We show calculations and analysis of I-D-V-G curves for carbon nanotube field effect transistors (CNTFETs). For 10 nm CNTFETs, it appears that there is a region where discretized energy levels in the channel have an effect on the ID-VG curve beyond the region where the complex band structure effect is important. This is because energy levels are pulled up from the valence band. We can still have this effect for longer channel CNTFETs, but because the spacing between energy levels is decreased, the energy discretization effect becomes less significant, and it appears as a region with small current oscillation. Also, because the tunneling through band gap states becomes unimportant and the complex band structure effect may even disappear, we separate the I-D-V-G curve into three parts: ON, OFF, and ON-II region for longer channel CNTFETs. The switching behavior from OFF to ON-II region may suggest interesting applications. (c) 2005 Elsevier Ltd. All rights reserved.