Hardware Reference
In-Depth Information
Theinvention ofthetransistor wasoneofthemostimportant developments leading tothe
personalcomputerrevolution.Thetransistorwasinventedin1947andannouncedin1948
by Bell Laboratory engineers John Bardeen and Walter Brattain. Bell associate William
Shockley invented the junction transistor a few months later, and all three jointly shared
theNobelPrizeinPhysicsin1956forinventingthetransistor.Thetransistor,whichessen-
tially functions as a solid-state electronic switch, replaced the less-suitable vacuum tube.
Becausethetransistorwassomuchsmallerandconsumedsignificantlylesspower,acom-
putersystem builtwithtransistors wasalsomuchsmaller,faster,andmoreefficient thana
computer system built with vacuum tubes.
The conversion from tubes to transistors began the trend toward miniaturization that con-
tinues to this day. Today's small laptop PC (or netbook, if you prefer) and even Tablet PC
systems, which run on batteries, have more computing power than many earlier systems
that filled rooms and consumed huge amounts of electrical power.
Although there have been many designs for transistors over the years, the transistors used
in modern computers are normally Metal Oxide Semiconductor Field Effect Transistors
(MOSFETs). MOSFETs are made from layers of materials deposited on a silicon sub-
strate. Someofthelayerscontainsiliconwithcertain impurities addedbyaprocesscalled
doping
or
ion bombardment
, whereas other layers include silicon dioxide (which acts as
aninsulator),polysilicon(whichactsasanelectrode),andmetaltoactasthewirestocon-
nect the transistor to other components. The composition and arrangement of the different
types of doped silicon allow them to act both as a conductor or an insulator, which is why
silicon is called a
semiconductor
.
MOSFETs can be constructed as either NMOS or PMOS types, based on the arrangement
ofdopedsiliconused.Silicondopedwithboroniscalled
P-type
(positive)becauseitlacks
electrons, whereas silicon doped with phosphorus is called
N-type
(negative) because it
has an excess of free electrons.
MOSFETs have three connections, called the
source
,
gate
, and
drain
. An NMOS transist-
or is made by using N-type silicon for the source and drain, with P-type silicon placed in
between (see
Figure 1.2
). The gate is positioned above the P-type silicon, separating the
source and drain, and is separated from the P-type silicon by an insulating layer of sil-
icon dioxide. Normally there is no current flow between N-type and P-type silicon, thus
preventing electron flow between the source and drain. When a positive voltage is placed
on the gate, the gate electrode creates a field that attracts electrons to the P-type silicon
betweenthesourceanddrain.ThatinturnchangesthatareatobehaveasifitwereN-type
silicon, creating a path for current to flow and turning the transistor “on.”
Figure 1.2
Cutaway view of an NMOS transistor.
