The successful experimental demonstration of large-scale and in-situ growth of horizontal, dense and aligned carbon nanotube (CNT) arrays is presented. This new CNT growth process is developed for the direct integration of nanotubes into gas sensing devices. The device electrodes are patterned in a TiN/Al2O3/TiN stack which is used as a support for the CNT growth catalyst. Thereafter, iron is locally deposited on the stack’s vertical sidewall as a catalyst material. With the use of a chemical vapor deposition (CVD) growth process selective with respect to the catalyst underlaying material, dense and thin arrays of horizontally aligned CNTs are directly grown from the Al2O3/Fe surface and bridge the electrode gap. This process enables for the wafer scale integration of dense CNT arrays as basic building blocks in devices such as micro-electromechanical systems (MEMS) for gas sensing applications. These CNT arrays, when contacted in between electrodes, show good electrical characteristics and a sensitive conductance response when exposed to a gas such as NO2. Although further optimization is required, these devices can be expected to show enhanced sensitivity for potential gas identification sensor systems due to their large detection surface.