Why do the electron loose energy when it moves from base to collector region? In a npn transistor first emitter base junction is forward biased so electrons move to the base region . The electrons are minority carriers for base .now when base collector region is revered biased ,as the electrons injected in the base move towards the CB junction it gets easily transmitted to the collector region since the reverse bias produces a strong field which helps the electrons to move.so my question is the electron after reaching the CB junction feels a field i.e a force which further helps it to move to the collector region.so the electrons should be gaining energy and also reverse bias depletion region will not act as as a resistor.
The electric field between the CB junction and the Collector contact pulls the electrons to the Collector contact because it has a positive voltage compared to the Base and CB junction. The electrons loose potential energy as they travel from CB junction to the Collector contact.
Perhaps you are thinking about the speed of the electrons but that is irrelevant. You should think about the potential energy instead. Pulling an electron back to the CB junction would work against that electric field so would cost energy. So an electron traveliing to the collector contact looses potential energy.