Carbon nanotube transistors operate as unconventional "Schottky barrier transistors," where transistor action occurs primarily by varying the contact resistance rather than the channel conductance. The study shows that the device characteristics are calculated for both idealized and realistic geometries, and scaling behavior is demonstrated. The results explain experimental observations, including the effects of doping and adsorbed gases. The electrode geometry is crucial for good device performance. Carbon nanotubes show promise for nanoscale field-effect transistors (FETs). However, theoretical understanding remains incomplete. Initially, it was assumed that the gate voltage modified the nanotube conductance, but increasing evidence suggests that Schottky barriers at the contacts play a central role. Ordinary FETs require ohmic contacts for effective switching, while reasonable work-function estimates suggest significant Schottky barriers at NT-metal contacts. The study shows that when there is a substantial Schottky barrier at the contact, NT-FETs operate as unconventional Schottky barrier transistors, where switching occurs primarily by modulation of the contact resistance rather than the channel conductance. SB-FETs have been considered as a possible future silicon technology due to their potential to operate at extremely small dimensions. The characteristics of such NT devices are calculated and shown to explain key experimental observations, including the effect of dopants and non-ideal switching behavior. The effect of adsorbed gases can be explained simply by their effect on workfunctions, without any doping. The geometry of the contact electrode plays a central role, primarily by scaling the required gate voltage. The study calculates the characteristics of the device shown in Fig. 1a, consisting of a NT embedded in a dielectric between a top gate and a ground plane, with two metal electrodes as source and drain. The current is given by the Landauer formula. The conductance is calculated for different SB heights, showing a symmetric dependence on the gate voltage when the metal Fermi level falls in the middle of the NT bandgap. The underlying mechanism for the transistor action is illustrated in Fig. 1c, where increasing the voltage difference between source and gate electrodes leads to a large electric field at the contact, reducing the width of the Schottky barrier and allowing thermally-assisted tunneling. The study concludes that the transistor action in carbon nanotube FETs can be understood based on transport across a Schottky barrier at the metal-NT contact. The gate induces an electric field at the contact, which controls the width of the barrier and hence the current. A sharper contact leads to focusing of the electrical field, allowing operation at lower gate voltages. Changes in workfunction, such as by adsorbed gases, affect the Schottky barrier and hence the device characteristics. By comparing calculations with experimental data for FETs exposed to oxygen or doped with potassium, the study suggests that the main effect of oxygen exposure is to change the workfunction ofCarbon nanotube transistors operate as unconventional "Schottky barrier transistors," where transistor action occurs primarily by varying the contact resistance rather than the channel conductance. The study shows that the device characteristics are calculated for both idealized and realistic geometries, and scaling behavior is demonstrated. The results explain experimental observations, including the effects of doping and adsorbed gases. The electrode geometry is crucial for good device performance. Carbon nanotubes show promise for nanoscale field-effect transistors (FETs). However, theoretical understanding remains incomplete. Initially, it was assumed that the gate voltage modified the nanotube conductance, but increasing evidence suggests that Schottky barriers at the contacts play a central role. Ordinary FETs require ohmic contacts for effective switching, while reasonable work-function estimates suggest significant Schottky barriers at NT-metal contacts. The study shows that when there is a substantial Schottky barrier at the contact, NT-FETs operate as unconventional Schottky barrier transistors, where switching occurs primarily by modulation of the contact resistance rather than the channel conductance. SB-FETs have been considered as a possible future silicon technology due to their potential to operate at extremely small dimensions. The characteristics of such NT devices are calculated and shown to explain key experimental observations, including the effect of dopants and non-ideal switching behavior. The effect of adsorbed gases can be explained simply by their effect on workfunctions, without any doping. The geometry of the contact electrode plays a central role, primarily by scaling the required gate voltage. The study calculates the characteristics of the device shown in Fig. 1a, consisting of a NT embedded in a dielectric between a top gate and a ground plane, with two metal electrodes as source and drain. The current is given by the Landauer formula. The conductance is calculated for different SB heights, showing a symmetric dependence on the gate voltage when the metal Fermi level falls in the middle of the NT bandgap. The underlying mechanism for the transistor action is illustrated in Fig. 1c, where increasing the voltage difference between source and gate electrodes leads to a large electric field at the contact, reducing the width of the Schottky barrier and allowing thermally-assisted tunneling. The study concludes that the transistor action in carbon nanotube FETs can be understood based on transport across a Schottky barrier at the metal-NT contact. The gate induces an electric field at the contact, which controls the width of the barrier and hence the current. A sharper contact leads to focusing of the electrical field, allowing operation at lower gate voltages. Changes in workfunction, such as by adsorbed gases, affect the Schottky barrier and hence the device characteristics. By comparing calculations with experimental data for FETs exposed to oxygen or doped with potassium, the study suggests that the main effect of oxygen exposure is to change the workfunction of