Fabrication and characterization of gallium nitride based high electron mobility transistors for mm-wave applications

In recent years High Electron Mobility Transistors (HEMTs) based on the nitride material system have successfully proven their potential as excellent high-power and high-frequency devices. While nitride-based HEMTs have been used commercially in the mobile communication technology for a few years, their optimization for millimeter-wave applications is still object of present research. Especially the application of nitrides as base material system for terahertz-frequency-multiplier-chains is a very promising but until now barely investigated research topic. The objective of this work is to optimize the standard process technology for the fabrication of nitride-based HEMTs of the Peter Grünberg Institute 9 (PGI-9) at the Research Center Jülich to allow for their application in the millimeter-wave regime. In order to identify performance limiting factors the influence of sub-100-nm gate lengths and the gate cross sectional structure were experimentally investigated. Furthermore T-gate processes were developed. In this context the influence of reactive ion etching with fluorine gases on the electrical properties of the used nitride material system was studied. Additionally a chlorine based dry-etch process was developed for gate-recessing purpose. Advantages and disadvantages of this chlorine-based dry etch process were compared with the advantages and disadvantages of a second gate-recess approach, which is based on sputter etching. Fully passivated HEMTs of this work achieved an extrinsic current-gain cut-off frequency of up to 60GHz and a maximum frequency of oscillation of 125 GHz. By means of the developed chlorine-based dry etch process HEMTs were built, which achieved an extrinsic transconductance of greater than 400 mS/mm, an extrinsic maximum frequency of oscillation of 105GHz and an extrinsic current-gain cut-off frequency of 35 GHz while having a gate length of 370 nm. As an example of a millimeterwave application monolithically integrated oscillator structures with a nitride-HEMT as an active device were designed and fabricated. The oscillators achieved a frequency of oscillation of 39.9 GHz and an output power of 10 dBm at 13.3 GHz