A potential barrier V volts exists across a P-N junction. The thickness of the depletion region is ‘d’. An electron with velocity ‘v’ approaches P-N junction from N-side. The velocity of the electron crossing the junction is

There exists an electric field from N side to P side. As electron experience a force opposite to the direction of electric field, it's velocity decreases, as it moves from N to P side; from conservation of energy12mv2-eV=12mv'2v'2=v2-2eVmv'=v2-2eVm