Low-frequency high-efficiency oscillations in germanium IMPATT diodes

Pulsed operation of germanium IMPATT diodes has produced oscillations from 10 MHz to 12 GHz, with efficiencies exceeding 40 percent for frequencies between 2 and 3 GHz. Recorded waveforms show that IMPATT oscillations are required to initiate the lower frequency high-efficiency modes. The diodes are epitaxial diffused junction n-p-p⁺mesa structures, with depletion widths ∼ 5 microns and breakdown voltages ∼ 60 volts. Typical diode area is cm². Static I-V curves, obtained with circuit conditions which do not permit any oscillations, exhibit positive incremental resistance. The usual IMPATT mode would be expected to be between 6 and 12 GHz. Operation at frequencies below the IMPATT frequency requires circuit conditions suitable for IMPATT oscillations to be present to initiate the lower frequency, higher efficiency mode. This mode is characterized by a sudden decrease in diode voltage and a simultaneous increase in current, similar to that reported for silicon devices [1]. Reproducible current and Voltage waveforms have been recorded for four distinctly different low-frequency modes of operation which result only from changes in the ac circuit seen by the diode.