Solar Power Class: Semiconductors and the P/N Junction

Holiday’s over, class, and it’s time to get back to our solar power lessons.  Today, let’s talk about semiconductors, and a slew of other things along the way!

What is a semi-conductor?  Pretty much what it sounds like.  If a substance which conducts electricity (think copper wires) is a conductor, and one that does not (rubber) is called an insulator, then a semi-conductor is right in between. Basically, atoms with only 1 or 2 electrons in their valence (outermost) orbit conduct electricity more easily than those which have a full or almost full outer valence (7 or 8).  The first are conductors, the latter are insulators.  So semiconductors are those with 3, 4, or 5 valence electrons, which means that they are neither particularly inclined nor disinclined toward conductance.

By taking silicon (which has 4 valence electrons) and doping it, as it is called, with an element which has either 3 or 5 electrons, materials with a net positive or negative charge can be created. This has to do with silicon’s natural crystal forming tendencies.  If the material used had 3 valence electrons, then the overall material has a positive charge and it is called a P-type material.  If it had 5, then the material holds a negative charge, and is called an N-type material.

All interesting enough, but things really get going when you place the two together.  The P/N Junction is the innovation that pretty much ushered in the electronic age.  The particular application of this material to solar power was actually one of the first experiments done with it, by Bell Labs back in the 50s.  When a P-type material and an N-type material are placed back to back, and a circuit is completed wiring the two pieces together, the sun excites electrons in the negatively charged layer (remember, this layer has extra electrons) and as the voltage (or pressure) builds to a point at which they overcome barrier resistance, they begin to jump to the positively charged layer where holes in the crystal structure await them.  From there, the circuit exits the P-type material and travels back to the N-type material via the wire circuit to restore the electron balance in each material.  If we hook up a battery amongst the wire circuitry, it is those bounced electrons which we store and call energy.

Okay, time for a break.  Join me back here tomorrow for the next installment of our solar power class.  In the meantime, for your reading pleasure, check out The Light Revolution, a great book about the necessary influence of the sun in our health and buildings.  Full review soon!

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