Q. Define Integrated circuits?
For the fabrication of semiconductor circuits, there are three distinct technologies employed:
1. Discrete-component technology, in which each circuit element is an individual component and circuit construction is completed by interconnecting the various components.
2. Monolithic technology, in which all the parts (such as transistors, resistors, capacitors, and diodes) needed for a complete circuit (such as an amplifler circuit) are constructed at the same time from one silicon wafer (which is typically 5 mils or 0.005 inch in thickness).
3. Hybrid technology, a combination of the preceding two technologies, in which various circuit components constructed on individual chips are connected so that the hybrid IC resembles a discrete circuit packaged into a single, small case.
Integrated circuits (ICs), in which several transistors, resistors, wires, and even other components are all fabricated in a single chip of semiconductor, are ideal building blocks for electronic systems. Space, weight, cost, and reliability considerations gave much impetus for the development of ICs. The ability to place circuit elements closer on an IC chip helps in extending the frequency range of the devices.Whereas the IC technology involves the use of only solid-state devices, resistors, and capacitors, the elimination of inductors is necessitated by the fact that typical semiconductors do not exhibit the magnetic properties needed to realize practical inductance values.
ICs are made by microfabrication technologies. The low cost of IC production is a result of planar processing in which fabrication begins with a very flat disc of silicon wafer, 5 to 10 cm in diameter and only 0.5mmthick. The small electronic structures to be built on it are then produced photographically. The technique is known as photolithography, in which a photosensitive lacquer (known as photoresist), which has the property of hardening when struck by light, is used. The fabrication method requires a series of masks, photoetching, and diffusions.
MOSFET chips generally utilize either a p-channel or an n-channel device; hence, these chips are known as PMOS and NMOS, respectively. Alternatively, both p-channel and n-channel devices are used to form compound devices, in which case they are known as complementary MOS (CMOS).Whereas the CMOS has the advantage of low power consumption, only a smaller number of devices can be placed on the chip. MOS technologies are popularly used in computer circuits due to their higher packing densities. Bipolar technologies, however, are used in high-speed applications because they respond more quickly. The device fabrication methods are too involved to be presented in this introductory text.
Small-scale integration (SSI) is used typically for a 20-component op amp, whereas large- scale integration (LSI) puts an entire microprocessor, typically with 10,000 components, on a single chip. The chief beneflts from integrating many components on an IC are low cost, small size, high reliability, and matched characteristics. Of the many IC packaging technologies, the most popular is the dual-in-line package (DIP), which consists of a rectangular plastic or ceramic case enclosing the IC, with protruding pin terminals. While an op amp is commonly supplied in an 8-pin DIP for insertion into some larger circuit, a microprocessor may have a 40- to 64-pin DIP to accommodate the many external connections needed for an LSI chip.