the electron
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The Electron
By a Gentleman
Insulators and Conductors
Conduction
All conduction is due to the movement of free electrons.
+ -
I’m free
In a Semiconductor the electrons are fixed until they receive a little energy
The Silicon, Si, Atom
Silicon has a valency of 4 i.e. 4 electrons in its outer shell
Each silicon atom shares its 4 outer electrons with 4 neighbouring atoms
These shared electrons – bonds – are shown as horizontal and vertical lines between the atoms
This picture shows the shared electrons
Intrinsic Semiconductors Conduction half way between a
conductor and an insulator Crystals of Silica
I’m free
A photon releases an electron that now can carry current
Intrinsic Semiconductors
A photon releases an electron that now can carry current
Heating Silicon
We have seen that, in silicon, heat releases electrons from their bonds…
This creates electron-hole pairs which are then available for conduction
Intrinsic Conduction
If more heat is applies the process continues…
Slide 8
More heat…
More current…
Less resistance…
The silicon is acting as a thermistor
Its resistance decreases with temperature
The Thermistor Thermistors are used to
measure temperature
They are used to turn devices on, or off, as temperature changes
They are also used in fire-warning or frost-warning circuits
Thermistor Symbol
Light Dependent Resistor (LDR) The LDR is very similar to
the thermistor – but uses light energy instead of heat energy
When dark its resistance is high
As light falls on it, the energy releases electron-hole pairs
They are then free for conduction
Thus, its resistance is reduced
LDR Symbol
Two semiconductor devices
2) Thermistor – resistance DECREASES when temperature INCREASES
1) Light dependant resistor – resistance DECREASES when light intensity INCREASES
Resistance
Amount of light
Resistance
Temperature
THE VARIATION OF THE RESISTANCE OF A THERMISTOR WITH TEMPERATURE
Thermistor
Digitalthermometer
10°C
Water
Heat source
Ω
Glycerol
Method1. Set up the apparatus as shown.2. Use the thermometer to note the
temperature of the glycerol and thermistor.3. Record the resistance of the
thermistor using the ohmmeter.4. Heat the beaker.5. For each 10 C rise in temperature,
record the resistance and the temperature using the ohmmeter and the thermometer.
6. Plot a graph of resistance against temperature and join the points in a smooth, continuous curve.
Precautions
Heat the water slowly so temperature does not rise at end of experiment
Wait until glycerol is the same temperature as water before taking a reading.
Extrinsic Semiconductors Doping is adding an element of different
valency to increase conductivity of semiconductor
Extrinsic Semiconductors P-type have more holes (Add
Group3)
The Boron Atom
Boron is number 5 in the periodic table
It has 5 protons and 5 electrons – 3 of these electrons are in its outer shell
Extrinsic Semiconductors N-type have more electrons (Add
Group5)
The Phosphorus Atom
Phosphorus is number 15 in the periodic table
It has 15 protons and 15 electrons – 5 of these electrons are in its outer shell
Extrinsic Conduction – p-type silicon
A current will flow – this time carried by positive holes
Note:
The positive holes move towards the negative terminal
Junction Diode
Two types grown on the same crystal
P-type N-type
Junction Diode
Near the junction some electrons from the ‘N’ fill the holes in the ‘P’ crystal.
N-typeP-type
Junction Diode
This creates area in the middle where there are no carriers so no conduction
P-type N-type
This barrier is called the DEPLETION LAYER
Junction Diode
When the diode is in FORWARD BIAS the depletion layer disappears. The diode conducts.
P-type N-type+ -
Junction Diode
When the diode is in REVERSE BIAS the depletion layer increases. The diode acts as a barrier or insulator.
P-type
N-type
- +
2009 Question 12 (b) [Higher Level] A semiconductor diode is formed when small
quantities of phosphorus and boron are added to adjacent layers of a crystal of silicon to increase its conduction.
Explain how the presence of phosphorus and boron makes the silicon a better conductor.
What happens at the boundary of the two adjacent layers?
Describe what happens at the boundary when the semiconductor diode is forward biased
Describe what happens at the boundary when the semiconductor diode is reverse biased.
Give a use of a semiconductor diode.
