This paper reports on the fabrication and performance of single-wall (SWNT) and multi-wall (MWNT) carbon nanotube (CNT) field-effect transistors (FETs). The study shows that transport through CNTs is dominated by holes and, at room temperature, appears to be diffusive rather than ballistic. By varying the gate voltage, the conductance of a single-wall device was modulated by more than 5 orders of magnitude. Multi-wall nanotubes typically show no gate effect, but structural deformations, such as a collapsed tube, can make them operate as FETs. Carbon nanotubes are unique carbon structures with exceptional electrical and mechanical properties. They can be single-walled (SWNTs) or multi-walled (MWNTs), and their electronic properties depend on their diameter and structure. SWNTs can be metallic or semiconducting, and their band gap decreases with increasing diameter. The study fabricated SWNT-based FETs and explored whether MWNTs can be used as active elements in CNT-FETs. Despite their large diameter, structurally deformed MWNTs can be used in NT-FETs. The carrier density and transport mechanism of the devices were evaluated based on their output and transfer characteristics. The SWNTs used were produced by laser ablation of doped graphite. They were cleaned with an H2SO4/H2O2 solution. MWNTs were produced by arc discharge and used without further treatment. The nanotubes were dispersed and spread on a substrate with predefined electrodes. The devices consist of either an individual SWNT or MWNT bridging two electrodes. The output characteristics of a SWNT-FET showed a linear I-VSD curve at Vg=0 V, with a resistance of 2.9 MΩ. For Vg < 0 V, the curves remained linear, while for Vg > 0 V, they became increasingly nonlinear. The transfer characteristics showed a similar behavior to a p-channel MOSFET, with the source-drain current decreasing with increasing gate voltage, indicating hole transport. The hole density was estimated to be about 1 hole per 250 carbon atoms in the NT, which is higher than in graphite. The hole mobility was found to be around 20 cm²/Vs, which is close to that in heavily p-doped silicon but lower than in graphite. For MWNTs, structural deformations can lead to a significant gate effect. The conductance modulation was about a factor of 2. The hole density was similar to that in SWNTs, but the mobility was higher, suggesting a reduced number of scatterers. This may be due to the lack of acid treatment and less deformation of MWNTs compared to SWNTs. In conclusion, the study demonstrates that SWNT-based FETs can be modulated by more than 5 orders of magnitude in conductance.This paper reports on the fabrication and performance of single-wall (SWNT) and multi-wall (MWNT) carbon nanotube (CNT) field-effect transistors (FETs). The study shows that transport through CNTs is dominated by holes and, at room temperature, appears to be diffusive rather than ballistic. By varying the gate voltage, the conductance of a single-wall device was modulated by more than 5 orders of magnitude. Multi-wall nanotubes typically show no gate effect, but structural deformations, such as a collapsed tube, can make them operate as FETs. Carbon nanotubes are unique carbon structures with exceptional electrical and mechanical properties. They can be single-walled (SWNTs) or multi-walled (MWNTs), and their electronic properties depend on their diameter and structure. SWNTs can be metallic or semiconducting, and their band gap decreases with increasing diameter. The study fabricated SWNT-based FETs and explored whether MWNTs can be used as active elements in CNT-FETs. Despite their large diameter, structurally deformed MWNTs can be used in NT-FETs. The carrier density and transport mechanism of the devices were evaluated based on their output and transfer characteristics. The SWNTs used were produced by laser ablation of doped graphite. They were cleaned with an H2SO4/H2O2 solution. MWNTs were produced by arc discharge and used without further treatment. The nanotubes were dispersed and spread on a substrate with predefined electrodes. The devices consist of either an individual SWNT or MWNT bridging two electrodes. The output characteristics of a SWNT-FET showed a linear I-VSD curve at Vg=0 V, with a resistance of 2.9 MΩ. For Vg < 0 V, the curves remained linear, while for Vg > 0 V, they became increasingly nonlinear. The transfer characteristics showed a similar behavior to a p-channel MOSFET, with the source-drain current decreasing with increasing gate voltage, indicating hole transport. The hole density was estimated to be about 1 hole per 250 carbon atoms in the NT, which is higher than in graphite. The hole mobility was found to be around 20 cm²/Vs, which is close to that in heavily p-doped silicon but lower than in graphite. For MWNTs, structural deformations can lead to a significant gate effect. The conductance modulation was about a factor of 2. The hole density was similar to that in SWNTs, but the mobility was higher, suggesting a reduced number of scatterers. This may be due to the lack of acid treatment and less deformation of MWNTs compared to SWNTs. In conclusion, the study demonstrates that SWNT-based FETs can be modulated by more than 5 orders of magnitude in conductance.