add missing $

src/14_doping_and_devices.md

@@ -112,7 +112,7 @@ If we keep on adding impurities, then at some point the weakly bound states will
 The overlap will create an effective tight-binding model that leads to a formation of an "impurity" band which breaks our approximation.
 We must therefore prevent the overlap of impurity bound states.
 From the previous section, we know that the extent of the bound state is roughly 4 nm and thus the distance between impurity atoms cannot exceed that. 
-As a result, the impurity concentration is bounded to $N_D \lesssim (1/4\textrm{nm})^3 \sim 10^{-5}.
+As a result, the impurity concentration is bounded to $N_D \lesssim (1/4\textrm{nm})^3 \sim 10^{-5}$.
 
 ## Number of carriers