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This paper presents a direct-analysis technique for transistors with a long-finger structure. The analysis technique is an incorporated simulation between the finite-difference time-domain electromagnetic (EM) and semiconductor device simulations. The co-simulation method can consider various EM couplings and phase shifts on finger electrodes of transistors. The method was applied for InGaP/GaAs HBTs with various finger lengths to investigate gain degradation characteristics as a function of the finger length. As the first step, circuit simulations were done instead of a semiconductor device simulation using SPICE models of the HBT. Both large- and small-signal equivalent-circuit parameters were extracted by measurements to estimate nonlinear and linear characteristics, respectively. Using the extracted small-signal parameters, the gain degradation was estimated. The co-simulation results showed the same tendency as measurement results. Additionally, it was numerically shown that a resistive loss was mainly affected for the gain degradation from a comparison between gold and lossless electrodes.