Update 13_semiconductors.md - small change

docs/13_semiconductors.md

@@ -57,7 +57,7 @@ Before proceeding further, let us remind ourselves of important band structure p
 Descibes how quickly electrons move within the lattice. 
 * Effective mass $m^* = \hbar^2\left(d^2 E(k)/dk^2\right)^{-1}$. 
 Tells us how hard it is to *accelerate* the particles and is related to the curvature of the band.
-* Density of states $g(E) = \sum_{\textrm{FS}} (dn/dk) \times (dk/dE)$. 
+* Density of states $g(E) = \sum (dn/dk) \times (dk/dE)$, where the sum runs over all states at the same energy. 
 The amount of states per infinitesimal interval of energy at given energy. 
 The quantity is vital in order to calculate any bulk property of the material such as conductivity, heat capacity, etc.