My samples are undoped and REO doped titania. The flat band potentials vs RHE are more negative for undoped titania. I calculated ND using MS equation and showed that for undoped, REO1-doped and REO-doped the ratio is 3:1.6:1. The conductivity measurement shows that REO1 and REO2-doped are more conductive by a factor of about 40. I am confused why ND is less for more conductive material. Please help me resolve this issue. I will be grateful if I get link to literature that can help solve this problem.