SummaryA sensor for two types of ammonia (NH3) and carbon monoxide (CO) is fabricated on silicon substrate by using the drop‐coating method and the sensor performance is investigated. Polypyrrole, an electro‐conductive plastic, is used as the sensing material of the gas sensor. The gas sensor functions to maintain a constant gate voltage in a state that facilitates measurement of the conductivity of the gate, source, and drain having a source electrode and the drain electrode of a field effect transistor channel between the structures. The sensor exhibits an increase/decrease of the resistance upon exposure to a low concentration of NH3 and CO gas. This approach can be applied for highly sensitive room‐temperature sensing of CO and NH3 at sub‐ppm concentrations and for the development of very low‐power devices. The synthesized conducting polypyrrole was characterized via field emission scanning electron microscopy (FE‐SEM) and Fourier transform infrared spectroscopy (FTIR). The electronic transport properties of the sensors are measured in a field effect transistor geometry under ambient conditions. The sensing properties of the conducting polypyrrole gas sensor at sub‐ppm gas concentrations are described.