Applied Physics
After 1945
Transistors and Lasers
Last time…matter sorted out
 Matter and anti-matter (1929)
 Cosmic rays - many new particles identified
(1930-50s)
 Hadrons = particles interacting by strong force
 Quarks make up all hadrons
 Leptons = particles interacting by weak force
 Standard model proposed by Gell-Mann (1963)
 All matter (3 families) made of 6 quarks and 6 leptons
 Four fundamental forces, transmitted by exchange of
particles
 Photons, gauge bosons, gluons, gravitons
 Unification of forces (next week)
Task of today’s lecture
 From reductionism to complex systems
 Greek atomists, Newtonian mechanics, QM, particle
physics all reductionist
 Field theories, kinetic theory of gases, condensed
matter physics, chaos theory all complex systems
 From the Manhattan Project to interdisciplinary,
applied research after 1945
 Nuclear and thermonuclear weapons
 Bell Labs and the transistor
 Many industrial labs and lasers and masers
Condensed matter physics
 Explaining the physical behavior of solids
 Magnetism, heat capacity, electrical properties, physical
properties (e.g., hardness, cleaving, transparency)
 Conductors, insulators, semiconductors
 Known since Faraday (1831)
 Semiconductor conductance affected by temperature and
impurity atoms (1 ppm), unlike conductors & insulators
 “Electron gas theory” of Thomson (1900) fails
• Assume an electric field gives velocity to free electrons and that kinetic
theory of gases can be applied to free electrons
• Could not predict measured conductivites or heat capacities of metals
 Quantum theory of solids (1928) succeeds
• Assume free electron energies in solids are quantized in bands
• Assume electrons move as waves through atomic lattice
Finding an amplifier
 Major technological need (reverse salient)
 Repeaters for long-distance telephone
 Radio receivers
 Radar receivers during World War II
 Lee De Forest’s triode amplifier, 1906
 Employed light-bulb technology
 Add third element (grid) to cathode ray tube
 AT&T purchases all patent rights, first
transcontinental phone line in 1915
 Makes possible development of sound motion
pictures (many contributions by De Forest)
Triode amplifier (“valve”)
Heater
Cathode ray tube
Grid
Cathode
+
+
Anode
Vout
Vin
Battery
Battery
+
Currentin
Currentout
Research at Bell Labs
 Problems with the triode
 Large, fragile, short lived, hot, energy needs
 Ineffective at higher frequencies (microwave)
 Cat’s whisker diode, 1874
Metal
Semiconductor
 Bell Lab’s post-war interdisciplinary
work
 Created condensed matter section
 Mission: “… to obtain new knowledge that can be used to develop new and
improved components for communication systems.”
 Theoretical and experimental physicists, physical chemist, electronics
expert, technicians worked together in total freedom (Manhattan Proj model)
 Many Bell Lab people attended courses in quantum physics at Columbia
University
2 types of semiconductors
found 1939 at Bell Labs
 N-type

28Si
14 dopped
(75As33)
with arsenic
+4
+4
+4
+4
+4
+4
+4
+5
+4
+4
+4
+4
+4
+4
+4
+4
+4
+3
+4
+4
 Valence electrons
 Extra electrons
 P-type
 Silicon dopped with boron
(11B5)
 Extra ‘holes’ (missing
electrons)
 N-P junction = diode
 1-way current flow
-
N -
+ P
+
(current flow)
Shockley’s failure, 1947
 Applied quantum theory of electrons and
sought a “semiconductor triode” -- failed
Metal contacts
Electric field acting as
“grid”
Vin
Semiconductor
+
Battery
+
Battery
Vout
Bardeen & Brattain’s point
contact transistor, 1947
 Submerged Shockley’s triode in liquid
 Found unexpected amplification
 Tried many liquids, geometries, replaced
liquid with another semiconductor
 Amplified … they won Nobel Prize in 1956
Narrow gap!
+
Vin
P-type
N-type
Vout
+
Flow of holes modulated
by Vin regulates flow of
current in Vout
Marketing the transistor
 Transistors combine several P-N junctions
 Slow development to 1952
 Transistors 8x more expensive than vacuum tubes
 Transistors could not be manufactured reliably
 No civilian applications except hearing aids
 Military purchases, 1952-64 ($50 million)
 Navy study shows 60% tube failure in wartime
 Nuclear missile program requires miniaturization
• Silicon Valley emerges to meet military need
• Manufacturing costs drop
• Illustrates role of government orders in civilian economy
 First transistor radios, 1959 ($50 each!)
Lasers = coherent light
Energy
 Light Amplification of Stimulated Emission
of Radiation = LASER
 Stimulated emission in excited atoms
Excited state
E=hf
2 photons out, in phase (each with hf)
Photon in with E=(hf)
Ground state
Coherent
light!
 Hypothesized by Einstein in 1916, but not
explored further
Theory of the laser
Mirror
Optical crystal
Partial mirror
Atoms
Pump light in to excite atoms
One excited atom emits photon parallel to axis, starting cascade
of stimulated emission (in phase) as photons move toward 1 end
Cascade amplified as photons are reflected from end mirrors
When amplification is great enough, coherent beam passes through
partially reflecting end mirror creating “light amplification”
From WWII to masers to
lasers
 Charles Townes at Bell Labs and Columbia
 Radar work during World War II (microwaves)
 Study molecular structure with microwaves
 Needed shorter-λ microwaves, built MASER, 1954
• Microwave Amplification of Stimulated Emission of Radiation
 Race to build lasers, 1954-62
 Townes-Gould idea 1958 (consulting for Bell Labs)
 Ruby crystal laser, Hughes Research Labs, 1960
• Powered by Edgerton flash lamp, gave red light
• News releases called it a “killer-ray gun”
 Helium-neon gas laser, Bell Labs 1960
• Excited helium excites neon, emits red light
 Semiconductor laser, IBM, GE, RCA 1962
• Used in pocket pointers, DVD-drives, laser printers
Lasers--billion $/yr industry
 Six Nobel Prizes in physics have involved lasers
 Industrial labs developed the technology
 Maser patent to Research Corp. of Smithsonian Inst
with royalties to Townes
 Laser patent to Bell Labs, legal war ensues and
Gould (the graduate student) wins control
 Applications of the laser everywhere
 Cutting (everything, from metals to eyeballs),
information transfer, light sources
 Modulated at 1011 cycles/sec in optical cables!