Download N-type semiconductor

Exam Tonight, 7:30 PM, Muenzinger Psychology E-0046
Bring calculator, pencils, and note cards (two 3x5 cards)
About the Exam:
•
Format same as Exam 1: Part multiple choice, part essay
•
Exam focuses on material covered since Exam 1, but you should review
previous material since we have been building on physics from beginning of
semester.
•
best preparation- go over homework and solutions, also class notes
(particularly questions in class), and be sure you understand answers.
•
Note Cards: good way to study is to make list of most important ideas and
formulas needed.
•
Homeworks to focus on: Sound Waves, Circuits (Audio
Amplifiers and Diodes), TVs, Sunlight and Color
TV …
Create beam of electrons accelerating towards screen.
Hit screen, excite phosphor, create light.
Big V
GRID
ANODE, high voltage,
lots of excess positives
Phosphor
Accelerating Coating
Anode, lots of
excess
positives
+
+
+
+
+
+
+
+
+
+
+
CATHODE: heated a bunch,
boil off electrons … so now free in
vacuum tube.
+
Each electron hits screen creates a photon of light. Grid controls number of
electrons in beam. How and why?
a. Add a bunch of excess negative charges on grid. Electrons trying to
escape pushed back inside.
b. Add a current through grid to create magnetic field. Electrons trying to
escape pushed back inside.
c. Add a bunch of excess positive charges on grid. Electrons sucked up by
grid.
TV …
Create beam of electrons accelerating towards screen.
Hit screen, excite phosphor, create light.
GRID
Phosphor
Accelerating Coating
Anode, lots of
excess
positives
+
ANODE, high voltage,
lots of excess positives
+
+
+
+
+
+
+
+
+
+
-- - -- - CATHODE: heated a bunch,
boil off electrons … so now free in
vacuum tube.
+
Answer is a. Add bunch of negatives to grid! Pushes electrons back
towards cathode … fewer into beam.
Control where beam hits screen with magnetic fields!
TV’s…. Some questions on interaction of magnetic fields and
electrons
Review Magnetic Field Lines –Magnetic field lines for Bar Magnet
How does the magnetic field interact
with electrons and other magnets?
a. A stationary electron placed at
point A will move down.
b. A stationary electron placed at
point A will move to the right.
c. A small bar magnet place at A will
rotate to line up with the magnetic
field
d. Both a and c
e. Both b and c
N
A
S
Permanent Bar magnet
S
Answer is c only.
N
Magnetic field line points in direction of force on a North Pole. So at A, north pole
feels force down, south pole feels force up.
No force on stationary electrons!
Magnetic fields only put forces on MOVING electrons, as in electron beams in TVs.
(Why? … Because that’s how nature works … we observe, describe observations.)
Forces on moving electrons by magnetic fields:
An electron is moving at speed V through a uniform magnetic field.
In which case is the force exerted by the magnetic field on the moving electron
greatest?
C)
A)
Greatest B)
V
e-
force
V
e-
Some force, part
of velocity is
perpendicular to
magnetic field
V
e-
No force on
electron, velocity
is NOT AT ALL
perpendicular to
magnetic field
D) The electrons in A, B, and C all experience the same size force
because the magnetic field is the same strength in all cases.
E) Constant magnetic fields cannot exert forces on charges.
Correct answer is A.
Force
Amount of force on Electron
electron depends on Velocity
how perpendicular
velocity is to
Magnetic field
magnetic field.
e-
V
TV …
Create beam of electrons accelerating towards screen.
Hit screen, excite phosphor, create light.
GRID
Phosphor
Coating
+
+
Deflecting
Coils (A)
+
+
+
+
+
+
+
+
+
Deflecting
Coils (B)
+
TV’s…. Some questions on interaction of magnetic fields and
moving electrons
Can you create a steady magnetic field with a coil of
wire?
N
a. Sure, Spin coil or move coil up and down rapidly
b. Sure, Send steady current through coil
c. Sure, Send steadily changing current through coil
d. No, It is not possible
S
Answer is B. Steady current produces steady magnetic field. (Remember
creating electromagnet in lab.. Picking up washers.
S
Larger current, stronger magnetic field
Smaller current, weaker magnetic field
No current, no magnetic field
Reverse current, magnetic field in opposite direction …
N
TV …
Create beam of electrons accelerating towards screen.
Hit screen, excite phosphor, create light.
+
+
Deflecting
Coils (A)
GRID
Phosphor
Coating
+
+
+
+
+
+
+
+
+
Deflecting
Coils (B)
Current through Coils A
(same current, same
direction, through both)
Vertical magnetic field.
Beam deflected horizontally
+
Electromagnetic Radiation Review:
Generated by accelerating charged particles…
oscillating electrons in a TV antenna or
oscillating electrons at the surface of the sun.
Wave of changing electric field … electric field puts force on an electron
(and magnetic field) traveling away from source. Characterized by a
wavelength and frequency.
