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The Electron. By a Gentleman. -. +. 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. Intrinsic Semiconductors. I’m free. Conductivity half way between a conductor and an insulator
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The Electron By a Gentleman
- + 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
Intrinsic Semiconductors I’m free • Conductivity half way between a conductor and an insulator • Crystals of pure Silica • A photon releases an electron that now can carry current
Extrinsic Semiconductors • These contain doping with atoms of different valency Doping is adding a material of a different valency to increase the conductivity.
Digital thermometer 10°C Ω Water Thermistor Glycerol Heat source THE VARIATION OF THE RESISTANCE OF A THERMISTOR WITH TEMPERATURE
Junction Diode • Near the junction some electrons from the ‘N’ fill the holes in the ‘P’ crystal. P-type N-type
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
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
Diode as Valve • Only allows current in one direction Forward Bias Reverse Bias
Characteristic Curve I/A In reverse Bias No conduction V/v Junction voltage (0.6V) Must be Overcome before Conduction starts
VARIATION OF CURRENT (I) WITH P.D. (V) mA Diode in forward bias + 6 V - V
VARIATION OF CURRENT (I) WITH P.D. (V) A Diode in Reverse bias + 6 V - V
Rectifier • Uses this to turn AC to DC Mains Resistor • This is called half wave rectification
Rectifier • We use a capacitor to smooth the signal to get something more like DC
Thermionic Emission • Electrons leaving the surface of a hot metal e- e- e- e- e- Hot Metal
A N O D E e- High Voltage Cathode Rays • First we heat the cathode to make the electrons jump off by Thermionic Emission We can use a voltage to accelerate the electrons to form a stream C A T H O D E e-
A N O D E e- 2000v Electron Energy • We calculate the energy of each electron first in electron volts EnergyGained = 2000eV C A T H O D E e-
A N O D E e- 2000v Electron Energy • Then we convert this to joules ( Charge on the electron = e = 1.6x10-19 C) Energy Gained = e.V = 1.6x10-19 . 2000 = 3.2x10-16 Joules C A T H O D E e-
A N O D E e- 2000v Electron Velocity • All the energy on an electron must be kinetic energy. Energy Gained = 3.2x10-16 = 0.5mv2 C A T H O D E e-
High Tension Voltage 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.-about90% X-rays very penetrating, fog film, not effected by fields.