Is resistance what makes a diode have a voltage drop?
Or is it impedence?
If it is electrical energy being converted to light, please tell me what this effect is called.
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The forward biased semiconductor junction takes a voltage level to be able to push the electronic charges over the P-N zone. Think of it as similar to how you have to "lift" each marble up to the table top from on the floor. In addition to the energy level difference needed to transport charge across the junction there is also a resistive part of the diode that drops some voltage as well. The resistive drop in the diode will be dependent upon the amount of current flow allowed through the junction.
This is due to the inherent Depletion Region in the PN Junction. http://en.wikipedia.org/wiki/Depletion_region
It's a little confusing, but essentially it is due to a loss of carriers (electrons and holes). Holes are simply the place where an electron "Could" be, but think of it like a small zone where there is No charge carriers, or like a small strip of insulation.
You need voltage(force) to push the electrons over this area.
When you reverse bias a diode, this depletion region becomes even BIGGER.