DE102005008051A1 - Gas sensor with field effect transistor, has porous gas sensitive layer directly applied to insulation of FET located directly above channel region - Google Patents

Abstract

A gas sensor includes an open-pore gas-sensitive layer (8) whose electrical characteristics are read by a FET (11). The sensitive layer is applied directly to the insulation of the FET, passing around an air gap. The insulation is located directly above the channel region of the FET or cooperates indirectly with the FET via a floating gate electrode (20). An independent claim is included for operating a gas sensor.

Description

The The invention relates to a gas sensor for detecting a single gas or a gas mixture in an unknown gas atmosphere. The reading the gas sensor is done by means of field effect transistor (FET).

conventional Solid state gas sensors are based on the evaluation of changes the electrical conductivity of semiconducting metal oxides, the evaluation of electrochemical Reactions of the gases to be detected or the detection of the liberated Reaction enthalpy upon combustion of the target gases. All these functional principles It is common that generally high temperatures are used, which requires a high heat output and the variety of detectable Gases very limited is.

In promising in this regard are gas sensitive field effect transistors because with these at low operating temperatures, down to room temperature, can be worked, and depending on the used gas-sensitive Material a variety of gases can be detected. As a measuring signal For this sensor type, the change of the Work function in the interaction of the gas-sensitive material used with the surrounding gases.

So far, two fundamentally different ways of constructing a gas FET are described, see 8A - 8D , The classic way is to use gas-sensitive metals directly as gate metal. This was already described in 1975 in: [1] Lundström, A hydrogen-sensitive Pd gate MOS transistor; KI Lundström, MSShi varaman, CMStevenson; J. Appl. Phys. 46 (1975) 3876-3881. The hydrogen-sensitive Pd in this case is used as the gate metal, cf. 8A - 8D applied directly to the gate oxide. It is essential for the function that the hydrogen can penetrate into the Pd and also diffuse out again. The diffused hydrogen now generates at the interface for channel isolation, a potential which drives the transistor and affects a change in the source-drain current. Pt can also be used instead of Pd, whereby here too the in and out diffusion of hydrogen is decisive for the formation of the signal at the FET. Thus, this type of gasFET is limited to the measurement of hydrogen or some hydrogen-containing gases, for example ammonia. The use of other than platinum metals is not possible because in such diffusion of hydrogen is not sufficient.

A Modification of this sensor principle is described in: [2], A new split-gate MOS transistor structure for detection of gases; K. Dobos, G. Zimmer, Transducers '85, Int. Conf. on Solid-State Sensors and Actuators. Digest of Technical Papers, June 11-12, 1985. The gate metal is structured so that the gas passes through openings can get into the area of the gate oxide metal surface. This increases the number of measurable gases, but the problem persists, essentially only the platinum metals and e.g. Gold usable are. For all others Metals is the surface with a stable oxide layer, a passivation, or occupied not chemically stable.

The another possibility of the construction of GasFETs introduce those known as SGFET and CCFET Variations in which a gas-sensitive material over a Air gap acts on a FET structure.

Known here is the variant SGFET (suspended / lifted gate), which starts from an FET structure and an air gap between the gate electrode and a present on the Si base body channel passivation of the FET is and the gate electrode is coated with a sensitive material; see [3], DE 4239319.C2 , as well as accompanying 8C and 8D ,

In the variant CCFET, the signal of the work function change is generated in a capacitor with an air gap, of which one electrode is coated with the gas-sensitive material. The potential of the work function change is coupled via the capacitance to the other electrode, which is designed as an electrically floating electrode, and is supplied by this to a separately and usually offset FET transistor, see [4]. DE 4333875.C2 ,

The state of the prior art described is in the 8A - 8D played. There, the respective schematic representation of the previously known variants is outlined.

8A and 8B show the use of a gas-sensitive compact gate 4 (eg Pd). In 8A is outlined the principle structure and in 8B the function. It diffuses sample gas H2 through the Pt electrode and generates at the interface to the channel insulation 3 a potential which drives the transistor.

