on a chip its value comes entirely from a material's sheet resistance combined with a drawn length-to-width ratio, its tolerance and temperature behavior are deliberate tradeoffs picked by which material was chosen, and its role in the circuit — pull-up, divider, current limit, or termination — is what actually explains why that particular value was selected in the first place.