A metal-oxide gas sensor has a dielectric base carrying two electrodes that are typically embodied as an interdigital structure (IDT) and a gas-sensitive element whose electrical conductivity varies according to the concentration of a measured gas component. Two electrodes form the interdigital structure and are connected to the gas-sensitive element. The base also carries a resistive heater element having energizing electrodes. All the electrodes terminate at one edge of the base, which typically is formed as a basically rectangular plastic plate.
Such a sensor is described for instance in DE 196 06 272 of O. Kiesewetter. Its heater element surrounds the interdigital structure, the actual sensor element being formed by means of the interdigital structure and the sensor element in contact with it. Compared to such sensors where the heater element and the interdigital structures are mounted on different faces of the base or insulator, production engineering advantages result due to the provision of the interdigital structure, the sensor element, and the heater element on the same face of the base or insulator. However, heat distribution is not uniform because the resistive heater element surrounds the interdigital structures. Very frequently this causes sensor behavior that is too non-specific, rendering it difficult or sometimes impossible to reliably control corresponding concentrations of measured gas values. Another disadvantage of this arrangement is that, given a required minimum surface area of the sensor element, the space needed on the base is greater than this minimum surface area and requires that the sensor unit be relatively large.