Evaluation of Aftereffects in Impatt Oscillators with Transient Ionizing Radiation

Aftereffects in IMPATT oscillators under transient ionizing radiation were evaluated using half watt Si and GaAs X-band IMPATT diodes in a resonant cap waveguide cavity. During the radiation testing, the RF impedance was varied by changing the length of a sliding short behind the diode and the bias impedance was varied independently by inserting lengths of coaxial cable between a fixed R-C bias network and the diode. From the experimental. results we conclude that: 1. the aftereffects are clearly related to a device-circuit interaction, which occurs in the resonant cap waveguide circuit only if the sliding short is one guide wavelength, rather than one-half guide wavelength, from the resonant cap diode structure; 2. the principal cause of the aftereffects is not improper bias circuit impedance resulting in bias circuit oscillations, although bias circuit oscillations can cause diode failure if they are allowed to occur; and 3. the GaAs Schottky diode is more prone to the aftereffects, occurring at power levels above 200 mW and dose rates above 2 x 109 rads/sec for the diode tested. The aftereffects appear to be triggered by a looping RF circuit impedance locus combined with the change in large signal IMPATT impedance with enhanced leakage current.