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

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

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Analog Electronics
Optocouplers combine

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

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