Mikael Löfdahl

Using a MISiC-FET sensor for detecting NH/sub 3/ in SCR systems

One way to decrease the emitted levels of NO/sub x/ from diesel engines is to add NH/sub 3/ in the form of urea to the exhausts after combustion. NH/sub 3/ will react with NO/sub x/ in the catalytic converter to form N/sub 2/ and water, which is called selective catalytic reduction (SCR). The amount of NH/sub 3/ added may be regulated through closed-loop control by using an NH/sub 3/ sensor. The metal-insulator silicon-carbide field-effect transistor (MISiC-FET) sensor has previously been tested for this application and has been shown to be sensitive to NH/sub 3/. Here, the sensors have been further studied in engine SCR systems. Tests on the cross sensitivity to N/sub 2/O and NO/sub 2/, and studies concerning the influence of water vapor have been performed in the laboratory. The difference between Ir and Pt films, with regard to catalytic activity, has also been investigated. The sensors were found to be sensitive to NH/sub 3/ in diesel engine exhausts. The addition of urea was computer controlled, which made it possible to add NH/sub 3/ in a stair-like fashion to the system and detect it with the MISiC-FET sensors. The presence of water vapor was shown to have the largest effect on the sensors at low levels and the NH/sub 3/ response was slightly decreased by a background level of NO/sub 2/.

Gas response dependence on gate metal morphology of field-effect devices

The dependence of the gas response on the gate metal morphology of field-effect gas sensors has been investigated in a new systematic way by using a scanning light pulse technique (SLPT) together w ...

Difference in hydrogen sensitivity between Pt and Pd field-effect devices

An explanation is given for the large differences in the hydrogen sensitivity in air observed for gas sensitive field-effect devices with palladium and platinum gates, respectively. It is demonstrated that this difference is mainly due to a difference in the hydroxyl formation rate between the two metals. The water production rates are, however, almost the same for the two metals. The considerably smaller sensitivity of platinum devices in air is then due to the much lower steady-state hydrogen atom concentration on the platinum surface compared with the palladium surface. This leads to a smaller coverage of hydrogen atoms at the metal–oxide interface and thus a smaller response of the device at a given hydrogen concentration in air.

Investigations on the possibilities of a MISiCFET sensor system for OBD and combustion control utilizing different catalytic gate materials

Different catalytic materials, like Pt and Ir, applied as gate contacts on metal insulator silicon carbide field effect transistors—MISiCFET—facilitate the manufacture of gas sensor devices with differences in selectivity, devices which due to the chemical stability and wide band gap of SiC are suitable for high temperature applications. The combination of such devices in a sensor system, utilizing multivariate analysis/modeling, have been tested and some promising results in respect of monitoring a few typical exhaust and flue gas constituents, in the future aiming at on board diagnostics (OBD) and combustion control, have been obtained.

A combinatorial approach for field-effect gas sensor research and development

A continuous two-dimensional variation of the properties of the gas sensitive layer of a metal-insulator-semiconductor structure has been analyzed with the scanning light pulse technique. This technique allows a lateral resolution of the local gas response and is, therefore, well suited for analyzing a device where each point of the gas sensitive layer has unique properties. The results show that this method has the potential to optimize the thickness combination of the metal films for a double-layer component to improve important sensor properties such as sensitivity, selectivity, and stability using a drastically reduced number of components.

MISiCFET chemical sensors for applications in exhaust gases and flue gases

A chemical gas sensor based on a silicon carbide field effect transistor with a catalytic gate metal has been under development for a number of years. The choice of silicon carbide as the semiconductor material allows the sensor to operate at high temperatures, for more than 6 months in flue gases at 300degreesC and for at least three days at 700degreesC. The chemical inertness of silicon carbide and a buried gate design makes it a suitable sensor technology for applications in corrosive environments such as exhaust gases and flue gases from boilers. The selectivity of the sensor devices is established through the choice of type and structure of the gate metal as well as the operation temperature. In this way NH3 sensors with low cross sensitivity to NOx have been demonstrated as potential sensors for control of selective catalytic reduction (SCR) of NOx by urea injection into diesel exhausts. Here we show that sensors with a porous platinum or iridium gate show different temperature ranges for NH3 detection. The hardness of the silicon carbide makes it for example more resistant to water splash at cold start of a petrol engine than existing technologies, and a sensor which can control the air to fuel ratio, before the exhaust gases are heated, has been demonstrated. Silicon carbide sensors are also tested in flue gases from boilers. Efficient regulation of the combustion in a boiler will decrease fuel consumption and reduce emissions.

Substrate Bias Amplification of a SiC Junction Field Effect Transistor with a Catalytic Gate Electrode

The drain current-voltage (I-d-V-D) characteristics of a chemical gas sensor based on a catalytic metal insulator silicon carbide field effect transistor (SiC-FET) were measured in H-2 or O-2 ambient while applying negative substrate bias, V-sub, at temperatures up to 600degreesC. An increase in the negative V-sub gives rise to an increase of the drain voltage at a given drain current level, which can be used to adjust the device baseline. In addition, we found that the difference in drain voltage between H-2 and O-2 ambient at a given drain current level (the gas response to H-2) increases for an increased negative substrate bias. By modifying an equation for the drain current in a SIT (static induction transistor), the influence of substrate bias on the amplification factors, mu and eta, was estimated using the temperature dependence of the I-d-V-D characteristics. From this, the effect of substrate bias on the gas response to hydrogen was calculated. It was clarified that the increase in the gas response caused by the negative substrate bias is due to a substrate bias dependence of the amplification factor of the short channel device.

MISiC-FET devices with bias controlled baseline and gas response

The drain current-voltage characteristics of a chemical gas sensor based on a catalytic metal insulator silicon carbide field effect transistor (MISiC-FET) was measured in H/sub 2/ and O/sub 2/ ambient while applying negative substrate bias at temperatures up to 600/spl deg/C. It is reported that the gas sensitivity can be amplified and the position of the base-line controlled by applying a negative substrate bias to the MISiC-FET device, which is a buried short channel device. This is possible in a wide range of drain current levels and over a large temperature range.

MISiCFET sensor arrays for on line diagnosis

Catalytic Metal Insulator Silicon Carbide Field Effect Transistor (MISiCFETs) sensors can be processed as a sensor array on a SIC-chip. Different selectivity of the sensors is achieved by the use of different combinations of catalytic metals and insulators. The response patterns are used to evaluate for example different states of a combustion (low/high emissions, efficient/less efficient combustion) or monitoring different components in the exhausts from the combustion.

2D-evaluation of the gas response of a RhPd-MIS device

A two dimensional variation of the properties of the gas sensitive layer of a metal-insulator-semiconductor (MIS) structure has been analysed with the Scanning Light Pulse Technique (SLPT). This technique allows a lateral resolution of the local gas response for such a device. The results indicate that this method can be used in order to choose an appropriate thickness combination of the metal films for a double layer component so that the sensor properties can be optimised. Furthermore, the results indicate that optimal properties of the gas sensitive layer differ from one gas to another. This is exemplified with hydrogen and ethanol.