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 Thermistors are used to measure temperature They are used to turn devices on, or off, as temperature changes They are used to turn devices on, or off, as temperature changes They are also used in fire-warning or frost- warning circuits 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 Digital thermometer 10°C Water Heat source Ω Glycerol
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.
Precautions Heat the water slowly so temperature does not rise at end of experiment 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. 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-typeN-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-typeN-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-typeN-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.
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 the p-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 BiasReverse Bias
LED An LED (Light Emitting Diode) works in the same way. We use it for pin lights. Forward BiasReverse Bias
Characteristic Curve - Diode V/ v I/ A Junction Emf (0.6V) Must be Overcome before Conduction starts In reverse Bias No 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 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 3A 1A 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-e- e-e- e-e-
Cathode Rays (Really Electrons) First we heat the cathode to make the electrons jump off by Thermionic Emission First we heat the cathode to make the electrons jump off by Thermionic Emission CATHODECATHODE e-e- e-e- We can use a high voltage to accelerate the electrons to form a stream ANODEANODE 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. CATHODECATHODE e-e- e-e- Energy Gained = 1 eV ANODEANODE 1v
Electron Energy We calculate the energy of each electron first in electron volts CATHODECATHODE e-e- e-e- Energy Gained = 2000eV ANODEANODE 2000v
Electron Energy Then we convert this to joules ( Charge on the electron = e = 1.6x10 -19 C) CATHODECATHODE e-e- e-e- Energy Gained = e.V = 1.6x10 -19. 2000 = 3.2x10 -16 Joules ANODEANODE 2000v
Electron Velocity All the energy on an electron must be kinetic energy. CATHODECATHODE e-e- e-e- Energy Gained = 3.2x10 -16 = 0.5mv 2 electron mass = 9.1 × 10 -31 kg ANODEANODE 2000v
Electron Velocity CATHODECATHODE e-e- e-e- Energy Gained = 3.2x10 -16 = 0.5mv 2 electron mass = 9.1 × 10 -31 kg 3.2x10 -16 = 0.5 (9.1 × 10 -31 ) v 2 V 2 =7x10 15 V= 2.6x10 7 m/s ANODEANODE 2000v
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
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.
Albert Einstein Uncle Albert was already a published scientist but the relativity stuff had not set the world alight. 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. 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
The Photoelectric Effect However 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 f 0 =Threshold Frequency Energy of incident photon = h.f = h. f 0 + KE of electron Work Function, Energy to release Electron Energy left over turned into 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.