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Graphene provides a unique platform for hosting high quality 2D electron systems. Encapsulating graphene with hexagonal boron nitride (hBN) to shield it from noisy environments offers the potential to achieve ultrahigh performance nanodevices, such as photodiodes and transistors. However, the absence of a bandgap at the Dirac point presents challenges for using this system as a useful transistor. In this study, we investigated the functionality of hBN-aligned monolayer graphene as a field effect transistor (FET). By precisely aligning the hBN and graphene, bandgaps open at the first Dirac point and at the hole-doped induced Dirac point via an interfacial moir\'e potential. To characterize this as a submicrometer scale FET, we fabricated a global bottom gate to tune the density of a conducting channel and a local top gate to switch off this channel. This demonstrated that the system could be tuned to an optimal on/off ratio regime by separately controlling the gates. These findings provide a valuable reference point for the further development of FETs based on graphene heterostructures.

In Bernal-stacked bilayer graphene (BBG), the Landau levels give rise to an intimate connection between valley and layer degrees of freedom. Adding a moir\'e superlattice potential enriches the BBG physics with the formation of topological minibands - potentially leading to tunable exotic quantum transport. Here, we present magnetotransport measurements of a high-quality bilayer graphene-hexagonal boron nitride (hBN) heterostructure. The zero-degree alignment generates a strong moir\'e superlattice potential for the electrons in BBG and the resulting Landau fan diagram of longitudinal and Hall resistance displays a Hofstadter butterfly pattern with a high level of detail. We demonstrate that the intricate relationship between valley and layer degrees of freedom controls the topology of moir\'e-induced bands, significantly influencing the energetics of interacting quantum phases in the BBG superlattice. We further observe signatures of field-induced correlated insulators, helical edge states and clear quantizations of interaction-driven topological quantum phases, such as symmetry broken Chern insulators.