Dear SHAIK,

Your question is a basic one concerning the variation of the recombination rate U from the bulk to the electrodes.In principle the recombination rate depends on the excess minority carrier density, say delta n, and the minority carrier lifetime ,Tau and specifically it is expressed by U = delta n/ Tau. The lifetime Tau is a material parameter and can vary from point to point in the material depending on density of the recombination centers in that point. Deep laying impurities and crystallographic defects may act as effective recombination centers. A homogeneous semiconductor material has constant density of recombination centers in the bulk and therefore constant minority carrier lifetime. HOWEVER, at the surface and at interfaces with metallic electrodes, there are normally high density of recombination centers because the interruption of the malarial and the presence many unsaturated dangling bonds.

Concerning the excess minority carrier profile, it is controlled by the injection, diffusion, and recombination of the minority carriers. It is determined by solving the semiconductor equations in the regions of interest in the device with the proper boundary conditions. It is always so that the minority carrier excess will be largest at the injection boundary and zero or very small at the metallic contact. The zero at the metallic contact is the consequence of the assumed infinite recombination rate at the metallic electrode as explained previously. When the recombination rate is infinite, the excess must tend to zero.

The surface recombination rate can be controlled by passivating the surface of the semiconductor. Also, the minority carrier in the bulk can be controlled by adding or removing deep lying impurities and or crystallographic defects.

Knowing delta n and Tau one can determine the recombination rate profile.

For more detail refer to the link

www.researchgate.net/publication/236003006_Electronic_Devices?ev=prf_pub

wish you success.

Book Electronic Devices

Abdelhalim Zekry explained it all very well, all I can do is offer a conciser version:

- lifetime is decreased near the contacts because of metal/semiconductor contact is a plane of effectively infinitely fast recombination.

- this has nothing to do with mobility.