The derivation of the continuity equation is trivial starting from
equation (3.29). Inserting the weight
results in a vanishing
Poisson bracket because of (A.9) as well as a vanishing field term.
Thus, the continuity equation for carriers reads
(3.37)
The systematically derived equation is what can also be expected from a
phenomenological point of view: The increase of carriers within a certain
volume has to be equal to the influx minus the net recombination rate within
this volume.