This thesis describes the fabrication and properties of novel organic/silicon hybrid field-effect transistor gas sensors. Whilst most of the work used the emeraldine base form of the conductive polymer polyaniline, the response of a device incorporating a metal-free phthalocyanine is also reported. Arrays of p-type transistors in which the gate electrodes were replaced by 'charge-flow' capacitors were fabricated using standard semiconductor processing techniques. Each array consisted of four devices in which the width of metallisation removed from the gate electrode (total width 72 µm) varied from 0 µm (i.e. the control device) to 35 µm. Thin films of the gas-sensitive organic materials were deposited by spin-coating, and chemically patterned within the holes in the gate metallisation. A delay, referred to as the 'turn-on' response, was observed in the drain current on application of a gate voltage. This was shown to depend on the temperature, level of humidity and the presence of certain gases. The electrical operating characteristics of the hybrid device with and without the polyaniline were examined. These included capacitance-voltage measurements, the 'turn-on' response at different temperatures and the variation of threshold voltage with temperature. From these results an understanding of the effect of integrating polyaniline within a p-channel transistor structure was obtained. The 35 µm gate-hole sensor incorporating polyaniline was found to be sensitive to NO(_x) and SO(_2) at room temperature at concentrations as low as 1 or 2 ppm. Decreasing the gate-hole area, and therefore the surface area of polyaniline, reduced the sensitivity of the device. The reactions were found to be reversible, although complete recovery required approximately eight hours. A similar sensor incorporating a metal-free phthalocyanine compound was reversibly sensitive to 2 ppm NO(_x) with a more rapid recovery of five hours. There was no observable response to SO(_2) or H(_2)S up to 30 ppm.