Structures in which magnetic and electronic materials are combined offer a variety of possibilities for realization of devices with improvement functionality or performance compared to conventional devices. We have designed, characterized, and analyzed a novel hybrid magnetoelectronic device: a monolithic field-effect-transistor-amplified magnetic field sensor in wh ich a granular tunnel magnetoresistive (TMR) thin film, consisting of Co nanoparticles embedded in an insulating SiO2 matrix, is incorporated into the gate of a p-channel Si metal-oxide-semiconductor field-effect transistor. In this structure, current flow through the TMR film leads to a buildup of electronic charge within the gate due to the Coulomb charging energy of the Co nanoparticles, and consequently to a transistor threshold voltage shift that varies with applied external magnetic field. In a prototype demonstration device, the relative current change induced by application of a 6 kOe external magnetic field at room temperature was amplified from 5% for the current through the TMR film to 21% for the transistor subthreshold current. The absolute current response in the saturation regime increased by a factor of about 500 compared to that of the TMR film alone. A detailed analysis of the device operation and of methods for optimization of performance are presented. It is anticipated that substantially better performance should be achievable with relatively straightforward improvements in device design and processing. This device concept is shown to compare particularly favorably with Hall bar sensors and thus may be very beneficial in sensor applications for medium field ranges up to about 1 T.