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An electrical circuit assembled from separately made components, such as individual transistors, resistors and capacitors. Several functions can share a package without forming a monolithic integrated circuit; packaging alone does not establish how the components were fabricated.

An integrated circuit combines interconnected electrical circuit elements in a fabricated structure. A monolithic integrated circuit forms its elements in or on one piece of semiconductor, rather than merely putting separately manufactured components into one enclosure. Transistors supply control in electronic amplifiers; resistors and capacitors can supply bias and timing. Jack St. Clair Kilby's patent describes making such functions from a common semiconductor body with compatible fabrication steps.^[ref]

The useful change is to fabricate connections and functions together. Shrinking an enclosure still leaves individual parts to position and join. Integration makes their relative arrangement part of a repeatable fabrication pattern. The distinction is functional as well as geometric: one small piece can implement a whole electrical circuit, while its external terminals expose only the connections needed by its surroundings.

A monolithic integrated circuit has its circuit elements fabricated in or on one semiconductor die. Their connections are essential: an unconnected collection of transistors does not implement the intended electrical circuit.

An original ideal example separates function from fabrication. Connect two resistors, $R_1=10\mathrm{k\Omega }$ above $R_2=20\mathrm{k\Omega }$, in a series circuit across an ideal $3\mathrm{V}$ voltage source. With no load electric current drawn from their meeting point, Ohm's law and the Kirchhoff cycle law giveThese equations apply to either a discrete circuit or an idealized monolithic integrated circuit realizing those same electrical resistances. Integration changes construction; the electrical circuit equations still determine the output. A real load, unwanted capacitance or finite resistor tolerance changes the ideal calculation. Manufacturing many functions together also couples their fate: a fatal defect can discard an entire semiconductor die, motivating the Poisson defect yield model.

The planar process fabricates semiconductor devices from a common surface while retaining a protective oxide over the exposed ends of their p-n junctions. Jean Hoerni's patent describes selective openings in silicon dioxide for semiconductor doping and contacts, with oxide left over the p-n junctions. Protection and access are designed together; the surface is not simply stripped after fabrication. This was a distinct enabling contribution to the monolithic integrated circuit.^[ref]