The transistor has now been electrically isolated and its three regions have been formed. To
be useful, it must be connected. Once again, the
wafer is oxidized and photolithography is used
to open up windows as shown in Figure 13-10
on page 392. These expose the connection
points for the emitter, base, and the collector.
Aluminum is evaporated and then deposited
onto the surface of the wafer to make contact
through the windows. Photolithography is used
to pattern the metal layer. Etching removes the
unwanted aluminum and Fig. 13-10(c) and (d)
shows what remains. Complex ICs can have
two or even three separate aluminum layers
separated by dielectric layers.While the transistors are being formed,
diodes are also being formed. Figure 13-11 on
page 392 shows a P-N junction diode in an IC.
Notice that it looks a lot like the transistor of
Fig. 13-9. The collector-base junction is used as
a diode, so no emitter diffusion is needed.Figure 13-12 shows how a capacitor might
be formed. The N type region acts as one plate,
an aluminum layer as the other, and silicon
dioxide serves as the insulator. Another approach is to use a reverse-biased P-N junction
as a capacitor. Both methods are used.
Figure 13-13 illustrates resistor formation.
Different values of resistance are realized by
controlling the size of the N channel and the
level of doping. Once again, heavy doping produces less resistance.
An MOS transistor is shown in Figure 13-14.
Notice the insulating (SiO2) layer between the
gate and the channel. MOS transistors take up less space than BJTs and are often preferred for
that reason.
IC components have certain limitations
when compared with discrete components:
• Resistor accuracy is limited. However,
resistors in hybrid ICs can be laser
trimmed to overcome this.
• Very low and very high resistor values
are not practical.
• Inductors are usually not practical.
• Only small values of capacitance are
practical.
• PNP transistors tend to not perform as
well as discrete types.• High voltage components are not practical.
• Power dissipation is usually limited to
modest levels.