A new direct extraction method to determine the parasitic capacitances for HBTs is presented in this paper. The main advantage is that base parasitic capacitance Cpb can be extracted by using three different size HBTs with the same pad profile. This method is based on an improved small-signal model, which takes into account the distribution effects of the base and collector feedlines. Good agreement is obtained between the measured and modeled results for the 1 x 3 x 12 mu m(2), 2 x 2 x 20 mu m(2) and 1 x 3 x 40 mu m(2) (number of emitter fingers x emitter width x emitter length) GaAs HBTs up to 40 GHz.