Christian Bur

Gas mixing apparatus for automated gas sensor characterization

We developed a computer-controlled gas mixing system that provides automated test procedures for the characterization of gas sensors. The focus is the generation of trace gases (e.g. VOCs like benzene or naphthalene) using permeation furnaces and pre-dilution of test gases. With these methods, the sensor reaction can be analyzed at very low gas concentrations in the ppb range (parts per billion) and even lower. The pre-dilution setup enables to cover a high concentration range (1:62?500) within one test procedure. Up to six test gases, humidity, oxygen content, total flow and their variation over time can be controlled via a LabVIEW-based user-interface.

Increasing the selectivity of Pt-gate SiC field effect gas sensors by dynamic temperature modulation

Based on a diode coupled silicon carbide field effect transistor (FET) with platinum as catalytic gate material, the influence of dynamic temperature modulation on the selectivity of gas analysis sensors FETs has been investigated. This operating mode, studied intensively for semiconductor gas sensors, has only recently been applied to FETs. A suitable temperature cycle for detection of typical exhaust gases (CO, NO, C3H6 , H2, NH3) was developed and combined with appropriate signal processing. The sensor data were evaluated using multivariate statistics, e.g., linear discriminant analysis. Measurements have proven that typical exhaust gases can be discriminated in backgrounds with 0, 10, and 20% oxygen. Furthermore, we are able to quantify the mentioned gases and to determine unknown concentrations based on training data. Very low levels of relative humidity below a few percent influence the sensor response considerably but for higher levels the cross interference of humidity is negligible. In addition, experiments regarding stability and reproducibility were performed.

Detecting Volatile Organic Compounds in the ppb Range With Gas Sensitive Platinum Gate SiC-Field Effect Transistors

In this paper, the use of a platinum gate gas-sensitive SiC field-effect transistor (SiC-FET) was studied for the detection of low concentrations of hazardous volatile organic compounds (VOCs). For this purpose, a new gas mixing system was realized providing VOCs down to sub-parts per billion levels with permeation ovens and gas predilution. Benzene, naphthalene, and formaldehyde were chosen as major indoor air pollutants and their characteristics are briefly reviewed. Measurements have shown that the selected VOCs can be detected by the SiC-FET in the parts per billion range and indicate a detection limit of ~1 ppb for benzene and naphthalene and ~10 ppb for formaldehyde in humid atmospheres. For 10-ppb naphthalene at 20% r.h., the sensor response is high with 12 mV, respectively, a relative response of 1.4%. Even in a background of 2-ppm ethanol, the relative response is still 0.3%. Quantification independent of the humidity level can be achieved using temperature cycled operation combined with pattern recognition, here linear discriminant analysis. Discrimination of benzene, naphthalene, and formaldehyde is also possible.

Hierarcical strategy for quantification of NO x in a varying background of typical exhaust gases

Silicon carbide based metal insulator field effect transistors (MISiC FET) with a catalytic gate metallization were used in temperature cycled operating mode (TCO) in order to improve the selectivity of the sensor. This approach obtaining multiple data from a single sensor, therefore known as a virtual multisensor, was originally developed for metal oxide sensors but earlier work proved the suitability for MISiC FETs as well. This strategy was now tailored to the quantification of NOx in a mixture of typical exhaust gases (CO, HC, plus NH3). Data was evaluated with multivariate statistics e.g. Linear Discriminant Analysis. In order to suppress the influence of varying background a hierarchical approach was used. Results show that quantification of NOx is possible even in a changing background.

Silicon Carbide Field Effect Transistors for Detection of Ultra-Low Concentrations of Hazardous Volatile Organic Compounds

Gas sensitive silicon carbide field effect transistors with nanostructured Ir gate layers have been used for the first time for sensitive detection of volatile organic compounds (VOCs) at part per billion level for indoor air quality applications. Formaldehyde, naphthalene, and benzene have been used as typical VOCs in dry air and under 10% and 20% relative humidity. A single VOC was used at a time to study long-term stability, repeatability, temperature dependence, effect of relative humidity, sensitivity, response and recovery times of the sensors.