8C and 8D show variants with air gap. 8C shows the basic structure and 8D the function that the gas 5 diffuses into the air gap and on the surface of the sensitive material causes a potential which acts capacitively on the conductivity in the FET channel.

The second variant has the advantage here that a large number different materials used as a gas-sensitive layer is, so a big one Bandwidth of gases can be detected. It exists here advantageous construction variants, e.g. also the use of thick films allow, as transistor and gate separately from each other Herge and only at the end z. B. together by flip-chip bonding together become.

The The main problem with this design, however, is an air gap with a height from 1-2 μm with a Variation of <50 % to realize. This is necessary because the height of the air gap is an essential parameter of the coupling the change the work function on the FET signal (source - drain current or its amendment) certainly. This causes considerable problems, especially with thick layers Problems, because layers with a roughness or bulge of below 1μm are to produce. Also, the use of thick films with a Layer thickness of more than 20 microns is possible with the flip chip technology only with great effort. It However, even with these small airgap dimensions, this remains Problem that is on the surface of the sensitive layer resulting potential across the air gap capacitive into the channel, which means that only about 1/100 the signal resulting from the gas adsorption effectively for control the channel conductivity is used.

The The problem of currently developed GasFETs lies in the fact that that depends on Construction principle the application or the signal reading is limited.

If on the one hand, the sensitive material directly as a gate electrode without Air gap is used, so is the selection of the sensitive layer materials limited and thus the application is limited to the detection of certain gases.

on the other hand is the sensitive layer, which with a certain distance to Transistor channel surface, So with air gap, applied, although any choice allow the sensitive layer and thus open up a very wide range of applications, wherein however, the signal reading over This air gap is very difficult. Is the air gap not small enough is the proportion of the resulting potential change in the transistor channel low coupled, resulting in very small sensor signals and u.U. to leads to a strong noise, because effects that occur very close to the transistor channel, very strong can couple and superimpose the actual gas signal.

Of the Invention is based on the object at field effect transistors for reading gas reactions on gas-sensitive materials the principle of work function measurement problems of detection weaker Eliminate signals.

The solution happens by the combination of features of claim 1 and of claim 12.

advantageous Embodiments are to be taken from the subclaims.

Of the Invention is based on the finding that a special construction principle and a particular method of making a gas sensitive FET to the solution the task can be provided.

• – Eine offenporig poröse Schicht eines gassensitiven Materials ist in beliebiger Dicke in direktem Kontakt mit der Isolierung eines gatelosen FET-Transistors dargestellt, wie in den 1A und 1B sichtbar. Die Aufgabe wird ebenso durch den direkten Kontakt des gassensitiven Materials in beliebiger Dicke mit der Isolierung der „floating Gate" Elektrode eines CCFET gelöst, bei dem eine Kapazität vorhanden ist und die „floating gate"-Elektrode einem räumlich getrennten Auslesetransistor das betrachtete Potential zuführt, wie in 2 dargestellt.
• – Auf der Rückseite des porösen, sensitiven Materials ist eine Elektrode vorgesehen, welche das sensitive Material mit einem bestimmten elektrischen Potential beaufschlagt und zur Steuerung des Transistors dient.
• – Das poröse Sensormaterial ist offenporös ausgebildet, so dass ein weit reichender Kontakt mit der umgebenden Atmosphäre besteht, dass sich das Messgas durch vertikale oder laterale Diffusion in der porösen Schicht des gassensitiven Materials verteilen kann.

The result is a GasFET, which is characterized by a new sensor structure:

• An open porous layer of a gas sensitive material is shown in any thickness in direct contact with the insulation of a non-gated FET transistor as shown in FIGS 1A and 1B visible, noticeable. The object is also solved by the direct contact of the gas-sensitive material in any thickness with the isolation of the "floating gate" electrode of a CCFET in which a capacitance is present and the "floating gate" electrode supplies the considered potential to a spatially separated read-out transistor, as in 2 shown.
• - On the back of the porous, sensitive material, an electrode is provided, which acts on the sensitive material with a certain electrical potential and serves to control the transistor.
• The porous sensor material is of open-pore design, so that there is extensive contact with the surrounding atmosphere, that the sample gas can be distributed by vertical or lateral diffusion in the porous layer of the gas-sensitive material.

