Operating principles of bipolar transistor magnetic sensors

Dual-collector bipolar transistor structures have been evaluated and found to be sensitive to an applied magnetic field. In particular, the magnetic field results in unequal currents to the two collectors. The output signal is taken as a differential voltage developed across the output load resistors. The mechanism giving rise to the unequal collector currents appears to be emitter-injection modulation caused by the applied magnetic field. The magnetic field causes the injection from one portion of the emitter to be enhanced while suppressing injection elsewhere thus setting up unequal currents to the two collectors. A simplified theory is presented to explain the behavior of these devices. The theory is found to be in general agreement with experimental results obtained from a number of different device configurations.