Deep levels related to iron in n-type silicon have been investigated using thermally stimulated capacitance (TSCAP) combined with minority carrier injection. The TSCAP measurement reveals two traps of EV+0.31 and EV+0.41 eV. The trap of EV+0.41 eV is a donor due to interstitial iron. The trap of EV+0.31 eV, due to a complex of interstitial iron and hydrogen, is observed in the sample etched chemically with an acid mixture containing HF and HNO3 and annihilates after annealing at 175 °C for 30 min. It is demonstrated that interstitial 3d transition metals such as vanadium, chromium, and iron tend to form complexes with hydrogen in n-type silicon, and the complexes induce donor levels below the donor levels of the isolated interstitial species. This trend is related to the interaction between the metals and hydrogen in the complexes. © 1997 American Institute of Physics.