Several parasitic effects that penalize III-V IC and RF transistors performances are directly linked to deep level trapping mechanisms into the structure. The analysis of these phenomena is correlated to the presence of traps signature related to intrinsic and/or process dependent defects. We propose in this paper a survey of a non-linear measurement method namely the isothermal relaxation method used to extract trap signatures. We improve the effectiveness of the mathematical part of the method to obtain more precise results in a more efficient way. An application of our software to Metamorphic HEMTs and a comparison to results obtained with the DLTS method is proposed.