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Transistors and Logic Gates
References:
http://www.st-and.ac.uk/~www_pa/Scots_Guide/info/comp/active/BiPolar/page1.html
PHY 201 (Blum)
Transistors
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There are various kinds of transistors:
bipolar, field-effect, etc.
They differ in stability, energy usage, and so
on, but they serve a similar purpose.
They are used to amplify a signal or to act
as a switch.
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It is as switches that they are used in computers.
The change from vacuum tubes to transistors
meant dramatic differences in the physical size of
computers and ultimately dramatic differences in
their speed, capacity and ubiquity.
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Generations
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Computers are thought of as belonging to
generations.
For the ENIAC and other computers of the first
generation, the processor was comprised of
vacuum tubes.
The processor’s individual vacuum tubes were
replaced by individual transistors in the second
generation of computers.
Several transistors could be placed close together on
an integrated circuit (IC) or chip. In third
generation computers the processor consisted of
several IC’s.
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Processor  Microprocessor
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Eventually the entire processor was placed
on a single chip. When this became
standard computers were said to enter the
fourth generation.
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In this case, the processor is known as a
microprocessor.
Some large machines today may have the
processor spread over more than chip. But
PCs typically have a single processor.
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Tubes
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A computer of the first generation consisted
of tubes.
Tubes started with an effect Thomas Edison
noticed while experimenting with light bulbs.
John Ambrose Fleming discovered that
one could exploit the effect to detect radio
waves and convert them to electricity, but the
signal was too small.
Lee de Forest added to the device, making
the triode; Edwin Armstrong pointed out it
could be used to amplify signals.
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Transistors
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A computer of the second generation
consisted of transistors.
William Shockley, John Bardeen and Walter
Brattain developed the transistor while
working at Bell Labs in 1947. (Nobel Prize
1956)
The transistor could play the same role as the
vacuum tube but was significantly smaller –
and thus faster and less power consuming.
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Integrated
Circuit
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A computer of third generation consisted of
integrated circuits.
The problem with computers is that they required so
many transistors connected to one another – the socalled “tyranny of numbers.”
This problem was solved by the “monolithic idea”
– the idea the many circuit elements (mainly
transistors) could be placed on the same piece of
semiconductor, i.e. an integrated circuit (IC).
In 1958 Jack Kilby of Texas Instruments invented
the IC. In 1959 Robert Noyce of Fairchild
Semiconductor independently developed a betterdesigned IC. (Nobel Prize 2000.)
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Microprocessor
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A computer of fourth generation has a
microprocessor.
What distinguishes a microprocessor from
other integrated circuits is that a
microprocessor can be programmed.
So along with the idea of the microprocessor
comes the idea of the instruction set – the set
of actions the programmer can have the
microprocessor perform.
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Intel
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Noyce
Moore
Intel was founded by Bob Noyce and
Gordon Moore who formerly worked for
Fairchild Semiconductor.
In a collaboration between Busicom (a
Japanese firm) and Intel to make calculators,
Ted Hoff devises a plan to put all the main
circuitry on one chip instead of the original
plan of twelve (in 1968).
The microprocessor was born.
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The Microprocessor is born
Intel made the 4004 (the
first microprocessor)
for Busicom. Knowing
they had a good thing,
they bought the rights to
the 4004 from Busicom
for $60,000.
Ted Hoff
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4004
One of the first
commercially
available
microprocessors.
But returning
to the matter
at hand
transistors.
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Diode Review
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Recall that a pn junction — the joining
together of p-doped (“too few”
electrons) and n-doped (“extra”
electrons) — makes a diode.
A diode is a circuit element that allows
current to flow in one direction
(forward bias) but not in the other
(reverse bias).
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Diode Review (Cont.)
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Recall that a semiconductor had a full
valence band and an empty conduction
band separated by a gap.
We could improve the conductivity of a
semiconductor by doping it.
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N-doping put some electrons into the conduction
band which were free to move about.
P-doping freed up some places in the valence
band – these “holes” were also free to move
about.
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Diode Review (Cont.)
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In a diode one bring together n-doped and pdoped material.
Some of the “extra” electrons from the n
side’s conduction band fill the empty levels in
the p side’s valence band, forming a region in
which the valence band is filled and
conduction band is empty.
