A bipolar RRAM selector with designable polarity dependent on-voltage asymmetry

Bipolar RRAM memory cells show different extents of asymmetry in set/reset voltages. Recently, our group has demonstrated a symmetric Si epitaxy based 4F² punch-through diode based NPN selector. Here, we demonstrate that a modification of the doping profile of the NPN diode selector produces designable asymmetry in the I-V characteristics. We demonstrate based on a TCAD study that replacing the uniform p region in the symmetric NPN structure by an asymmetric p/p-bi-layer could lead to an asymmetry in the turn-on voltage |V_on+|/|V_on-| by a factor of ~1:2. Our calculations show that the on-current (few μA at 20nm technology node or >1MA/cm²) and on-voltage asymmetry control is adequate for various promising bipolar RRAM memory elements from literature. Finally, the on-voltage asymmetry control is experimentally demonstrated based on a low temperature (<;700°C) Si epitaxy process; the measured asymmetry in the new experimental devices is about |V_on+|/|V_on-|=1:2.1 compared to the a|V_on+|/|V_on-|=1:1.2 in the symmetric devices.