The sensitivity of 'top-down' fabricated Si nanochannel field effect transistor (FET) biosensors has been analyzed quantitatively, as a function of the channel width and doping concentration. We have fabricated 130-, 150-, and 220 nm-wide Si FET channels with 40 nm-thick p-type silicon-on-insulator (SOI) layers doped at 8 x 10(17) and 2 x 10(18) cm(-3), and characterized their sensitivity in response to the variation of surface charges as hydrogen ion sensors within buffer solutions of various pH levels. Within the range of channel width and doping concentration investigated, the pH sensitivity of Si channels is enhanced much more effectively by decreasing the doping concentration than by reducing the channel width, which suggests a practical strategy for achieving high sensitivity with less effort than to reduce the channel width. Similar behavior has also been confirmed in the immunodetection of prostate specific antigen (PSA). Combined with excellent reproducibility and uniformity of the channel structure, high controllability of the doping concentration can make the 'top-down' fabrication a very useful approach for the massive fabrication of high-sensitivity sensor platforms in a cost-effective way.