Thermal detectors and thermal sensing cells can include a region of a support layer or support structure. Within the region can be reaction surfaces or other reaction regions, as well as contact pads and circuitry connecting the contact pads to other components. Also, a cell region can include a structure with reaction regions, contact pads, and control/detection circuitry connected to the contact pads; the control/detection circuitry controls occurrence of reactions in response to control signals, such as by drop merging, and also allows electrical detection of thermal signals from the reaction regions. The control/detection circuitry can include reaction control components such as drop merger electrodes and also thermal sensors such as thermistors, or it can include control/sensor elements such as semiconductor slabs that perform both functions. Each cell in an array can have control/detection circuitry that does not extend or connect outside the cell except through contact pads.