Low-temperature operation of silicon dynamic random-access memories

An investigation of commercial 16-, 64-, and 256-kb dynamic random-access memories (DRAMs) from five manufacturers for operations at low temperatures is discussed. The lowest temperature at which proper circuit operation was observed was 89 K for the 256-kb units. It is believed that below this temperature the sense circuits fail. Above a critical temperature, typically about 240 K, but dependent on the device family, the minimum refresh frequency is determined from the Shockley-Hall-Read electron generation rate, which is strongly temperature dependent. Below this temperature, the minimum refresh frequency is temperature independent but strongly voltage dependent and results from band-to-band tunneling in the Si depletion region under the capacitor. Times between required refresh cycles as long as 4.6 days have been observed at 183 K. These results suggested that DRAMs specifically designed for low-temperature operation can be operated at or near liquid-nitrogen temperature and as static RAMs with a reduction in circuit complexity and in power dissipation