1. The Electron
By a Gentleman

2. Insulators and Conductors

3. 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

4. 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

5. Intrinsic Semiconductors
Conductivity half way between a conductor and an insulator
Crystals of pure Silica
I’m free
A photon releases an electron that now can carry current

6. Intrinsic Semiconductors
A photon releases an electron that now can carry current

7. 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

8. Intrinsic Conduction If more heat is applies the process continues…
More current…
Less resistance…
The silicon is acting as a thermistor
Its resistance decreases with temperature
Slide 8

9. 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

10. 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

11. Two semiconductor devices
1) Light dependant resistor – resistance DECREASES when light intensity INCREASES
2) Thermistor – resistance DECREASES when temperature INCREASES
Resistance
Amount of light
Resistance
Temperature

12. THE VARIATION OF THE RESISTANCE OF A THERMISTOR WITH TEMPERATURE
Digital
thermometer
10°C
Water
Heat source Ω
Glycerol

13. Method 1. 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.

14. 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.

15. Extrinsic Semiconductors
Doping is adding an element of different valency to increase conductivity of semiconductor

16. Extrinsic Semiconductors
P-type have more holes (Add Group3)

17. 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

18. Extrinsic Semiconductors
N-type have more electrons (Add Group5)

19. 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

20. Extrinsic Conduction – p-type silicon
A current will flow – this time carried by positive holes
Note:
The positive holes move towards the negative terminal

21. Junction Diode
Two types grown on the same crystal
P-type
N-type

22. Junction Diode P-type N-type
Near the junction some electrons from the ‘N’ fill the holes in the ‘P’ crystal.
P-type
N-type

23. This barrier is called the DEPLETION LAYER
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

24. - + Junction Diode P-type N-type
When the diode is in FORWARD BIAS the depletion layer disappears. The diode conducts. - +
P-type
N-type

25. - + Junction Diode P-type N-type
When the diode is in REVERSE BIAS the depletion layer increases. The diode acts as a barrier or insulator. - +
P-type
N-type

26. 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.

27. Homework
2004 HL Q12(d)

28. The p-n Junction – no potential
0.6 V
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

29. 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

30. The Reverse Biased P-N Junction
Thus, the depletion layer ( INSULATOR ) is widened and no current flows through the 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

31. 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

32. 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

33. The Forward Biased P-N Junction
At the junction electrons fill holes
Both disappear as they are no longer free for conduction
They are replenished by the external cell and current flows
This continues as long as the external voltage is greater than the junction voltage i.e. 0.6 V

34. 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
this….
The arrow shows the direction in which it conducts current
The p-n junction is called a DIODE and is represented by the symbol…

35. Diode as Valve Only allows current in one direction Forward Bias
Reverse Bias

36. LED
An LED (Light Emitting Diode) works in the same way. We use it for pin lights.
Forward Bias
Reverse Bias

37. Characteristic Curve - Diode
I/A In
reverse
Bias No
conduction
V/v
Junction Voltage (0.6V)
Must be Overcome
before Conduction starts

38. VARIATION OF CURRENT (I) WITH P.D. (V)mA
Diode in forward bias +
6 V - V

39. VARIATION OF CURRENT (I) WITH P.D. (V)
Diode in Reverse bias +
6 V - V

40. Rectifier Uses this to turn AC to DC
Mains
Resistor
This is called half wave rectification

41. Rectifier
We use a capacitor to smooth the signal to get something more like DC

42. 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.

43. Transistors
Small changes in the input signal greatly changes the size of the depletion layer
The current increases if the D.P. is small 3A 1A
10mA
30mA

44. Signal Amplification
So small changes in input signal create large charges in output.

45. Thermionic Emission Hot Metal
Electrons (as named by G. Stoney) leaving the surface of a hot metal e- e- e- e- e-
Hot Metal

46. Cathode Rays (Really Electrons)
First we heat the cathode to make the electrons jump off by Thermionic Emission
We can use a high voltage to accelerate the electrons to form a stream A N O D E
High Voltage C A T H O D E e- e-

47. 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.
Energy Gained = 1 eV A N O D E 1v C A T H O D E e- e-

48. Electron Energy
We calculate the energy of each electron first in electron volts
Energy Gained = 2000eV A N O D E
2000v C A T H O D E e- e-

49. Electron Energy Energy Gained = e.V = 1.6x10-19 . 2000
Then we convert this to joules ( Charge on the electron = e = 1.6x10-19 C)
Energy Gained = e.V = 1.6x
= 3.2x10-16 Joules A N O D E
2000v C A T H O D E e- e-

50. Electron Velocity Energy Gained = 3.2x10-16 = 0.5mv2
All the energy on an electron must be kinetic energy.
Energy Gained = 3.2x10-16 = 0.5mv2
electron mass = 9.1 × kg A N O D E
2000v C A T H O D E e- e-

51. Electron Velocity Energy Gained = 3.2x10-16 = 0.5mv2
electron mass = 9.1 × kg
3.2x10-16 = 0.5 (9.1 × 10-31) v2
V2=7x1015
V= 2.6x107 m/s A N O D E
2000v C A T H O D E e- e-

53. CRT and Demo

54. 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?

55. What is the speed of the electron at the anode
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

56. H/W
2005 OL Q10

57. X-Rays - + Thermionic Emission High Tension Voltage
Electrons jump from the surface of a hot metal cathode–
Thermionic Emission
Accelerated by high voltage they smash into tungsten target anode to produce x-rays
Most of the electron energy is lost as heat.-about 90%
X-rays very penetrating, fog film, not effected by fields.

58. Photons
Bohr first suggested a model for the atom based on many orbits at different energy levels E2 E1

59. Photons
If the electron in E1 is excited it can only jump to E2. E2 E1

60. Photons
Then the electron falls back. The gap is fixed so the energy it gives out is always the same E1
A small amount of energy in the form of an e-m wave is produced E2

61. Photons E2 –E1 = h.f Where f=frequency h= Planck’s constant
So Max Planck said all energy must come in these packets called photons.
He came up with a formula for the frequency
E2 –E1 = h.f
Where f=frequency
h= Planck’s constant E2 E1

62. Internet demo

63. X-ray Tube Energy Gained = e.V = hf = hc/λ
1.6x = (6.6 x 10-33)(3x108)/λ

64. 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.

65. H/W
HL 2010 Q9

67. Now show them the spectra of different lights using linear disperser

69. Demo Light Emission

70. 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.

71. The Problem
If you shine light on the surface of metals electrons jump off e
Polished Sodium Metal
Electrons emitted
This is The PHOTOELECTRIC EFFECT

72. 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

73. We can also prove this with the experiment below

74. Vary intensity by moving lamp back and forth

75. The Photoelectric Effect
The more intensity you gave it the more electrical current was produced
Current
(# of electrons)
Light Intensity
(# of photons)

76. Use of photocell
Light meter
Burglar alarms

77. The Photoelectric Effect
However something strange happened when you looked at frequency
Frequency of light
Electron Energy
Newtonian Physics
could not explain this

78. 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

79. Einstein’s Law Energy of incident photon = h.f = h. f0+ KE of electron
Frequency of light
Electron Energy
f0=Threshold Frequency
Energy
left over
turned
into
velocity
Energy of incident photon =
h.f = h. f0+ KE of electron
Work Function,
Energy to release Electron

80. 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 (Greater energy than work function)

81. 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.

82. H/W
2003 HL Q 9
2005 HL 12(d)

83. Lets do Homework –oh goody
2004 HL Q12(d)
2005 OL Q10
2010 HL Q9