A solution of the continuity equations is obtained for the space charge distribution by assuming (1) that deviations from neutrality are small, and (2) that the space charge fields, which are a consequence of the terms containing ▿·Ein these equations give rise to pure diffusion and pure ``drift‐wave'' terms with time dependent coefficients. It is, for equal numbers of holes and electrons initially injected,e[(p−p0)−(n−n0)]=−&tgr;&rgr;∇E·J[1−exp(−t/&tgr;&rgr;)], where &tgr;&rgr;equalsK/4&pgr;&sgr;, the relaxation time,Jis the total current density including diffusion, as well as drift, and∇Emeans that the divergence does not operate onE(the electric field is held constant). In the differentiation ∂n/∂xand ∂p/∂xare considered to be equal, as are ∂2n/∂x2and ∂2p/∂x2.Working independently, H. Brooks and W. van Roosbroeck arrived at expressions for both the ambipolar diffusion coefficient and the ``group mobility'' of the drift of a pulse of excess carriers. This theory yields the same results, and, in addition, transient expressions are gotten for each.