For transistor action

(1) Base, emitter and collector regions should have similar size and doping concentrations.

(2) The base region must be very thin and lightly doped.

(3) The emitter-base junction is forward biased and base collector junction is reverse based.

(4) Both the emitter-base junction as well as the base collector junction are forward biased.

(2) and (3)

For transistor action, the base region must be very thin and lightly doped. Also, the emitter-base junction is forward biased and base-collector junction is reverse biased.