The Great White LED Mystery

Have you come to any conclusions about the process of illumination from the white LED? Maybe, you say, a wide conduction band with a wide valence band? Let us find out by doing a few experiments.

One hypothesis may be that the white LED is made up of three different LEDs: red, green, and blue and constructed so that when the LED is turned on, the three LEDs light up simultaneously. We can use the data from a previous experiment to test this by gradually turning up the voltage supplied to the LED to see if that is the case.

Does the white color LED turn on at the lowest of the threshold voltages of the three colors, red, green, and blue?

Find any similarities in the threshold voltage of white LED and another color LED.

We find that the white color LED turns on not at the lowest of the three voltages, but the highest, namely blue. So the solution is not as simple as just a combination of three LEDs. Can you infer any hypotheses from this observation?

We know that this white LED phenomenon probably has something in connection with a blue LED. So let us perform another observation. Look closely at the white LED when it is barely lit. What color do you see in the middle of the LED assembly?

You should have seen a spot of blue light in the middle surrounded by a circle of white light. Wait a minute, blue light? Isn't this a white LED? Isn't a white LED supposed to emit white light and only white light? Well, these white LEDs obviously do not follow the model of traditional LEDs. These white LEDs work by having blue LED material in the middle surrounded by fluorescence material. Remember fluorescence? Fluorescence materials absorb light energy to emit light. It is this phenomenon that produces the bright white color with a tint of blue in the center.