In this work we report the development of back-gated carbon nanotubes field-effect transistors (CNT-FET) and their electrical characterization for sensing applications. Different kinds of CNTs (MWCNTs produced by different techniques and SWCNT) have been used as channel layer in the FET structure and this allows the investigation of the role of defects on the sensing properties of the devices. In particular, defects due to the growth process or induced by chemical treatment on the CNT walls have been investigated.