Scaling Challenges of Floating Gate Non-Volatile Memory and Graphene as the Future Flash Memory Device: A Review

Abstract

With the increasing number of electronic consumer and information technology, demands for non-volatile memory rapidly grow. This study comprehensively reviewed the challenges related to the floating gate transistor for each component of the gate stack that consists of the tunnel oxide layer, inter-poly dielectric (IPD) oxide layer and poly-Si floating gate until reaching its bottleneck as the floating gate thickness scale to 7 nm. By going through the development of flash memory from its early year, the issues of the floating gate structure are identified. In resolving these issues, several approaches and upcoming flash memory devices are proposed. The prospect of carbon-based floating gate devices covering the graphene flash memory (GFM) and the vertical CNTFET floating gate are thoroughly discussed; reviewing its device performance and transient characteristics, which were extracted from experimental works and compared with recent upcoming technologies such as the HFG and the three-dimensional (3D) vertical flash memory. Reports indicate that graphene flash memory (GFM) capable of rivaling with the upcoming hybrid floating gate (HFG) device. It is found that the carrier concentration, transient characteristics and memory window of GFM are very much comparable to the HFG. Attributes to the high density of states (DOS) of graphene, GFM managed to lower its operating voltage while achieving comparable memory window as HFG. In the case of CNTFET with nanocrystal floating gate, the device performance is lesser than the 3D vertical flash memories in terms of its short channel effect and gate capacitance ratio. But considering its nanometer structure that has ballistic properties, better device performance with a proper definition of metal contact and synthesis can be achieved. Finally, the advantages of using graphene as floating gate are concluded and future work to improve the CNTFET floating gate is proposed.