Hardening Electronic Devices against Very High Total Dose Radiation Environments

The possibilities and limitations of hardening silicon semiconductor devices to the high neutron and gamma radiation levels (greater than 1016 neutrons/cm 2 (i Mev eq.) and greater than 108 rads (Si) required for the NERVA nuclear engine development are discussed. A comparison is made of the high dose neutron and garmna hardening potential of bipolar, metal insulator semiconductor (MIS), and junction field effect transistors (JFET). Experimental data is presented on device degradation for the high neutron and gamma doses. Previous data and comparisonsl, 2,3 indicate that the JFET is much more immune to the combined neutron displacement and gamma ionizing effects than other transistor types. Presently, hardened JFET's degrade about 50% at 1016 neutrons/cm 2. Theoretically they can be improved, but the resulting hardened JFET's have low breakdown voltages (less than 20 volts). Experimental evidence (5 volt threshold shift at 108 rads) is also presented, which indicates that p channel MOS devices, made by Hughes Aircraft may be able to meet the requirements. Electrical characteristics compromises caused by device hardening and the ability to satisfy requirements on a practical reliable basis are discussed.