P-N junction voltage under polarization

by Fernsanz
Tags: junction, polarization, voltage
 P: 58 All the books that deal with the p-n junction under applied bias asume that the same equations used for the equilibrium case (no bias) can be used for the biased case provided that the juntcion voltage $V_{ic}$ is substituted by $V_{ic}-V$, where $V$ is the applied bias. Although this can seem plausible I don't see any reason for why the junction voltage drop should be the $V_{ic}-V$, i.e., why the voltajes should obey a sort of superposition principle. Is there any reason on the ground of fundamental electrostatic / thermodynamics? Thanks.