Influence of parasitic capacitances on the performance of passivated InAlAs/InGaAs HEMTs in the millimeter wave range

High performance in the millimeter wave range have been already reported for InAlAs/InGaAs High Electron Mobility Transistors (HEMTs) on InP substrates. High frequency performance is essentially limited by two elements : the feedback capacitance Cgd and the gate resistance Rg. On the first hand, many different gate geometries such as T-gate or Γ-gate have been developed to improve Rg. On the other hand, for operation stability and better reliability of MMIC´s using HEMTs, devices must be coated with a dielectric layer. However, the passivation layer increases the parasitic capacitances and then, decreases the device performance. In this paper, we present a study of the influence of parasitic capacitances such as feedback capacitance on the high-frequency characteristics of InP-based HEMTs with T-gate supported by a Si₃N₄ passivation layer. Extraction of S-parameters from 1 to 40 GHz, and noise measurement at 94 GHz have been performed on 0.12-0.15 μm gate length samples. First, we investigate the consequence of the cap layer underetch on the parasitic capacitances. Secondly, the effect of the dielectric coating layer under the top of the T-gate is examined