Photocopiers
Virtual tour
of the Duane Dungeons…..
Reminders/Updates:
Register your clicker!
Day 4:
HW 1 due Friday at 5pm
Photocopiers: Bloomfield 10.2
HW 2 available tomorrow
Photoconductors 12.1
Help sessions : Today 2-4pm, Friday 1-3pm.
Don‟t struggle alone! 1
2
Photocopiers
Photocopiers
The top selling industrial product of all time
Static electricity and semiconductors
1. Coat a photoconducting sheet with a layer of negative charge
2. Use light to selectively remove some charge, creating a charge image of
original
3. Toner sticks to charged areas only, creating a visible image of original
4. Transfer toner to new piece of paper
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1.
- -- ---------- - - --------------- --------------------- ------------------------
2.
------------ -- -- -- ---- -- -- -- ---- -- -- -- ---- -- -- -- --- - - - - - - - - --
-- -- -- -- - - -------- ---------- ---------- ---------- ---------- ---
Which picture best represents neutral chalk dust sticking to the
negatively charged transparencies?
“Photocopying”
demo
a)
-
b)
+ +
+ +
- ++
- +
- +
-
d)
c)
3.
------------ -- -- -- ---- -- -- -- ---- -- -- -- ---- -- -- -- --- - - - - - - - - --
CU
+- +- ++
-
+
+ - +- +
-
e) None of the above
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1
Which picture best represents neutral chalk dust sticking to the
negatively charged transparencies?
Photocopiers
Static electricity and semiconductors
Neutral chalk dust can be polarized:
- Negative charges in chalk get repelled to far side of dust particle
- Force of attraction/repulsion between point charges decreases with distance
 Attraction between opposite charges bigger than repulsion between like charges
 Net force of attraction between neutral chalk and transparency
1. Coat a photoconducting sheet with a layer of negative charge
transparency
finger
2. Use light to selectively remove some charge, creating a charge image of
original
- Just like charged balloon and neutral wall.
Chalk dust
(see sweater simulation)
c)
- ++
- +
- +
3. Toner sticks to charged areas only, creating a visible image of original
-
4. Transfer toner to new piece of paper
http://phet.colorado.edu
But where does light (the „photo‟ bit) come in?
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Photocopying 1:
Making a Charge Image
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Photocopying 2:
Transferring Charge Image to Toner and Paper
Photoconductor
Grounded metal
Attract toner to
charge image
Charge photoconductor
Corona wire
Roller and
brush
toner
velocity
+++
+++++
Document
Light
reflected
off
document
Selective
discharge
+ +++ + +
Black image
Positive
toner particle
+++++
Charge image
+++++
+ ++ +
++ + +
+
++ +
+
+
+ + +
+
Release toner
Light
Charge image
+
- - -+++ +
+++++
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Photoconductor at heart of photocopier:
- Enables you to make a charge image using light
- Behaves like an insulator when in the dark
- Where light hits, R low, electrons flow away,
- When add toner, sticks only where charges are left.
Toner
attracted to
paper
+
+ + +
+ +++ +
+
+ ++
--------- -- -----------Heat
-
-
+ ++
-
-
Straightforward. All just
good old physics of attraction
between opposite charges, plus
photoconductor physics.
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Semi-conductor physics
Photocopying 2:
Transferring Charge Image to Toner and Paper
Charge Paper
+
+++
-
-
-
-
First have to understand what determines electrical resistance of a material:
a) insulators (wood, ceramic, plastic)- very high resistance.
Copy
b) conductors (metals)- very low resistance
c) Semiconductors - in the middle. Resistance depends on temp.,
light, cleanliness.
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12
2
What determines electrical resistance of a
material
- Charged particles (almost always electrons) carry
current inside materials
- Resistance of material depends on
a) Number of charged particles that are free to move and carry
current
b) Number of obstacles that charged particles might bump into
What determines electrical resistance of a
material
- Charged particles (almost always electrons) carry
current inside materials
- Resistance of material depends on
a) Number of charged particles that are free to move and carry
current
b) Number of obstacles that charge carriers might bump into
Can be controlled in a semiconductor
- Because electrons have to obey the weird rules of quantum mechanics
- At the heart of all modern electronics!
- Another system in which electrons are controlled by rules of QM?
Review of electrons, atoms and QM
Nucleus
- Protons and neutrons
- Positively charged
- Very small and dense
Electron cloud
- Negatively charged
- MUCH larger than nucleus
Quantum Mechanics
- Weird physics of very small things (like electrons)
- Electrons behave as both particles and waves
- As waves, they can only move in certain ways and have certain
amounts of energy (occupy certain allowed energy levels)
- Only 2 electrons per energy level (Pauli Exclusion Principle)
particle
Atomic structure
If the nucleus was the size of a ping-pong ball, how big
would the circumference of the atom be?
A) Size of desk
B) Size of physics building
C) Size of Campus
D) Size of Boulder
E) Size of USA
wave
Atomic structure of solids and energy bands
Atomic structure
many atoms
one atom
A) Size of desk
B) Size of physics building
C) Size of Campus
D) Size of Boulder
E) Size of USA
Energy
If the nucleus was the size of a ping-pong ball, how big
would the circumference of the atom be?
About 1mile radius, or 100,000 times bigger than nucleus
Discreet energy levels
for electrons
Electron energy levels get
shifted and shared between
all atoms and electrons
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3
Microscopic look at different materials
In solid, billions of atoms, electrons, and energy levels!!
