To investigate the cause of the superior RF output power performance of GaAs-FETs with a field-modulating plate, the impact of a field-modulating plate on the surface-trap-induced gate depletion region is evaluated using pulsed I-V characteristics of GaAs-FETs with and without a field-modulating plate. In conventional FETs (without a field-modulating plate), the very high density of the trapped electrons (similar to 10(12) cm(-2)), which are localized at the gate edge on the drain side, generates a surface depletion region extending deeply in the vertical direction. In FETs with a field-modulating plate, on the other hand, this detrimental phenomenon is significantly suppressed. Finally, this investigation shows that both RF saturated output power measured and that calculated using pulsed I-V characteristics are reasonably consistent. (c) 2006 Elsevier Ltd. All rights reserved.