http://spectrum.ieee.org/semiconductors/materials/the-toughe...
Highlights:
> In the GaN FET, on the other hand, the two-dimensional electron gas already exists naturally. So a positive voltage applied to the drain immediately pushes current from source to drain. Thus the amount of current is varied by applying a negative voltage to the gate, which restricts the number of electrons available to flow from source to drain. A large enough negative voltage turns off the flow altogether. Thus in contrast to a silicon FET, which is normally off, a GaN FET is normally on.
(Somewhat misleading; silicon FETs that are "normally off" are enhancement-mode transistors. Of course, there are depletion-mode MOSFETs, and of course JFETs. There are even power MOSFETS that are depletion mode, see here: http://www.ixys.com/documents/appnotes/ixan0063.pdf In any case, this confirms the new transistor to be a depletion mode device. This has implications for biasing which could be inconvenient in some applications. In any case, it means it's not simply drop-in replacement for enhancement-mode MOSFETs in existing designs.)
> One of us (Mishra) has succeeded in making bipolar GaN transistors. But they are not yet as reliable as the FETs because at the moment it is very difficult to make p-type material good enough to use in a bipolar transistor. Applying electrical contacts to the material, as is necessary to connect the device into a circuit, often wiped out the semiconductor's p-type character.
If these challenges are solved, the idea of a new kind of BJT is exciting.