Update 13_semiconductors.md - fix

src/13_semiconductors.md

@@ -65,6 +65,7 @@ Heat capacity $C_v = \tfrac{d}{dT}\int_{-\infty}^\infty E\times g(E) \times dE\t
 
 A completely filled band carries no electric current:
 $$
+\begin{align}
 j = 2e \frac{1}{2\pi} \int_{-\pi/a}^{\pi/a} v(k) dk = 2e \frac{1}{2\pi \hbar} \int_{-\pi/a}^{\pi/a} \frac{dE}{dk})\times dk = \\
 2e \frac{1}{2\pi \hbar} [E(-\pi/a) - E(\pi/a)] = 0
 \end{align}