p-n junction
By DarthVader
Date: 2022-06-14
Topic: 129 see comments
Post views: 1069
p-n junction
semiconductors are made from silicon, germanium, and gallium arsenide.
Silicon atoms have 4 electrons in their valence shell, they bond to four other silicon atoms forming a crystaline structure.
Higher temperatures even those such as room temperature can provide enough energy to break some of the bonds and free up some electrons to travel through the structure.
Doping the silicon with phosphorous atoms which contain 5 electrons in their valence shell, provides more free electrons and thus more negative charge carriers. This is called n-type dopant.
Doping the silicon with boron which contains 3 outer electrons in its valence shell, creates more gaps known as ‘holes', where an electron can sit. This provides more positive charge carriers and is known as p-type dopant.
How a p-n junction works
- When an n-type material and p-type material are joined, they form a special region in the center where they meet.
- At this region electrons will flow from the n-type material over to the p-type material to fill the ‘holes’ which are created by atoms searching for a free electron to bond with.
- This means there is now a net positive charge in the n-type material, and a net negative charge in the p-type material which creates a potential difference.
- This creates an electric field between the 2 sides. (negative & positive).
- This electric field then forces the mobile electrons in the n-type material to the periphery and similarly also forces the mobile ‘hole's' to the periphery of the p-type material. Effectively pumping apart any seperated charge within the depletion zone.
- This creates an area known as the ‘depletion zone’ in the center where the 2 materials meet.
- In order to allow a current to flow across the depletion zone, an anode must be connected to the p-type region and a cathode connected to the n-type region, once connected to a circuit, a voltage high enough to overcome the electric field in the depletion zone must be applied.
- Current can flow in the forward bias across the depletion zone but in the reverse bias, the depletion zone is widened, and current cannot flow. (This is the basis of a diode)
PV Cell
When a semiconductor is exposed to a source of photons from light, some of the photons are absorbed and their energy is used to release some electrons from their parent atoms, creating pairs of free electrons and holes.
At the same time, other free electrons are reunited with holes and the energy that was previously used to free these electrons is released, usually in the form of heat.
As long as the rate of electron-hole production is equal to the recombination rate, the situation remains steady.
In order to make a working PV device, it is necessary to find a way of preventing this rapid recombination of eletrons and holes so that the electrons can be diverted into an external ciruit.
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