M.Sc Thesis

M.Sc StudentShifman Evgeny
SubjectHysteresis and Sensing in Carbon Nanotube Field-Effect
DepartmentDepartment of Electrical and Computer Engineering
Supervisors DR. Yuval Yaish
PROF. Nir Tessler


Carbon nanotubes and especially Carbon nanotube field effect transistors (CNTFETs) are widely studied due to their excellent electric properties. However, one of the major challenges regarding CNTFETs performance is the noticeable gate hysteresis. The hysteresis phenomenon is often attributed to surface charges movement and more precisely to water-mediated charge mobility.

 In the first part of this study, we have attempted to improve our understanding of the hysteresis in CNTFETs, and see whether this hysteresis phenomenon can be used for sensing.

 Since the hysteresis involves mobility of charge, we have first developed a quantitative method to measure lateral and temporal charge density on dielectric layers using electrostatic force microscopy (EFM). Using this approach, we found that upon applying gate voltage compliance in the vicinity of grounded electrodes, a water-assisted surface charge redistribution is induced and results in screening of the electric field originates from the gate electrode. Then we provided experimental evidence that the hysteresis in suspended CNTFETs and on-surface CNTFETs operating at low gate voltages, is based on the described above phenomenon of charge redistribution, and it is not related to charge injection from the CNT itself as previously believed. Finally, we report gate-induced modification of water adsorption and CNTFETs-based measurements which assist us to extract the actual surface humidity.

In the second part, we have repeated the research for various solvents adsorption (methanol, ethanol and isopropanol) in addition to water. We have shown that the hysteresis phenomenon can be also caused by these solvents, similar to the hysteresis caused by water. Using CNTFETs-based measurements we have shown the dependence of the time constants of CNTFET current decay as function of vapor percentage in the environment of the device, thus revealing this sensing possibility.