A biosensor based on a nanowire field-effect transistor is demonstrated on a bulk silicon wafer for low-cost applications. The silicon nanowire is fabricated using a simple reactive-ion etching technique known as the Bosch process. The sensor operation of the fabricated device is confirmed as a proof of concept by detecting the negatively and positively charged polymers on the nanowire surface in real time. The drain current of the device is clearly modulated by the charge polarity of the polymers. In addition, the specific binding of the antigen and the antibody for avian influenza is also detected by a real-time label-free electrical method for practical applications. Control experiments support that a charged species only on the nanowire surface leads to a significant change in the drain current of the sensor. The proposed approach in bulk nanowire biosensors paves the way for the application of complementary metal-oxide-semiconductor technology for low-cost and miniaturized biosensors without external transducers; this approach is attractive in disposable and portable applications.