Transport properties of drift plasma near the surface of a semiconductor under the influence of electromagnetic (EM) waves were investigated both theoretically and experimentally. First, a general expression for the omega- and kappa-dependent effective permittivity of the drift plasma was derived, using the fluid model. Then, interdigital admittance of the drift plasma was calculated using the Green's function approach for space harmonics. In both of the collision-dominant (CD) case for microwave region and the nearly collision free (NCF) case for THz region, negative conductance values were predicted for GaAs and InP, which became remarkably large in the NCF case. In order to confirm the existence of surface plasma wave interactions, experiments were carried out in the CD case at microwave frequencies by using GaAs and InP samples with interdigital metal electrodes. S-parameter measurements revealed marked change of admittance in both samples near velocity synchronization between carrier drift and fundamental EM wave of the interdigital structure. Net negative conductance was not obtained most probably due to shielding by interface state admittance. (C) 2004 Elsevier B.V. All rights reserved.