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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.

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  • \$\begingroup\$ Looks like a duplicate of electronics.stackexchange.com/questions/427679/…. \$\endgroup\$ – TimWescott Jun 8 '20 at 18:45
  • \$\begingroup\$ I am curious regarding the ratio of thickness of base region and other regions. The ratio doesn't matter much, what matters is that the base is thin. I suggest you study how transistors are constructed. The collector is often quite large and the base and emitter regions are doped areas within that large collector. How thin the base can be made depends on the manufacturing process. A high voltage BJT will have a thicker base than a low voltage RF transistor. \$\endgroup\$ – Bimpelrekkie Jun 8 '20 at 18:52
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Figure 7.4 here indicates a width of 5-20um. However, this is a design parameter, and will vary based on product and process.

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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.

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