Normally, we use a "conductor" made out of iron or copper to allow the electricity to flow from one gizmo to another. Sometimes we cross our wires in such a way that it might not be entirely predictable which direction electricity will flow. To ensure it flows in the direction we want it to, we use something called a "diode." A diode is a "semi-conductor," which means sometimes it allows electricity to flow, and sometimes it does not. In order to accomplish this, we use two ionic compounds (material that might have more electrons than protons or vice versa) with opposite charges that, together, may allow elecricity to flow one way, but not the other. The idea is to connect the two compounds, but not let them react with each other, otherwise, they'd not do what we want, so we mix silicon to accomplish this. If you mix boron (3 outer electrons) with silicon (4 outer electrons), you end up with a compound that's P-type (positive) by 1 missing electron. If you mix phosphorus (5 outer electrons) with silicon, you end up with 9 outer electrons (rather, 1 extra on the outside) which becomes N-type (negative). Stick them together, and you end up with something interesting callde a "diode."

So, the P-type as a single electron sized hole per atom pair, while the N-type has an extra electron per atom pair. If the ground side is connected to the P side of the diode, the p-side is going to try to get the electron from the ground, so the "hole" will float to the ground side, and the vice versa to the otherside. This means that, although the electrons try to go from the negative side to the positive side of the circuit, with the same amount of effort the electrons and holes in the diode are going the opposite direction (towards the pools that they're attracted to), which creates a "reverse bias" voltage of the same degree. Basically, it's like pushing putting both your hands together and comitting to a pushing match: neither arm has enough strength to beat the other, since it's the same body, thus current cannot flow.

But, cutting the wire would have the same effect, so you would assume, then that flipping the diode around would then allow electricity to flow. Basically, since opposites attract (creating the problem above), non-opposites repel, which pushes the hole and extra electrons to the middle (the "junction"), where things are going with the grain instead of against it.

Please note, excessive voltage can create a scenario where the diode's natural ability to reverse bias is too little, thus it ends up going the other way, and at said voltage, usually the diode ends up getting fried from excessive heat produced by the conflict. Semi-conductors are "voltage sensitive" for this reason.

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