Transcript Ch.42- p-n junctions, LED, solar cell, MOSFETs and superconductivity

Announcements
Start reading Chapter 43 (Nuclear Physics)
New terminology and applications of quantum
mechanics and special relativity.
Today: p-n junctions, transistors and
superconductivity
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QM/Solid State Physics conceptual question
Question: An isolated zinc atom has a ground state
electron configuration of filled 1s, 2s, 2p, 3s, 3p and
4s subshells.
Z=30, 1s22s22p63s23p64s23d10
How can zinc be a conductor if its valence subshell is
full ?
Ans: Band gaps form with large number of interacting
zinc atoms in a lattice – electronic properties are
modified by the Pauli principle with a large number of
electrons.
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Origin of energy bands and band gaps
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QM/Solid State Physics conceptual question
Question: Speeds of molecules in a gas vary with
temperature whereas speeds of electrons in the
conduction band of a metal are nearly independent of
temperature ? Why ?
Ans: In a gas, molecules move freely with energy
3/2kT = ½ mv2. However, the energy of electrons in the
conduction band depends on the Fermi energy and
depends only weakly on temperature (see figure).
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The p-n junction
University of Colorado simulation of p-n junction at
https://phet.colorado.edu/en/simulation/legacy/semicon
ductor
I = I S (eeV /kT -1)
Here IS is the saturation
current, V is the voltage, T
is the temperature and k is
Boltzmann’s constant.
Note that V increases, the current goes up exponentially.
For reverse bias, the voltage is negative, the current IS
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The p-n junction
• A p-n junction is the boundary in a semiconductor
between a region containing p-type impurities and
another region containing n-type impurities.
Very important: Many devices including transistors,
integrated circuits and diodes use p-n junctions
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More on the p-n junction
• A p-n junction is the boundary in a semiconductor
between a region containing p-type impurities and
another region containing n-type impurities.
Reverse biased, big energy
gapno current flows across
the junction except by tunneling.
“Depletion region” forms
Question: What happens
when the p-n junction is
biased as shown ?
-
+
p
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n
More on the p-n junction
• A p-n junction is the boundary in a semiconductor
between a region containing p-type impurities and
another region containing n-type impurities.
Question: What happens
when the p-n junction is
biased as shown ?
+
Electrons or holes now flow easily.
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p
n
Currents through a p-n junction
• Figure 42.30 below shows a p-n junction in equilibrium.
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Applications of a p-n junction
LED (Light Emitting Diode) is an example of
a technological application of a p-n junction.
Question: How does this work ? Is an LED
monochromatic ?
Ans: N.B. the junction is forward biased. When
electrons and hole recombine, they emit photons.
Yes,the photon energy corresponds to the band gap.
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Comments
A modern white LED lightbulb converts more than 50
percent of the electricity it uses into light. Compare to the
4 percent conversion rate for incandescent bulbs.
The 2014 Nobel Prize in Physics was for the invention of
the blue LED.
Shuji Nakamura (University of California at Santa
Barbara),
Hiroshi Amano (Nagoya),
Isamu Akasaki (Nagoya)
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Applications of a p-n junction
A photovoltaic cell is an example of a
technological application of a p-n junction.
Question: How does this work ? Do the incoming
photons have to be monochromatic ?
Ans: The incoming photon can dislodge an electron and (and
create a hole). No. The electron are in continuous energy
bands so the incoming light need not be monochromatic.
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The transistor invented at Bell Labs, NJ in 1947
William Shockley, John Bardeen* and Walter
Brattain. (1956 Nobel Prize in Physics)
*John Bardeen will crop up again in this chapter.
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Transistors (p-n-p sandwiches)
• The figure shows a p-n-p transistor in a circuit.
VC>>VE so the power
dissipated in the
resistor may be much
larger than the power
supplied to the emitter
by VE.
“Voltage amplifier”
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Transistors (p-n-p sandwiches)
The figure (right) shows a common-emitter circuit.
A large current IC
controlled by a small
current Ib
A “current amplifier”
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MOSFET (Metal Oxide Semiconductor Field Effect Transistor)
• The figure shows a field-effect transistor (Current from
source to drain controlled by the potential difference
between the source and an drain and by the charge on the
gate).
Without charge on the gate, one of the junctions is
reverse biased and no current flows.
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MOSFET
Question: Suppose a negative charge is placed on the gate
of the MOSFET, will a substantial current flow between the
source and the drain ?
Ans: No, the gate will repel electrons
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MOSFET
Question: Why does QM tunneling limit the miniaturization
of MOSFETs ?
Ans: If the barrier between the source and drain becomes
too small, it will become easy for electrons to tunnel from
the source to the drain. The device will leak current even
when turned off. (QM limits electronics miniaturization)
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Integrated circuit
• An integrated circuit can contain millions of transistors
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Superconductivity
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Superconductivity: History
Bardeen**, Cooper and Schrieffer explained it in 1956
1911: Kamerlingh-Onnes discovered that
some metals at low temperature become
perfect conductors. The resistance was
lower than could be measured (still true
today!).
Resistivity
Temperature
Zero!
1933: Meissner discovered that
superconductors expel a magnetic field.
They can be levitated by the magnetic
repulsion.
This does not happen
in a superconductor.
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How to avoid the Pauli exclusion principle
Pauli and two unidentified
men
No two electrons can be in the same quantum mechanical
state.
Yet in a superconductor, all the electrons are in the lowest
energy state.
Question: What gives ? How do we explain this ?
Ans: Due to the interaction of electrons with nearby
positive ions in the lattice. Pairs of electrons with opposite
spins can form “Cooper pairs”, which have spin zero and
are not subjected to the Pauli exclusion principle.
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Superconductivity
Bardeen, Cooper and Schrieffer explained all this in 1956
Resistivity
The BCS physics in a (small) nutshell:
At low temperatures, the electrons in
superconductors bind into pairs that have
zero spin – the pairs are “composite
bosons.” They love being in the same state
and they condense into it.
Temperature
Zero!
A boson condensate can flow without
scattering, it’s a charged “superfluid.”
Meanwhile a gap (not a band gap) opens up
and there are no single-electron states left
to accelerate and experience scattering. 
Zero Resistance!
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This does not happen
in a superconductor.
Demo
1972 Nobel Prize in Physics
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