Gallium nitride

Gallium nitride is a wide band gap semiconductor, and almost everything it is used for follows from that one number. Its band gap is 3.4 electron volts where silicon's is 1.1, and the gap sets both the colour of light a material can emit and the electric field it can withstand before breaking down.

The colour half gave the world the blue LED, and with it white light — a blue LED behind a yellow phosphor is what every white LED and every LED-backlit screen actually is. Red and green LEDs had existed since the 1960s and blue had not, and without blue there is no white, so lighting stayed incandescent and fluorescent for thirty years for want of one material.

The voltage half is the current one. GaN withstands roughly ten times the electric field silicon does, so a device can be a tenth as thick for the same voltage, which makes it faster and lower-loss. That is why a modern phone charger is a third the size of the one from five years ago: it switches at a much higher frequency, so the transformer inside it can be much smaller.

A wide band gap is the whole story, read three ways.

Light. An electron falling across the gap emits a photon of that energy, so the gap sets the colour. 3.4 electron volts is ultraviolet, and alloying gallium nitride with indium tunes it down through violet and blue into green — which is how one material system covers the short end of the visible spectrum.

Voltage. The same strong bonding that makes the gap wide makes the material hard to ionise, so it withstands about ten times silicon's electric field before avalanche breakdown. A device blocking 600 volts can therefore be built from a much thinner layer, and a thinner layer has less resistance and less stored charge — which means lower conduction loss and much faster switching.

Heat. The wide gap also means far fewer thermally generated carriers, so GaN keeps working at temperatures where silicon starts conducting when it should not.

The reason it took thirty years is growth rather than physics. GaN decomposes rather than melting, so it cannot be pulled from a melt the way silicon is, and until recently there were no sizeable GaN wafers to grow it on. It has to be deposited from vapour onto a foreign substrate — sapphire, silicon carbide or silicon — whose atomic spacing does not match, which fills the layer with dislocations.

Isamu Akasaki and Hiroshi Amano solved it in the late 1980s with a low-temperature buffer layer that absorbed the mismatch, and then solved p-type doping, which had defeated everyone: the magnesium dopant was being passivated by hydrogen, and an electron beam or a thermal anneal drove the hydrogen off and switched the conductivity on. Shuji Nakamura, working at a small Japanese chemical company, turned it into a manufacturable bright blue LED by 1993. The three shared the 2014 Nobel Prize in Physics.

Lighting, which is where its mass impact is. Essentially every white LED is a GaN blue emitter behind a phosphor, and the displacement of incandescent and fluorescent lighting by LED is among the largest single energy efficiency changes of the century — lighting fell from roughly a fifth of world electricity use towards a small fraction of that.

Displays. The blue subpixel of an LED-backlit or micro-LED screen, and the blue laser that reads a Blu-ray disc.

Power conversion. Chargers, adapters, data-centre power supplies and increasingly onboard vehicle chargers, at voltages up to about 650 volts, where GaN's speed shrinks the magnetics around it.

Radio frequency. GaN has taken most of the high-power RF market from gallium arsenide and from travelling-wave tubes: 5G base stations, radar, and satellite transmitters, where it delivers far more power per millimetre of device width.

What it has not taken is high-voltage traction inversion in electric vehicles, which is silicon carbide's. Above about a kilovolt GaN's advantage over SiC narrows and SiC's vertical device structure and thermal conductivity win.

GaN's commercial story is about substrates rather than about the compound.

Growing it on sapphire was how LEDs were made and sapphire is expensive and small. Growing it on silicon — which is very difficult, because the two materials expand at different rates and a cooling wafer cracks — is what made GaN power devices cheap, because it lets them be made in existing silicon fabs on 200-millimetre wafers. That single manufacturing change is why GaN chargers went from exotic to ordinary in about five years.

The gallium itself is a by-product of aluminium refining, recovered from Bayer process liquor, and is produced overwhelmingly in China, which introduced export controls on it in 2023. It is not scarce in the crust and there is no gallium mine anywhere; supply is a function of somebody else's alumina production and of whether recovery is switched on, which is a distinctive kind of vulnerability.