Update 13_semiconductors.md - put eq on two lines

src/13_semiconductors.md

@@ -61,7 +61,10 @@ 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:
 
 $$
-j = 2e \frac{L}{2\pi} \int_{-\pi/a}^{\pi/a} v(k) dk = 2e \frac{L}{2\pi \hbar} \int_{-\pi/a}^{\pi/a} (dE/dk)\times dk = 2e \frac{L}{2\pi \hbar} [E(-\pi/a) - E(\pi/a)] = 0
+\begin{align}
+j = 2e \frac{L}{2\pi} \int_{-\pi/a}^{\pi/a} v(k) dk = \\
+2e \frac{L}{2\pi \hbar} \int_{-\pi/a}^{\pi/a} (dE/dk)\times dk = 2e \frac{L}{2\pi \hbar} [E(-\pi/a) - E(\pi/a)] = 0
+\end{align}
 $$
 
 ## From electrons to holes