PROCEDURE FOR MEASURING THE CONCENTRATION OF A GAS
[0001] The invention relates to a procedure for measuring the
concentration of a target gas, wherein a gas sensor is provided
whose sensor signal is dependent on a target gas concentration.
[0002] Such a procedure is disclosed in DE 43 33 875 C2. With
this procedure, a gas sensor is provided which has a silicon
substrate in which a field effect transistor is integrated. The
field effect transistor has a gate electrode which is
conductively connected to a sensor electrode, over which is
arranged a gas sensitive layer, which gas sensitive layer is
separated from the sensor electrode and capacitively coupled to
the sensor electrode by an air gap. A cover electrode is mounted
on the back side of the gas sensitive layer facing away from the
sensor electrode. A surface zone of the gas sensitive layer
facing towards the sensor electrode is brought into contact with
a target gas, which is adsorbed on the surface zone upon contact
therewith. With a change in the concentration of the target gas,
the electron affinity in the surface zone of the gas sensitive
layer changes. Because the sensor electrode is capacitively
coupled to the surface zone, the electric potential on the gate
electrode also changes. The current flow between a drain
connector and a source connector of the field effect transistor
is controlled as a function of the change in potential. In such a
gas sensor the measurement signal is approximately logarithmic to
the target gas concentration, i.e., the measurement sensitivity
decreases as the target gas concentration increases. Because the
presence of oxygen may also interfere with the measurement signal,
the measurement of higher target gas concentrations with this
procedure is problematic.
[0003] The object is therefore to provide a procedure for
measuring the concentration of a target gas that enables a high
measurement precision.

[0004] This object is achieved by provision of a gas sensor whose
sensor signal at a constant temperature is dependent on the
target gas concentration and which has a lower measurement
sensitivity in a first modulation range than in a second
modulation range, wherein the gas concentrations allocated to the
modulation ranges are dependent on the temperature, and wherein
the temperature of the gas sensor is controlled so that the
sensor signal is essentially independent of the target gas
concentration and lies within the second modulation range, and
wherein the temperature of the gas sensor is a measurement for
the target gas concentration.
[0005] With different target gas concentrations, the working
point of the gas sensor is thus advantageously always located in
the modulation range with the greater measurement sensitivity,
whereby different target gas concentrations can be measured with
the same measurement sensitivity. The temperature control is thus
effected so that, with a change in the target gas concentration,
the sensor signal of the gas sensor more or less maintains its
value, so that the target gas concentration can then be
determined from the set temperature. The modulation ranges can be
experimentally measured and/or determined beforehand with the aid
of a mathematical model.
[0006] In a preferred embodiment of the invention, the
temperature is controlled so that the gas sensor is operated at a
working point at which the gas sensor has its greatest
sensitivity to the target gas. An even more precise measurement
of the target gas concentration is then possible.
[0007] It is advantageous if the controlling of the temperature
of the gas sensor takes place only if the detected temperature
lies within a prespecified temperature range, and if the
concentration of the target gas is determined outside this
temperature range as a function of the sensor signal of the gas
sensor and parameters. In a target gas concentration range in
which the gas sensor still has sufficient measurement accuracy
even without temperature control and/or in which controlling the

temperature of the gas sensor is too complex, the temperature
control can thus be deactivated. In this way the procedure can be
carried out more easily and with less energy expenditure.
[0008] In a preferred embodiment of the invention, the
temperature outside the temperature range provided for the
temperature control is set to a constant temperature value.
Measurement inaccuracies caused by fluctuations in ambient
temperature while the temperature control is deactivated can thus
be avoided.
[0009] It is advantageous if the temperature range provided for
the temperature control lies above 60 °C, particularly above 70 °C,
and where appropriate above 80 °C. The temperature of the gas
sensor is then easily adjustable by heating.
[0010] The sensor signal is advantageously captured by measuring
the electron affinity of a gas sensitive layer. Preference is
given to the gas sensitive layer being covered with an
electrically insulating coating that is inert to the target gas,
adhesively bound to the gas sensitive layer, and configured so
that it is permeable to the target gas whose concentration is to
be measured and another gas differing from the target gas and
capable of being adsorbed on the surface zone. The coating can
have different diffusion constants for the target gas and the
other gas, wherein the diffusion constants, the target gas, and
the other gas are selected with respect to each other so that the
sensitivity of the gas sensor to the target gas increases if the
target gas concentration exceeds a prespecified concentration
threshold in the absence of the other gas.
[0011] Preference is given to the sensor signal being
capacitively measured across an air gap on the gas sensitive
layer. In the procedure of the invention, however, the sensor
signal can also be captured by measuring the electrical
resistance of the gas sensitive layer. For doing so, the gas
sensitive layer can be a metallic oxide layer which changes its
resistance upon exposure to gas.

