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The effects of fast neutrons with fluences up to 1016 n/cm2 on both dc and microwave characteristics of nearly abrupt X-band avalanche diodes have been studied. Good forward dc characteristics were observed up to 2Ã1015 n/cm2. At higher doses, the forward characteristics are degraded by the formation of a thin intrinsic layer at the metallurgical PN junction. The thickness of the intrinsic layer and its effects on device behavior could be reduced by reducing the depth of the diffused junction. At neutron fluences exceeding 4Ã1015 n/cm2, the avalanche breakdown voltage has a negative temperature dependence which results in a marked degradation in device reliability. The negative temperature coefficient is thought to be a result of an increase in the reverse saturation current or of changes in the field profile. Microwave oscillator characteristics - including oscillation threshold, power output, frequency, efficiency, and FM and AM noise - were virtually unchanged up to 1015 n/cm2. At neutron fluences greater than 1015 n/cm2, the oscillation threshold decreased and power output increased significantly (by 2 to 6 dB). The improvements are attributed to widening of the drift-field region and to a reduction in width of the unswept epitaxial parasitic resistance region. The greatest improvements occurred for the lowest-output power devices. These results suggest the use of nuclear radiation to tailor the defect concentrations and the field profiles of avalanche diodes to achieve optimal structures.