  Substantial progress on field effect transistors "FETs" consisting of
semiconducting single wall carbon nanotubes "s-SWNTs" without detectable traces
of metallic nanotubes and impurities is reported. Nearly perfect removal of
metallic nanotubes is confirmed by optical absorption, Raman measurements, and
electrical measurements. This outstanding result was made possible in
particular by ultracentrifugation (150 000 g) of solutions prepared from SWNT
powders using polyfluorene as an extracting agent in toluene. Such s-SWNTs
processable solutions were applied to realize FET, embodying randomly or
preferentially oriented nanotube networks prepared by spin coating or
dielectrophoresis. Devices exhibit stable p-type semiconductor behavior in air
with very promising characteristics. The on-off current ratio is 10^5, the
on-current level is around 10 $\mu$A, and the estimated hole mobility is larger
than 2 cm2 / V s.
