The potential barrier of a P-N junction diode is 20 mV, when an electron having energy 400 meV travels from P to N, after crossing the junction the energy of the electron is

Electric field in the depletion layer is directed from N to P. Electrons experience a force opposite to the direction of electric field. so when an electron moves from P side to N side it gains energyso final energy = initial + gained energy in the electric field = 400+20 = 420 meV