||The adsorption of gas molecules on MOS (metal-oxide-semiconductor) structure changes the work function difference between metal and semiconductor and causes a corresponding change in threshold voltage. A MOS structure with a suitable gas-sensitive gate material can therefore work as a gas sensor by monitoring the changes in threshold voltage upon gas adsorption. In this thesis, a surface chemistry based model is proposed to describe the gas sensing ability of the MOS structure. This model is derived from the device physics and surface chemistry. We have applied our model to three sensor systems, namely the adsorption of H2 on Palladium gate, the adsorption of NH3 on Platinum gate and the adsorption of CO on Palladium gate. We found that the model correctly describes each sensor system. This model is incorporated into the popular SPICE circuit analysis program. The performance of an integrated gas sensor, which comprises the front-end sensor and the signal-conditioning circuitry, can therefore be simulated in the same manner as conventional integrated circuits. A chopper-stabilized circuit applied to a gas sensor was analyzed by the circuit model. Simulation results showed that the sensitivity of the gas sensor can be improved. A second area covered in this thesis is the design of experimental gas sensors used by the Integrated Sensor Group in EEE Department of HKUST. It is the first gas sensor technology development vehicle at HKUST. The design was named "IGSOO1" which stands for the first design of Integrated Gas Sensor.