Negative bulk mobility devices - what next?

The transferred-electron (=TE) type negative differential bulk mobility (=NDBM) of the Gunn Effect in GaAs has formed one of the cornerstones of a rapidly growing solid-state microwave technology. Reliable solid-state microwave oscillators with several hundred milliwatts of low-noise c.w. power at X-band are commercially available, as are lower-power oscillators up to at least 70 GHz, and amplifiers to at least half the latter frequency. While these developments have by no means reached their inherent limits yet, new approaches to NDBM are already being explored, to take over where the present ones leave off. In this paper we wish to review briefly three such approaches, in the order of an increasingly speculative nature.