Steady state gamma testing of a 4K NMOS dynamic RAM

Samples of 4096-bit dynamic random-access memory (4K RAM) were tested in the ionizing environment of a Ce-13 source in support of digital multiplexing and rate buffering equipment being designed for the NIMBUS-G satellite. The 4K RAM was selected for use in a data rate buffer due to size, weight, and power advantages over bipolar or magnetic devices. A major concern in using a NMOS silicon gate RAM in the satellite application was the ionizing radiation which the RAM must tolerate in earth orbit. The 4096 memory bits were arranged in a 64 row by 64 column matrix with one sense amplifier associated with one column and 64 rows to detect the low signal level. A brief discussion of the memory operation is presented. Test results indicate that 24 hr after irradiation, 4 of the 5 devices tested were again functional. Three possible failure mechanisms are postulated: a loss of charge in the storage capacitors, a change in the ratio of the stray capacitance to the storage capacitance, and a shift in threshold voltage.