A parameter extraction procedure for determining an improved T-topology small signal equivalent circuit of a metallic collector-up HBT with composite base is presented. The proposed T-small signal equivalent circuit includes base and collector impedances modelled by parallel RC circuits. The new technique employs analytically derived expressions for direct calculation of HBT T-Model equivalent circuit element values in terms of the measured S-parameters. This approach avoids errors due to uncertainty in fitting to large, over determined equivalent circuits and does not require the use of test structures and extra measurement steps to evaluate parasitics. Physically realistic results are demonstrated under various biasing conditions for the n-p-n InP/InGaAsP/InGaAs composed base HBT with metallic collector-up structure (Cup MHBT). The agreement between the measured and model-produced data is excellent over the large frequency range and for several polarizations conditions for devices. Crown Copyright (C) 2008 Published by Elsevier Ltd. All rights reserved.