SiC, due to its chemical and physical inertness and large band gap, is an interesting substrate for the fabrication of electronic devices that have to work at high temperature and in aggressive environments, working conditions of most of the gas sensors. However, the lower maturity of the SiC technology, as compared to the Si one, requires the development and improvement of the technological steps.
The working principle of these sensors is the decomposition of the gas in the gate and the diffusion of some of the resulting chemical species towards the oxide, which is increased for porous gates, and that gives rise to modification of the electrical characteristics of the devices.
MOS tunnel diodes and capacitors have been fabricated on SiC substrates for their implementation as gas sensors. The devices use a thin Pt/TaSi_x catalytic gate, formed by sputtering, and an oxide, with thickness varying between 1 and 40 nm. The devices are mounted on alumina plates with a printed Pt heater.
The as-fabricated devices cannot be operated above 200^oC and show limited sensing properties. Annealing at 600-800^oC for 3 minutes in an RTP furnace allows the devices to be operated at temperatures in excess of 400^oC and gives rise to an improved sensitivity for CO and NO₂ gases, with a maximum gas sensitivity at around 250^oC for CO and 320^oC for NO₂. A further improvement in the sensitivity to CO can be obtained by treating the samples in alternate pulses of 1% propane and 1% oxygen in argon gas at 600^oC, which is believed to increase the porosity of the gate material. In addition, the device becomes sensitive to propane, but no longer senses the presence of NO₂.
The sensing behaviour of these SiC devices in the presence of different gases will be presented as a function of the technological steps employed in their fabrication and correlation with the structural and chemical properties of the layers will be discussed.

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