A bulk GaAs semiconductor negative resistance device has been operated in a relaxation mode analogous to tunnel diode operation. The output consists of a closely-packed train of narrow, high-voltage pulses. The circuit consists of an inductor in series with the parallel combination of the active device and the load resistor, driven by a low impedance generator. By shunting the device with a suitable load resistor, avalanche breakdown, which would normally occur, is prevented and the generated voltage pulses are impressed directly across the load. The circuit is constructed in a coaxial line and operates without a cavity or other tuning elements. By driving this circuit with a pulse of about 100 ns duration, an output pulse train having an amplitude of over 200 V (10 times Gunn threshold) was obtained across a 3000 Ω load. The output pulses were of about 1 ns duration and spaced about 10 ns apart. Since the pulse spacing varies with the amplitude of the drive voltage, pulse frequency modulation is possible. The ramifications of a time-variant characteristic caused by the formation of a traveling Gunn domain during part of the cycle are discussed. © 1969.