The main differences between integrated and discrete circuits which perform the same function are that the integrated circuit uses transistors and diodes as liberally as possible. This is because resistors and capacitors occupy more space in the chip than do transistors and they
In the fabrication of a complete integrated circuit all the components, active and passive, required to make up the circuit are formed at the same time. The components are then interconnected as required by means of an aluminium pattern which is deposited on the top of the silicon slice. As an example, suppose the simple circuit shown in Fig.15(a) is to be integrated. Fig. 15(b) shows the three components of the circuit diffused into a p-type substrate. The components are each isolated from the substrate by a reverse-biased p-n junction and are connected together in the required manner by an aluminium pattern which is deposited on to the surface of the chip.
The problem of interconnecting the various components in an IC increases with increase in the number of components. Fig. 16 shows the top view of an IC that contains ten bipolar transistors, six diodes and six resistors. There are also six terminals to be brought out of the package that can be used for the connection of external components and power supplies. A metallization pattern must be deposited on the top of the chip to form all of the required interconnections between the components. Clearly, it can be difficult to work out the best pattern and a computer with the appropriate software is often used to aid in this task.
A large number of circuits are simultaneously produced in a single silicon wafer, and after formation they are separated into individual chips and then sealed within a suitable package. The majority of integrated circuits are available in one or more of two kinds of package; these packages are the TO circular packages and (he dual-in-line (DIL). The two packages are illustrated by Figs. 17(a) and (b), the latter being much the more popular.
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