Temperature Cycled Operation of SiC Field Effect Gas Sensors: Increasing the Selectivity for Improved Sensor Systems

In order to detect and quantify nitrogen oxides (NOx) in a mixture of typical exhaust gases a diode coupled FET has been investigated using Temperature Cycled Operation. This approach, originally developed for metal oxide gas sensors, is quite new for GasFETs but preliminary studies proved that it is suitable for GasFETs as well. In this paper the basic concept was improved by a temperature cycle tailored to NOx detection. Multivariate statistics have been used to evaluate the sensor data. Measurements have shown that with a piecewise extraction of features, a quantification of NO with additional NO2 is possible in the background of exhaust gases. Thus, the detection of NOx and especially the determination of the concentration can be improved.

Influence of a Changing Gate Bias on the Sensing Properties of SiC Field Effect Gas Sensors

Field effect transistors based on silicon carbide have previously been used with temperature cycled operation to enhance the selectivity. In this study the influence of a changing gate bias on the sensing properties of a platinum gate FET has been studied in order to extend the virtual multi-sensor approach. The sensor exhibits gas specific hysteresis when changing the gate bias indicating that additional information regarding selectivity is contained in the transient behavior. Measurements also showed that especially the shape of the sensor signal changes dramatically with different gas exposures (e.g. H2, CO or NH3) during relaxation after step changes of the gate bias. The changing shape primarily reflects the gas itself and not the concentration so that the selectivity of the sensor is increased.

Exploring the gas sensing performance of catalytic metal/ metal oxide 4H-SiC field effect transistors

Gas sensitive metal/metal-oxide field effect transistors based on silicon carbide were used to study the sensor response to benzene (C6H6) at the low parts per billion (ppb) concentration range. A combination of iridium and tungsten trioxide was used to develop the sensing layer. High sensitivity to 10 ppb C6H6 was demonstrated during several repeated measurements at a constant temperature from 180 to 300 °C. The sensor performance were studied also as a function of the electrical operating point of the device, i.e., linear, onset of saturation, and saturation mode. Measurements performed in saturation mode gave a sensor response up to 52 % higher than those performed in linear mode.

Detecting Volatile Organic Compounds in the ppb range with platinum-gate SiC-Field Effect Transistors

In this work, the use of a platinum gate gas-sensitive SiC Field Effect Transistor (SiC-FET) was studied for the detection of low concentrations of hazardous Volatile Organic Compounds (VOC). For this purpose, a new gas mixing system was built providing VOCs down to sub-ppb levels by permeation ovens and gas pre-dilution. Measurements have shown that benzene, naphthalene and formaldehyde can be detected in the ppb range and indicate a detection limit of 1-2 ppb for benzene and naphthalene. The sensitivity is high with a response of 5.5 mV for 10 ppb naphthalene in a humid atmosphere (at 20% relative humidity) and with additional 2 ppm ethanol the response to naphthalene was still 1.3 mV. Formaldehyde can be detected down to approximately 100 ppb under humid conditions. This is the first time that a metal gated SiC-FET was used to detect hazardous VOCs in the low ppb range making SiC-FETs suitable candidates for indoor air quality applications.

New method for selectivity enhancement of SiC field effect gas sensors for quantification of NOx

A Silicon Carbide based enhancement type field effect transistor with porous films of Iridium and Platinum as gate metallization has been investigated as a total NOx sensor operated in a temperature cycling mode. This operating mode is quite new for gas sensors based on the field effect but promising results have been reported earlier. Based on static investigations we have developed a suitable T-cycle for NOx detection in a mixture of typical exhaust gases (CO, C2H4, and NH3). Significant features describing the shape of the sensor response have been extracted allowing determination of NOx concentrations in gas mixtures. Multivariate statistics (e.g. Linear Discriminant Analysis) have been used to evaluate the multidimensional data. With this kind of advanced signal processing the influence of sensor drift and cross sensitivity to ambient gases can effectively be reduced. Thereby, we were able to detect NOx and furthermore determine different concentrations of NOx even in mixtures with typical exhaust gases. It can be concluded that the performance of field effect gas sensors for NOx determination can be enhanced considerably.