The open-pore sensor material can consist of a non-conductive Material or consist of an electrically conductive material.

For the function of the structure:
Reactions of the gas with the sensitive layer cause a change in the electrical properties of the porous sensitive layer, which are read out by the FET structure. These changes in electrical properties consist of either electrical potentials, which are generated predominantly at the interfaces of the porous layer for isolation or to the electrode, or else Changes in the dielectric properties of the porous layer such as the dielectric constant or the electrical residual conductivity. The claimed structure is schematically in the 1 and 2 shown.

materials for the Electrodes are preferably inert metals such as gold, nickel, silver, Aluminum or chrome. Of great importance are structure and structure the porous one Layer for the signal quality.

Between the sensitive material and the contacts for source and drain is a utmost Insulation resistance required because so a near-current Reading the potential changes to be detected is possible and leakage currents through skipping Source or drain potentials for the deterioration of the gas signal to lead would.

All grains The thick layer should be well connected and good Contact to the electrode on the back to have. The electrode is preferably made of metal.

The Pores are all consistent connected with each other and with the gas atmosphere of the environment.

The Size of the pores must be such that the in and out diffusion of the gases does not obstruct excessively is and thus the response time of the sensor is not affected.

• – An den Grenzflächen der porösen Sensorschicht entsteht durch die Beaufschlagung mit dem Zielgas über die Austrittsarbeitsänderung ein elektrisches Potential. Dieses wirkt dann so, als ob der FET mit einer zusätzlichen „Gate"-Spannung beaufschlagt worden wäre und ändert den Source Drain-Strom, wodurch das Sensorsignal darstellt wird, oder
• – durch die Gasbeaufschlagung ändern sich Raumladungszonen der porösen gassensitiven Schicht und damit die relative Dielektrizitätskonstante des Sensormaterials, d.h. die kapazitive Ankopplung der rückseitigen Elektrode an den FET wird verändert. Wenn nun diese Elektrode mit zeittransienten Potentialen, im einfachsten Fall mit einer sinusförmigen Wechselspannung, beaufschlagt wird, so wirkt sich die Stärke der kapazitiven Ankopplung direkt auf den Source-Drain-Strom aus, wodurch das Sensorsignal darstellt wird. Diese Betriebsart erfordert sehr gute elektrische Isolatoren als sensitive Schicht.

For the operation of a sensor according to the invention, there are two modes of operation:

• - At the interfaces of the porous sensor layer is formed by the application of the target gas on the work function change an electrical potential. This then acts as if the FET had been charged with an additional "gate" voltage and changes the source drain current, thereby representing the sensor signal, or
• By the application of gas, space charge zones of the porous gas-sensitive layer and thus the relative dielectric constant of the sensor material change, ie the capacitive coupling of the rear electrode to the FET is changed. Now, if this electrode with time-transient potentials, in the simplest case with a sinusoidal AC voltage, is applied, the strength of the capacitive coupling acts directly on the source-drain current, whereby the sensor signal is displayed. This mode requires very good electrical insulators as a sensitive layer.

For the business The above structure is a readout with either one single mode of operation or a combination thereof possible.

• – Die Signaleinkoppelung wird deutlich verbessert, da der Luftspalt entfällt. Gegenüber einem herkömmlichen Aufbau mit Luftspalt ist ein um bis zu zwei Größenordungen stärkerer Sensoreffekt zu erwarten.
• – Der Aufbau des GasFET vereinfacht sich, da nicht mehr der große Aufwand zur präzisen Einstellung eines exakten Luftspaltes erforderlich ist. Insbesondere entfallen die sehr hohen Anforderungen an die Qualität der Oberflächenstruktur der Sensorschicht des Luftspaltaufbaus hinsichtlich Rauhigkeit und Aufwölbung.
• – Die bisherigen Anforderungen an eine präzise Definition sowie die Beschränkung der Schichtdicke der Sensorschicht, im Luftspalt durch die Aufbautechnologie gegeben, entfällt.
• – Es können verschiedenste Materialklassen, wie Leiter/Nichtleiter, Metal/Metalloxid/organische Materialien, verwendet werden. Diese ermöglichen die Ausbildung einer porösen Struktur.