[0012] Illustrative embodiments of the invention are explained in
greater detail in the following, with reference to the drawing.
Shown are:
[0013] Fig. 1 a cross-section of a gas sensor, which has a
SGFET, whose channel zone is capacitively coupled by an air gap
to a gas sensitive layer provided with a passivation coating,
[0014] Fig. 2 a cross-section of a gas sensor, which has a
CCFET, whose sensor electrode is capacitively coupled by an air
gap to a gas sensitive layer provided with a passivation coating,
[0015] Fig. 3 a cross-section of a gas sensor configured as a
Kelvin probe, in which the gas sensitive layer has a passivation
coating,
[0016] Fig. 4 a graphic illustration of a sensor signal (top
curve) and the target gas concentration (bottom curve) of an
illustrative embodiment of the gas sensor, wherein the time t is
graphed on the abscissa and the amplitude S of the sensor signal
of an electric potential sensor is graphed on the left ordinate
and the target gas concentration k is graphed on the right
ordinate,
[0017] Fig. 5 an illustration similar to Fig. 4, wherein,
however, the temperature of the gas sensor is less than that in
Fig. 4,
[0018] Fig. 6 an illustration similar to Fig. 5, wherein,
however, the temperature of the gas sensor is less than that in
Fig. 5,
[0019] Fig. 7 a graphic illustration of a threshold for the
target gas concentration of the gas sensor, wherein the
temperature is graphed on the abscissa and the threshold is
graphed on the ordinate, and
[0020] Fig. 8 a schematic illustration of a control mechanism.
[0021] In a procedure for measuring the concentration of a target
gas, a gas sensor 1 configured as a SGFET, as a CCFET, or as a
Kelvin probe is provided.
[0022] As can be discerned in Fig. 1, the gas sensor configured
as a SGFET has a substrate 2 in which an electric potential

sensor 27 is integrated. The electric potential sensor 27 has a
drain 3 and a source 4, which are arranged in an n-doped
transistor tub. The drain 3 and the source 4 can be composed of,
for example, p-doped silicon. The drain 3 is connected via
electrical conductor paths to a drain connector, which is not
shown in any greater detail in the drawing. In an analogous
manner the source 4 is connected to a source connector. A channel
zone 5 on which is arranged an electrically insulating thin oxide
layer serving as a gate dielectric is formed in the substrate 2
between the drain 3 and the source 4.
[0023] A gas sensitive layer 7, which is preferably composed of a
noble metal, particularly platinum or palladium, is arranged on a
mounting element 6 above the channel zone 5 and; separated from
said channel zone 5 by an air gap 8. A surface zone 9 of the gas
sensitive layer 7 facing towards the channel zone 5 is
capacitively coupled by the air gap 8 to said channel zone 5.
[0024] The mounting element 6 is connected to the substrate 2 on
both sides of the gas sensitive layer 7 via an electrical
insulation layer 10. It is clearly discernible in Fig. 1 that
the mounting element 6 and the gas sensitive layer 7 form a
suspended gate.
[0025] The air gap 4 is connected to the atmosphere surrounding
the gas sensor 1 by at least one opening, which is not shown in
any greater detail in the drawing. The surface zone 9 of the gas
sensitive layer 7 is brought into contact via this opening with a
target gas to be detected, namely hydrogen, and another gas,
namely an electronegative gas, for example atmospheric oxygen.
Upon contact with the surface zone 9, the target gas and the
other gas are adsorbed on the surface zone 9. Upon adsorption of
the target gas, the electron affinity in the surface zone 9
changes, which leads to a change of the electric potential in the
channel zone 5.
[0026] In the illustrative embodiment according to Fig. 1, the
channel zone 5 is configured as an open channel (ISFET) and
capacitively coupled directly to the gas sensitive layer 7 by the

