Description FET-Based Gas Sensor Lately, gas sensors using the work function modification of sensitive materials as physical value, have experienced ar. increased interest- The reasons for this lie in the possibilities of being able to operate thesa gas sensors with low operating energies (lew-power), in a cost-efficient production and assembly technology of such gas sensors (low-cost), as well as in a broad range of gases, which can be detected with this platform technology (high versatility)/ because numerous different detection substances can be integrated in such superstructures. Assembly and operating methods are known/ e.g., from the following documents: German Patent Application No. 19814857, DE-C-19956744, DE~ C-19849932, DE-C-19956306 or DE-C-19926747. The basic assembly of the gas sensors, hereinafter referred to as GasFET, is schematically illustrated in Fig, 4. When the gas, which is to.be detected, is present, an electrical potential, which corresponds to the modification of the work -junction of the sensitive material/ for example 50-100 mV, is created at the sensitive layer, with which, e.g. the lower side of the detached gate electrode is coated. This potential acts upon the channel of an FET structure and modifies the source-drain current. The mcdified source-drain current, for example, is read directly. In the alternative, the modification of the source-drain current is reset, for example to zero, by applying an additional voltage to the detached gate (U^ate) cr to the transistor well (Uweli) . The additionally applied voltage thereby represents the read-out signal, which, correlates directly with the work function modification of the sensitive layer. The modification of the work function signal can be read with different variants of a GasFET with air gap, as is illustrated in Figures 5, 6. On the one hand, this can be effected by means of a SGFET or, on the other hand, by means of a CCFET. It is emphasized that with all of these superstructures, what is read is not the modification of the work function of riiie sensitive layer alone during gas application, but always only the difference of the modification of the work function of the sensitive layer and of the oppositely located passivation layer of the transistor of the reference layer. Figures 5, 6 represent two different embodiments of a. GasFET'for reading the work function signal. A basic problem of all gas sensors and also of the described variants lies in the limited selectivity. This means that rhe sensors possibly not only react to the target gas, but also to other gases, which is associated with the terra cross-sensitivity. In some applications, the overlapped gas signals thereby lead ~o a situation, in which the definition of the target gas concentration cannot be made from the sensor signal with sufficient; validity, because the cross-sensitivities distort the target gas concentration in =n unacceptable manner. Or, the one hand, the cause for this cross-sensitivity can ce a characteristic of the sensitive layer itself. On the other hand, however, ether areas, fcr example at the channel surface of uhe transistor, where the reference layer is placed, can also generate distorting signals. These regions then act: like a second sensitive layer and -he gas signal, which is to be read, is greatly influenced. New cross-sensitivities can thus first be created or the existing undesirable reactions of the sensitive layer can be amplified. Furthermore, a concurrent reaction at a reference layer on the target gas can lead to a decrease of :r.e measuring signal, which is determined by the difference :f the two reactions, as well as to an apparent noticeable slowdown of the response cf the sensor. -.- present, a final solution for operating a GasFET without ;ross-sensitivities does not yet exist. - With the use of reference layers, which are as inert as possible, such as LPCVD nitride, direct analyses show -hat in response to critical gases, such as NH3, N02 or in respor.se tc the use of very high humidity, a gas reaction also occurs here. - Even with an ideal gas-insensitive reference layer, cross reactions of the sensitive layer cannot be prevented. present improvements: - By means of application-specific signal evaluation, cross-sensitivities can be corrected to some, extent, which, however, can only be accomplished for simple applications. The use of an additional sensor, which only reacts to the undesired gas, and the signal of which is used for compensating the cross-sensitivity of the actual gas sensor by means of signal processing. However, this clearly increases the system cos~s. .