Consider a system of $N$ "non-interacting" electrons $/ \mathrm{cm}^3$, each of which can occupy either a bound state with energy $\varepsilon=-E_d$ or a free-particle continuum with $\varepsilon=\frac{p^2}{2 m}$. (This could be a semiconductor like Si with $N$ shallow donors $/ \mathrm{cm}^3$.)
(a) Compute the density of states as a function of $\varepsilon$ in the continuum.
(b) Find an expression for the chemical potential in the low temperature limit.
(c) Compute the number of free electrons (i.e., electrons in the continuum) as a function of $T$ in the low temperature limit.