A fine structure of phonon replicas in the current-voltage characteristic of a resonant-tunneling diode has been investigated experimentally. A detailed study of the diode I-V curves in magnetic fields of different orientations has allowed to determi
ne the origin of the features in the fine structure. The voltage positions of the features are shown to coincide with calculated that in the frame of two models: LO-phonon assisted tunneling and resonant tunneling of polarons.
