For instance, the MMBT3904 and MMBT3906 BJTs are listed as NPN/PNP Switching Transistors, and the datasheets mention the switching times, while the BC846 and BC856 BJTs are listed as NPN/PNP general purpose transistors (and the switching speed would have to be deduced by looking at the transition frequency ft?)
Besides the obvious (higher ft for switching transistors): Is there a difference in the way these are designed and manufactured? Can one type generally be used in the other application, but not vice versa?
What about things like miller capacitance, linearity and noise?
Are there certain tricks in the geometry on the silicon, or concentration of dopants?
Related, for FETs: What's the difference between field-effect transistors (FETs) marketed as switches vs. amplifiers?
Answer
From what I remember in reading through the Motorola transistor data book a few months ago, switching transistors, as you stated, have a faster ft and because of this, they have a smaller linear region. Small signal transistors have a slower ft, but a larger linear region. I recently took a VLSI class that unfortunately only focused on MOSFETs. From this, I can only assume that the length of the N region in n PNP or the length of the p region in an NPN in a switching transistor is smaller so it's easier to make the depletion region large enough to make the transistor conduct. I would also assume the opposite is true for small signal transistors.
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