We have fabricated single-walled carbon nanotube (SWNT) arrays, about 85% of which are semiconducting. Fe nanoparticles, which were used as a catalyst, were produced simply and cheaply from Wistar rat blood. The SWNT arrays generally grow parallel to the gas flow. Hundreds of devices with varying SWNT density in the channel were measured and we found that the on-off ratio for such devices with the quasi-parallel, semiconducting-rich SWNTs in the channel can be easily increased via an electrical breakdown method. Thus, large-scale fabrication of FET devices is possible simply by controlling the width of the channel. Finally, we determined that the mobility of the devices reached 3900 cm(2) V(-1) s(-1).