In an intrinsic Semiconductor number of free electrons 'n' = No. of holes 'p'
np = (Ni^2) . where Ni is intrinsic concentration.
but i came to know that this product holds good in case of p and n type materials ..where as addition of n type materials increases no of electrons ..
and holes conc. is almost the same . then how can it be constant ?

 

Well each time an electron moves a hole is left behind. The number of "free" electrons is a constant, albeit a large one, so it stands to reason that the addition of an "impurity" element leaves the resulting alloy electrically neutral. So the number of electrons and holes remains the same.