Transistors and Logic Gates

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2 Νοε 2013 (πριν από 3 χρόνια και 9 μήνες)

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PHY 201 (Blum)

Transistors and Logic Gates

References:

http://www.st
-
and.ac.uk/~www_pa/Scots_Guide/info/comp/active/BiPolar/page1.html

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Transistors


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
.


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


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


Eventually the
entire processor

was placed
on a single chip
. When this became
standard computers were said to enter the
fourth generation
.


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


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


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


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 so
-
called
“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 better
-
designed IC. (Nobel Prize 2000.)

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Microprocessor


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


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.

Noyce

Moore

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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


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.)


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.


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.)


In a diode one bring together n
-
doped and p
-
doped 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


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

-

-

-

-

-

-

+ + +

+ + +

n
-
doped

+ + +


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Applying a voltage II


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

-

-

-

-

-

-

+ + +

+ + +

n
-
doped



+ + +

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Bipolar transistors


A bipolar transistor starts with two back
-
to
-
back diodes (pn junctions).


There are two kinds: NPN and PNP


The middle region is usually small

N
-
doped

P
-
doped

N
-
doped

P
-
doped

N
-
doped

P
-
doped

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Third lead


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


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

In bipolar
transistors,
the various
regions are
referred to as
the
collector
,
base

and
emitter
.

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Collector, base, emitter

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Connecting the transistor


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.

C (N)

B (P)

E (N)

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No flow


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.)


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.

C

B

E

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Flow


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


The arrow on an
NPN points from
base to emitter
indicating the
forward
-
bias
direction that turns
the transistor “on.”

C
B

E

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Characteristics of the Off State

B

C

E

Little to no current going into the base and a large
voltage drop across the collector/emitter leads.

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Off


Base
-
emitter circuit


Little to no current flowing.


Most of the voltage dropped across the base
-
emitter as opposed to the resistor in the circuit.


Collector
-
emitter circuit


Little to no current flowing


Most of the voltage dropped across the collector
-
emitter 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


Base
-
emitter circuit


Current flowing.


Most of the voltage dropped across the resistor as
opposed to the base
-
emitter in the circuit.


Collector
-
emitter circuit


Current flowing.


Most of the voltage dropped across the resistor as
opposed to the collector
-
emitter in the circuit.

PHY 201 (Blum)

Logic gates


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

Circuit Symbol

NOT

AND

OR

NAND

NOR

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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

A

B

Out

0

0

1

0

1

0

1

0

0

1

1

0