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.

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

+ - p n + - Forward bias - + Reversed bias - + p n

Experiment PN Junction Diode Characteristics

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

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

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

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