We show that AES depth profiling extended by a simple profile simulation technique allows characterization of details in the Ge concentration gradients for SiGe hetero-bipolar transistors (HBTs). Using the mixing-roughness-information depth (MRI) model to simulate the experimental data allows us to reveal concentration steps with a precision of about +/-2 at.% and small deviations from linear concentration gradients. The obtainable high lateral resolution of AES facilitates an application for process optimization and control in small microelectronic structures. (C) 2003 Published by Elsevier B.V.