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Semiconductors and Electromagnetic Waves


Semiconductors and Electromagnetic Waves * * * * * * * * * * * * * Figure 18.2 The electromagnetic spectrum. The wave shown is not to scale. In reality the wavelength ... – PowerPoint PPT presentation

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Title: Semiconductors and Electromagnetic Waves

Semiconductors and Electromagnetic Waves
23.5 Semiconductor Devices
Semiconductor devices such as diodes and
transistors are widely used in modern
electronics. Technology has clearly
revolutionized society, but solid-state
electronics is revolutionizing technology itself.
The Electron volt
  • Small particles use small amounts of energy.
  • The electron volt (eV) is the magnitude of the
    amount of energy it take for one electron to move
    through a potential difference of one volt.
  • 1 eV 1.6 x 10-19 Joules

  • Silicon is the most common material used as a
    semiconductor (germanium is also used).
  • It has 4 valence electrons and forms a stable
    lattice structure.
  • All electrons are used in the bonding process.
    None are free to move through the lattice
    structure, therefore pure Si is a poor conductor.

Band Gap
  • The band gap (EG) is the minimum amount of energy
    required for an electron to break free of its
    bound state.
  • When the band gap energy is met, the electron is
    excited into a free state, and can therefore
    participate in conduction
  • The band gap determines how much energy is needed
    from the sun for conduction, as well as how much
    energy is generated.
  • A hole is created where the electron was formerly
    bound. This hole also participates in conduction.
  • The band gap energy of Si is 1.1 eV
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23.5 Semiconductor Devices
The semiconducting materials (silicon and
germanium) used to make diodes and transistors
are doped by adding small amounts of an impurity
  • Small amounts of a material with 5 valence
    electrons added to the silicon (e.g phosphorus).
  • Extra electron is a mobile negative charge
    carrier which increases overall conductivity.
  • The n-type semiconductor is electrically neutral.
    The doping process increased conductivity only.

n-type semiconductor doping
  • Small amounts of a material with 3 valence
    electrons are added to the silicon (e.g boron).
  • Extra electron hole is a mobile positive
    charge carrier which increases overall
  • Note that the p-type semiconductor is
    electrically neutral, just like the n-type

p-type semiconductor doping
23.5 Semiconductor Devices
What do you get when you put an p-type and an
n-type semiconductor together?
overall neutral, but with moving, positive holes
overall neutral, but with moving, negative
You get a p-n junction, of course!
  • Mobile electrons from the blue, n-type material
    move left to fill the holes in the pink p-type
    material ( left, in Fig a). One may think of the
    square electron holes as moving right.
  • The layer at the end of p-type material becomes
    negative and vice versa. This results in an
    electric field, pointing from n-type material to
    p-type material (Fig b).
  • The resulting structure is called a diode.
  • No current flows because the diode is
    electrically neutral.

PN junction demo
Connect a voltage source with a diode.
A solar cell is a diode.
  • Photons in sunlight hit the solar panel.
  • The energy ionizes atoms in the charge layers.
  • Electrons are ejected from their atoms, allowing
    them to flow through the material to produce
  • Due to the composition of solar cells, the
    electrons are only allowed to move in a single
    direction. As a result, the solar cell develops
    a positive and negative terminal, much like a

A solar cell is a diode.
When the energy of a photon is equal to or
greater than the band gap of the material, the
photon is absorbed by the material and excites an
electron into the conduction band. Both a
minority and majority carrier (i.e electron and
hole) are generated when a photon is
absorbed. The generation of charge carriers by
photons is the basis of the photovoltaic
production of energy.
Light-generated current
  • Two key processes
  • Absorption of a photon with energy greater than
    EG creates an electron-hole pair. However, If
    this pair recombines, then there will be no

Light-generated current (contd)
  1. Separation of carriers at the p-n junction due to
    the electric field. If the light-generated
    minority reaches the p-n junction, it is swept
    across the junction by the electric field at the
    junction, where it is now a majority carrier.
    The majority carrier is prevented from crossing
    the pn junction so travels through the external
    circuit to recombine.

Light-generated current
Note that in this animation, the blue is the
p-type, and pink is the n-type Blue carriers are
positive holes, and red are negative electrons.
24.1 The Nature of Electromagnetic Waves
This picture shows an electromagnetic wave, such
as a light wave, or radio wave. An EM wave is a
transverse wave that does not need a medium, e.g.
air, or water, to propagate.
  • In 1865, long before experimental evidence, the
    English physicist Maxwell correctly predicted
    that, in a vacuum

e0 8.85 x 10-12 C2/(N m2
µ0 4p x 10-7 T m/A.
24.3 The Speed of Light
  • The American physicist Albert Michaelson improved
    on attempts to measure the speed of light.
  • By placing his mirrors on top of 2 Southern
    California mountains, he obtained a value of c
    that was less than 0.0014 different that the
    currently accepted value.
  • He definitely got a A on that lab.

