At zoro bias, Fermi levels of p and n type semiconductor match with each others making bend bending which stops the motion of electrons and holes. At reverse bias, p side is made more negative making it uphill for electrons.Fermi level of p side goes up and n side down increasing band bending and hence majority charge carries can not move.  On the other hand, at forward bias, p side is made more positive making downhill for electrons. Fermi level of p side goes down and n side up manking band bending small. This allows the flow of majority charge carriers across the junction.

sir you want to say that in case of reverse bias there is more negative charge due to battery in p side because of that hole potential energy decreases and  conduction and valance moves up  and electron potential energy decreases due to positive charge of battery and conduction and valance band moves down  .sir please clear it more.

My dear Gautam,

In reverse bias, the positive voltage applied to n-type material attracts electrons towards the positive electrode away from the junction while the holes move towards the negative electrode away from the junction. The net result is that the depletion layer grows wider due to lack of electrons and holes and therefore high impedance path is created. The result is that a high potential barrier is created making the conduction and valence band energy up in p type and down in n type as compared to unbiased condition. I hope it is understandable now.

i want to know that whatever i have written in above paragraph is not a correct explanation ? please explain in terms of potential energy 

I think that your thinking in terms of decrease of potential energy of holes in p type and electrons in n side is logical.

The so-called energy band diagram, which is very helpful in understanding the basic mechanism in any semiconductor device, be it a p-n junction or a Schottky barrier or an MOSFET channel, is a plot of the total (potential plus kinetic) electron energy as a function of the x-axis, which is the direction of the applied electric field or potential. The   electron energy is (-qV), where -q is the charge of the electron and V is the applied potential. For a p-n junction, reverse bias means the p-type semiconductor (neutral region) has a lower potential than the n-type semiconductor (neutral region). Consequently, the p-type has a higher electron potential energy than the n-type semiconductor (neutral region). In other other words, for electrons, negative potential translates to positive potential energy, because V gets multiplied by the charge of the electron, which is -q. 

Dear Gautam,

I see some misinterpretation because of the definition of the potential and potential energy. Both potential and potential energy needs a reference for measurement. So, it is more helpful to speak from potential differences and potential energy differences.

Let us now return to a pn junction diode. A pn junction diode has under any bias three different regions: the neutral n region, the neutral p region and the space charge dipole region around the interface between the two sides.

Under no bias, the potential difference is equal to the contact difference of potential phi which is equal the band bending as the colleagues explained.

At reverse bias Vr, the diode must absorb the applied voltage, the only way is space charge widens to accommodate the reverse voltage, therefore the net potential difference will become phi+Vr,

This happens that as the reverse voltage is the applied the n side will be at the positive potential of the applied source while the negative side will be at the p-side. The positive pole attracts electrons from the n-side leading to the widening of the n side of space charge region. The same happens with the holes where they will be attracted to the negative pole winding the p-side of the space charge region.

The opposite happens with the forward biasing.

For more information please refer to the link: Book Electronic Devices

Best wishes

Good morning,

When two materials, one p-type and another n-type, are joined, a diffusion process takes place and majority carriers of one region occupy a portion in the other region (e.g. electrons in n-type material are diffused in p-type material and vice-versa).

This process creates a space charge region where fixed charges are remained (holes in n-type and electroncs in p-type) and an electric field is caused by these charges, the process finishes when created electric field compensates charges movement, thus when electrons in the conduction band of the n region and holes in p region see the same potential barrier, called built-in potential, Vbi, which may be expressed as the difference between the intrinsic Fermi levels in the p and n regions (this because bands of the materials are shifted to reach the same fermi level). The EQUILIBRIUM are reached.

When a positive potential, Vr, is applied between the n and the p regions the equilibrium condition no longer holds and the potential barrier is increased (see attached files ). Electrons in n region are further kicked out from the neutral region and the same happens for the holes in the opposite verse, fermi levels are not the same for both materials and, in particular their difference is qVr. This dues widening of space charge region and, indeed, a further shift of the bands.

Contrari-wise, happened in the forwad bias where the applied voltage reduces the potential barrier and, briefly, causes conduction of the pn junction.