fT vs. Ic Curves (50 and 113 IBM GHz SiGe HBTs)
Here are published curves showing the fT vs. Ic curves for three emitter sizes of the 50 GHz generation of IBM SiGe HBT. Observe that the peak of the curve is going down with the area of the emitter. Shown to the right is a published fT vs. Ic curve for
The peak for this curves moves back up but also by just a factor of two vs. the same sized emitter device at 50 GHz. Hence the power delay product will remain the same even though the current has gone up.