Gallium nitride

Gallium nitride, GaN, is a hard, clear crystal that is a semiconductor. Its key property is a wide band gap, about 3.4 electron volts: electrons need a big jump of energy to start conducting, and when they fall back they can give off ultraviolet light, or, with some indium added, blue. That made it the material that finally delivered bright blue LEDs.

It can't simply be melted and grown like silicon. Instead it's built up as thin layers from gases, a method called metal-organic chemical vapor deposition (MOCVD). Inside a reactor, gallium arrives as the vapor of an organic gallium compound, trimethylgallium, and ammonia supplies the nitrogen. Over a sapphire wafer heated to around 1,000 °C, the molecules break down and gallium and nitrogen atoms join into crystal, layer upon layer. Small amounts of silicon or magnesium can be added so the layer carries extra electrons or holes.

In a blue LED chip, gallium nitride layers carry current to and from the light-making region. It's also the host crystal for indium gallium nitride, the layers that glow. Beyond LEDs, gallium nitride transistors switch power very efficiently, which is why small, powerful phone chargers often advertise GaN.