Homework
2004 HL Q12(d)
The p-n Junction – no potential
As the p-type has gained electrons – it is left with an overall negative charge…
As the n-type has lost electrons – it is left with an overall positive charge…
Therefore there is a voltage across the junction – the junction voltage – for silicon this is approximately 0.6 V
0.6 V
The Reverse Biased P-N Junction
Take a p-n junction
Apply a voltage across it with the
p-type negative
n-type positive
Close the switch
The voltage sets up an electric field throughout the junction The junction is said to be reverse – biased
The Reverse Biased P-N Junction
Negative electrons in the n-type feel an attractive force which pulls them away from the depletion layer
Positive holes in the p-type also experience an attractive force which pulls them away from the depletion layer
Thus, the depletion layer ( INSULATOR ) is widened and no current flows through thep-n junction
The Forward Biased P-N Junction
Take a p-n junction
Apply a voltage across it with the
p-type postitive
n-type negative
Close the switch
The voltage sets up an electric field throughout the junction
The junction is said to be forward – biased
The Forward Biased P-N Junction
Negative electrons in the n-type feel a repulsive force which pushes them into the depletion layer
Positive holes in the p-type also experience a repulsive force which pushes them into the depletion layer
Therefore, the depletion layer is eliminated and a current flows through the p-n junction
The Forward Biased P-N Junction
At the junction electrons fill holes
They are replenished by the external cell and current flows
Both disappear as they are no longer free for conduction
This continues as long as the external voltage is greater than the junction voltage i.e. 0.6 V
The Forward Biased P-N Junction
If we apply a higher voltage…
The electrons feel a greater force and move faster
The current will be greater and will look like
The p-n junction is called a DIODE and is represented by the symbol…
The arrow shows the direction in which it conducts current
this….
Diode as Valve
Only allows current in one direction
Forward Bias Reverse Bias
LED An LED (Light Emitting Diode) works in the
same way. We use it for pin lights.
Forward Bias Reverse Bias
Characteristic Curve - Diode
V/v
I/A
Junction Emf (0.6V)Must be Overcome
before Conduction starts
In reverse
BiasNo
conduction
VARIATION OF CURRENT (I) WITH P.D. (V)
mA
V
+6 V
-
Diode in forward bias
VARIATION OF CURRENT (I) WITH P.D. (V)
+6 V
-
Diode in Reverse bias V
A
Rectifier
Uses this to turn AC to DC
This is called half wave rectification
Mains
Resistor
Rectifier
We use a capacitor to smooth the signal to get something more like DC
Amplification
On 16 December 1947 William Shockley, John Bardeen and Walter Brattain built the first practical transistor at Bell Labs
Despite hardly talking to each other.
Transistors Small changes in the input signal
greatly changes the size of the depletion layer
10mA
3A1A
30mA
The current increases if the D.P. is small
Signal Amplification So small changes in input signal
create large charges in output.
Thermionic Emission
Electrons (as named by G. Stoney) leaving the surface of a hot metal
Hot Metal
e-e-
e- e- e-
Cathode Rays (Really Electrons) First we heat the cathode to make the
electrons jump off by Thermionic Emission
CATHODE
e-e-
We can use a high voltage to accelerate the electrons to form a stream
ANODE
High Voltage
Electron Energy Units We calculate the energy of each
electron first in electron volts. The energy gained when an electron crosses a potential difference of 1Volt.
CATHODE
e-e-
Energy Gained = 1 eV
ANODE
1v
Electron Energy We calculate the energy of each
electron first in electron volts
CATHODE
e-e-
Energy Gained = 2000eV
ANODE
2000v
Electron Energy Then we convert this to joules ( Charge on the
electron = e = 1.6x10-19 C)
CATHODE
e-e-
Energy Gained = e.V = 1.6x10-19 . 2000
= 3.2x10-16 JoulesANODE
2000v
Electron Velocity All the energy on an electron must be kinetic
energy.
CATHODE
e-e-
Energy Gained = 3.2x10-16 = 0.5mv2
electron mass = 9.1 × 10-31 kg
ANODE
2000v
Electron Velocity
CATHODE
e-e-
Energy Gained = 3.2x10-16 = 0.5mv2
electron mass = 9.1 × 10-31 kg
3.2x10-16 = 0.5 (9.1 × 10-31) v2
V2=7x1015
V= 2.6x107 m/s
ANODE
2000v
CRT and Demo
2003 Question 9 List two properties of the electron. Name the Irishman who gave the
electron its name in the nineteenth century.
Give an expression for the force acting on a charge q moving at a velocity v at right angles to a magnetic field of flux density B.
An electron is emitted from the cathode and accelerated through a potential difference of 4kV in a cathode ray tube (CRT) as shown in the diagram.
How much energy does the electron gain?
What is the speed of the electron at the anode? (Assume that the speed of the electron leaving the cathode is negligible.)
