In an n-type semiconductor, the thermal equilibrium electron concentration (n0=1015/cm3) and the intrinsic carrier concentration (ni=1010/cm3). The mean life time of minority carriers is 1us. The steady state excess hole concentration due to constant light illumination is 4*105/cm3. Under illumination, the steady state electron-hole concentration is-
At steady state since generation rate is equal to recombination rate, I found out recombination rate by dividing excess hole concentration by the given hole lifetime. I got 400 cm-3/ns but the answer provided by the instructor is 500 cm-3/ns.
If I add thermal equilibrium hole value to excess hole concentration then I get the required answer of 500. But recombination rate is given by excess minority concentration. So why I need this addition?