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

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

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Analog Electronics
Optocouplers combine

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

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