Fun Electron Tricks
Semiconductor Devices
npn junction
• Put another n-type semiconductor on the other side of the p-type
semiconductor
• No matter which way I apply potential difference, one p-n junction is
reverse biased, and electrons entering the p-type region quickly
combine with holes, creating more negative charge
MOSFET
(Metal-Oxide-Semiconductor, Field-Effect Transistor)
• If, however, I apply a positive potential to one side of the
p-type semiconductor, without allowing another path for
electrons to flow out of the device, I will create a channel
for e- to get from one n-side to the other.
n-type
p-type
n-type
MOSFET
• Now, if I bias the device in either direction, current will
flow, electrons toward the positive potential, and
conventional positive current toward the negative potential
Gate
n-type
p-type
n-type
MOSFET
(Metal-Oxide-Semiconductor, Field-Effect Transistor)
• The potential difference between drain and source is
continually applied
• When the gate potential difference is applied, current flows
Gate
Drain
Source
n-type
p-type
n-type
Bipolar Junction Transistor
Emitter
Base
Collector
increasing
electron energy
increasing hole
energy
n-type
p-type
n-type
Bipolar Junction Transistor
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans.html#c1
How do transistors fit in?
For now, view transistor as switch:
If switch is “on,” current can pass
If switch is “off,” no current can pass
We can use this simple device to construct
complicated functions
NOT Gate - the simplest case
Put an alternate path (output) before a switch.
Output
Input
Switch
Dump
If the switch is off, the current goes through the
alternate path and is output.
If the switch is on, no current goes through the
alternate path.
So the gate output is on if the switch is off and off
if the switch is on.
NAND - a variation on a theme
NAND gate returns a signal unless both of its two
inputs are on.
Put an extra switch after a NOT device
Output
Input
Switch
Input
Switch
Dump
If both switches are on, current is dumped.
Otherwise the current goes to the output.
AND - slightly more complicated
AND gate returns a signal only if both of its two
inputs are on.
Use the NAND output as input for NOT
Output
Switch
Input
Switch
Input
Switch
Dump
If both inputs are on, the NOT input is off, so the
AND output is on.
Else the NOT input is on, so the output is off.
Interference of Waves and the
Double Slit Experiment
• Waves spreading out from two points, such as
waves passing through two slits, will interfere
l
d
Wave crest
Wave trough
Spot of
constructive
interference
Spot of
destructive
interference
The Double-slit experiment for
particles
• Particles do not diffract; they either go through a slit or
they don’t
• Particles passing through a slit hit a screen only in a small
area; if they all have the same initial velocity, they will all
hit at the exact same point
• Particles passing through two slits will form two maxima
in front of the two slits
What Happens if Electrons Pass
Through Small Openings?
What does that tell you about
electrons?
The Plot Thickens
An experiment called the
“photoelectric effect” also gives
unexpected results!
The Photoelectric Effect,
Pictorially
• Light shining on a material may be absorbed by electrons in that
The
energy
of the
material
If ankinetic
electron
absorbs
electron
will be to
equal
to the
enough energy
break
energy absorbed by the
free of its
bonds,
can
electron
minus
theitenergy
leave the
needed
to material
free it, provided
the electron does not lose
any energy in collisions
Wave theory predicts . . .
• the energy of emitted electrons should
depend on the intensity of light
• electrons will need to soak up energy from
wave for period of time before being ejected
• the frequency of light won’t affect the
maximum kinetic energy of electrons
The Photoelectric Effect,
Experimentally
• As a given color (frequency) of light enters the black box-like
photoelectric head, it falls on a plate of electron-emitting material
inside
• Emitted electrons are collected on another plate nearby, producing
an electric potential difference between the two plates (like a
capacitor)
• When the capacitor is fully charged and no more electrons can be
added, the potential energy of the capacitor equals the maximum
kinetic energy of the electrons trying to leave the original plate
• The potential difference on the capacitor at this point is called the
stopping potential Vs for the electrons, and it is proportional to the
maximum kinetic energy of electrons emitted by the light:
K = eVs = Eabsorbed - F
Work function (energy needed
to remove electron)
Do the Photoelectric Experiment
Upon what does the energy of emitted
electrons appear to depend?
Experiment sees . . .
• the energy of emitted electrons does not depend on
the intensity of light
• electrons are ejected immediately
• the frequency of light does affect the maximum
kinetic energy of electrons; kinetic energy is
linearly dependent on frequency
• intensity of light determines number of emitted
electrons (photocurrent)
Einstein to the Rescue
• Einstein suggested that light was emitted or absorbed in
particle-like quanta, called photons, of energy, E = hf
If that energy is larger than
an electron
absorbs
theIfwork
function
of the one
of these
photons,can
it gets
metal,
the electron
leave;
if not,hf
it of
can’t:
the entire
energy.
Kmax = Eabs – F = hf - F
Einstein’s Photoelectric Theory
eVs = Kmax = hf – F
• Kmax  f
• Is this consistent with what you saw in the experiment?
• Electrons are ejected as soon as a photon strikes the
material.
• Is this consistent with what you saw in the experiment?
Einstein’s Photoelectric Theory
eVs = Kmax = hf – F
• If hf < F, no electrons are emitted; cutoff
frequency
• What should the slope of a K vs. f plot
yield? Is that what you got?
The Conflict
• Wave theory accurately describes interference and
diffraction, along with other behavior of light,
such as dispersion and refraction
• The particle theory accurately describes
photoelectric effect, black body radiation, and
other experimental results
 Is light a particle? Or is it a wave?
 Is a platypus a duck? Or is it a beaver?
 Am I my mother? Or am I my father?
The Resolution
• Light is not either a particle or a wave
• Light exhibits wavelike properties when traveling
• Light exhibits particlelike properties when
interacting with matter
• deBroglie suggested that traditional “particles”,
like the electron, also exhibit wavelike properties
• p=h/l, so large (macroscopic) momentum means
small (undetectable) wavelength
The interpretation
• Light and “particles” propagate through space as
probability waves
• I cannot say for certain where a particle is, where
it was, or how it got to wherever it might have
been
• I can, however, say where it is most likely to be
found, where it most likely was, and how likely it
is that it took a particular path
• This behavior is described by a wave function
Y(x,y) which obeys Schrödinger’s equation
Before the next class, . . .
• Homework 20
• Do Activity 19 Evaluation by Midnight
Monday
• Read Chapters 7 and 8 in Turton.
• Do Reading Quiz