Update 13_semiconductors_solutions.md - typo

src/13_semiconductors_solutions.md

@@ -120,7 +120,7 @@ $$ E_{vb} \approx -t_{vb}(ka)^2 $$
 3.
 For the density of states for both the electrons and holes we find
 $$
-g_e(E) = \frac{1}{\pi a \sqrt{t_{cb}(E-E_G}}, \:\: \text{if} \:\: E > E_G,
+g_e(E) = \frac{1}{\pi a \sqrt{t_{cb}(E-E_G)}}, \:\: \text{if} \:\: E > E_G,
 $$
 and 
 $$