24.2 The Electromagnetic Spectrum
Like all waves, electromagnetic waves have a
wavelength and frequency, related by
Fig. 18-2, p.430
24.2 The Electromagnetic Spectrum
Example 1 The Wavelength of Visible Light Find
the range in wavelengths for visible light in the
frequency range between 4.0x1014Hz (red) and
7.9x1014Hz (violet).
These wavelengths correspond to 0.75 µm
(microns) and 0.38 µm, respectively .
The Crab Nebula is a remnant of a star that
underwent a supernova. This event was recorded
in the year 1054 A.D (see Anasazi pictograph,
below). The Crab Nebula is located at a distance
of 6.0 x 1016 km away from the earth. How long
ago did the supernova happen?
The Crab Nebula is a remnant of a star that
underwent a supernova. This event was recorded
in the year 1054 A.D (see Anasazi pictograph,
below). The Crab Nebula is located at a distance
of 6.0 x 1016 km away from the earth. How long
ago did the supernova happen? - 7300 years ago
from 2010
24.4 The Energy Carried by Electromagnetic Waves
Electromagnetic waves, such as the microwaves
shown below, carry energy, much like sound waves
EM/Solar Radiation
  • Radiation is the heat-transfer mechanism by which
    solar energy reaches our planet.
  • Energy transferred by radiation is called
    electromagnetic radiation and can travel through
    a vacuum. This radiation is NOT radioactive!
  • All radiation travels at the speed of light in a

When radiation strikes an object
  • Transmission (no change in direction or
  • Scattering and Reflection (transmission in
    another direction)
  • Absorption, which is accompanied by change of
    temperature for object absorbing the radiation.

Solar Radiation in the Atmosphere
Reflection and Albedo
  • Reflectionelectromagnetic radiation bouncing of
    from a surface without absorption or emission, no
    change in material or energy wavelength
  • Albedo proportional reflectance of a surface
  • a perfect mirror has an albedo of 100
  • Glaciers snowfields approach 80-90
  • Clouds 50-55
  • Pavement and some buildings only 10-15
  • Ocean only 5! Water absorbs energy.

Typical Albedos of Materials on the Earth
Absorption and Emission
  • Absorption of radiation electrons of absorbing
    material are excited by increase in energy
  • Increase in temperature physical/chemical change
  • Examples sunburn, cancer
  • Emission of radiation excited electrons return
    to original state radiation emitted as light or
  • Example earth absorbs short wave radiation from
    sun (i.e. visible light) and emits longwave
    (infrared or heat) into the atmosphere.

Laws Governing Radiation
  • All objects at a temperature greater than 0 K
    emit radiant energy. This includes the Earth,
    and its polar ice caps.
  • For a given size, hot object emit more energy
    than cold objects (Stefan-Boltzmann Law)

Laws Governing Radiation
  • The hotter the radiating body, the shorter the
    maximum wavelength (Wiens Law).
  • The Sun is a very hot body. Although it radiates
    in all parts of the spectrum, much of its
    radiation is short-wave radiation. The much
    cooler Earth radiates in longer wavelengths
    called (!) long-wave radiation .

Electromagnetic Spectrum
  • Note the distinction between short-wave and
    long-wave radiation.

EM radiation from Sun and Earth
Laws Governing Radiation
  1. Objects that are good absorbers of radiation are
    good emitters as well (Kirchoffs Law). The
    Earth and the Sun absorb and radiate with nearly
    100 efficiency for their respective temperatures
  2. The gases of the atmosphere not so much. They
    absorb some wavelengths and then re-emit them.
    They let other wavelengths pass through with no

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The Greenhouse Effect
  • Sun emits EM radiation of all wavelengths, but
    primarily shortwave (i.e. visible).
  • Earths surface absorbs this energy
  • Most is re-emitted upward, as IR (longwave)
  • greenhouse gases (water vapor, carbon dioxide,
    methane, etc.) let shortwave energy pass, but
    absorb longwave energy radiated upward by the
  • this longwave energy is re-radiated in all
    directions, some of it returning to the Earths
    surface. This is what keeps our atmosphere at a
    livable temperature of about 15 degrees C (59
    degrees F).

