Oscillations due to travelling Gunn domains have been observed in gated and ungated GaAs m.e.s.f.e.t.s (channels) made on v.p.e. and l.p.e. active layers. The doping was 1-2 × 1017cm−3and the thickness 0.1-0.2 μm. In channels with non-negativeI/Vslope the oscillators were possible only with pulsed bias and decayed 1–3 ns after switching on the bias. In f.e.t.s the oscillations were self sustained, delivered approximately 60 μW r.f. power on 50 Ω and their frequency in the range 16–27 GHz was predicted, as in the channels, by the length of the drain to source gap, The domain formation is sensitive on the cross section of the epilayer and the microstructure of the ohmic contacts.