Transcript Review:

Review:
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Most semiconductors are made of a poor
conductor that has had impurities (atoms of
another material) added to it. The process of
adding impurities is called doping.
There are two types of impurities:
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N-type doping (negative charge)
P-type doping (absence of electrons create the
effect of a positive charge )
Transistors and diodes have a lot in common.
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A transistor is created by using three layers
rather than the two layers used in a diode.
You can create either an NPN or a PNP
sandwich. A transistor can act as a switch or
an amplifier.
A transistor looks like two diodes back-to-back.
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You'd think that no current could flow through a
transistor because back-to-back diodes would
block current both ways. And this is true.
However, when you apply a small current to the
center layer of the sandwich, a much larger
current can flow through the sandwich as a
whole.
This gives a transistor its switching behavior. A
small current can turn a larger current on and
off.
Transistors explained
Types of transistors
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There are two types of
standard transistors, NPN
and PNP, with different
circuit symbols.
Most transistors used today
are NPN because this is the
easiest type to make from
silicon.
The leads are labelled
base (B), collector (C)
and emitter (E).
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If a negative voltage is applied to the collector of
the transistor, then ordinarily no current flows
BUT there are now additional holes at the
junction to travel toward point 2 and electrons
can travel to point 1, so that a current can flow.
It can be shown that most of the current flows
between points 1 and 2. In fact the amplitude
(magnitude) of the collector current in a
transistor is determined mainly by the emitter
current which in turn is determined by current
flowing into the base of the transistor. Consider
the base to be a bit like a tap or faucet handle.
Function
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Transistors amplify current, for example they
can be used to amplify the small output current
from a logic chip so that it can operate a lamp,
relay or other high current device. In many
circuits a resistor is used to convert the changing
current to a changing voltage, so the transistor is
being used to amplify voltage.
A transistor may be used as a switch (either
fully on with maximum current, or fully off with
no current) and as an amplifier (always partly
on).
Its Use in computers
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Transistors are basic components in all of
today's electronics.
They are just simple switches that we can use
to turn things on and off. Even though they are
simple, they are the most important electrical
component.
For example, transistors are almost the only
components used to build a Pentium processor.
A single Pentium chip has about 3.5 million
transistors.
Sources
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http://www.electronicstutorials.com/basics/transistors.htm
http://www.kpsec.freeuk.com/components/tran.htm
http://www.101science.com/transistor.htm
http://amasci.com/amateur/trshort.html
http://computer.howstuffworks.com/diode1.htm