ELECTRIC-FIELD-DRIVEN MOTION OF SEMICONDUCTOR NANOSTRUCTURES: FROM ORIENTATION TO PROGRAMMABLE PROPULSION

This dissertation establishes the motion of semiconductor devices in fluids as a con- tactless probe of their electronic and interfacial properties. Three related systems are investigated. First, p–n silicon nanowires orient in a DC electric field. Their measured permanent dipole is much larger than expected from the depletion region and scales with nanowire length and radius in a manner explained by doping-dependent surface charge. Second, silicon and germanium diodes propel in aqueous suspension under an AC field, while silicon photodiodes propel under illumination. Half-cycle-resolved current measure- ments, damaged-diode controls, pH-indicator experiments, and flow visualization show that propulsion is driven by rectified forward current or photocurrent through a self- electrophoretic mechanism, rather than by the reverse-bias potential alone. Third, the dissertation extends this mechanism to MOSFETs, for which source and drain exchange roles when the applied AC field reverses. A centered-gate device with a floating body does not propel, confirming the symmetry constraint. Directed motion is obtained by two distinct symmetry-breaking strategies: displacing the gate from the channel center, which produces a tunable net channel current, and tying the body to the source, which activates the internal body diode. Numerical simulations and a macroscale 4-terminal MOSFET demonstrate the geometric mechanism, while NMOS and PMOS experiments show opposite propulsion directions and confirm electronic rectification. Together, these results establish transistor propulsion as a new class of programmable AC micromotor and identify the conditions required for its use in contactless characterization. The work provides a framework for using optical orientation and propulsion measure- ments to infer surface charge, rectification, threshold voltage, and ON-state conductance across large populations of devices. Extending the measurements to lithographically fab- ricated microscale SOI devices and bottom-up-synthesized nanowire transistors remains an important next step toward high-throughput characterization and sorting of nanoscale electronics.