After leaving the anode, the electron travels at a constant speed and enters a magnetic field at right angles, where it is deflected. The flux density of the magnetic field is 5 × 10–2 T.
Calculate the force acting on the electron. Calculate the radius of the circular path
followed by the electron, in the magnetic field. What happens to the energy of the electron
when it hits the screen of the CRT? mass of electron = 9.1 × 10–31 kg; charge on
electron = 1.6 × 10–19 C
H/W
2005 OL Q10
X-Rays
Electrons jump from the surface of a hot metal –
Thermionic Emission
Accelerated by high voltage they smash into tungsten
Most of the electron energy is lost as heat.-about 90%
X-rays very penetrating, fog film, not effected by fields.
High Tension Voltage
Photons Bohr first suggested a model for
the atom based on many orbits at different energy levels
E1E2
Photons
If the electron in E1 is excited it can only jump to E2.
E1E2
Photons Then the electron falls back.
The gap is fixed so the energy it gives out is always the same
E1
E2
A small amount of energy in the form of an e-m wave is produced
Photons So Max Planck said all energy must
come in these packets called photons. He came up with a formula for the
frequency
E1E2
E2 –E1 = h.f
Where f=frequency
h= Planck’s constant
2006 Question 12 (d) [Higher Level] The first Nobel Prize in Physics was awarded in
1901 for the discovery of X-rays. What are X-rays? Who discovered them? In an X-ray tube electrons are emitted from a
metal cathode and accelerated across the tube to hit a metal anode.
How are the electrons emitted from the cathode?
How are the electrons accelerated? Calculate the kinetic energy gained by an
electron when it is accelerated through a potential difference of 50 kV in an X-ray tube.
Calculate the minimum wavelength of an X-ray emitted from the anode.
H/W
HL 2010 Q9
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
Now show them the spectra of different lights using linear disperser
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
Demo Light Emission
Albert Einstein Uncle Albert was already a
published scientist but the relativity stuff had not set the world alight.
He set his career in real motion when he solved a problem and started the science of Quantum Mechanics that the old world Jew in him could never come to terms with.
The Problem If you shine light on the surface
of metals electrons jump off
Polished Sodium Metal
e
e
e
e
e
• Electrons emitted• This is The PHOTOELECTRIC
EFFECT
A charged Zinc plate is attached to an Electroscope
When a U.V. lamp is shone on the plate the leaf collapses as all the electrons leave the surface of the zinc
We can also prove this with the experiment below
Vary intensity by moving lamp back and forth
The Photoelectric EffectThe more intensity you gave it the
more electrical current was produced
Current
(# of electrons)
Light Intensity
(# of photons)
Use of photocell Light meter Burglar alarms
The Photoelectric EffectHowever something strange
happened when you looked at frequency
Frequency of light
Electron Energy
Newtonian Physics could not explain this
So we define the Photoelectric effect as:-
Electrons being ejected from the surface of a metal by incident e-m radiation of a suitable frequency.
Albert used Planck’s theory that as energy came in packets each packet gives energy to 1 electron only
A small packet would not give the electron enough energy to leave
Low frequency light had too small a parcel of energy to get the electron free. Energy of each
photon = h.f
Einstein’s Law
Frequency of light
Electron Energy
f0=Threshold Frequency
Energy of incident photon =
h.f = h. f0+ KE of electron
Work Function,Energy to release Electron
Energy left over
turnedinto
velocity
Einstein's Explanation
Waves come in packets called photons Energy of a photon only depends on it’s
frequency One photon gives all it’s energy to one
electron If the energy is greater than the work
function the electron escapes Incident Photon must be above a
threshold frequency
2004 Question 9 [Higher Level] Distinguish between photoelectric emission and
thermionic emission. A freshly cleaned piece of zinc metal is placed on the
cap of a negatively charged gold leaf electroscope and illuminated with ultraviolet radiation.
Explain why the leaves of the electroscope collapse. Explain why the leaves do not collapse when the zinc is
covered by a piece of ordinary glass. Explain why the leaves do not collapse when the zinc is
illuminated with green light. Explain why the leaves do not collapse when the
electroscope is charged positively. The zinc metal is illuminated with ultraviolet light of
wavelength 240 nm. The work function of zinc is 4.3 eV. Calculate the threshold frequency of zinc. Calculate the maximum kinetic energy of an emitted
electron.
H/W
2003 HL Q 9 2005 HL 12(d)
Lets do Homework –oh goody 2004 HL Q12(d) 2005 OL Q10 2010 HL Q9
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