Bias circuit oscillations in Gunn devices

A theoretical and experimental study of low-frequency oscillations in the bias circuit of short (nominally 12 µ) Gunn devices is presented. Sinusoidal oscillations and relaxation oscillations including damped sinusoids and exponentially decaying pulses have been observed. The frequency, pulse width, and repetition rate are adjustable with bias voltage, bias circuit impedance, and the impedance of the microwave circuit. This behavior, as well as the conditions for stable bias, is explained in terms of an average terminal i-v characteristic for those devices which exhibit a terminal current drop. The oscillation amplitude is determined by the shape of this i-v curve. Pulses have been observed with subnanosecond rise times, peak voltages of 35 volts and adjustable pulse widths ranging from 3 to 200 ns for typical driver pulse widths of 100 to 500 ns. The sinusoidal frequencies were adjustable in the range of 300 to 550 MHz. The microwave output was in X-band and was self modulated by the bias circuit oscillations. The measured waveforms agree well with theory.