We measured by internal photoemission the conduction-band discontinuity Delta E(c) in p-In0.53Ga0.47As/n-InP heterojunctions with a Si delta layer (1x10(12) cm(-2)) inserted in InP at 10 Angstrom from the interface. The n-type Si delta doping induced an inhomogeneous and temperature-dependent conduction-band offset reduction as revealed by two onsets in the spectral response. The first one was absent in room-temperature data and was due to the Si intralayer presence. The second correlated with the conduction-band discontinuity value for heterojunctions without delta doping and its presence served as an indication of the inhomogeneity of the Si delta layer. The measured value of the modification was 0.11+/-0.04 eV in good agreement with the calculated one. Current-voltage measurements confirmed that the Si delta layer modified the transport parameters of the heterojunction only at low temperature. (C) 1995 American Institute of Physics.