The constant growth of electric consumption leads to considerable progress in power conversion. Recent studies have shown that using Gallium Nitride (GaN) as a technological building bloc permits to develop converter operating at high frequency with reduced volume and weight. Furthermore, it is conceivable the monolithic co-integration of devices towards full-GaN switching cells. Therefore, characterization of GaN power devices is needed to provide accurate models in a wide frequency band in order to design new generations of converters. An innovative modeling method for GaN High Electron Mobility Transistor (HEMT) power transistors based on the use of Scattering parameters (Sparameters) and small-signal equivalent circuit was recently developed and validated in previous studies. Meanwhile, the demand concerning GaN diodes increases, pushing forward the need for dedicated electric model. Through S-parameters, current-voltage and current-collapse measurements, this paper presents the characterization of packaged GaN diodes with the aim to establish an accurate nonlinear model. The analyzed devices are still in the development phase, but initial results are very promising and get close to commercial SiC diodes available on the market.