UCLA

Gate bias in a field-effect transistor (FET) is not purely a tool for device applications; it also serves as a means to tune carrier density and explore transport physics in materials. This capability has made the FET an effective platform for two-dimensional (2D) materials research, and in fundamental science, gate-tunable carrier density has been used extensively to uncover exotic physics at cryogenic temperature, providing the basis for future quantum and semiconductor device applications. However, conventional high-k dielectric gating presents integration challenges with 2D semiconductors, and while ionic-liquid or ion-gel gating can supply large carrier densities, this doping is not switchable at cryogenic temperature. Here, this dissertation proposes a solid-state gating approach that induces carrier densities beyond what conventional methods achieve, exploiting the rich trap landscape of a metal halide perovskite to explore exotic transport phenomena in transition metal dichalcogenides (TMDCs). In a WSe2/two-dimensional lead-halide-perovskite (LHP) van der Waals heterostructure, the LHP's own uncompensated, self-compensation-protected trap states serve as a voltage-programmable charge reservoir: applying a gate voltage shifts the WSe2 band alignment relative to these trap states, and once the WSe2 Fermi level is brought into resonance with the trap manifold, electrons transfer from WSe2 into the LHP without a thermal activation barrier, leaving behind a corresponding hole density in the WSe2 channel. Because the mechanism involves only electronic, not ionic, charge transfer, it remains fully switchable to millikelvin temperature. Hall measurements demonstrate carrier densities in WSe2 reaching 8.5-9.0 x 1013 cm-2 at 0.5 K, more than six times the density expected from electrostatic gating alone, and approaching values conventionally achievable only with ionic gating at room temperature. Bias-dependent nonlinear transport establishes the microscopic mechanism underlying resonant versus off-resonant charge transfer, confirming that the enhanced conductance originates from gate-controlled band alignment rather than mobility modulation alone. Temperature-dependent Hall transport further demonstrates that this doping is static: the induced carrier density persists and remains gate-tunable down to 0.5 K, in direct contrast to the freeze-out behavior of ionic gating. Finally, two-dimensional bias-gate spectroscopy quantifies the capacity and energetic structure of the trap-based doping reservoir, revealing a discrete, high-density manifold of accessible states with a peak density of states of 9.7 x 1021 eV-1 cm-3, capable of sustaining volumetric doping densities approaching 1019-1020 cm-3. Extending transport measurements to high magnetic field further shows that this ultrahigh-density regime brings the WSe2 valence band to the verge of a band-to-band transition between two nearly degenerate valence-band valleys, populating two nearly degenerate Fermi surface pockets whose coexistence is a precondition for field-driven Fermi-surface nesting. On the verge of this crossover, an applied magnetic field completes the nesting condition and drives the system into a field-induced density-wave transition, observed with two distinct signatures depending on carrier density: the resistance increases at the transition in the free-carrier (high-density) regime, but decreases at the transition in the localized (low-density) regime.. Together, these results establish gate-driven interfacial band modulation, using the intrinsic defect landscape of a companion 2D material as an on-chip doping reservoir, as a general strategy for achieving ultrahigh, static, and fully switchable carrier densities in 2D heterostructure FETs, distinct from and complementary to chemical, ionic, and ferroelectric doping approaches.