NEC Corp has announced it prototyped a field effect transistor (FET) coating using carbon nanotube solution and achieved an on/off source-drain current (comparison of current when the power is on and off) of up to 107 by making channel length longer.

This study was unveiled at the 54th Spring Meeting 2007, the Japan Society of Applied Physics and Related Societies (held at Sagamihara Campus, Aoyama Gakuin University from March 27 to 30, 2007). As an FET channel, NEC used film coating of carbon nanotubes, which were formed using laser ablation and dissolved in an organic solvent.

When producing semiconductor carbon nanotubes using laser ablation, metal carbon nanotubes are contained. The metal carbon nanotubes cause on/off current ratios to lower, which has been a challenge so far. NEC reduced the chance that metal carbon nanotubes cause a short in the source-drain circuit by extending channel length. When using 1- to 20-亮m length carbon nanotubes, on/off ratio rose at a channel length of 30 亮m or longer, according to the company. When channel length was set at 100 亮m, maximum on/off ratio reached 107.

NEC also examined the defect rates in the carbon nanotube FET coating through simulation. The company changed carbon nanotube length in a range of 5 to 70 亮m under a condition of a channel length of 50 亮m and width of 50 亮m with a metal and semiconductor carbon nanotube ratio of 1:2 to obtain the rates of defect with a short or an open in the source-drain circuit. As a result, the company confirmed that the shorter carbon nanotube length gets, the lower rates of defective units become. For example, if carbon nanotube length is 10 亮m, rates of defect could decline to 0.01% or lower, according to NEC.

Given low temperature during manufacturing process, the carbon nanotube FET coating is expected for application to flexible devices using plastic substrates. NEC also announced that it coated a PEN (polyethylene naphthalate) substrate with carbon nanotube FET at 100-degree processing temperature and confirmed that the FET can drive an organic EL device. NEC said that mobility is estimated to have been over 100 cm2/Vs and an on/off ratio was between 10 and 103 then.