In this work, we present several strategies of making robust carbon nanotube (CNT) n-type field-effect transistors (NFETs). One approach uses low-work function metal contacts to enable electron injection into CNT for NFET operation while using pre-defined oxide trenches to protect the sidewalls of such easily oxidized contacts. The sidewall-protected NFETs are shown to be stable over several months. The contact length scaling study reveals a large contact resistance that increases rapidly as the contact size shrinks, probably due to the poor interface wetting. A thin Ti layer is shown to significantly enhance contact wetting on CNT and improve device performance. Another approach involves charge-transfer doping from layers deposited on top of CNT channel. Results from different doping layers and deposition conditions are presented and compared. For applications like CNT thin film transistors (TFTs, where transport is not limited by contacts), such channel doping techniques will be required to form NFETs.