The construction according to the invention without an air gap achieves enormous advantages over the prior art, that is to say a gas FET with an air gap:

• - The signal coupling is significantly improved because the air gap is eliminated. Compared to a conventional construction with an air gap, a sensor effect that is stronger by up to two orders of magnitude is to be expected.
• - The structure of the gas FET simplified because no longer the great effort for precise adjustment of an exact air gap is required. In particular, the very high demands placed on the quality of the surface structure of the sensor layer of the air gap structure with regard to roughness and bulge.
• - The previous requirements for a precise definition and the limitation of the layer thickness of the sensor layer, given in the air gap by the construction technology, is eliminated.
• - It can be used a variety of material classes, such as conductors / non-conductors, metal / metal oxide / organic materials. These allow the formation of a porous structure.

• – Durch die niedrige Betriebstemperatur kann der Energieverbrauch soweit gesenkt werden, dass ein Batteriebetrieb oder der direkte Anschluss an Datenbusleitungen möglich ist.
• – Ohne große Probleme können GasFETs mit unterschiedlichen gassensitiven Eigenschaften durch die Auswahl der sensitiven Schicht realisiert werden.
• – Aufgrund der kleinen Bauform können ohne Schwierigkeiten Sensorarrays mit einer großen Zahl von unterschiedlichen Einzelsensoren unterschiedlichster Sensitivität realisiert und auf einem gemeinsamen Sensorchip integriert werden. Sensorarrays in Kombination mit einer adäquaten Auswertung über beispielsweise Chemometrie oder neuronale Netze, sind ein Weg, das Problem der Querempfindlichkeit zu vermindern, die Messgenauigkeit und den Messbereich zu erhöhen und die Zuverlässigkeit zu verbessern.
• – Eine Integration der Auswerteelektronik bzw. eines Teiles der Elektronik in den Gassensor ist ohne Schwierigkeiten möglich.

In addition, there are the general advantages of gas sensors based on GasFET:

• - The low operating temperature can reduce the power consumption to the extent that battery operation or direct connection to data bus lines is possible.
• - Without much difficulty GasFETs with different gas-sensitive properties can be realized by selecting the sensitive layer.
• - Due to the small size of the sensor arrays can be realized with a large number of different individual sensors of different sensitivity and integrated on a common sensor chip without difficulty. Sensor arrays in combination with an adequate evaluation via, for example, chemometrics or neural networks, are a way to reduce the problem of cross sensitivity, to increase the measuring accuracy and the measuring range and to improve the reliability.
• - An integration of the transmitter or a part of the electronics in the gas sensor is possible without difficulty.

in the The invention will be described below with reference to the accompanying drawings not restrictive Figures embodiments described:

1A B show the basic structure of a gas sensor according to the invention starting from an SGFET, but without an air gap; the structure in 1A and how it works in 1B .

2 shows the basic structure of a gas sensor according to the invention starting from a CCFET structure with direct application of a porous gas-sensitive layer 8th on the canal insulation 3 .

3A , B show design variants in flip-chip technology with different connection variants for the sensitive layer 8th contacting electrode 9 using a non-conductive substrate 141 .

4A , B show construction variants in flip-chip technology with different connection variants for the sensitive layer contacting electrode using a conductive substrate 142 .

5 shows a construction variant in flip-chip technology with a gas-sensitive layer 8th enclosing form of the substrate for mechanical stabilization of the layer using a non-conductive substrate 141 .

