Integrated Circuits, also called ICs or informally, "microchips" or just "chips", are miniature electronic circuits built onto a single (very small) piece of silicon embedded in a package that allows it to be connected to a cicuit. They can fulfill many functions, ranging from simple logic gates and other simple devices such as counters and shift registers, to computer processors.
Any device built on a single "chip" of silicon is said to be "monolithic", from the Greek for "single stone".
Advantages of ICs
When ICs were introduced, they represented a revolutionary paradigm shift in the construction of electronic device. Rather than everything having to be made from basic components, certain standard modules could be plugged in in the form of a "microchip".
This reduces cost (a simple IC of the kind used in this Wikibook costs between 10p and £1), as opposed to the equivalent cost of potentially upwards of tens, or more likely, hundreds, of discrete components.
Also, as the devices are highly miniaturised, they take up less space, use less power, and operate faster with less noise, due to the components inside being very close together.
Modern ICs can fit up to 1,000,000 transitors onto a piece of silicon with an area of 1mm².
Form of an IC
All ICs have the same basic parts. At the heart of the IC is the die. This is the piece of silicon that the circuit is built on. It is generally smaller than 1mm on a side and is usually located in the geometric centre of the device.
The die is connected by fine wires (generally 20μm or thinner) to the pins. Pins are metal protrusions that are used to make contact with the outside circuitry. Pins usually take to form of the "legs" seen sticking out from the sides on most common microchips. Pins can also be vertical pins or tiny solder balls on the bottom surface of the IC.
The dies and the pins are set into the carrier. This is the body of the IC and is generally made of plastic or ceramic. To orientate the device, a notch, dot, or cut-off corner is usually included on the carrier.
For more information on IC package designs, see "Practical Electronics Packages" and "Howto identify integrated circuit(IC) chip packages".
Scales of Integration
The number of transistors in an IC determines the scale of integration of the IC. The higher the scale, the more complex the IC. The table below shows a rough outline of the division between scales. Bear in mind that there is no hard-and-fast rule governing the naming of the scales, and as semiconductor technology grows, the number of transistors seen as "very large" today will seem small.
|Scale of Integration||Number of Transistors||Example|
|SSI||Small-Scale Integration||10+||555 Timer|
|VLSI||Very Large-Scale Integration||100,000+||Computer processor|
|ULSI||Ultra Large-Scale Integration||Used to describe ICs with a significantly larger number of transistors than common at the time.|
|WSI||Wafer-Scale Integration||ICs that use the entire silicon wafer (over 300mm in diameter). These would contain billions of transistors, but are genrellar viewed as being inpractical and have not been widely pursued by IC manufacturers.|
The layout of the components in ICs has changed dramatically since ICs were first introduced. As more modern processes have allowed smaller and more precise components to be etched in the die, different kinds of electronics could be used, resulting in continuously changing and improving performance from the ICs. Below is a very brief summmary of the main IC families. For more information on the specifics of each family, visit the linked pages.
The earliest ICs used resistors and transistors, in a form called resistor-transistor logic (RTL). This was very prone to noise with more than three inputs, and as such was only suitable for very small-scale devices.
Next came diode-transistor logic (DTL). This used diodes to supplement the resistors used in RTL, but still suffered many problems, including a very long propagation delay (the time taken to change state).
Transistor-transistor logic (TTL) was a very popular format for ICs and is still in use in places today. This basically uses only transistors to fulfil its function.