M.Sc Thesis

M.Sc StudentNussinson Dan
SubjectField Effect Transistors Based on Carbon Nano-Tubes
DepartmentDepartment of Physics
Supervisors DR. Yuval Yaish
ASSOCIATE PROF. Michael Reznikov


Carbon nano-tubes are hollow cylindrical molecules made purely of carbon. A collection of remarkable traits makes them an extremely appealing material for future electronics, opto-electronics, biological sensors and nano-electro-mechanical systems (NEMS).  In this work, the fabrication process of single-wall carbon nanotubes field-effect-transistors (SWNT-FETs) is described and various electrical measurements of these devices are presented, some exhibit a new type of behavior.

A theoretical model we developed, proposes an explanation to the measured data.

A computer simulation based on this model complies, in most cases, with previously reported measurements and simulations . Yet, under certain parametric conditions (e.g., CNT chirality and contacts work function) the simulation reproduces the peculiar gate dependence that we observed.

Typical SWNT-FETs exhibit two types of characteristic current behavior as a function of gate voltage (I-Vg curve), depending on the SWNT classification:  (a) semi-conducting, (b) metallic.  Additionally, the type of metal used to contact the SWNT determines if it will exhibit P-type or N-type behavior. P-type (N-type) FETs based on Semi-conducting SWNT are 'o' at high negative (positive) gate voltages, and show constant current at high positive (negative) gate voltages, respectively.  FETs based on metallic SWNT show finite current for both high gate voltages and low gate voltages. A current dip is present between these two transport regimes.  The on-current at negative voltages is higher than the on current at positive voltages for P-type devices and vice versa for N-type devices.

Yet,  On  some  devices,  particularly  of  the  metallic  type,  a  current reduction was observed when the gate voltage was decreased beyond the current saturation point. On most of these devices the current dropped to about 50% of its peak value.  To the best of our knowledge, this peculiar current behavior has never been reported in the literature before.

Our model is based on simple assumptions which are delineated in this work. Unlike in most simulations published to date, the exact and complete band structure of the SWNT was considered rather than the density of states alone.  This, along with the assumption that inter-band transitions have negligible contribution to the current, enabled the simulation to reproduce the new observed behavior.  Furthermore, the simulation provides a more complete electrostatic solution to surrounding-gate device geometry, thus enabling prediction of device behavior for different lengths of SWNTs, and larger values of source biases than reported before.