$[25$ Pts$.]$ The effective recombination lifetime, given by

$\frac{1}{{}_{\text{e}\text{f}\text{f}}}=\frac{1}{{}_B}+\frac{1}{{}_S}$;${}_B=\frac{1}{{}_n{v}_{th}{N}_T},{}_S=\frac{d}{2s_r}$

is plotted below as a function of impurity density $N_T.$ Knowing that the effective lifetime is dominated by bulk recombination while the defect density is high and by the surface recombination while the defect density is low.

Determine ${}_n$ and $s_r,v_{th}={10}^{7}c\frac{m}{s},d=500m$