Radiation Effects in a CMOS/SOS/AL-GATE D/A Converter and On-Chip Diagnostic Transistors

This paper presents the results obtained from total dose and transient radiation tests on a CMOS/SOS/Al-Gate D/A converter and on-chip diagnostic transistors. Samples were irradiated by Cobalt-60 gamma rays under worst-case conditions, and by 10-MeV electron pulses of 50-ns and 4.4-ps duration. Devices were fabricated with three different insulators; the two discussed here are standard wet oxide and a pyrogenic oxide. Test transistors on the D/A chips made it possible to diagnose the failure modes of the converter and to evaluate some special designs. These consisted of standard edge p- and n-channel transistors, edgeless units, edgeless tetrode transistors, and an edgeless type transmission gate with a diode clamp from substrate to gate. The total dose results indicate that the pyrogenic oxide increased the failure dose of the operational amplifier portion of the converter from 103 rads (Si) to 2×l06 rads (Si); however, the sample and hold failed after exposure to a low level of 103 rads (Si). Test devices indicated this to be due to the radiation-induced leakage current of the transmission gate which discharges the sample and hold capacitor. The diode clamp decreased the threshold voltage shifts and the leakage currents. The edgeless devices improved the device performance because of a more abrupt turn-on. Narrow-pulse test data indicated that the edgeless units produced less photocurrent than the edge units by about a factor of three to four. Converter upset levels are <109 rads/s due to precision requirements which make a few millivolt transients untenable.