diff --git a/src/13_semiconductors.md b/src/13_semiconductors.md
index 6f8325eda9862f39eaa4979ac1467129cd50264a..584989b7f080a267b9c9cba386d5ec753a86a85b 100644
--- a/src/13_semiconductors.md
+++ b/src/13_semiconductors.md
@@ -61,7 +61,7 @@ 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:
 
 $$
-j = 2e \int_{-\pi/a}^{\pi/a} v(k) dk = 2e \int_{-\pi/a}^{\pi/a} (dE/dk)\times dk = 2e [E(-\pi/a) - E(\pi/a)] = 0
+j = 2e \frac{L}{2\pi} \int_{-\pi/a}^{\pi/a} v(k) dk = 2e \frac{L}{2\pi \hbar} \int_{-\pi/a}^{\pi/a} (dE/dk)\times dk = 2e [E(-\pi/a) - E(\pi/a)] = 0
 $$
 
 ## From electrons to holes