This region (called the depletion zone) is a
poor conductor.
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Applying a voltage
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Connecting a p-n junction to a battery as shown below
adds positive charges on the n-doped side making the
region of poor conductivity larger.
This is called reverse bias.
Current doesn’t flow in this direction.
p-doped
--- +++
--- +++
+++
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n-doped
Applying a voltage II
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Connecting a p-n junction to a battery as shown below
adds positive charges on the p-doped side making the
region of poor conductivity smaller (oversimplified).
This is called forward bias.
Current does flow in this direction.
p-doped
+++
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--- +++
--- +++
n-doped
Bipolar transistors
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A bipolar transistor starts with two back-toback diodes (pn junctions).
There are two kinds: NPN and PNP
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The middle region is usually small
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N-doped
P-doped
P-doped
N-doped
N-doped
P-doped
Third lead
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So far the device seems useless; two
back-to-back diodes wouldn’t conduct in
either direction.
But we add a third lead (connection)
directly to the middle portion.
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Not symmetric
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The transistor would seem to be
symmetric with the two N-doped
regions being the same, but actually
these regions differ in their amount of
doping and serve different purposes in
the transistor.
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Collector, base, emitter
Collector
Base
Emitter
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In bipolar
transistors,
the various
regions are
referred to as
the collector,
base and
emitter.
Collector, base, emitter
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Connecting the transistor
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C (N)
B (P)
E (N)
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Imagine applying a
potential difference
(voltage) across the
base-collector leads
with the collector
higher, this reverse
biases that pn
junction so there
would be no current
flow.
No flow
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There is no flow because of the
depletion zone (the region in which the
valence band is filled and the
conduction band empty).
Reverse bias voltages tend to make the
depletion zone a bit larger.
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Connecting the transistor (Cont.)
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C
B
E
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Now consider
applying a (smaller)
voltage across the
base-emitter leads
with the base
higher, this forward
biases that pn
junction so current
will flow.
Flow
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Forward biasing a pn junction tends to
eliminate the depletion zone (in this case
putting electrons into the conduction
band).
Because the transistor has one shared
depletion zone that has been eliminated by
the base-emitter forward bias, both
currents (collector-base and base-emitter)
can flow.
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NPN in a circuit
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C
B
E
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The arrow on an
NPN points from
base to emitter
indicating the
forward-bias
direction that turns
the transistor “on.”
Characteristics of the Off State
C
B
E
Little to no current going into the base and a large
voltage drop across the collector/emitter leads.
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Off
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Base-emitter circuit
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Little to no current flowing.
Most of the voltage dropped across the baseemitter as opposed to the resistor in the circuit.
Collector-emitter circuit
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Little to no current flowing
Most of the voltage dropped across the collectoremitter as opposed to the resistor in the circuit.
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Characteristics of the On State
C
B
E
Current through the base lead, small voltage drop across
collector/emitter leads.
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On
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Base-emitter circuit
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Current flowing.
Most of the voltage dropped across the resistor as
opposed to the base-emitter in the circuit.
Collector-emitter circuit
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Current flowing.
Most of the voltage dropped across the resistor as
opposed to the collector-emitter in the circuit.
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Unusual feature
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One feature that people find unusual when
learning about transistors is that when the
transistor is “on” the collector-emitter
voltage is less than the base-emitter
voltage.
This is because in the collector-emitter, one
is going from n-doped material to n-doped
material, whereas in the base-emitter, one
is going from p-doped to n-doped.
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Logic gates
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The on-off nature of diodes and
transistors make them ideal for building
logic gates.
Logic gates have input which is
interpreted as a logic value (0 or 1, low
or high, false or true) and have output
which can also be interpreted logically.
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Logic gates
Logic
NOT
AND
OR
NAND
NOR
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Circuit Symbol
AND Gate
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OR Gate
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NOT Gate
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NOR Gate
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NAND Gate
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Logic Gate Example (00)
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Logic Gate Example (01)
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Logic Gate Example (10)
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Logic Gate Example (11)
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NOR
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A
B
Out
0
0
1
0
1
0
1
0
0
1
1
0