An implementation of a particular electronic-circuit function in which all the individual devices required to realize the function are fabricated on a single chip of semiconductor, usually silicon. The individual devices normally consist of semiconductor diodes, transistors, and resistors.
In MOS integrated circuits the active devices are MOSFETs, which operate at low currents and high frequencies. A very large number of MOSFETs can be packed together on one silicon chip, i.e. MOS circuits have a high packing density. They also consume very little power. The development of MOS technology has allowed extremely complex functions to be performed on a single chip.
In bipolar integrated circuits the components are bipolar transistors and other devices that are fabricated using the p-n junction properties of semiconductors. Compared with MOS circuits, bipolar circuits have higher operating speeds but have the disadvantages of high power consumption and low packing density. They are also less simple to fabricate than MOS circuits.
The improvement in the fabrication technology of integrated circuits has made possible the construction of a huge number of components on a single chip. These may be combined on the chip to make a wide variety of digital and analog circuits. The complexity of a digital circuit produced on a single chip is usually described in terms of the number of transistors involved, or sometimes of the number of logic gates. This formerly led to the following differentiation:
VLSI very large-scale integration;
LSI large-scale integration;
MSI medium-scale integration;
SSI small-scale integration.
These terms are now not widely used because all modern chips are VSLI.
Digital integrated circuits are often represented by their logic function rather than their electronic function in order to ease their understanding. See also hybrid integrated circuit.