6A B show a construction variant of the invention in which the porous gas-sensitive layer 8th is applied directly on the Si chip and then an electrode layer is deposited; the example of the CCFET with a readout transistor; corresponding 6A with the overall structure with gas-tight and structured electrode, which is structured so that it represents a coherent area, eg in the form of a grid and accordingly 6B with details of the electrode when using a thin electrode which does not fill the pores of the gas-sensitive layer, but nevertheless has a coherent structure over the two-dimensional surface,

7A , B show construction variants with additional gas-active layer; 7A with cover layer of the surface of the Si chip, using the example of the CCFET as a readout transistor and 7B with additional gas-active layer, here as covering layer, which prepares the measuring gas before it is fed to the actual gas sensor, ie filters,

8A D show schematic representations of the prior art; 8A with the use of a gas-sensitive compact gate, eg from Pd, with the sketch of the basic structure or after 8B , with the function, and 8C , D with variants with air gap; 8C shows the basic structure and 8D the function.

The invention is in the 1 and 2 explained in detail. 1A shows a cross section through a readout transistor consisting of bulk silicon 1 , Transistor tub 2 , Source S and Drain D, the channel insulation 3 , as well as the directly on the channel insulation applied gas-sensitive layer 8th , Furthermore, an electrode applied on the back of the sensitive layer 9 seen. The functioning of this construction results from 1B , where the gas 5 , a measurement gas containing one or more target gases, can diffuse into the porous-porous layer. In the 1A , B are the gas-sensitive layers applied directly to the channel insulation, so that no air gap is present, which would be produced with great effort,

2 also shows a FET according to the invention, the function of which again comes into play when reading out a sensor signal in the gas detection by a gas-sensitive layer. The readout transistor 11 is in this case laterally offset from the gas-sensitive layer with the rear electrode 9 in the arrangement of bulk silicon 1 and insulation 12 positioned. The sensor signal generating effects in conjunction with the sensitive layer 8th become a floating gate electrode 20 directed to the readout transistor. A control electrode 10 is in the range of the gate electrode 20 and the sensitive layer present.

The 3A and 3B show construction variants of described gas sensors whose sensor signal is read out by means of FET. In 3A is inside the bulk silicon 1 the transistor tub 2 available, as well as an integrated electronics 16 , The gas-sensitive layer lies above the FET, being on the channel insulation 3 is applied without the representation of an air gap. Furthermore, there is a backside electrode 9 shown in the case of 3A , B with a non-conductive substrate 141 connected is. In order not to obstruct the gas diffusion is at least one gas inlet 13 formed in the substrate. The contacting of the electrode 9 happens either via conductive flip-chip bumps 152 or via a bonding wire 18 ,

The 4A , B correspond largely to the 3A , B, but with a conductive substrate 142 is used. Consequently, on the via 19 corresponding 3B be waived.

5 shows an embodiment with a non-conductive substrate 141 , where the electrode 9 via a via 19 and a bonding wire 18 again with the electrode 17 connected is. The flip-chip bumps 151 are not conductive in this case. If the sensitive layer is not sufficiently mechanically stable can accordingly 5 a comprehensive stabilization by the inclusion of the sensitive layer 8th in one hand, the channel insulation and on the other hand, the non-conductive substrate are made.

8A -D represent the previously known prior art. It will be in the 8A . 8B each gas-sensitive FETs shown their gas-sensitive layer 4 consists of a solid compact material, for example Pd or PT, wherein the sample gas can diffuse in and generates a corresponding potential. In the 8C . 8D is the sensitive layer 6 with a gate electrode 7 combined and there is an air gap.

6A , B show a section through a gas-sensitive FET with floating gate electrode 20 , open-pored layer 8th and structured electrode 91 , as well as a detail in the form of the porous grain structure 22 with surface metallization 21 represents.

7A , B show CCFETs with additional gas-active elements 23 . 231 , These can be positioned in such a way that they are located upstream of the gas-sensitive layer relative to the measurement gas stream 8th or between gas-sensitive layer 8th and the insulation 12 lie.

For the construction of superstructures offers on the one hand, the hybrid structure by means of flip-chip technology. In the 3A and 3B becomes the construction with a non-conductive substrate 141 shown with metallization by way of example. First, the non-conductive substrate 141 prepared with metallization and porous gas-sensitive layer, on the other hand in standard Si technology, the gatelose readout transistor is shown. Both are then connected in a flip-chip process so that the porous gas-sensitive layer rests directly on the insulation of the transistor channel.

