I-NPN: A sub-60mV/decade, sub-0.6V selection diode for STTRAM

In this paper, we propose a new 2 terminal selection device for STTRAM at 0.6 V, with high on current (>10MA/cm²) and high on-off ratio (10⁴). STTRAM has recently been proposed as an SRAM alternative with fast program erase (0.5-5ns) and low voltage (0.2V) and high-current (<;20MA/cm²) [1]. An epitaxial Si NPN structure has been explored by simulations for greater than 1.5V operation [2] and experimentally demonstrated [3-4], which is very high voltage for STTRAM. E.g. for efficient cross-point, the selector threshold voltage (V_T) must be slightly above set voltage (V_set) of STTRAM (or any bipolar RRAM) as shown in Fig. 1. If the STTRAM has V_set=0.2V and I_set=20MA/cm², (a) for successful set/reset operation, the selector needs to pass on current (I_on=I_set) at V_T=0.6V for a cross-point voltage V=V_T+V_set=0.8V (same as V_Dd for compatibility with SRAM power supply). (b) For low off current at cross-point voltage V/2=0.4V, the voltage must drop mainly across the selector which will pass off-current (I_off) such that Ion/Ioff is high (~10⁴). Thus, a steep sub-threshold slope (SS) is required for I_on/I_off modulation in V_T-V/2=0.2V which implies that SS<;50mV/decade. Thus such a selector has 3 requirements for STTRAM replacement of SRAM, (i) on-voltage <;0.6V, (ii) on-current density >20MA/cm² and (iii) SS <;50mV/decade. In this paper, we present the concept and explore the performance of such a selector for STTRAM/bipolar RRAM.