Computers and How They Work

Vacuum Tubes and CRT
Individual Transistors
Integrated Circuit (IC)
PCB Board
Masks, Ingot, Wafers
Chips, Boards
Video: Transitorized
The Bipolar NPN Transistor
The base-emitter junction is forward biased, the
base-collector junction is reverse biased.
The transistor is bipolar because both charge
carriers (holes and electrons) are present.
Fabricating a transistor on a single
piece of doped silicon
Basic Amplifier Circuit using the
NPN Bipolar Transistor
Model of first Transistor c. 1947
The Field Effect Transistor (FET)
flows from
the Source
to the Drain
when an
electric field
is applied to
the Gate.
Field Effect Transistor (FET)
FETs are tiny
switches that can
open and close
very quickly.
A negative voltage
will turn this pchannel transistor
on. ‘On’ means
there is a
between the
Source and Drain.
Before the transistor, there was the
Vacuum Tube
► Also
used to
amplify signals
► Also used as a
Electrons are “boiled” off the cathode and
attracted to the anode because of the applied voltage.
A grid controls the electron flow.
The signal to be amplified is connected
to the grid. A small signal controls
a much current.
Cathode Ray Tube (CRT)
Still is use Today
► Vacuum
Tube, Electron Gun, Fluorescent
Screen, and Coils which accelerate and
deflect the electron beam
► Magnetron
► Power Amplifiers
ENIAC in 1946
First electronic
► Designed for the Army
► $500,000
► 19,000 Vacuum Tubes
► 1000 Relays
► 200 KW of power
► Filled multiple rooms
(700 sq. ft)
► Soldered and
constructed by hand
by the University of
► Slow
► Unreliable
Hearing Aids - the first commercial
devices that used transistors
c. 1953
► Individual
► Integration of parts required hand wiring
Early Sony Transistor Radio
Richard Feynman
As told to American Physical Society in 1959,
'People tell me about miniaturization, and how far it has progressed today. There is a
device on the market by which you can write the Lord's Prayer on the head of a pin.
But that's nothing . Why can we not write the entire 24 volumes of the
Encyclopedia Britannica on the head of a pin?
Feynman asked why wires couldn’t be 90 nm
The birth of the integrated circuit
1947- using silicon as a transistor is discovered
1959- Texas Instruments put 10 transistors on one piece of
silicon. First company to use Silicon instead of Germanium
Used in Apollo Program to achieve moon landing –
transistors integrated on pieces of silicon offered lower
power and weight. 1961-69
Moore’s Law (Gordon Moore – Intel Founder)
Intel 4004
The World’s first
Microprocessor, made in
Computer on a chip!
It had 2300 transistors and
ran at 740 KHz.
It could execute 45
Could execute 96,000
instructions per second
As powerful as the ENIAC
Where’s the chip?
Chips can have up to a
billion transistors built
on a small piece of
CMOS stands for ‘Complimentary
Metal Oxide Semiconductor’. This
means CMOS chips contain both
pMOS and nMOS transistors.
Integrated Circuit
Manufacturing Process
Clean Room
Masks, Photoresist, & a developing
process called Photolithography
transfers patterns to the wafer
What do computers do?
► Very
simple operations
 Every computer has a small set of simple
instructions it can perform (Instruction Set)
► Operate
on 1’s and 0’s (binary data)
 Numbers are represented with 1’s and 0’s
 Letters, symbols, & keystokes are represented
with 1’s and 0’s.
► Do
the operations very fast
► Complex operations are broken down into
many VERY simple operations.
Modern Computers might process
64 bits at a time
Binary Codes are base-2 numbers
2097  2 X 103 + 0 X 102 + 9 X 101 + 7 X 100 = 2,097
1011  1 X 8 + 0 X 4 + 1 X 2 + 1 X 1 = 11
(American Standard Code for Information Exchange)
Each letter, punctuation mark, etc. has a unique 8-bit code.
00101110  FUN
The Declaration of Independence, your love letters and emails,
the Bible are huge numbers.
All data can be digitized with out degradation over time.
Analog sound
waves can be
stored directly in
their analog form,
e.g. by cutting
grooves on vinyl.
Photographic picture is an analog
image. The photo represents the
original image. Note there is a
continuous range of hues and shades
in negative film. Copying from
analog data usually results in loss of
data. Copies from copies do not look
as good as originals.
A digital image is
made up of many
pixels, each pixel
having one color out
of a fixed number of
possible colors.
Each pixel is a digital
number. Copying
does not result in loss
of data.
Data can be converted to binary
(digitizing) using an
analog to digital converter
Periodically take
a sample, at a
rate of at least
½ the
frequency. The
more samples
the more
information is
captured. The
more bits used
the closer the
the original.
Logical ‘AND’
Series Circuit with battery, two
switches and a light bulb.
Logical ‘OR’
Two switches in parallel, with a
bulb and battery.
Inverter or NOT gate
Series circuit with
switch in parallel with
Not-OR  OR
Easier to make than AND and OR
MOSFET Transistor as a Switch
PNP type – negative voltage (a ‘0’)
turns this transistor “on”
NPN type – a postive voltage (a
‘1’) turns this transistor “on”
Transistors perform logical operations
Build a Truth Table for this Circuit:
Exclusive OR (XOR)
•Inequality Tester
•1-Bit Adder
Only 16 transistors
XORs and NOR make an
equality tester
How could this circuit be used to implement your
word processors ‘search’ function? How would
you find “FUN” in your document?
XORs, ANDs, and OR make an
Adder Circuit
 Transistors act like on/off switches
 Computers operate on long words
(64 bits) of 1’s and 0’s
 Sophisticated operations can be
broken down into many very simple
 Complex circuits are built from
simpler circuits (Equality Tester
made from XOR and NOR)
 Basic logic operations are done with
a few transistor.
 Modern computers could be made
with mechanical relays.

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