The gas 5 has sideways, in the places where no flip-chip bump 151 . 151 is admitted to the gas-sensitive layer. To improve the gas transport in the sensitive layer substrates 141 . 142 be used with reduced thickness and etched openings. The substrates are connected to the chip by flip-chip bonding using gold bumps or polymer bumps. The electrical connection of the electrode 17 the substrate to the Si chip or the electrical circuit is carried out either by conductive flip-chip bumps, such as gold, by metal-filled polymer bumps or by a wire bond 18 , corresponding 3B , where then a via 19 in the substrate 141 is needed.

It is advantageous to integrate the electrical circuit directly into the Si chip, as in the 3A and 3B indicated. The electrical circuit here optionally includes a drive and readout electronics for the FET, a signal amplification electronics, a signal processing electronics, for example, to linearize the characteristic or drift compensation or even signal digitizations to output a digital output signal.

In a further construction variant are conductive substrates 142 , for example of Si or of metals, as a carrier for the sensitive layer 8th used. In this case, the substrate itself fulfills the role of the gas-sensitive layer contacting electrode and an additional metallization can be omitted. For the electrical connection to the electronics, in turn, the two variants discussed above can be selected.

Some gas-sensitive materials may not exhibit good mechanical stability or lack of film adhesion when prepared as an open-pored layer. Therefore, it makes sense to incorporate a cavity into the substrate receiving the gas-sensitive layer into which the gas-sensitive layer is then introduced. This serves for the mechanical stabilization of the sensitive layer during preparation and also during sensor operation. The substrate may also be shaped such that it itself rests directly on the Si chip in order to keep mechanical loads away from the gas-sensitive layer sensitive to mechanical stresses during the flip-chip joining process. In addition, recesses may be provided in the substrate in order to take into account the space requirement of the flip-chip bonding sites. A schematic drawing can be found in 5 ,

All Construction variants can for different Transistor variants, both for the SGFET construction, for the CCFET setup as well as for possible others Variants of the Auslese FETs are used.

The 6A shows a structure without substrate for the metal electrode. This structure can be selected if the gas-sensitive thick film sufficiently well adheres, and in itself has good mechanical stability, so that a mechanical African pressing is unnecessary. The starting point here is the Si chip on which the porous gas-sensitive layer is prepared in a first step. Subsequently, the metal electrode is applied as a thin film (sputtering, vapor deposition) or as a thick film (screen printing technique). To improve the gas transport in the layer, it is structured, ie with openings 13 Mistake. Through these openings, the gas penetrates into the porous structure of the gas-sensitive layer 8th a, the metal layer itself may be made gas-tight and about 1-5μm thick or thin, typically be made 50-200nm thick. In the latter case, although the surface of the porous layer is provided with a continuous conductive electrode, the metallization is too thin to close the open pores of the structure. The fact that the pores remain open, is an optimal gas access to the gas-sensitive layer ge guaranteed.

• – Es bietet sich auch dann an, anstelle von relativ inerten Metallen wie Gold auch katalytisch wirksame Metalle wie z. B. Platinmetalle als Elektrode zu verwenden, um damit die Gassensitivität zu beeinflussen. Hiermit können gezielt reaktive Gase abreagiert oder träge Gase aktiviert werden,
• – Ersatz der Standard-Transistorisolierung durch eine gasaktive Schicht 23. An der gassensitiven Grenzfläche zwischen dem porösen gassensitiven Material und der Si-Chip Oberfläche bzw. der Elektrodenoberfläche können zusätzlich reaktive Schichten eingebracht werden. Hiermit wird die Gassesensitivität des Grenzflächenpotentials im Einzelfall optimiert, wie es in 7B dargestellt ist.

Improvement of gas sensitivity:
In order to achieve an improvement in gas sensitivity beyond the choice of suitable gas-sensitive material, two approaches are provided:

• - It is also appropriate, instead of relatively inert metals such as gold and catalytically active metals such. As platinum metals to use as an electrode, so as to influence the gas sensitivity. Hereby reactive reactive gases can be reacted off or inert gases can be activated,
• - Replacement of the standard transistor isolation by a gas-active layer 23 , Reactive layers can additionally be introduced at the gas-sensitive interface between the porous gas-sensitive material and the Si-chip surface or the electrode surface. In this way, the gas sensitivity of the interface potential is optimized on a case by case basis 7B is shown.

