The behaviour of an InP/InGaAs heterojunction bipolar transistor stack in a magnetic field is described here with reference to various surface modifications. These include surface passivation with sulphur-based species, surface amorphisation with plasma-induced ion bombardment and inclusion of three-dimensional ferromagnetic thin-film structures on the surface of the device. The structure, fabrication and modelling of the device is described in brief, followed by the results of transport and transduction experiments on the hybrid ferromagnet-bipolar transistor device. The results clearly show the important role played by surface conditions on surface and near-surface electron interactions brought about by Lorentz deflection of transport current in bipolar transistors and the potential for using these effects to measure the strength of magnetic fields. A new type of compound semiconductor integrated ferromagnet-bipolar transistor magnetic field sensor, based on these processes, is presented. (C) 2009 Elsevier Ltd. All rights reserved.