thin-layer oxide and the air gap 8. It is clearly discernible
that the channel zone 5 is arranged on the side of the air gap 8
positioned opposite to the gas sensitive layer 7.
[0027] In the illustrative embodiment according to Fig. 2, the
field effect transistor is configured as a CCFET in which the
channel zone 5 is laterally positioned to the gas sensitive layer
7 in the substrate 2 and capped with a gate electrode 11. To
capacitively couple the channel zone 5 to the gas sensitive layer
7, the gate electrode 11 is connected via an electrical
connecting line 12 to a sensor electrode 13, which is arranged on
an insulation layer 10 on the substrate 2 on the side of the air
gap 8 positioned opposite to the surface zone 9 of the gas
sensitive layer 7. The insulation layer 10 can be, for example, a
Si02 layer. The configuration of the suspended gate of the SGFET
corresponds to that in Fig. 1.
[0028] In the illustrative embodiment shown in Fig. 3, the gas
sensor 1 is configured as a Kelvin probe. The gas sensitive layer
7 is arranged on an electrically conductive mount 14 and has on
its side facing away from the mount 14 a surface zone 9, on which
the target gas can be adsorbed. The surface zone 9 is separated
by an air gap 8 from an electrode 15, with which it forms an
electrical capacity.
[0029] The electrode 15 can be set into oscillation by an
actuator, which is not shown in any greater detail in the drawing.
The electrode 15 thus moves alternately towards and away from the
gas sensitive layer 7, as indicated by the arrow Pf. The
electrode 15 and the mount 14 or the gas sensitive layer 7 are
connected to connectors 16 of an evaluation and modulation device
17. The latter has an electric potential sensor (not shown in any
greater detail), which is connected to the connectors 16 for
measuring the voltage between the gas sensitive layer 7 and the
electrode 15. The evaluation and modulation device 17 also has an
adjustable voltage source which is control-connected to the
electric potential sensor, and via which a countervoltage is
applied between the electric potential sensor and the electrode

15 and/or the mount 15. The countervoltage is selected so that
the electric potential measured by the electric potential sensor
in the center is equal to zero.
[0030] In the gas sensors 1 described above, the surface zone 9
of the gas sensitive layers 7 is always continuously covered by
an electrically insulating polymer coating 18, which is inert to
the target gas and which is preferably composed of
polymethylmethacrylate (PMMA) or polyimide. The coating 18
adheres tightly to the gas sensitive layer 7. The coating 18 is
configured as a thick layer with an approximately constant
thickness, which preferably measures between 0.5 urn and 2.5 urn.
[0031] The coating 18 is permeable to the target gas as well as
to the other gas. The diffusion constants, the target gas, and
the other gas are adapted to each other so that the sensitivity
of the gas sensor 1 to the target gas strongly increases when the
concentration of the target gas exceeds a threshold 19 in the
absence of the other gas. The position of the threshold 19 is
dependent on the temperature.
[0032] It can be discerned in Figs. 4-6 that, at constant
temperature and target gas concentrations 21 lying within a first
concentration range whose upper limit is delineated by the
concentration threshold 19, the sensor signal 20 of the electric
potential sensor 27 always at first increases approximately
logarithmically with the target gas concentration 21. In the
first concentration range, the sensor signal 20 of the electric
potential sensor 27 lies within a first modulation range 29.
[0033] In a second concentration range, which at its lower end
borders the concentration threshold and is considerably narrower
than the first concentration range, the sensor signal 20 strongly
increases at constant temperature. In the second concentration
range, the sensor signal 20 lies within a second modulation range
30, in which the measurement sensitivity of the gas sensor 1 is
greater than in the first modulation range 29. In a third
concentration range, which lies above the second concentration
range and borders it, the sensor signal 20 of the electric

potential sensor 27 at constant temperature remains essentially
constant at a value bordering the second concentration range.
[0034] In Fig. 7 it can be discerned that the concentration
threshold 19 is dependent on the temperature of the layer
sequence formed from the gas sensitive layer 7 and the coating 18
and continuously increases with increasing temperature. The
increase is approximately exponential to the temperature. Where
appropriate, the exponential increase can be linearly
approximated in the relevant range for the concentration
measurement.
[0035] The gas sensors illustrated in Figs. 1-3 in each case have
a thermostat 22, which is only schematically illustrated in Fig.
8, by means of which the temperature of the gas sensitive layer 7
and the coating 18 is adjustable. A control input of the
thermostat 22 is connected to a control signal output 23 of a
control mechanism, which is used to adjust the temperature of the
gas sensitive layer 7 and the coating 18 so that the sensor
signal 20 of the electric potential sensor 27 is always
essentially independent of the target gas concentration 21 and
lies within the second modulation range 30.
[0036] The control mechanism has a comparator 24, which has an
actual value input connected to the electric potential sensor 27
and a target value input connected to a director 25. An output of
the comparator 24 is connected via a modulator 26 to the control
signal output 23. By means of the director 25, a target value 28
is applied to the target value input, which lies within the
second modulation range 30 and thus corresponds to a value that
the sensor signal 20 of the electric potential sensor 27 has at a
target gas concentration 21 above the concentration threshold 19.
[0037] In a first operating mode of the gas sensor 1, the
modulator 26 always controls the thermostat 22 so that in the
event of a deviation between the sensor signal 20 of the electric
potential sensor 27 and the target value 28, the temperature of
the gas sensitive layer 7 and the coating 18 is changed so as to
reduce the deviation. If the sensor signal 20 of the electric