Source
Periodic oscillations
in the electric field.
e-
What do radio waves and uv light have in common?
a. Both are propagating waves of changing electric field.
b. Both travel at the speed of light through space
c. Both are generated by accelerating charged particles
d. All of the above
Travels away
from source at
the speed of
light
(c = 3 x 108 m/s)
Electromagnetic Radiation Review:
Generated by accelerating charged particles…oscillating electrons in a TV
antenna or oscillating electrons at the surface of the sun.
Wave of changing electric field (and magnetic field) traveling away
from source. Characterized by a wavelength and frequency.
Source
Periodic oscillations
in the electric field.
e-
Travels away
from source at
the speed of
light
(c = 3 x 108 m/s)
What do radio waves and uv light have in common?
d. All of the above. These are characteristics common to all forms of electromagnetic
radiation (Radio Waves, Microwaves, Infrared (heat), Visible light, UV light, X-rays,
Gamma rays).
What is different between these forms of EM radiation?
Wavelength. Frequency.
Amount of energy per packet of radiation (photon).
Which of the following statements are true:
1. All EM (electromagnetic) radiation comes in small packets called photons.
2. Every blue photon has the same amount of energy as every other photon of
that exact same blue color.
3. It is possible for a red photon to have the same amount of energy as a blue
photon.
4. If I measure the energy of the red light emitted by a red LED, it is possible to
measure any amount energy 0 J and higher.
a. 1
b. 1 and 2
c. 1 and 3
d. 1, 2, and 4
e. 1, 3, and 4
Which of the following statements are true:
1. All EM (electromagnetic) radiation comes in small packets called photons.
True: A photon is a unit of light energy. Can have one photon, two photons,
etc. Cannot have 1.5 photons of red light.
2. Every blue photon has the same amount of energy as every other blue photon.
True: A photon of a specific color of light has a specific energy. All photons of
that exact color of light will have that same amount of energy… no more, no
less.
3. It is possible for a red photon to have the same amount of energy as a blue
photon.
False: For each frequency of electromagnetic radiation (Radio waves,
Microwaves, visible, UV), the energy in a photon of EM radiation at that
frequency is different. This energy is given by
Energy of photon = Planck’s constant x frequency = 6.626x10^-34 x frequency
Speed of light = frequency x wavelength
4. If I measure the energy of the red light emitted by a red LED, it is possible to
measure any amount energy from 0 Joules and higher.
False: Detect one photon of energy, two photons, three photons, 1,000,000
photons, but never 1,000,000.1 photons.
a. 1
b. 1 and 2
c. 1 and 3
d. 1, 2, and 4
e. 1, 3, and 4
White paint has little particles of transparent material in it to
make paint look white. The main reason this works is
because:
a. They add texture to the surface it is not smooth
b. They each reflect little bit of light because speed of light in
particles is slower than in base material of paint.
c. They each reflect all of the light that hits them, but
oriented in different directions so on average wall looks
white.
d. Little particles have no function.
magnify
Answer is b. Light changes speed going into particles… a
little bit of light reflects off each particle, enough particle in
paint to reflect all the light. Any light not reflected is
absorbed (dye absorbs light). Energy turns into heat!
Circuits: understand where electrons are flowing and why.
Components: Resistors, Transistors, Capacitors, Diodes
LED
Audio Amplifier
+9V Power Source
+
50 Ohm
Resistor
B
A
Gate
-
Electron
Flow
N
P
C D
S
N
Gate
Permanent
Magnet
E
Ground
Diode:
Joined P-Type
and N-type
semiconductor
Diode Review:
• Pass current in only one direction.
• Junction of P doped and N doped semiconductors
Pure Semiconductor N-type semiconductor
empty
empty
full
full
Mostly Silicon w/
tiny fraction Phosphorus
P-type semiconductor
empty
full
1 empty level
per boron
atom
Mostly Silicon w/
tiny fraction Boron
Si, 4 electrons go into
valence band, just fill
What is required for material to be a good conductor?
a. A small energy gap between the bands
b. A band that is completely full of electrons so a lot of electrons can flow
c. Empty energy levels very, very close in energy to the highest energy electrons
Diode Review:
• Pass current in only one direction.
• Junction of P doped and N doped semiconductors
Pure Semiconductor N-type semiconductor
empty
empty
full
full
Mostly Silicon w/
tiny fraction Phosphorus
P-type semiconductor
empty
full
1 empty level
per boron
atom
Mostly Silicon w/
tiny fraction Boron
Si, 4 electrons go into
valence band, just fill
What is required for material to be a good conductor?
Answer is c. Empty energy levels very, very close in energy to the highest energy
electrons. Both N-type and P-type semiconductors satisfy this requirement.
Diode Review:
• Electrons flow N to P only. One direction.
