Integrated Circuits

Integrated Circuits

Introduction

A collection of one or more gates fabricated on a single silicon chip is called an integrated circuit. Here we will discuss the elements of an integrated circuit and how those element function. Digital systems have incorporated integrated circuits for many years because of their small size, high reliability, and low cost.

The terms “chip” and “integrated circuit” refer to semiconductor circuits that use collections of microscopic transistors that are all co-located on the same small piece of silicon. A transistor is a semiconductor device used to amplify and switch electronic signals and electrical power. Chips have been designed to do all sorts of functions, from very simple and basic logical switching functions to highly complex processing functions. Some chips contain just a handful of transistors, while others contain several hundred million transistors. Some of the longest surviving chips perform the most basic functions. These chips, denoted with the standard part numbers “74XXX”, are simple small-scale integration devices that house small collections of logic circuits. For example, a chip known as a 7400 contains four individual NAND gates, with each input and output available at an external pin.

As shown in Fig. 1 below, the chips themselves are much smaller than their packages. During manufacturing, the small, fragile chips are glued (using epoxy) onto the bottom half of the package, bond- wires are attached to the chip and to the externally available pins, and then the top half of the chip package is permanently affixed. Smaller chips may only have a few pins, but larger chips can have more than 500 pins. Since the chips themselves are on the order of a centimeter on each side, very precise and delicate machines are required to mount them in their packages.

Smaller chips might be packaged in a “DIP” package (DIP is an acronym for Dual In-line Package) as shown below in Fig. 1. Typically on the order of 1" x 1/4", DIP packages are most often made from black plastic, and they can have anywhere from 8 to 48 pins protruding in equal numbers from either side. DIPs are used exclusively in through- hole processes. Larger chips use many different packages—one common package, the “PLCC” (for Plastic Leaded Chip Carrier) is shown below. Since these larger packages can have up to several hundred pins, it is often not practical to use the relatively large leads required by through-hole packages. Thus, large chips usually use surface mount packages, where the external pins can be smaller and more densely packed.

Figure 1. Different integrated circuit chip packages.

On schematics and on the circuit board, chips are often shown as square boxes denoted with a “U__” or “IC__” reference designator. Some chips are loaded into sockets so that they can be easily removed or replaced. Chips, even in their plastic packages, are quite fragile and are subject to damage from a variety of sources, including electrostatic discharge, or ESD. Care should always be taken not to touch the leads on chips, as it is likely this will cause permanent damage.

Important Ideas

  • Both chips and integrated circuits refer to semiconductor circuits that use collections of microscopic transistors that are all co-located on the same small piece of silicon.
  • Smaller chips might be packaged in a “DIP” package, they can have anywhere from 8 to 48 pins protruding in equal numbers from either side. DIPs are used exclusively in through-hole processes.
  • Larger chips use many different packages—one common package, the “PLCC”. Large chips usually use surface mount packages, where the external pins can be smaller and more densely packed.

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