The thermal behavior of bipolar junction transistors (BJTs) With full dielectric isolation is investigated in both time and frequency domain by means of thermal impedance measurements and calibrated electro-thermal simulations. The influence of layout design parameters like area and aspect ratio of the emitter stripe, as well as distance between emitter and trench sidewalls, is extensively analyzed and quantified. For the first time, the influence of mu m-thick AlN layers acting as heatspreaders is studied under dynamic conditions from both the thermal-only and electrothermal viewpoint. It is shown that employing AlN layers - besides effectively lowering the self-heating thermal resistance - also yields a faster transient thermal behavior. Equivalent RC thermal networks are automatically extracted from the experimental thermal impedance versus time to perform electrothermal transient simulations of AlN-covered devices and to analyze their thermal response in the frequency domain. (C) 2010 Elsevier Ltd. All rights reserved.