Analog Electronics
The ICBO is the electric current that flows when some DC voltage is applied

  in the forward direction to the collector junction with emitter open.
  in the forward direction to the emitter junction with collector open.
  in the reverse direction to the emitter junction with collector open.
  in the reverse direction to the collector junction with emitter open.

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Analog Electronics
For transistor action

  the emitter should be heavily doped.
  the base region must be very thin and lightly doped.
  Je must be forward biased and Jc should be reverse biased.
  all of these.

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