Under electron spin resonance conditions, changes of the capacitance of vertical field-effect transistors are observed, due to spin-dependent trapping of charge carriers by defects at the interface between the substrate and the channel region. The spectra obtained by capacitively detected magnetic resonance show the presence of two different defects, tentatively assigned to defects introduced by processing and complexes involving transition-metal impurities. Using a quantitative model, the number of defects resonantly charged by this trapping is estimated. It is shown that the possible cross talk of spin-dependent changes of the conductivity in the substrate is, in fact, suppressed by the large impedance of the space-charge layer. © 2000 American Institute of Physics.