How do electrons in n type (conduction band) fall into holes in p type (valence band with lower energy) in a pn junction? I was studying about the semiconductor physics.I learned about the concepts of holes and all.If the  electrons present in n type is in conduction band how can they fall into holes in valence band of p type which is of much lower energy state.
If it is by losing energy while  crossing the depletion region then how in the first place ,on joining p and n type materials, the electrons on n side combine with the holes on the other side forming the depletion layer itself.
The question may be utter foolish but correct me if i am wrong.
 A: I'm hoping the diagram at the bottom may help a little, it's one I made when I was writing up to show the basics of what happens in the p-n junction (without any bias). I would highly recommend also the website pveducation.org, which takes you through step-by-step what happens on the formation of a p-n junction and how the depletion region is formed.
The formation of the depletion region occurs chiefly through diffusion of carriers - i.e. the electrons, which are the majority carrier in n-type materials diffuse towards the p-type side, and vice versa for holes. As a result the 'ion' cores of the opposing charge are left behind, which causes the build up of an electric field between the cores (positive in n-type, negative in p-type).

The recombination of the electrons with the holes can happen through many pathways - for example through defects in the material. It doesn't happen straight away, and the electron lifetime is an important property in a semiconductor!
I hope I have helped, if there is anything I've missed do let me know.
(Edit as I realised I had my band labels the wrong way around)
