# Semiconductor - the mass action law question

I was told that the mass action law $$np = n_i^2$$ only applies to compensated semiconductors and intrinsic semiconductors, and it does not apply to n-type or p-type semiconductors. Is this true? I also heard the argument that the hole concentration decreases as the electron concentration increases in case of a n-type doping, therefore the mass action law is still applicable. I am not sure which is right, but I am sure I am missing something here.

When solving problems such as the one below

The electron concentration in silicon at $$T=300\text{ K}$$ is $$n_0 = 2\times 10^5 \text{ cm} ^{-3}$$.

(a) Determine the position of Fermi level with respect to the valence band energy level.

(b) Determine $$p_0$$.

(c) Is this n- or p-type material?

Source: Semiconductor physics and devices, Donald A. Neamen

I am not sure whether I can apply the mass action law or not. For example, could part (b) solved using the mass action law since the electron concentration is given? $$p_o = n_i^2/n_o$$