-.r. object cf the invention is to provide a gas sensor on The basis of field-effect transistors, which is as free of cross-sensitivities as possible. The solution of this object occurs with the feature combination of claim 1. Advantageous embodiments can be gathered from the subclaims. In response to the use of a GasFET with selected target gas, sensitive layer, and reference layer, the invention utilizes the specific selection of this reference layer in • such a manner that reactions to gases, which cause cross-sensitivities cancel each other out in an extreme case. The reference layer is a layer at the transistor channel surface, which presently is not considered to be gas-sensitive in any way. A single gas sensor can thus be designed to be free of cross-sensitivities - The invention is based on the insight that, during the detection of gas signals by means of a GasFET, it must be taken into consideration that the raodif icatiori of the work • function signal/ which is measured by means of a GasFET, is not only the reaction of the gas-sensitive layer alone, but also the difference of the reaction caused by the target gas between the sensitive layer and the transistor channel. surface or the reference layer, in the active region of the GasFET. See Fxg . 1 . 2ft A<[> ls represents the mod if ication of the work function, at the sensitive layer, while & 2R expresses the modification cf the work function at the reference layer of the transistor channel during a gas application. The mutual cocrdinaticn in the material selection for the gas-sensitive layer and for the reference layer is to result in the basis for an improved differentiability between target signal and interference signal. For this, the ratio of the sums of these signals is adapted to a maximum distinction possibility. Here, the elimination of the signal on the basis of cross-sensitivities is an extreme case. The modification of the source/drain current (IDS) or cl the control variables Ug^e °r Ufcran5weii are thus directly influenced only by A !A) total/ not by individual work function modifications A$l or Below, exemplary embodiments will be described by means cf schematic figures, which do not limit the invention. Figure 1 Fiqura 2 Figure 3 shows the reaction of the target gas at the sensitive layer and at the reference layer in the active region of the GasFET, shows the reaction of the target gas at both layers, sensitive layer and reference layer, which have the same modification of the work function, shows the reaction of the target gas at bozh layers, sensitive layer and reference layer, whereby different signs or orientations are present, Figure 4 shows a schematic design of a GasFET according to the state of the art, Figures 5, 6 represent two different embodiments of a GaslriT for reading the work function sign: according to the state cf the art, Figure Figure 8 shows an example of a compensation of cross-sensitivities with, the use of a second GasFEt with adapted reference layer, shews a'work function signal of a C02-sensitive layer, for example BaliOs/CuO mixed oxide, under COj and NH3 gas application, Figure 9 shows a C02 GasFET sensor signal under C02 gas and NH3 gas exposure with PECVD nitride as reference material, Figure 10 shows a C02 GasFET sensor signal under C02 gas and NH3 gas exposure with the same sensitive layer according to Figure 8 and with. aluminusi/0 . 5% Cu as adapted reference material . Case Studies First case: For the detection of the target gas it Is essential that the raodif ication of the work function at the sensitive layer is greater than the nodification of the work function at the reference layer. If the reference layer is enxbodiea in such a manner that it does not react with the. target gas, the reference layer thus being inert in this regard/ A3> 2R = 0, then the work function difference corresponds only to the gas reaction at the sensitive layer. => A (a*) total = A A (A*) total = 0 The reaction of the cas at both layers, sensitive layer and reference layer, hereby has the same modification of the work function A$ . Due to the fact that the target gas and the gas, which causes the cross-sensitivity, are not identical, case 1 and case 2 exclude one another . Due to the correct combination of sensitive layer and reference layer it is possible that the target gas doss net react with the reference layer, but only with the sensitive layer. Contrary thereto, the interfering gas reacts with the reference layer and with the sensitive layer and there causes a modification of the work function, which is used for compensation purposes. Third case: Improvement of the target gas sensitivity of the Ga^sFET: 3y means of the correct combination of the gas-sensitive reference layer ar.d the gas-sensitive layer, the reaction of the target gas can cause a modification of the work function with different signs at both surfaces. The reference layer is not inert, but reacts with the target gas. Howev&r, contrary to the reaction at the sensitive layer, the modification of the work function at the reference layer has a sign, which is negative thereto; see Fig. 3. total = A