Microelectromechanical resonator sensors are crucial in the cutting-edge technologies used in our everyday communication, timekeeping and computing systems. Their extreme sensing capabilities make them ideal candidates for the innovation of future technologies. However, with our ever-growing desire for faster communication, more sensitive systems, and more advanced technologies comes the need for a new generation of resonator sensors. This next generation will have to be faster, more accurate, and just as cheap as their predecessors if they are to enable the rapid growth of our technological needs. In this thesis, we investigate recently fabricated state-of-the-art extreme aspect ratio membrane resonators. The characteristics of extreme aspect ratio membrane resonator sensors are researched, and
the effects of nonlinear forces on their operation are explored. Some of these nonlinear attractive forces, such as the Casimir effect, are common to the extreme dimensions of these resonators. Another common nonlinear attractive force in MEMS, the electrostatic force, and its effects on resonator operation and output are investigated as well. Analytical models are fashioned and a FEM model is produced and validated using experimental results, showing it reflects reality. FEM simulations show that for these extreme aspect ratio resonators, the nonlinear softening effect is solely responsible for the change in the eigenfrequency which proves to be able to boost the responsivity of these resonators by factors of hundreds to thousands. Models are investigated for both conductors and dielectric resonators with different geometries and different material parameters, which all show these results. Responsivities of 133.2 kHz/kPa and 1.6 kHz/nm are found, which exceed the state-of-the-art. The negative effects of nonlinear forces such as pull-in are considered, investigated, and models are produced which predict them to prevent device failure. Furthermore, the role of crucial resonator parameters is investigated to aid future research in leveraging this potential new technique of enhancing sensor capabilities.