A semiconductor based gas sensor was developed by integrating a gas sensitive polymer with a floating-gate metal-oxide-semiconductor (FGMOS). The integration of the gas sensitive polymer with a semiconductor device enables the design of a large sensor array in a single chip for electronic sensing application. A new FGMOS structure was designed to reduce the number of post-processing steps during electrochemical polymer deposition. In this new design, the top metal layer of a standard CMOS process was used as an extended sensor pad which was connected to the floating gate. A sensor chip was designed using a standard 0.35 µm CMOS process. A polymer film was selectively deposited on the top metal layer (sensor pad) of a FGMOS using two post processing steps. The sensor response was measured by exposing the sensor in different concentration of water and methanol vapor. A short pulse measurement technique was introduced to measure the sensor response which was not affected by the measurement parameters.