Semiconductor optical amplifiers (SOAs) and laser diodes (LDs) are important components for optical systems. The structure and principle of SOAs and LDs are similar. In this paper, we have proposed a unified equivalent circuit model, which includes time delay circuit and different boundary conditions, for the simulations of traveling wave (TW) SOAs, Fabry-Perot (FP) SOAs and LDs. The effect of amplified spontaneous emission on FP-SOAs is also taken into account. Using this circuit model, our simulation results are in good agreement with published results for SOAs. This model has also been applied to analyze the static and dynamic behavior in LDs. A good agreement between our simulated results and the theoretical calculations is observed for LDs. (C) 2007 Elsevier Ltd. All rights reserved.