update table

src/14_doping_and_devices.md

@@ -127,6 +127,8 @@ As a result, the impurity concentration is bounded to $N_D \lesssim (1/4\textrm{
 | $n_h$ | Concentration of holes in the valance band |
 | $n_D$ | Concentration of electrons in the donor bound state|
 | $n_A$ | Concentration of holes in the acceptor bound state|
+| $N_D$ | Concentration of donor impurities| 
+| $N_A$ | Concentration of acceptor impurities|
 
 We now have the necessary tools to determine how the Fermi level changes with doping. 
 The algorithm to determine the Fermi level of a semiconductor was outlined in the previous lecture and we continue to use it here.