A complete circuit, including active and passive electronic devices and their interconnections, that is made on a single substrate. A hybrid integrated circuit consists of several separate component parts attached to a ceramic substrate and interconnected either by wire bonds or a suitable metallization pattern. The individual parts are unencapsulated and may consist of diffused or thin-film components or one or more monolithic circuits. An IC is much smaller than a circuit made from discrete packaged components and once fabricated an individual component cannot be altered without destroying the entire circuit.
A monolithic integrated circuit has all the circuit components made into or on top of a single chip of semiconductor. The components are then connected by one or more levels of metallization deposited onto the surface of the IC in an appropriate pattern (see multilevel metallization). Making the circuit directly in the semiconductor enables circuits and structures to be made, and electronic functions to be generated, that would be impossible to realize in discrete form. This is because the reliability of the many individual components required for the circuit is not sufficient to allow the complex circuit formed therefrom to be guaranteed to operate, whereas using the same technology the reliability of the complete IC would be similar to that of each component.
Silicon is used in the majority of commercial integrated circuits. Bipolar integrated circuits are based on bipolar junction transistors, and are used for high-speed analogue and digital circuits and for lowest-noise ICs. MOSFETs are used in the highest-density ICs, such as microprocessors and memory ICs, as the small size and low power consumption of the individual transistors permit very complex circuits to be made. Combinations of technologies are used in particular applications; for example, BiCMOS technology combines bipolar transistors for output power capability and CMOS transistors for logic operation, while BiFET technology combines bipolar circuits with JFETs for low-noise high-speed analogue ICs. Gallium arsenide is used for monolithic microwave integrated circuits (MMICs) for specialist applications at microwave frequencies.
The complexity of a (digital) monolithic IC is described by the number of components that form the circuit; often this is counted in terms of the number of logic gates per IC. SSI (small-scale integration) refers to simple circuits of up to about ten components; many GaAs MMICs fall into this category. MSI (medium-scale integration) describes circuits of 10–100 components. LSI (large-scale integration) refers to circuits of 100 to a few thousand gates; many high-speed silicon and GaAs digital circuits are LSI. VLSI (very large scale integration) describes all ICs that are larger than a few thousand gates; this includes almost all microprocessors, memory, and digital signal processing ICs currently available. Gate counts in high-end commercial multicore processors now exceed billions of gates.
https://learn.sparkfun.com/tutorials/integrated-circuits A tutorial on integrated circuits, on the sparkfun website