1. Conductors – many electrons free to move
Insulators – few free electrons
Semi-conductors – insulators when pure but can be made conductive by adding impurities (doping), stimulating with heat, light, or a voltage
Intrinsic (pure ) semi-conductor has 4 electrons in the outer shell all form bonds unless there is a disruption to the crystal pattern.

2. Doping
p – type semiconductor
n – type semiconductor
Charge carrier - electron
+ -
Charge carrier - hole
+ -

3. + - p n +
- Forward bias -
+ Reversed bias - + p n

4. Experiment PN Junction Diode Characteristics

5. Light emitting diodes
Forward biased pn junction
When electron hole pairs recombine they give out energy E = hf e + - p n
If the energy is large enough the frequency of the light will be visible

6. Photodiode – reacts very quickly to light
Photovoltaic mode (forward biased)
When light shines on the junction electrons are given enough energy to escape the atom forming electron hole pairs. This gives a voltage across the junction. There will be no other power source in the circuit.
Used in solar cell for calculators etc.
Photoconductive mode (reverse biased)
When light shines on it the resistance decreases as electrons are given enough energy to escape the atom so the number of charge carriers increases. The current in the circuit will increase.
Current is proportional to irradiance

7. Transistors
NPN
PNP b
+0.7 V
e at 0V
c connected to +Vs I b
- 0.7V
c connected to - Vs
e at 0V I
MOSFET
Metal Oxide Semiconductor Field Effect Transistor
drain
source
gate

8. MOSFET – when the potential difference between the source (0V) and the gate is greater than a threshold value (usually about 2V) it allows current to flow.
p-type substrate n
source
gate
drain
metal
Oxide layer
semiconductor