We propose and demonstrate the operation of a monolithic field-effect-transistor-amplified magnetic field sensor device, in which a tunnel-magnetoresistive (TMR) material is incorporated within the gate of a Si metal–oxide–semiconductor–field-effect transistor. A fixed voltage is applied across the TMR layer, which leads charge to build up within the gate. Applying or changing an external magnetic field causes a change in the charge within the TMR layer and, consequently, a shift in the transistor threshold voltage, which leads to an exponential change in subthreshold current IDS sub and a quadratic change in saturation current IDS sat. The application of a 6 kOe magnetic field at room temperature leads in our device to an absolute change in IDS sub three times as large and in IDS sat 500 times as large as the corresponding change in current through the TMR layer alone. The relative change in IDS sub is a factor of four larger than that in the current through the TMR layer. © 1999 American Institute of Physics.