Deep-Level Spectroscopy, based on transformation and analysis of capacitance or charge transients, following the excitation of the sample by voltage pulses, yields information on electrically active defects in semiconductors, hardly obtainable by other methods. Our microscope performs Isothermal Charge-Transient Spectroscopy (IQTS). It samples the transients from 2micros to tens of ms and beyond with a resolution of hundreds of electrons. By means of a small heated stage the temperature of the sample can be varied between room temperature and about 200 degrees C. From the shift of IQTS peak maxima with temperature, the activation energy and capture cross-section of defects can be obtained. We have shown that quantitative determination of concentration is possible but it requires careful analysis and simulation, since the analysed volume depends besides on the experimental conditions also on the properties of the analysed structure.