Page 227 - A Practical Guide from Design Planning to Manufacturing

200 Chapter Seven

largely ignore the details of the behavior of the transistors that will ulti-
mately make up the microprocessor, but in the end the processor is pri-
marily just a collection of transistors connected by wires. No matter how
sophisticated or complicated the microarchitecture or logic design of the
processor, its functions must be built using transistors and wires.
Although bipolar junction transistors (BJTs) were invented first, today
all microprocessors use metal oxide semiconductor field effect transistors
(MOSFETs). Modern microprocessors contain many large arrays of
memory elements and MOSFETs are ideal for memory storage as well
as performing logic. In addition, MOSFET logic can operate at lower volt-
ages than required by BJTs. As reduced device sizes have required lower
voltages, this has become extremely important and MOSFETs have
come to dominate digital hardware design.
To make the job of designing a processor with hundreds of millions of
transistors manageable, circuit design must be broken down into very
small elements. This chapter describes how MOSFETs are used to build
logic gates and sequential elements, which are connected together to form
the desired logic. For logic design, transistors can be thought of as simple
switches that are either on and conducting or off and not conducting.
However, circuit design must consider not just getting the correct logical
answer but also the trade-offs of the circuit implementation. Different cir-
cuits provide the same answer but with very different speed, cost, or
power. Nonideal behavior of transistors must be considered as well. There
will be electrical noise on signal wires, leakage currents from off tran-
sistors, and constraints on not only the maximum delay (maxdelay)
allowed but also the minimum delay (mindelay). The circuit designer
must take all these factors into account while still delivering a logically
correct implementation. This means circuit designers must understand
the details of not only the behavior of the logic they are to implement, but
also the behavior of the transistors themselves.

MOSFET Behavior
As described in Chap. 1, each MOSFET has three terminals called the
gate, source, and drain. When the transistor is on, current flows from
the source to the drain, and when the transistor is off, no current flows.
MOSFETs are made in two basic types using N-type and P-type dopants.
NMOS transistors use N-type dopant for the source and drain separated
by P-type. This creates a MOSFET that will turn on when the gate volt-
age is high. The high voltage draws the negative carriers from the source
and drain under the gate. PMOS transistors use P-type dopant for the
source and drain separated by N-type. This creates a MOSFET that will
turn on when the gate voltage is low. The low voltage draws the posi-
tive carriers from the source and drain under the gate. See Fig. 7-1.