The gas-active layer 231 can also over the electrode 91 be applied, as well as filters for catalysis. In this case, it is the gas sensitive area through the electrode 91 electrically shielded, which is why the occurring there, possibly unwanted potentials are not included.

• – katalytische Eigenschaften und verändert das dem Sensor zugeführte Messgas, durch z.B. Wegoxidation reduzierend wirkender Gase die Querempfindlichkeiten bewirken, wie z.B. Lösungsmittel, chemische Aktivierung reaktionsträger Gase, damit diese besser vom Sensor nachgewiesen werden können. Materialien sind Dispersionen von Katalysatoren auf einem Träger,
• – Gasadsorbierende Eigenschaften, d.h. Störgase werden an Ihr adsorbiert und gelangen nicht zur porösen gassensitiven Schicht. Materialien hierfür sind z.B. Aktivkohle oder Zeolithe.

In variants with porous or apertured electrode, an additional gas-permeable gas-active layer may be provided above the electrode, facing the sample gas 231 to be upset, cf. 7B , This has at operating temperature of the sensor:

• - Catalytic properties and changes the sensor gas supplied to the sensor, for example, by path oxidation of reducing gases cause cross-sensitivities, such as solvents, chemical activation of reaction gases, so that they can be better detected by the sensor. Materials are dispersions of catalysts on a support,
• - Gasadsorbierende properties, ie interfering gases are adsorbed to her and do not reach the porous gas-sensitive layer. Materials for this are, for example, activated carbon or zeolites.

The Gas-active layer is in this case by the gas-sensitive area by the electrode electrically shielded, which is why the occurring at her u.U. undesirable Potentials are not included.

• – Die Transistoreigenschaften, sowie die gassensitiven Eigenschaften sind abhängig von der Betriebstemperatur. Daher ist es sinnvoll zur Kompensation dieser Effekte in der Sensorsignalverarbeitung einen Temperatursensor vorzusehen, der in den Si-Chip integriert ist.
• – Zur Vermeidung von Temperatureinflüssen, zur Optimierung der Funktion mancher Sensormaterialien sowie zur Vermeidung von Betauung auf dem Sensor ist es u.U. sinnvoll die Temperatur auf einem Niveau über der Umgebungstemperatur zu stabilisieren. Daher ist die Verwendung eines Heizelementes vorzusehen. Dieses ist entweder in den Si-Chip oder aber in das Substrat zu integrieren.
• – Transistoraufbauten mit in Kontakt mit der Umgebung stehenden elektrisch aktiven Flächen neigen zu Drifterscheinungen. Daher ist es. sinnvoll für jeden mit der gassensitiven Schicht versehenen Messtransistor einen Referenztransistor vorzusehen, der mit einer nicht gassensitiven Schicht versehenen ist und damit nur die Drifteffekte anzeigt. Durch Kombination beider Signale können die Drifteffekte weitgehend eliminiert werden, genauso, wie Temperatureinflüsse.
• – Wie schon in den 3A, B erwähnt ist es ggf. sinnvoll, in den Si-Chip eine Elektronik zu integrieren. Diese verstärkt die schwachen Sensorsignale, verarbeitet die Sensorsignale, wie Driftkompensation, Kennlinien Linearisierung, führt Kompensationen bei Verwendung von Referenztransistoren durch verarbeitet das Signal des Temperaturfühlers, steuert die Heizung an.

