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Digital Logic Structures:
Chapter 3
COMP 2610
Dr. James Money
COMP 2610
Transistors

Most microprocessors are constructed out of
what are called MOS transistors

MOS stands for metal-oxide semiconductor

For us, we just assume these work according
to their specs
Transistors
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The Pentium IV CPU has over 32 millions
MOS transistors

The IBM PowerPC 750 FX has 38 millions
MOS transistors

We are going to study how these connect
together to get larger units
Transistors


There are two types of transistors
–
p-type
–
n-type
These both work logically similar to wall
switches in our homes
Transistors

Let’s first consider the wall switch example:
Transistors

Instead of the wall switch, you could insert a
n-type or p-type MOS transistor:
Transistors

In (a), the MOS transistor is by itself

In (b), it is combined with the wall switch
circuit

Note there are three terminals in the diagram
Transistors

Three terminals:
–
Source – input voltage
–
Drain – output voltage
–
Gate – controls output of transistor, depends on
types of MOS
Transistors
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What is important is for a n-type transistor, if
the gate is supplied with 2.9 volts, the source
to drain acts like a closed circuit

That is, the drain is 2.9 volts

If the n-type transistor gate has 0 volts, this is
an open circuit and connection between
source and drain is broken
Transistors

To conclude, for n-type MOS transistor in
figure (b):
–
If Gate has 2.9 volts or on, then voltage is
suppiled and the light comes on
–
If Gate has 0 volts or off, the voltage is not
supplied and the light is off
Transistors

Figure (c) is the shorthand version of the
circuit we will normally use

We only show the terminals of the power
supply

The triangle represents source and the
horizontal line represents drain
Transistors

A p-type transistor works in the opposite
fashion:
–
If Gate has 2.9 volts, the circuit is broken and
drain has 0 volts
–
If Gate has 0 volts, the circuit is closed and the
drain has 2.9 volts
Transistors

p-type transistor diagram:
Transistors

Note that n-type and p-type act in
complementary ways

Circuits with both types of MOS transistors
are called CMOS circuits because of this

CMOS – complementary metal-oxide
semiconductor