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 behavior of the surface depletion by using a very thin 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_gd with V_GS is measured and related to C_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_gd with V_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 μm devices. The reverse gate diode characteristics are improved, resulting in low leakage current and high breakdown voltage.