Analog Electronics
The emitter of the transistor is generally doped the heaviest because it

  is the first region of transistor.
  must possess low resistance.
  has to supply the charge carriers.
  has to dissipate maximum power.

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Analog Electronics
Optocouplers combine

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

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