Update 13_semiconductors_solutions.md - typo

src/13_semiconductors_solutions.md

@@ -142,7 +142,7 @@ Analoguous, we find for the hole density
 $$
 \begin{align}
 n_h &= \int_{0}^{\infty} f(E_h) g_h(E_h) dE_h\\
-&\approx \frac{e^{\beta \mu}}{4\pi a \sqrt{\pi t_{vb}k_B T}}.
+&\approx \frac{e^{-\beta \mu}}{4\pi a \sqrt{\pi t_{vb}k_B T}}.
 \end{align}
 $$