Page 38 - A Practical Guide from Design Planning to Manufacturing

14 Chapter One

closed to turn the transistor on or off. The voltage of the gate wire pro-
duces an electric field which determines whether current can flow from
the source to the drain or not. In the left figure, a low voltage at the gate
repels the negative charges in the source and drain and keeps them sep-
arated. In the right figure, a high voltage at the gate attracts the neg-
ative charges and pulls them together into a channel connecting the
source and drain. Where bipolar junction transistors require a constant
current into the base to remain switched on, MOSFETs require only a
voltage to be held at the gate. This allows MOSFETs to use less power
than equivalent BJT circuits. Almost all transistors made today are
MOSFETs connected together in integrated circuits.

The Microprocessor
The integrated circuit was not an immediate commercial success. By
1960 the computer had gone from a laboratory device to big business
with thousands in operation worldwide and more than half a billion dol-
2
®
lars in sales in 1960 alone. International Business Machines (IBM ) had
become the leading computer manufacturer and had just begun shipping
its first all-transistorized computer. These machines still bore little resem-
blance to the computers of today. Costing millions these “mainframe”
computers filled rooms and required teams of operators to man them.
Integrated circuits would reduce the cost of assembling these computers
but not nearly enough to offset their high prices compared to discrete tran-
sistors. Without a large market the volume production that would bring
integrated circuit costs down couldn’t happen. Then, in 1961, President
Kennedy challenged the United States to put a man on the moon before
the end of the decade. To do this would require extremely compact and
light computers, and cost was not a limitation. For the next 3 years, the
newly created space agency, NASA, and the U.S. Defense Department
purchased every integrated circuit made and demand soared.
The key to making integrated circuits cost effective enough for the gen-
eral market place was incorporating more transistors into each chip.
The size of early MOSFETs was limited by the problem of making the
gate cross exactly between the source and drain. Adding dopants to form
the source and drain regions requires very high temperatures that would
melt a metal gate wire. This forced the metal gates to be formed after
the source and drain, and ensuring the gates were properly aligned was
a difficult problem. In 1967, Fedrico Faggin at Fairchild Semiconductor
experimented with making the gate wires out of silicon. Because the sil-
icon was deposited on top of an oxide layer, it was not a single crystal

2
Einstein and Franklin, “Computer Manufacturing,” 10.