Performance of GaAs-based FETs is strongly affected by the electrical properties of passivated surfaces via the surface charges induced at interface states and at the traps within the passivation films. In order to study these two kinds of sources, a set of new characterization techniques are respectively developed for the GaAs system which is in a largely different situation from that for the SiO_2/Si interfaces. our optical technique based on electroreflectance allows accurate evaluation of interface states and is especially useful for the material systems with strong surface pinning. For the study of trapping centers in CVD dielectrics, use of thermally oxidized Si interfaces is found to be quite convenient. The experimental findings are further discussed in conjunction with FET characteristics and an important of time-domain control of the surface charges is emphasized in the power FET operation. This concept turns out to be the main motivation of a new MESFET structure with a filed-modulating plate (FPFET) which has achieved a state-of-the-art RF performance based on GaAs.
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