improve notation

docs/13_semiconductors.md

@@ -264,7 +264,7 @@ Therefore, the fourth step assumes that the Fermi level is far from both bands $
 As a result, the Fermi-Dirac distribution is approximately similar to Boltzmann distribution:
 
 $$
-f(E)_{e/h} \approx \exp\left[-(E_{e/h}\pm E_F)/k_BT\right].
+f(E)_{e/h} \approx \exp\left[(-E_{e/h}\pm E_F)/k_BT\right].
 $$
 
 Now we can move to the last step and calculate $n_e$ and $n_h$: