Graphene quantum capacitance varactors for wireless sensing applications

The low density of states in graphene makes it possible for the quantum capacitance to be of the same order of magnitude as the oxide capacitance for experimentally achievable gate dielectric thicknesses [1]. This property, combined with the fact that the density of states varies as a function of energy, means that the capacitance in a metal-oxide-graphene capacitor can be tuned by varying the carrier concentration [2]. The very high mobility and zero band gap in graphene also allow it to remain conductive throughout the entire tuning range, making graphene an idea material to realize a high quality factor (Q) variable capacitor (varactor). If combined with an on-chip inductor to form an LC oscillator circuit, graphene varactors could enable a new class of ultra-compact sensors with wireless readout capability. Compared to MEMS-based varactors [3], the extremely-large capacitance per unit area of graphene varactors should allow orders-of-magnitude improvement in scalability, a vital feature for numerous applications including in vivo sensing where small size is critical. In this abstract, the device concept is described and simulated performance projections are provided. The main findings in this study are that wide frequency tuning ratios (> 50%) and high Q (> 40 at 1 GHz) are possible using realistic assumptions for the graphene properties, device dimensions and parasitic resistances.