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The fast neutron irradiation induced degradation of junction field effect transistors has been studied in detail. A comparison of p-channel to n-channel devices has confirmed reported differences in hole and electron removal rates. The predicted increase of hardness with increasing channel dopant concentration is experimentally demonstrated, although channel dopant grading makes very heavily doped units difficult to attain in practice. Also, a device modification which allows attainment of breakdown voltages higher than those characteristic of the channel doping level is presented and experimentally verified. This modification made possible the construction of devices with breakdown voltages greater than fifty volts, which degraded by only 25% in transconductance at fluences of approximately l-3Ã1015 neutrons/cm2 (E > 10 kev, Triga). Thus, the technique offers promise in the construction of very radiation resistant devices with wide operating voltage ranges.