A light-emitting diode is related to a transistor. It turns electrical energy into light energy directly, without anything getting hot; in fact, they come close to the ideal of turning all of the electrical energy input into visible light. Their sensitivity to the direction of the current is inherent to the way they work; when you connect them to a battery the wrong way, no current flows and nothing happens at all. The bidirectional L.E.Ds (that give different colors for different polarity) are actually two diodes in one container.

The electrical behavior of a light emitting diode is not simple. It will not do anything at all unless the voltage is more than 1.6 V (in contrast, a light bulb with low voltage is still glowing, but very dim and red). We cannot define an electrical resistance for these devices -- we could claim that they have a resistance that changes with the applied voltage, but this is not a very useful concept.

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