Influence of a surface layer on DC- and RF-performance of AlInAs/GaInAs HFETs

A method of growing AlInAs/GaInAs HFETs on InP substrates that combines the low resistance of the doped surface with high breakdown the surface depletion by using a very highly doped cap layer that is just depleted by the surface potential is presented. As a tool to investigate the extension of the depletion region at the drain side of the gate with the electric field, the variation of C/sub gd/ with V/sub GS/ is measured and related to C/sub gs/. While in the case of the highly doped cap layer the boundary is essentially pinned by the n+-cap recess shape, resulting in a variation of C/sub gd/ with V/sub DS/ by a factor of two, the surface depleted structure allows for a wide movement of the depletion zone resulting in a variation by more than a factor of five and leading to the excellent cutoff frequency of 240 GHz for 0.3 mu m devices. The reverse gate diode characteristics are improved, resulting in low leakage current and high breakdown voltage.<
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