Analog Electronics
In a bipolar junction transistor the base region is made very thin so that

  electric field gradient in base is high.
  base can be easily fabricated.
  base can be easily biased.
  recombination in base region is minimum.

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Analog Electronics
Optocouplers combine

 Power transistor and silicon transistor.
 IGBTs and MOSFETs.
  Infrared light emitting diode and a silicon photo transistor.
 SITs and BJTs.

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