What happens if doping concentration is less than Intrinsic concentration? Will the semiconductor act as intrinsic or extrinsic? Please Explain
TL;DR: band structure basically stays the same since your dopant doesn't increase the electron/hole density enough.

Let's take an example: let's say \$n_i = 10^{-10}\$ and you have initially an intrinsic semiconductor. Then \$n_e = n_i = 10^{-10}\$. 

Let's say you dope N-type with density \$N_d = 10^{-11}\$. Assuming full ionization of the dopant, you get \$n_e = n_i + N_d\$ (approximately, I believe there's an equation for this).

This means that you have \$n_e = 1.1 \times 10^{-10} \$ and \$n_h = n_i^2 / n_e = 0.909 \times 10^{-10} \$. 

Basically the concentrations don't change enough to matter. This is why when you dope you dope much much greater than \$n_i\$ since if you don't dope enough nothing really happens.