In-Substrate Imaging of Diamond–hBN FET Current via Wide-Field Quantum Diamond Microscopy

We demonstrate wide-field magnetic imaging of current flow in hydrogen-terminated diamond field-effect transistors (FETs) through in-substrate nitrogen-vacancy (NV) centers. Hydrogen termination of the diamond surface induces a two-dimensional hole gas (2DHG), while an ensemble of near-surface NV centers located ∼1 μm below the surface enables operando magnetic imaging of current flow with micrometer-scale spatial resolution. The FETs were electrically characterized over a range of drain–source biases, V ds = 0 to −15 V, and gate voltages, V gs = +3 to −9 V, followed by in situ wide-field NV magnetometry during device operation. Magnetic field maps and reconstructed current-density distributions directly visualize current injection at the source–drain contacts and transport beneath the hBN-gated channel. Magnetic field maps reveal current-density variations in the channel region owing to non-uniformities or defects in the gate dielectric. In addition, we observe a pronounced enhancement of the drain current (∼600 to 900 μA) and a shift in the apparent threshold voltage during laser illumination, reflecting photo-induced changes in channel electrostatics. By correlating gate-dependent magnetic images with simultaneous electrical measurements, we directly link spatial current distributions to FET transfer characteristics, providing new insight into buried interface transport and non-uniform gating effects in the transistor channel. As the methodology is compatible with top-gated FETs, it can be used to map channel current distributions with micrometer resolution in emerging channel materials, such as 2D materials and wide-bandgap channels, and establish wide-field NV magnetometry as a powerful platform for probing charge transport in transistors and van der Waals dielectric heterostructures.