Hello Mukesh
Definition and basic idea is given above and i just want to add that With the increase in thermal noise SNR decreases.However you can go to the optiwave examples in which effect of thermal noise and shot has been investigated
Moreover if you need to know about basic calculation of all the noises in photodetector then visit this link
http://optical-technologies.info/noise-in-photodetectors/
For deep study i would refer you GP Aggarwal’s book on fiber communication.
Last link to understand performance of PIN and APD in same systems when calculated with thermal noise and without.
http://www.researchgate.net/publication/256820780_Performance_comparisons_between_PIN_and_APD_photodetectors_for_use_in_optical_communication_systems