I have created a schematic model for the BFP640 Infineon transistor by my own editing the SPICE file (.cir) obtained from the Infineon website and importing it to QucsStudio. Although the s-parameters simulation is performed quite well, the transient simulation for an input sine wave at 1 GHz during 20 ns does fail (see the attached snapshot). In order to check my model, I have used the same SPICE model in LTSpice and I have obtained the correct results (see the attached snapshot). Perhaps I doing something wrong in QucsStudio.
There’s indeed a bug in the substrate capacitance that causes this problem. If you set Cjs of the BJT to zero, it should work. You can add the Cjs value to Cj of the diode in order to reach the same value again.
Thanks for reporting the problem! It will be fixed in the next release.