What We’ve Learned So Far
A Review of Topics on Test 2
What Have We Learned About
Optical and Electric Storage?
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Laser light is focused through a (circular) lens
onto the surface of a CD
The central max of the diffraction pattern must be
no larger than one bit if data is to be resolved
d sin q = 1.22 l
tan q = y/D
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Capacitors store charge Q in proportion to the
voltage V between the plates:
C = Q/V = e0 A/d
Capacitors are used in RAM
What Have We Learned About
Magnetic Storage?
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Two domains magnetized in same direction is a 0
Two domains magnetized in opposite directions is
a1
Direction of magnetization changes at start of new
bit.
Magnetic data is written by running a current
through a loop of wire near the disk
As magnetic data passes by coil of wire, changing
field induces currents according to Faraday’s Law:
e
d B
dB
 iR  
 A
dt
dt
What Have We Learned About
Magnetoresistance?
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Charges traveling through magnetic field experience
magnetic force (provided velocity and field are not
aligned):
FB = qv x B
In a current-carrying wire, this force results in more
frequent collisions and thus an increased resistance:
Magnetoresistance
Electrons traveling through magnetized material undergo
spin-dependent scattering
When magnetic field is present in magnetic superlattice,
scattering of electrons is cut dramatically, greatly
decreasing resistance: Giant magnetoresistanced
What Have We Learned About
Atoms?
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ENERGY IS QUANTIZED
Electrons can absorb energy and move to a higher level;
they can emit light and move to a lower level
In hydrogen the emitted light will have energy
E = (13.6 ev)(1/nf2 – 1/ ni2)
The wavelength is given by l = hc/E = 1240(nm eV)/E
Energy levels of nearby atoms are slightly shifted from
each other, producing bands of allowed energies
Electrons move from the locality of one atom to the next
only if an energy state is available within the same band
What have we learned about
Resistance?
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In many, ohmic, materials, current is proportional
to voltage:
V = iR
Resistance is proportional to the length of an
object and inversely proportional to crosssectional area:
R = rL/A
The constant of proportionality here is called the
resistivity. It is a function of material and
temperature.
What Have We Learned About
Solids?
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In conductors, the valence band is only partially-full, so
electrons can easily move
In semiconductors and insulators, the valence band is
completely full, so electrons must gain extra energy to
move
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Conductors have a partially-filled valence band
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semiconductors have smaller band gap, insulators have
larger band gap
The primary effect of higher temperature on resistance is to
increase R due to more collisions at higher temperatures
Semiconductors have a completely-filled valence band
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The primary effect of temperature on resistance is due to this
requirement: the higher the temperature, the more
conduction electrons
What Have We Learned About
Semiconductors?
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Can dope semiconductors to increase conductivity: p-type
uses atoms with 3 valence electrons, so empty “hole”, ntype uses atoms with 5 valence electrons, so extra
conduction electron
In p-n junction, conduction electrons travel from n side
(and holes travel from p side) to p (n) side to combine with
holes (electrons)
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p-side becomes negatively charged; n-side becomes
positively charged: potential difference
If put negative terminal on p-side, increases potential
difference and get no current (reverse bias)
If put positive terminal on p-side, lower potential difference
and get current
What Have We Learned About
Semiconductor Devices?
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When electrons and holes combine at p-n junction, excess
energy emitted as light (LED)
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energy of light depends on bandgap
fairly monochromatic wavelength emitted in all directions
Can pump enough electrons to conduction band to achieve
population inversion, and could mirror surfaces to
encourage stimulated emission: this produces
semiconductor laser
see applet for more info about laser operation:
http://www.phys.ksu.edu/perg/vqm/laserweb/Ch3/F3s5p1.htm