First of all, in pin photo diode, intrinsic region is fully depleted and depletion region capacitance, Cdep = permittivity of the material * A/W 

where A= cross sectional area and W = width of the depletion region 

From the above equation we can find W and t_drift=W/ V_d where V_d=drift velocity

which is a function of applied electric field (depends on applied voltage and particular material system) 

Reference: https://www.amazon.com/Optoelectronics-Photonics-Principles-Safa-Kasap/dp/0201610876

(Chapter 5: photo detectors) 

I hope it will help you 

Thancks for your answer

bot how can find W(width of the depletion region) while we dont have A(cross sectional area)??

Depletion Width can be found in two ways: 

1. From experimental C-V curve 

2. Analytically if you know the doping and semiconductor parameters ( If you are doing experimental work, then you must have to verify it with your experimental data) 

I know these two techniques, there may be some other techniques in that respect that I might not know yet..... You can find it :) 

In case of cross sectional Area (A), it is the cross sectional area of your device on which you are experimenting/fabricating. So You are the only source who will know it :) 

You are welcome :) 

Thank you very much for your help

can you help me to find W from experimental C-V curve(with formulas)

Thanks again

Sorry for the late reply, did you edited your response, I didn't notice the question that you asked. 

Well I can suggest a book, i.e: its a heavy procedure, it can't  be described so easily in this comment section. 

https://www.amazon.com/Semiconductor-Physics-Devices-Basic-Principles/dp/0073529583

You have to read the chapter of 7 and 8 (diode chapters) thoroughly. I hope you will get your answer :)