InP Gunn diodes with current limiting cathodes have proven to be highly efficient, medium power devices at millimeter wave frequencies up to 110 GHz. Device and circuit designs have continously improved and cw efficiencies of 21% and 6.3% have recently been obtained at 35 GHz and 94 GHz respectively. Using these basic InP devices in planar microstrip circuits along with special flip chip GaAs varactors, wide band tunable planar VCO's have been developed for these millimeter wave frequencies. Also as a result of the current limiting design, these InP Gunn diodes work well as stabilized reflection amplifiers and stable amplifier designs have been made up to 110 GHz with output power levels sufficient to drive very high output power tubes.