1D Selection Device Using Carbon Nanotube FETs for High-Density Cross-Point Memory Arrays

A novel one-transistor-n-resistors (1TnR) array architecture is demonstrated as a cost-effective solution to the sneak path problem in large-scale cross-point memory arrays. In a 1TnR array, a single transistor (1T) with a 1D channel effectively controls a number of resistive switching nonvolatile memory (NVM) cells (nR) while limiting the sneak leakage current within the 1D channel without sacrificing the device density. To maximize these benefits, a carbon nanotube FET (CNFET) is employed as the 1D selection device, due to its near-ballistic electrical transport properties even at a small device width. Experimental demonstrations of the CNFET-based 1TnR concept are presented with two promising resistive switching NVM candidates: 1) resistive random access memory (RRAM) and 2) phase-change memory (PCM). Here, we report that the integrated bipolar Al₂O₃-based RRAM consumes programming energies as low as 0.1-7 pJ per bit and has a high programming endurance of up to 10⁶ cycles. The 1TnR RRAM cell also has self-compliance characteristics, because the semiconducting carbon nanotube (CNT) that serves as the bottom electrode limits the device current. The unipolar PCM cells integrated with CNFETs show uniform electrical characteristics with high ON-/OFF-resistance ratios of >10. Owing to the extremely small contact area between the phase change material, Ge₂Sb₂Te₅, and the CNT, remarkably low programming currents of <;1 μA are achieved.