A Low-Power Subthreshold-to-Superthreshold Level-Shifter for Sub-0.5V Embedded Resistive RAM (ReRAM) Macro in Ultra Low-Voltage Chips

Many mobile chips with low supply voltage (VDD) require low-voltage embedded nonvolatile memory (eNVM) to enable low-power read operations and zero-standby-current power-off storage. ReRAM has a lower write-voltage (V-W), smaller write-current (I-W), and larger resistance-ratio than other NVMs, making it a good candidate for low-VDD eNVM, as long as the following two challenges can be overcome: (1) the failure of level-shifters (LSs) to operate with a low input voltage (VDDL) during write operations, particularly under high converting voltages (VDDH); (2) the consumption of large DC current by LS resulting in degradation in VDDmin for the on-chip charge-pump, particularly in NVMs with a large number of rows. This study proposes a pseudo-diode-mirrored (PDM) LS to achieve low VDDL, while maintaining a small DC current. PDM LSs were fabricated in a 65nm 4Mb embedded ReRAM macro, which achieved 0.1V minimum-VDDL at VDDH=2V.