This paper presents an approach for the de-embedding and modeling of multi-port transistors. First, the proposed de-embedding method is an extension of a three step method (Pad-Open-Short) for accurate on wafer (MMIC) S-parameters measurements. The novelty of this approach lies in the fact that the proposed de-embedding method for multi-port devices takes into account the imperfections of the standards. Then, we present two approach for the modeling of 3 and 4 ports GaAs HEMT transistors. The non-linear model was developed from I-V and S-parameters measurements. The methodology for 3-port device modeling allows us to determine accurate non-linear model in high frequencies. The second approach is dedicated for the distributed modeling of a 4-port transistor. The original electrical models of multi-port transistors developed in this thesis aims to reduce the time and the design phases, and to make reliable the prototyping of microwave functions using these components. The work presented here is therefore dedicated to improving the electrical modeling of transistors focused as their application on the Ku band.