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A Study on the Behavior of Gate Recess Etch by Photoresist Openings on Ohmic Electrode in InAlAs/InGaAs mHEMT Devices

In the fabrication of InAlAs/InGaAs metamorphic high-electron-mobility transistor (mHEMT), the determination of whether etching has been completed to the desired gate recess depth is made by measuring whether the drain current through the channel layer has reached the target current. Non-uniformity of the etching rate occurs during wet etching with citric acid. In this study, the cause of that non-uniformity was investigated. We confirmed that an electrochemical potential caused by the electrolyte of the etching solution was induced between the ohmic electrode and the epitaxial layer of the recess region, resulting in a non-uniform etching rate. In particular, the case where the Au of an ohmic electrode is exposed by the monitor window for the measuring channel current was considered. The gate recess etch rate was changed by the presence, location and size of the photoresist openings on the ohmic electrodes.