This paper proposed a Hall cross plate for magnetic field detection with substrate biasing to improve the sensitivity. By adjusting the bias voltage of the reversed-biased pn-junction (V_PN) formed by the N-well and the P-well underneath, we can modulate the thickness of the depletion layer and, thus, the effective thickness of the cross plate formed by the N-well, eventually ameliorating the sensitivity. We verified the proposed substrate-biased Hall sensor with finite-element analysis and measurement from the cross plate prototyped in the 180-nm silicon-on-insulator CMOS process. From the measurement, the resistance of the cross-plate changes from 2.13 to 2.39 kΩ when the V_PN decreases from 0 to −0.6 V, indicating the variation in the sheet resistance, induced by the change in the thickness of the cross-plate. Moreover, the current-related sensitivity increased by 28.4% in the current mode when V_PN changed from 0 V to −0.6 V. The Hall plate achieves a current-related sensitivity of 0.3 V/mA·T with a power consumption of 6.5 mW.