Analog Electronics
The current gain of a bipolar transistor drops at high frequencies because of

High current effects in the base
The early effect
Transistor capacitances
Parasitic inductive elements

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Analog Electronics
Optocouplers combine

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

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