What was the difficulty in developing the blue LED? — JM, Hoboken, NJ
A light emitting diode (an LED) produces light when a current of electrons passes through the junction between its two pieces of semiconductor—from a n type semiconductor cathode to an p type semiconductor anode. The LED’s light is actually produced in the anode when an electron that has just crossed the p-n junction and is orbiting a positively charged region (called a “hole”) drops into the hole to fill it. In filling the hole, the electron releases energy and that energy becomes light through a process called fluorescence.
The energy in a particle of light (a photon) is related the color of that light—with blue photons having more energy than red photons. Here is where the difficulty in making blue LED’s comes in: to produce a blue photon, the electron in an LED must give up lots of energy as it fills the hole in the anode. This need for a large energy release places a severe demand on the semiconductors from which the blue LED is made. These semiconductors need an unusually large band gap—the energy spacing between two types of paths that electrons can follow in the semiconductor. It wasn’t until recently that good quality semiconductors with the appropriate electrical characteristics were available for this task.