Diodes are electronic components found in nearly every electronic device and serve as one-way regulator valves. Diodes allow electricity to flow in one direction through the device, but not the other. They can be used to divert damaging high voltage away from sensitive grid transformers. The diode operates somewhat like a regulator valve in plumbing.
The EMP device uses the point at which the diode can no longer block the high voltage, holds the voltage to that 'level,' shunting the excess current through itself, to the ground and away from the grid equipment in a controlled, non-destructive fashion. The voltage surges caused by EMPs are a hundred times faster than those caused by lightning, so experts doubt that existing devices designed to protect the grid against lightning strikes would be effective against an EMP.
Today's electric grid has a number of different protections. The timeframes of these protections range from very fast to very slow, and they're overlaid on the electric grid to ensure that an event cannot cause a catastrophic outage of the electric grid. The fastest protection typically found on the grid reacts against pulses in one millionth of a second, to protect against lightning. For EMPs, we're talking ten billionths of a second, which is a hundred times faster.
Fortunately, the new Sandia device can react that quickly. Part of what makes the diode special is that it is made from gallium nitride, the same basic material used in LEDs. Gallium nitride is a semiconductor, like silicon. But because of its chemical properties, it can hold off much higher voltage before it breaks down. The material itself also responds very quickly and therefore is a good candidate for achieving the fast response needed to protect the grid from an EMP.
At this point development of cost-effective fabrication processes is still underway. However, it now seems that it's just a matter of time before EMPs become a much smaller threat to our civilization.