P-type
N-type
empty
empty
full
full
To
+Voltage
Electrons can flow from
N-type to P-type
In other direction, voltage can’t give electrons enough
energy to jump band gap so current does not flow.
Light emitting diode.
One photon with each electron that passes across P, N junction.
If current is steady, bigger energy gap between bands will mean:
a. brighter light , b. dimmer light
c. different color light
d . Both a and c.
e. both b and c.
Important things to keep in mind about circuits:
Electrons always want to flow from lower voltage
to higher voltage (towards excess positive charge).
If no voltage difference, no reason for electrons to flow
Rate of electron flow (current) from lower to higher
voltage depends on resistance between these two points
Electrons are attracted to excess positive charge
Electrons will repel each other (like charges repel)
If both of these then force of attraction to
positives will balance force of repelling of each
other, else electrons will move (flow).
Voltage, Current, and Resistance
If R1 and R2 are both 50 ohms, then voltage at B is 4.5 Volts… electrons
have used up half of their 9 Volts of energy by the time they get to B
+9 V
What if R2 decreases, what changes?
a. Current increases, Voltage diff. across R1 increases,
and Voltage at B is lower
b. Current decreases, Voltage diff. across R1 decreases,
Voltage at B is higher
c. Current increases, Voltage diff. across R1 increases,
R1 =
and Voltage at B is higher
50 Ohms
d. Current decreases, Voltage diff. across R1 decreases,
Voltage at B is lower
B
R2 =
50 Ohms
GROUND
(0 V)
Voltage, Current, and Resistance
If R2 decreases, what changes?
a. Current increases because total
resistance of path is lower!
(Rtotal = R1 + R2). Rtotal less.
V=IR so I = Vtotal/Rtotal ; Vtotal = 9V.
+9 V
R1 =
50 Ohms
With larger current, Voltage Diff across
R1 increases. (each electron loses more
energy lost across R1 now.)
V_across R1 = IR.
If more voltage is lost across R1, Voltage
at B decreases
B
R2
GROUND
(0 V)
Transistors control current flow by changing resistance:
Control current by varying amount of positive charge at Gate (A)
A
Gate
 
Gate
NO Charge at Gate
Current valve closed
Infinite Resistance
I=0
+++
A
I Lots of positive
charge at Gate
Current Valve Open
Small Resistance
I is big
Adjust positive charge on Gate to get current in between I = 0 and I = big
NOTE: No current or electron flow across here.
There is an insulator there.
Voltage, Current, and Resistance
Adding more positive charge to Transistor’s Gate is like lowering
resistance of R2. Effect is increase current and lower voltage at B.
+9 V
+
A
Gate
What voltages are possible at B?
a. any voltage
b. any voltage greater than 0 V
c. 0 to 20 V
d. 0 to 9 V
R1 =
50 Ohms e. 1 to 8 V
Answer is d. 0 to 9 V.
B
0 V if transistor is wide open… no
resistance from transistor and current
limited by R1.
R2
9 V if transistor resistance is infinite… no
current flow at all.
GROUND
(0 V)
Capacitors – two metal plates that store charge;
insulator in between plates.
No current or electron flow
across here.
+9V “Above Ground”
Adjustable
Power Supply
GROUND
(0 V)
Capacitor Review:
What happens when I close the switch what happens to the bulb?
a. Bulb turns on and stays at constant brightness until open switch again.
b. Bulb turns on, dims steadily, then is off.
c. Bulb turns on and has steady brightness until off.
d. Bulb does not light, no current because no complete circuit.
-- -- -- -- -- -- -+++++++
+++++++
A B
+9V
-  -  - -
-  -  - -
-  -  - -
Adjustable
Power Supply
GROUND
(0 V)
-  -  - -
+9V “Above Ground”
Capacitor Review:
b. Electrons flow for brief period of time and then stop.
Current slows as voltage difference between capacitor
plate and ground decreases. (V = IR ; I =V/R)
Current stops when no voltage difference between
capacitor plate A and positive terminal of power supply
(both at +9 V) and no voltage difference between
capacitor plate B and Ground. (both at 0 V)
-- -- -- -- -- -- -+++++++
+++++++
A B
+9V
0V
+9V “Above Ground”
Adjustable
Power Supply
GROUND
Current
What happens when I close the switch?
0
time after voltage
hooked up.
Capacitor Review:
1) Lower Voltage of Power Supply
2) Plate A is at higher voltage than Supply.
Electrons flow towards Plate A until voltage
difference is 0. Excess positive charge on
Plate A will decrease.
3) Attraction of electrons on Plate B to
positive charge on Plate A has decreased.
Force of electrons repelling each other is
greater than attraction to Plate A, so some
electrons leave … flow back to Ground.
-  -  - -
+4V
9V “Above Ground”
Adjustable
Power Supply
-  -  - -
GROUND
-  -  - -
-  -  - -
-- -- -- -- -- -- -+++++++
+++++++
A B
+4V
+9V
+0V