Conduction rule: For electrons to move (when a voltage is applied) there
must be an empty energy level immediately above them
• Levels get shifted and shared for all
atoms and electrons
• So many individual levels, just talk
about bands of levels.
• 2 electrons per level until run out of
electrons
Conductor:
empty levels
very close
Semiconductor:
Half way in between.
Small jump to empties
empty
gap- no levels
Higher energy levels
are empty
Electron energy
Insulator:
Big jump
to empties.
empty
empty
Small gap
full
full
electron like ball
rolling on almost
flat ground
3
Lower energy levels –
2
mostly full of electrons
1
moves easily
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full
Electron
like a ball
in pit.
- Electron like ball
in shallow pit.
- Small boost
required to move.
Can‟t move
without big boost.
Question on energy levels in different materials
Worth “seeing” in practice
Consider 4 Different Materials W,X,Y,Z
The diagram shows the top 2 filled and lowest 2 empty bands
20
empty
full
25 eV
Energy
conductivity simulation
http://phet.colorado.edu
http://phet.colorado.edu/simulations/sims.php?sim=Conductivity
w
21
Which band structure goes with which material?
D=Diamond C=Copper G=Germanium (semi-conductor)
a. D=w, C=x, G=y
d. D=y, C= w, G=y.
b. D=z, C=w, G=y
e. D=w, C=x, G=y
y
22
D=Diamond C=Copper G=Germanium (semi-conductor)
c. D=z, C=y, G=x
z
0
Which band structure goes with which material?
empty
full
a. D=w, C=x, G=y
d. D=y, C= w, G=y.
b. D=z, C=w, G=y
e. D=w, C=x, G=y
empty
full
c. D=z, C=y, G=x
Energy
25 eV
Energy
25 eV
w
0
x
x
y
Big gap to
empty level.
Insulator
w
z
23
0
Small gap to
empty level.
Semi-conductor
x
No gap to
empty level.
Conductor
y
Big gap to
empty level.
Insulator
z
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4
Which band structure goes with which material?
a. D=w, C=x, G=y
d. D=y, C= w, G=y.
b. D=z, C=w, G=y
e. D=w, C=x, G=y
Review of semiconductors.
empty
full
D=Diamond C=Copper G=Germanium (semi-conductor)
empty
c. D=z, C=y, G=x
Like ball stuck in shallow pit
Little gap
full
e
Energy
25 eV
Big gap to
empty level.
Insulator
e.g. D
Small gap to
empty level.
Semi-conductor
w
No gap to
empty level.
Conductor
0
Big gap to
empty level.
Insulator
e.g. D
y
x
e.g. G
• Small gap between filled and empty energy levels.
• Gap is big enough to stop current (high R).
• Gap is small enough so humans can find ways to boost electrons up.
• Concept behind all modern electronics!
• Use semiconductors to control electric currents.
z
e.g. C
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Q: Which will decrease resistance of a semiconductor?
add heat, light,
shake,
add dirt
a.
T
T
T
T
b
T
T
F
F
c
T
T
F
T
d
F
T
F
F
e
F
T
F
T
Review of semiconductors.
empty
Review of semiconductors.
empty
Like ball stuck in shallow pit
full
Q: Which will decrease resistance of a semiconductor?
add heat, light,
shake,
add dirt
a.
T
T
T
T
b
T
T
F
F
c
T
T
F
T
d
F
T
F
F
e
F
T
F
T
Q: Which will decrease resistance of a semiconductor?
c
full
add heat,
T
light,
T
shake,
F
add dirt
T
Heat: gives electrons thermal energy to hop to upper band.
Light: photoconductor
Shake: energy does not go into individual electron: F
Add dirt: “doping” adds extra levels so electrons have empty levels nearby  Need
VERY pure material to be semiconductor.
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Photo-conductor: Kick electron to empty levels with light
empty
e
e
• Small gap between filled and empty energy levels.
• Gap is big enough to stop current (high R).
• Gap is small enough so humans can find ways to boost electrons up.
• Concept behind all modern electronics!
• Use semiconductors to control electric currents.
Little gap
Like ball stuck in shallow pit
Little gap
Little gap
full
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28
Photo-conductor: Kick electron to empty levels with light
empty
e
Little gap
E = energy of photon
= h x frequency (h)
= h x c / wavelength (hc/)
full
Q: If green light has just enough energy to excite an electron into an empty
level so it can conduct electricity
e
E = energy of photon
= h x frequency (h)
= h x c / wavelength (hc/)
Q: If green light has just enough energy to excite an electron into an empty
level so it can conduct electricity
a. both blue and red light will also make the PC a conductor
b. blue light will, red light will not
c. neither blue nor red light will make the PC into a conductor
b. blue light will , red light will not
Blue light: shorter , higher E than green,
more than enough energy to excite electron
Red light: longer , lower E than green,
Not enough energy to excite electron to conduction band
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Photon energy (hc/) needs to be bigger than energy of gap or it will
have no effect.
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Back to Photocopying
Photocopier summary
Photoconductor
Grounded metal
• Spread charge on photoconductor
• Use light to selectively remove some charge and make a
charge image of original
• Electrostatic attraction sticks toner to regions where charge
remains
• Use heat to bond toner to new piece of paper
• Copying is complete!
Charge photoconductor
Corona wire
velocity
+++
+++++
Document
Selective
discharge
Light
reflected
off
document
+++++
We will return to semi-conductors and their use in
electronics in a couple of weeks
Charge image
Now we understand
photoconductors and
how this step works
+++++
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