Transcript Chapter 23/24

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
Semiconductors
•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
• http://pveducation.org/pvcdrom/pn-junction/band-gap
23.5 Semiconductor Devices
SEMICONDUCTORS
The semiconducting materials (silicon and germanium)
used to make diodes and transistors are doped by
adding small amounts of an impurity element.
n-TYPE SEMICONDUCTORS
•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
http://www.ece.utep.edu/courses/ee3329/ee
3329/Studyguide/Shockwave/Fundamentals
/Demos/Donor.html
p-TYPE SEMICONDUCTORS
•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 conductivity.
•Note that the p-type
semiconductor is
electrically neutral, just
like the n-type material.
p-type semiconductor doping
http://www.ece.utep.edu/courses/ee3329/ee
3329/Studyguide/Shockwave/Fundamentals/
Demos/acceptor.html
http://pveducation.org/pvcdrom/pnjunction/equilibrium-carrier-concentration
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 electrons
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
http://www.ece.utep.edu/courses/ee3329/e
e3329/Studyguide/Shockwave/PNjunctions
/Demos/PNJunctionDiode.html
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 electricity.
•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 battery.
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.
http://pveducation.org/pvcdrom
/pn-junction/absorption-of-light
Light-generated current
Two key processes:
1. Absorption of a
photon with energy
greater than EG
creates an
electron-hole pair.
However, If this
pair recombines,
then there will be
no current.
Light-generated current (cont’d)
2. 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
http://pveducation.org/pvcdrom/solar-cell-operation/light-generated-current
Note that in this
animation, the
blue is the ptype, 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:
ε0 = 8.85 x 10-12 C2/(N m2
μ0 = 4π x 10-7 T m/A.
c
1
 0 0
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.
c  3.00 10 m s
8
24.2 The Electromagnetic Spectrum
Like all waves, electromagnetic waves have a wavelength and
frequency, related by:
c  f
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).
c 3.00 108 m s
7
 

7
.
5

10
m  750 nm
14
f
4.0 10 Hz
c 3.00 108 m s
7
 

3
.
8

10
m  380 nm
14
f
7.9 10 Hz
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 heattransfer 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
vacuum.
When radiation strikes an object
• Transmission (no change in direction or temperature)
• 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
• Reflection–electromagnetic 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 heat
– Example: earth absorbs short wave radiation from
sun (i.e. visible light) and emits longwave (infrared or
heat) into the atmosphere.
Laws Governing Radiation
1. All objects at a temperature greater than
0 K emit radiant energy. This includes
the Earth, and its polar ice caps.
2. For a given size, hot object emit more
energy than cold objects (StefanBoltzmann Law)
Laws Governing Radiation
3. The hotter the radiating body, the shorter
the maximum wavelength (Wien’s 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
4. Objects that are good absorbers of
radiation are good emitters as well
(Kirchoff’s Law). The Earth and the Sun
absorb and radiate with nearly 100%
efficiency for their respective
temperatures
5. 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
absorption.
The Greenhouse Effect
• Sun emits EM radiation of all wavelengths, but
primarily shortwave (i.e. visible).
– Earth’s 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 Earth.
– this longwave energy is re-radiated in all directions,
some of it returning to the Earth’s surface. This is
what keeps our atmosphere at a livable temperature
of about 15 degrees C (59 degrees F).
the Radiation Balance
• Sun emits EM radiation of all wavelengths,
but primarily shortwave (i.e. light).
– Earth’s 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 (infrared, “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 F).
Correlation of the rise in atmospheric carbon dioxide
concentration (blue line) with the rise in average
temperature (red line)
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:
a) Sound waves require a medium, whereas electromagnetic
waves do not.
b) 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.
 vrel 
f o  f s 1 

c 

if vrel  c
c) use plus if observer and source are moving together, minus if they
are moving apart.
d) vrel is a magnitude and therefore always positive.
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 stationary.
24.5 The Doppler Effect and Electromagnetic Waves
source frequency
fs = 8 x 109 Hz
frequency “observed”
by speeding car
vrel vrel 
2100 Hz  f o  f s  f o  ff s (1 f 1  ) f s
o
s c
  c 
reflected frequency observed
by police car
 v 
f o  f o 1  rel 
c 

Replace f’0 with term for f0 on the right side of equation:
v 
f o  f s  f o (1   rel )  f s
 c 
Replace f0 with fs on the right side of the equation and expand the square:
 vrel   vrel 
( f o  f s )  f s 1 
 1 
  fs
c 
c 


2


v
v


rel
rel

  fs
( fo  f s )  f s 1  2



c  c  


24.5 The Doppler Effect and Electromagnetic Waves
Continuing:
2


v
v



rel
rel

( fo  f s )  f s  2


 c  c  


vrel 
vrel 
( fo  f s )  f s
2 

c 
c 

but we can make the assumption that vrel<< c, so the last term becomes 2:
v

( f o  f s )  2 f s rel
c
 f o  f s   2100 Hz 
8
c  
vrel  
3
.
0

10
m s  39 m s

9
 2 f s   2 8.0 10 Hz 




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
 v 
f o  f s 1  rel 
c 

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
frequency.
We say light has been “blueshifted” for an object moving
closer, and “red-shifted” for
an object moving away.
if vrel  c
Red shifts and blue shifts: The Big Bang
 v 
f o  f s 1  rel 
c 

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.
if vrel  c
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 circumstances).
•When forward-biased, the threshold
voltage for silicon is about 0.7 volts.
•A diode is a non-ohmic device; it does
not obey Ohm’s 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.