The surface‐wave instability analyzed by Baraff and Buchsbaum in the limit of infinite ωcτ is investigated here for ωcτ large but finite. In the previous analysis, the carrier drift velocity required to establish the instability was found to be proportional to (K1-K2), the effective dielectric mismatch across the interface at which the surface wave was excited. Here we find that with ωcτ large but finite, this threshold velocity is increased by an amount proportional to (ωcτ)-1 and inversely proportional to (K1-K2). For a given value of ωcτ there thus exists an optimum choice of (K1-K2) which minimizes the required threshold velocity. These new results can be obtained from heuristic arguments which do not require the calculation of field components beyond lowest nonvanishing order in (ωcτ)-1. The heuristic arguments are here justified rigorously for the physically interesting case of small (K1-K2).