Page 16 - Semiconductor Manufacturing Handbook

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IC DESIGN

IC DESIGN 2.3

Collector Collector
p n

Base Base
n p

p n

Emitter Emitter

C C

B B

E E
FIGURE 2.2 BJTs—npn and pnp—symbolic representation.

Another type of transistor, and one that is used in a majority of digital integrated circuits today,
is called the metal-oxide-semiconductor-field-effect transistor (MOSFET). Instead of using the nota-
tion of base, collector, and emitter, the MOSFET uses the notation of gate, source, and drain (see
Fig. 2.3). By adding a layer of SiO to provide insulation between the metal contacts, the gate input
2
impedance is greatly increased over that of the base input impedance of the BJT, allowing for much
lower power consumption than BJT transistors.
A MOSFET that is configured in an n-p-n format similar to Fig. 2.3 is often referred to as an
“n channel” MOSFET because a small channel of electrons will form between the two n substrates when
an electrical stimulus is applied. Similarly, a MOSFET that has two p substrates embedded in an n sub-
strate (p-n-p) is referred to as a “p channel” MOSFET, due to the creation of a small channel of holes.
When MOSFETs are combined in certain configurations, the circuits can perform boolean
logic functions. By using both n and p channel MOSFETs in the same circuit, there is almost no

Metal
Drain Gate Source

SiO 2

n n

Si
p

FIGURE 2.3 n-channel MOSFET.

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