Page 176 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

Integrated Circuits ~ICs~ 157

Finally, gates fabricated using the gallium arsenide (GaAs) semiconductor as a
substrate are approximately eight times faster than their silicon equivalents, but
they are expensive to produce, and so are used for specialist applications only.
If an integrated circuit containing millions of transistors were constructed
entirely from high-power transistors, it could literally consume enough power
to incinerate itself (or at least melt itself down into an undistinguished puddle
of gunk). As a compromise, some integrated circuits use a combination of
technologies. For example, the bulk of the logic gates in a device may be
implemented in low-power CMOS, but the gates driving the output pins may
be constructed from high-drive TTL. A more extreme example is that of
BiCMOS (Bipolar CMOS), in which the function of every primitive logic gate
is implemented in low-power CMOS, but the output stage of each gate uses
high-drive bipolar transistors.

SUQQ~~ Voltages
Towards the end of the 1980s and the beginning of the 1990s, the majority
of circuits using TTL, CMOS, and BiCMOS devices were based on a 5.0-volt
supply. However, increasing usage of portable personal electronics such as
notebook computers and cellular telephones began to drive the requirement
for devices that consume and dissipate less power. One way to reduce power
consumption is to lower the supply voltage, so by the mid-to-late 19903, the
most common supplies were 3.3 volts for portable computers and 3.0 volts for
communication systems. By 2002, some specialist applications had plunged to
1.8 volts, with even lower supplies on the horizon. Unfortunately, lowering the
supply voltage can drastically affect the speed of traditional technologies and
greatly lessens any speed advantages of BiCMOS over CMOS. A relative
new low-voltage contender that appeared in the latter half of the 1990s was
BiNMOS, in which complex combinations of bipolar and NMOS transistors
are used to form sophisticated output stages providing both high spee
static power dissipation.

Equivalent Gates
One common metric used to categorize an integrated circuit is the number
of logic gates it contains. However, difficulties may arise when comparing
devices, as each type of logic function requires a different number of transistors.