Field-effect transistors are used as chemical or biological sensors for a wide range of applications since they offer a number of key benefits: they are small, they can be manufactured at low cost and they consume very little energy. Researchers at Fraunhofer EMFT are working on a new FET sensor concept for carrying out measurements in both liquid and gaseous media. The novel set-up is to simplify packaging and allow more flexible measurements.
Conventional FET-based gas sensors measure the electrical contact potential difference between the substrate and a sensor layer mounted opposite. However, this set-up is prone to drift effects in gaseous mixtures with a high level of humidity and has a low level of sensitivity due to the finite air gap thickness.
Instead of this capacitive structure, researchers are using a miniaturized Faraday cup (600 x 100 μm², 450 μm deep) as the sensor element in their new concept: this has an embedded and entirely electrically insulated floating electrode. The electric potential of the cup is precisely defined. But the potential of the interior volume, or on the interior surface, can change due to environmental influences, for example due to the charge of a liquid or due to changes in charge in a gas-sensitive layer due to physical or chemical factors. These changes in charge can be detected by the floating electrode which is in turn connected to the gate of a read-out transistor.