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the Radiation Balance
  • Sun emits EM radiation of all wavelengths, but
    primarily shortwave (i.e. light).
  • Earths surface absorbs this energy
  • Most is re-emitted, as heat (longwave)
  • Greenhouse Effect
  • greenhouse gases let shortwave energy (light)
    pass through, but absorb and emit longwave energy
    radiated by the Earth, keeping it the atmosphere

Fig. 18-7, p.433
  • Most solar energy is in the form of shortwave
    radiation (e.g. light, uv rays)
  • Earth absorbs this energy and re-emits as
    longwave radiation (infra-red, heat)
  • Greenhouse gases (CO2, CH4 H2O) in the atmosphere
    absorb infrared radiation
  • This natural process allows the Earth to maintain
    an average yearly temperature of about 150 C (600

Correlation of the rise in atmospheric carbon
dioxide concentration (blue line) with the rise
in average temperature (red line)
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How CO2 in atmosphere relates to temperature
EM Wave Intensity
  • Intensity defined previously for sound waves as
    power to area ratio Intensity P/A.
  • Intensity is inversely proportional to the square
    of the distance from the source of the wave.
  • Recall power is the amount of energy transported
    per second.

24.5 The Doppler Effect and Electromagnetic Waves
  • Electromagnetic waves also can exhibit a Dopper
    effect, but it
  • differs for two reasons
  • Sound waves require a medium, whereas
  • waves do not.
  • For sound, it is the motion relative to the
    medium that is important.
  • For electromagnetic waves, only the relative
    motion of the source
  • and observer is important.
  • c) use plus if observer and source are moving
    together, minus if they are moving apart.
  • d) vrel is a magnitude and therefore always

EOC 37
  • A distant galaxy is simultaneously rotating and
    receding from the earth. As the drawing shows,
    the galactic center is receding from the earth at
    a relative speed of uG 2.00x106 m/s. Relative
    to the center, the tangential speed is vT 5.00
    x105 m/s for locations A and B, which are
    equidistant from the center.

EOC 37
  • . When the frequencies of the light coming from
    regions A and B are measured on earth, they are
    not the same and each is different than the
    emitted frequency of 6.20 x 1014 Hz. Find the
    measured frequency for the light from region A
    and region B.

EOC 37
  • .Find the measured frequency for the light from
    region A and region B.
  • A 6.17 x 1014 Hz
  • B. 6.15 x 1014 Hz

24.5 The Doppler Effect and Electromagnetic Waves
Example 6 Radar Guns and Speed Traps The radar
gun of a police car emits an electromagnetic wave
with a frequency of 8.0x109Hz. The approach is
essentially head on. The wave from the gun
reflects from the speeding car and returns to
the police car, where on-board equipment measures
its frequency to be greater than the emitted wave
by 2100 Hz. Find the speed of the car with
respect to the highway. The police car is
24.5 The Doppler Effect and Electromagnetic Waves
source frequency fs 8 x 109 Hz
frequency observed by speeding car
reflected frequency observed by police car
Replace f0 with term for f0 on the right side of
Replace f0 with fs on the right side of the
equation and expand the square
24.5 The Doppler Effect and Electromagnetic Waves
but we can make the assumption that vrelltlt c, so
the last term becomes 2
  • Doppler weather radar uses the Doppler shift of
    reflected radar signals to measure wind speeds
    and gauge the severity of a storm.
  • This picture is off the coast of Florida.

Red shifts and blue shifts The Big Bang
  • For light coming from astronomical objects, this
    Doppler equation is no longer correct, but it is
    still true that the light coming from an object
    moving closer has a higher frequency, while the
    light coming from a receding object has a lower
  • We say light has been blue-shifted for an
    object moving closer, and red-shifted for an
    object moving away.

Red shifts and blue shifts The Big Bang
  • The light coming from the stars and galaxies
    around us is red-shifted, leading to our present
    belief that the galaxy is expanding.
  • Extrapolating back in time brings us to a point
    when the universe was contained in a volumeless
    point that exploded, aka The Big Bang.

23.5 Semiconductor Devices
There is an appreciable current through the diode
when the diode is forward biased. Under a
reverse bias, there is almost no current through
the diode.
23.5 Semiconductor Devices
  • The graph shows dependence of current on
    magnitude and polarity of voltage applied across
    an ideal p-n junction.
  • the arrow/bar is the symbol for diode (arrow
    shows the direction the diode allows conventional
    current to flow).
  • Reverse bias- regardless of how much voltage
    applied, no current flows.
  • Forward bias after some threshold voltage
    applied (here slightly more than 0.5 volts),
    current rises at an exponential rate.

23.5 Semiconductor Devices
  • A more realistic graph for a silicon diode.
  • When reverse-biased, a real diode lets in a very
    small amount of current.
  • If you apply enough reverse voltage (V), the
    junction breaks down and lets current through
    (shown at far-left unlikely in normal
  • When forward-biased, the threshold voltage for
    silicon is about 0.7 volts.
  • A diode is a non-ohmic device it does not obey
    Ohms Law.
  • If you apply more more voltage (bigger battery),
    the current through the diode will increase, but
    the voltage drop will always remain at the
    threshold value.