Abstract: The invention relates to a microelectronic sensor device and a method for the investigation of biological target substances (20), for example oligonucleotides like DNA fragments. In one embodiment, the device comprises a reaction surface (RS) to which target specific reactants (10) are attached and which lies between a sample chamber (SC) and an array of selectively controllable heating elements (HE). The temperature profile in the sample chamber (SC) can be controlled as desired to provide for example conditions for a PCR and/or for a controlled melting of hybridizations. The reactant (10) and/or the target substance (20) comprises a label (12) with an observable property, like fluorescence, that changes if the target substance (20) is bound to the reactant (10), said property being detected by an array of sensor elements, for example photosensors (SE).