Sensitivity control of carbon nanotube-based piezoresistive sensors by drain-induced barrier thinning

Abstract

The sensor characteristics of a membrane-based carbon nanotube (CNT) sensor showing maximum gauge factors of up to 810 is analyzed by a device study combining technological and theoretical approaches. Drain-induced barrier thinning (DIBT) is found to contribute significantly to this high sensitivity by a modification of the Schottky barrier width at the CNT-metal junctions. A high threshold voltage roll-off of (1370±130) mV⋅V-1 and degradation of subthreshold swing is observed even for channel length of 200 nm. The piezoresistive behavior of the CNT sensor running in the DIBT regime shows up as a complex and input-voltage dependent interplay of strain-dependent Fowler-Nordheim tunneling and the intrinsic thermionic resistance change. We demonstrate, that this interplay can be controlled by the applied bias voltages VGS and VDS in such a way, that the overall gauge factor is enhanced up to 150% with respect to the intrinsic effect. The control of the gauge factor via VDS is enabled by the DIBT effect, which appears for our CNT device at remarkably long CNT-channels.The experimental findings are retraced by a simplified transport model, which combines a numerical device solver with an electronic charge transport model for strained carbon nanotube based field-effect transistors (CNT-FETs) considering thermionic as well as tunneling contributions to the overall conductance. Strain dependent tunneling through the Schottky barriers appears to be the key contribution to the strain sensitivity in our model. Device characteristics have been derived from the model which reproduce the experimental findings emphasizing the significance of tunneling processes in combination with DIBT for the superior strain sensitivity of our device.