The electrical properties of carbon nanotube thin-film transistors (CNT-FETs) fabricated using plasma-enhanced chemical vapor deposition (PECVD) were studied by scanning probe microscopy. The measured results suggest the formation of an island structure in the subthreshold regime and the disappearance of the island structure at the ON state. These results were explained by the change in the effective number of CNTs that contributed to the electrical conduction due to the gate-bias-dependent resistance of the semiconducting CNTs. The results obtained by Monte Carlo simulation revealed similar results. The effects of metallic CNTs with defects and the scatter of the drain current in the subthreshold regime were also examined.