Question

The electron and hole concentrations in an intrinsic semiconductor are n_i and p_i respectively. When doped with the p type material, these change to n and p respectively, then
1. n + p = n_i + p_i
2. n + n_i = p + p_i
3. n p_i = n_i p
4. n p = n_i p_i

Answer 1

Correct Answer - Option 4 : n p = n_i p_i

The law of mass action is used to determine the minority carrier in a doped semiconductor

According to law:  np = n_i p_i 

For an intrinsic semiconductor, n_i = p_i 

∴ np = ni2

Under equilibrium, the total positive charge = the total negative charge

With acceptor concentration as N_A we get:

n + N_A ^- = p