I have never seen how does a BJT look like inside the plastic container. I know base region is pretty thin as compared to collector and emitter. I am curious regarding the ratio of thickness of base region and other regions.
Figure 7.4 here indicates a width of 5-20um. However, this is a design parameter, and will vary based on product and process.
Given Ftau of a bipolar is proportional to the time to cross the base region, 10Ghz silicon device needs a base region 1,000x thinner than a 10MHz device.
With charge carrier velocities of approximately 500 meters per second, or
- 1 micron in 1/500,000,000 second
we conclude a 1GHz device needs 0.5 micron THICKNESS of the base region.
SiGe devices have much faster carrier velocities.