Simscape Diode/Transistor modelling issue

Hi,

I am trying to model a control circuit in Simulink using Simscape components. My circuit was initially simulating perfectly but then I tried changing the diode properties to resemble those of a 4148 diode. This slows down my simulating time considerably, in fact, it seems that the simulation will not complete at all. I then swapped the diode model for an NPN Bipolar Transistor model and connected the base and collector in a diode configuration (with the emitter connected to the base). This again simulates fine until I try to adjust the parameters to resemble those of a 3904 transistor. Then the simulation starts at some ridiculously low time value (10^-300 seconds!) and increases very slowly. In other words, it wont finish. I normally use a spice simulator for my circuit simulations but I thought that I could benefit from using Matlab to reference circuit variables.

Regards

Rachel