what is law of mass action or constancy of pn product

 Mass action law or constancy of pn product

It states that ,under thermal equilibrium the product of concentration of free electrons and concentration of free holes is constant and the value of constant is equal to square of intrinsic carrier concentration of that material 

        

                                         pn = ni²

                        where  p = hole concentration
                                    n= electron concentration
                                    ni=intrinsic carrier concentration