Hybrid 2T nMOS/pMOS Gain Cell Memory with Indium-tin-oxide and Carbon Nanotube MOSFETs for Counteracting Capacitive Coupling

We demonstrate a back-end-of-line (BEOL) compatible 2T gain cell consisting of n-type indium-tin-oxide (ITO) MOSFET as a write transistor and p-type carbon nanotube (CNT) MOSFET as a read transistor. The opposite polarities help counteract the capacitive coupling in gain cells operating with voltage sensing: the drop of the storage node voltage due to the capacitive coupling with the write wordline (gate of a write nMOSFET) is fully recovered at the time of enabling the read wordline (source of a read pMOSFET) for readout. With the low leakage of an ITO nMOSFET and the opposite polarity of the read transistor, retention time of more than 500 s is achieved at zero-volt standby voltage. The results suggest that the hybrid combination of n-ITO and p-CNT is a promising approach to improve gain cell operation in terms of read margin, retention, and read speed for 3D integrated all-BEOL gain cell memory above the silicon FET logic layer.