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

The superior performance of membrane-based carbon nanotube (CNT) sensors showing maximum gauge factors of up to 800 is analyzed by a device study combining technological and theoretical approaches. Drain-induced barrier lowering (DIBL) is found to contribute significantly to this high sensitivity. A high subthreshold voltage roll-off of (750±200) 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 DIBL regime is shown as a complex and input-voltage dependent interplay of strain-dependent Fowler-Nordheim tunneling and the intrinsic thermionic resistance change. We show, that this interplay can be controlled by the applied bias voltages 𝑉GS and 𝑉DS in such a way, that the overall sensitivity is enhanced up to 150 % with respect to the intrinsic effect. The control of the sensitivity via 𝑉DS is enabled by the DIBL 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 model for strained carbon nanotube based field-effect transistors (CNT-FETs) covering thermionic as well as tunneling contributions. Strain dependent tunneling through the Schottky barriers (SBs) appears to be the key contribution to the strain sensitivity in our model. From the model device characteristics have been derived which reproduce the experimental findings emphasizing the significance of tunneling processes in combination with DIBL effects for the superior strain sensitivity of our device.