Hardware Reference
In-Depth Information
they generated, tubes were notoriously unreliable—in larger systems, one failed every couple of
hours or so.
The invention of the transistor was one of the most important developments leading to the personal
computer revolution. The transistor was invented in 1947 and announced in 1948 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 the Nobel Prize in Physics in 1956 for
inventing the transistor. The transistor, which essentially functions as a solid-state electronic switch,
replaced the less-suitable vacuum tube. Because the transistor was so much smaller and consumed
significantly less power, a computer system built with transistors was also much smaller, faster, and
more efficient than a computer system built with vacuum tubes.
The conversion from tubes to transistors began the trend toward miniaturization that continues to this
day. Today's small laptop PC (or Ultrabook) 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 substrate. Some of the layers
contain silicon with certain impurities added by a process called
doping
or
ion bombardment
,
whereas other layers include silicon dioxide (which acts as an insulator), polysilicon (which acts as
an electrode), and metal to act as the wires to connect 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 of doped
silicon used. Silicon doped with boron is called
P-type
(positive) because it lacks 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 transistor is made
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 silicon 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 between the source and drain. That in turn changes that area to behave
as if it were N-type silicon, creating a path for current to flow and turning the transistor on.
