If you look at a modern printed circuit board (PCB), you will almost certainly see at least two or three rectangular black packages with silver-colored terminals protruding from the edges. For example:
Electrical engineers will often refer to these packages as components or devices, and with good reason—they look like components, are purchased as components, and typically function as components. Nonetheless, engineers know that they are not actually components, if we interpret this word as denoting things such as resistors, capacitors, inductors, diodes, and transistors. Rather, these black packages are actually circuits.
An extraordinarily valuable characteristic of transistors is that they can be extremely small, which in turn allows us to condense sophisticated functionality into a small physical area. However, one microscopic transistor is not very useful in a macroscopic world: How would we plug it into a breadboard or measure its voltages? How would an assembly machine place it on a PCB?
The creation of integrated circuits (ICs) is a technique that serves as a bridge between the microscopic world of transistors and the macroscopic reality in which human beings must live. By integrating a circuit consisting of multiple transistors (and other components) into a single package that can be handled by people and machines, IC designers allow other engineers to benefit from the ease with which semiconductor devices can be miniaturized. ICs also ensure that we won’t have to continually solve problems that have already been solved: complex, high-performance designs can be quickly and easily incorporated into countless different systems, because engineers can purchase this design as a proven, thoroughly characterized IC instead of creating a customized circuit that does more or less the same thing.
The following diagram conveys the basic structure of an integrated circuit.
The die is a piece of semiconductor material (usually silicon) that has been transformed into a functional circuit by means of doping, chemical-vapor deposition, metallization, and photolithography. (This fabrication process actually produces a wafer composed of multiple circuits, which are then separated by dicing.) The die is enclosed in a package, and bonding wires form electrical connections between the package’s terminals (also called pins) and the corresponding nodes on the die.
Integrated circuits can be divided into two general categories based on their physical structure. Through-hole ICs have longer pins that extend through a PCB and are soldered from the bottom; surface-mount ICs have shorter pins that do not extend to the other side of the board. The following image shows a through-hole IC (on the right) and a surface-mount IC (on the left).
Nowadays it is common to see IC packages that don’t have protruding pins. These packages conserve PCB area, but they are also difficult or impossible to solder by hand. Here are two examples:
Integrated circuits have become an essential tool for electrical engineers working with applications as diverse as consumer electronics, aerospace systems, and medical equipment. In the rest of this chapter, we will explore the functionality provided by different categories of ICs. First, though, we should discuss the difference between analog circuits and digital circuits, and this will be the subject of the next tutorial.