Hole transport is a logical concept and it's not exist in practice. Holes are the empty space of excited electrons which could be filled with recombined electrons. Actually electrical current is generated with electron movements in a closed circuit, both in excited (temporary) state and recombination state.

Extracting holes from the semiconductor is just an allegory for injecting electrons into the valence band. When you extract electrons from the conduction band to the "electron-extracting" metal contact, you have to inject electrons to the valence band via the "hole-extracting" contact at the same time, otherwise your semiconductor would become more and more charged.

Thank you Nima and Sebastian, any one of you could you please tell me what would be the work function values of the two metal contacts (metal which can take electron from semiconductor and the other metal which can offer electron to valence band of semiconductor) with respect to valence band edge and conduction band edge of sandwiched semiconductor for proper charge transfer of both hole and electron?

Ideally, the work function of the two metal contacts should match the valence and conduction band edge energies of your semiconductor, respectiveley, so that ohmic-type contacts with negligible injection/extraction barriers are formed. However, also the formation of Schottky barriers can lead to effective charge injection/extraction, which is for example used in Schottky-type quantum dot solar cells.

Thank you very much Sebastian Wilken, your explanation is quite useful to me.

Dear Sateesh, Dear Colleagues,

I find the answer is okay. However let me add some comments which may be useful for this important topic:

Semiconductors are characterized by conduction through the two types of charge carriers, negative electrons in the conduction band and holes in the valence band.

P-type semiconductors and n-type semiconductors are exiting. Accordingly, holes are existing exactly like electrons.

Any semiconductor device, e,g, a solar cell is equipped by metallic electrodes to access it electrically. Such metal semiconductor contact thought for electrically accessing the semiconductor device must be ohmic and its contact resistance must be very small such that its voltage drop is negligible compared to the voltage drop on .the device itself.

Metal - semiconductor contacts on the other side may be rectifying exactly like the p-n junction diodes.In order to make them ohmic with low resistance we must make them tunneling by increasing the doping of the semiconductor to the degeneracy limit or we may increase the recombination rate at the M-S interface to very large values such that minority carrier acculturation is prohibited.

For the holes to flow without hinderness to the metal the valence band edge of the semiconductor must lay under the fermi-level of the metal in contact. And for electrons to flow without hinderness from the semiconductor to the metal, the conduction band edge must lay above the fermi-level in the metal.

Basically, under the electric field, electrons flow in the bottom of a more or less empty energy band and holes move on the top of a nearly filled band. The electrons are accelerated by acquiring adjacent upward empty energy levels while holes are accelerated by acquiring adjacent down ward filled energy levels.

For more details about conduction of electrons and holes you may see the link:

https://www.researchgate.net/profile/Abdelhalim_Zekry/publications/?pubType=book&ev=prf_pubs_book

Thank you Prof. Abdelhalim Zekry providing such a detailed explanation.