To improve functionality, the following elements may be provided:

• - The transistor properties, as well as the gas-sensitive properties are dependent on the operating temperature. Therefore, it is useful to compensate for these effects in the sensor signal processing to provide a temperature sensor which is integrated into the Si chip.
• - To avoid temperature effects, to optimize the function of some sensor materials and to prevent condensation on the sensor, it may be useful to stabilize the temperature at a level above the ambient temperature. Therefore, the use of a heating element is provided. This can be integrated either in the Si chip or in the substrate.
• - Transistor structures with standing in contact with the environment electrically active surfaces are prone to drift phenomena. Therefore, it is. makes sense to provide a reference transistor for each provided with the gas-sensitive layer measuring transistor, which is provided with a non-gas sensitive layer and thus indicates only the drift effects. By combining both signals, the drift effects can be largely eliminated, as well as temperature effects.
• - Like in the 3A , B mentioned, it may be useful to integrate electronics into the Si chip. This amplifies the weak sensor signals, processes the sensor signals, such as drift compensation, linearization characteristics, performs compensations when using reference transistors processed by the signal of the temperature sensor, controls the heater.

Literature:

• [1] Lundström, A hydrogen-sensitive Pd-gate MOS transistor; K.I. Lundström, M. S. Shivaraman, C. M. Stevenson; J. Appl. Phys. 46 (1975) 3876-3881,
• [2] K. Dobos, G. Zimmer, "A new split-gate MOS transistor structure for detection of gases "; Transducers' 85, Int. Conf. on Solid-State Sensors and Actuators. Digest of Technical Papers, June 11-12 1985th
• [3] DE 4239319 C2
• [4] DE 4333875 C2

Claims (18)

Gas sensor with field effect transistor (FET) for signal detection with a gas-sensitive layer, the changed electrical properties are readable with existing target gas through the FET, characterized in that the sensitive layer is open-pore porous and formed in any thickness and bypassing an air gap directly on an insulation a FET is positioned directly over the channel region of the FET or indirectly via a floating floating gate electrode ( 20 ) with the FET ( 11 ) works together. Gas sensor according to claim 1, wherein the back of the sensitive material, an electrode is provided in order to the sensitive material a bestimm create electrical potential. Gas sensor according to one of the preceding claims, in the open-pored sensitive layer of a non-conductive Material exists. Gas sensor according to claim 3, wherein the electrode made of an inert metal such as gold, copper, nickel, silver, aluminum or chromium exists. Gas sensor according to one of the preceding claims, in the sensitive layer of a firmly connected with each other Elements exists and a stable contact to the back electrode having. Gas sensor according to one of the preceding claims, in the porosity the sensitive layer is adapted so that the diffusion the measuring gas is not obstructed. Gas sensor according to one of the preceding claims, in the porous porous sensitive layer for their mechanical stabilization in one Room with stable walls is housed and these walls openings have to diffusion of the sample gas. Gas sensor according to one of the preceding claims, in the between the insulating layer and the porous layer an additional gas-active layer is introduced. Gas sensor according to one of the preceding claims, in the over the porous one Detection layer an additional gas-active layer is attached, which the sample gas before feeding to the modified actual gas sensor. Gas sensor according to one of the preceding claims, in as a carrier for the sensitive layer conductive Substrates are used. Gas sensor according to one of the preceding claims, in to compensate for drift, temperature or transverse sensitivities several FET gas sensors with porous sensitive layers are present. Method for operating a gas sensor accordingly one of the claims 1-11, characterized in that for generating a sensor signal an additional Gate voltage is detected, which is formed by by existing target gas at the interfaces of porous sensitive layer creates an electrical potential. Method for operating a gas sensor accordingly one of the claims 1-11, characterized in that for generating a sensor signal the capacitive coupling of a rear electrode to the FET is detected at existing target gas, since space charge zones the open-porous change sensitive layer and thus change the relative dielectric constant. The method of claim 13, wherein the backside Electrode with time transient potentials is applied, so that's the strength the capacitive coupling directly to a source-drain current of FET and generates the sensor signal. A method according to claim 13 or 14, wherein the Representation of the sensitive layer of high-quality insulators be used. Method according to one of claims 12-15, wherein a combination from the detection of an additional Gate voltage and the detection of the capacitive coupling of an electrode for generating a sensor signal is used. A method according to any one of claims 12-16, wherein a plurality of different individual sensors in a sensor arrangement a common sensor chip are integrated. The method of claim 17, wherein the evaluation in connection with a sensor arrangement using chemometry or use of neural networks happens.