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The last revolution happened ~ 60 years ago, when
vacuum tubes were replaced by semiconductor devices
1918
1960s
Vacuum tubes show high performance but are too big
Vacuum tubes can survive a nearby
nuclear explosion
What is a vacuum tube?
What vacuum tubes are for
Diode: a valve for electrons
+
gate
_
Triode: a switch or an amplifier
First digital computers
ENIAC (1946) weighed 30
tons, occupied 1800 square
feet and had 17,468 vacuum
tubes.
It could make 5000 additions
per second
First computer bug was a real bug
Harvard Mark II computer, 1947
1954-1963: SAGE Air Defense Project
• 2 32-bit computers
• Each contains 55,000 vacuum tubes, weighs
250 tons, and consumes 3 Megawatt
• Tracks 300 flights
• Total cost: $60 billion (double the price of
Manhattan Project!)
• Performance equivalent to $8 calculator
built on transistors!
Dangers of
forecasting
Computers in the future may
weigh no more than 1.5 tons.
(Popular Mechanics, 1949)
1940's - IBM Chairman Thomas
Watson predicts that "there is a
world market for maybe five
computers".
1950's - There are 10 computers in
the U.S. in 1951. The first
commercial magnetic hard-disk
drive and the first microchip are
introduced. Transistors are first used
in radios.
ENIAC (1946) weighed 30
tons, occupied 1800 square
feet and had 17,468 vacuum
tubes.
It could make 5000 additions
per second
1960's-70's - K. Olson, president,
chairman and founder of DEC,
maintains that "there is no reason
why anyone would want a computer
in their home." The first
microprocessor, 'floppy' disks, and
personal computers are all
introduced. Integrated circuits are
Semiconductor Revolution
“One should not work on
semiconductors, that is a filthy mess;
who knows whether they really exist.”
Wofgang Pauli 1931
Transistor invention: 1947
John Bardeen, Walter Brattain, and William Shockley
AT&T Bell Labs
Nobel Prize in Physics 1956
energy
Semiconductor or
dielectric
metal
Conduction band
Band gap
Valence band
The coolest thing about semiconductors: they can be doped
Even tiny fraction of impurities (0.00001) dramatically increases the ability to conduct
current (conductivity)
Energy diagram of doped semiconductors
Free electrons are supplied to the
empty conduction band
Holes are left in the full valence band
Both electrons and holes can now conduct current!
P-n junction
P-type
neutral
N-type
_
+
neutral
Forward bias
_
+
Reverse bias
_
+
Field-effect transistor
_
+
_
+
Voltage on gate controls the current of electrons from source to drain.
FET can be made very small using nanotechnology!
Commons.wikimedia.org
Current, Ohm’s Law, Etc.
dQ
i
dt
V
Ohm ' s Law : R  ; R  Const (independent of V )
i
l
R
A


j  E


E  j
The Continuity Equation for Steady
State Currents
Currents and current densities
are constant in time –

steady state. The flux of j out of any closed
surface must be zero.
 
 j  dS  0
Drift velocity
j
v
nq
For steady state situation
 
j

d
S

0

 
 E dr  0
1.Kirchhoff’s junction rule: The algebraic sum
of the currents into any junction is zero.
2.Kirchhoff’s loop rule: The algebraic sum of
the potential differences in any loop must be
zero.
Resistors in series:
R  R1  R2
Resistors in parallel:
1 1
1
 
R R1 R2
Circuit with capacitors
Joule’s Law
2
V
P  Vi  i R 
R
2
Why does a light bulb burn out when you switch the
light on and never when you turn it off?
The amount of current
i
drawn by a given device is determined by its power input
P
P  Vi  i 
V
The current in a 100-W light bulb is
P 100W
i 
 0.83 A
V 120 V
V 120 V
R 
 144
i 0.83 A
A kitchen circuit
An 1800-W toaster, a 1.3-kW electric
frying pan, and a 100-W lamp are
plugged into the same 20-A, 120-V
circuit.
a) What current is drawn by each device,
and what is the resistance of each
device?
b) Will this combination blow the fuse?
To prevent the fuse from blowing, you replace the fuse with one rated at 40 A.
Is this a reasonable thing to do?
Pi R
2
Wheatstone bridge
Charles Wheatstone
British scientist and inventor
1802-1875
Samuel Hunter Christie, British scientist and mathematician
Wheatstone English concertina
Electromotive force
The Magnetic Field
The force

F
on a charge q moving with a velocity

  
F  q( E  v  B)

v
The magnitude of the force
F  qvB sin 
[ B]  Newtons /(Coulomb  meter / sec)
1T (tesla)  1w / m  1Newton / C  m / s  10 gauss
2
4
4
BEarth  1Gauss  10 T
left-hand rule
right-hand rule