potential sensor 27 coincides with the target value 28, the
temperature of the gas sensitive layer 7 and the coating 18 is a
measurement for the target gas concentration 21.
[0038] In a second operating mode, the temperature of the gas
sensitive layer 7 and the coating 18 is set to a constant value
with the thermostat 22. Alternately, the thermostat 22 can also
be deactivated in the second operating mode, so that the
temperature of the gas sensor 1 then corresponds approximately to
the ambient temperature. The second operating mode is always
activated when the temperature detected by the modulator 2 6 falls
below a prespecified minimum temperature value. This value can be,
for example, ca. 60 - 80 °C.
[0039] In the second operating mode the target gas concentration
21 is determined as a function of the signal value of the sensor
signal 20 of the electric potential sensor 27 and as a function
of parameters, which can be in the form of, for example, a
characteristic line. In the second operating mode the signal
analysis is essentially analogous to that of a standard gas
sensor. As soon as the minimum temperature value is exceeded, the
sensor switches to the first operating mode, in order to
determine the target gas concentration 21 as a function of the
set temperature. The first operating mode is thus used for higher
target gas concentrations 21 and the second operating mode for
lower target gas concentrations 21.
[0040] Preference is given to selection of the first operating
mode if the concentration of the target gas is between 1% and 4%.
The corresponding concentration range can be determined
experimentally. In this range there is an approximately
exponential correlation between the temperature and the target
gas concentration 21. Compared with a standard gas sensor, the
gas sensor 1 of the invention thus enables a considerably better
resolution in this concentration range.

We Claims
1. Procedure for measuring the concentration of a target gas,
wherein provision is made for a gas sensor (1) whose sensor
signal (20) at constant temperature is dependent on a target gas
concentration (21) and has a lower measurement sensitivity in a
first modulation range (29) than in a second modulation range
(30), wherein the gas concentrations allocated to the modulation
ranges (29, 30) are dependent on the temperature, and wherein the
temperature of the gas sensor (1) is controlled so that the
sensor signal (20) is essentially independent of the target gas
concentration (21) and lies within the second modulation range
(30) , and wherein the temperature of the gas sensor (1) is a
measurement for the target gas concentration (21).
2. Procedure as in claim 1, wherein the temperature is controlled
so that the gas sensor (1) is operated at a working point at
which the gas sensor (1) has its greatest sensitivity to the
target gas.
3. Procedure as in claim 1 or claim 2, wherein the temperature of
the gas sensor (1) is only controlled when the detected
temperature lies within a prespecified temperature range, and
wherein the concentration of the target gas outside this
temperature range is determined as a function of the sensor
signal (20) of the gas sensor (1) and parameters.
4. Procedure as in any one of claims 1 through 3, wherein the
temperature outside the temperature range provided for the
temperature control is set to a constant temperature value.
5. Procedure as in any one of claims 1 through 4, wherein the
temperature range provided for the temperature control lies above
60 °C, particularly above 70 °C, and where appropriate above 80 °C.

6. Procedure as in any one of claims 1 through 5, wherein the
sensor signal (20) is captured by measuring the electron affinity
of a gas sensitive layer (7).
7. Procedure as in any one of claims 1 through 6, wherein the
sensor signal (20) is capacitively measured across an air gap (8)
on the gas sensitive layer (7).
8. Procedure as in any one of claims 1 through 7, wherein the
sensor signal (20) is captured by measuring the electrical
resistance of the gas sensitive layer (7).

In a procedure for measuring the concentration of a target gas, a gas sensor (1) is provided whose sensor signal (20) at constant
temperature is dependent on a target gas concentration (21) and has a lower measurement sensitivity in a first modulation range
(29) than in a second modulation range (30). The position of the modulation ranges (29, 30) is dependent on the temperature. The temperature of the gas sensor (1) is controlled so that the
sensor signal (20) is essentially independent of the target gas concentration (21) and lies within the second modulation range (30). The temperature of the gas sensor (1) is then a measurement for the target gas concentration (21).