Technology for Non-Recessed Short Gate Length E-Mode AlGaN/GaN High-Electron Mobility Transistors

Anirban Basu, Minjun Yan, Vipan Kumar, Ilesanmi Adesida
Micro and Nanotechnology Laboratory and Department of Electrical and Computer Engineering
University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA

In the fabrication of AlGaN/GaN HEMTs, silicon nitride (SiNx) is often used for a variety of purposes, such as passivation layer and field-plate dielectric layer to name a few [1,2]. Nitride deposition, as a processing step, is usually done before gate fabrication. Subsequent gate fabrication is carried out by removal of the nitride from the gate footprint area [3]. The nitride removal, being a highly anisotropic process, is effectively performed using fluorine-based plasma etching in RIE plasma systems. In order to ensure complete removal of nitride, significant over etching is done. During this over etching period the epilayer is subject to fluorine plasma bombardment, which has been found to have multiple advantageous effects on the performance of AlGaN/GaN transistors. Foremost is the positive shift of threshold voltage as a consequence of CF4 bombardment. SIMS has shown an abundance of fluorine atoms in the epilayer. The incident fluorine ions, speculated as negatively charged, can effectively raise the AlGaN barrier potential and hence can deplete the channel charge thereby moving the threshold voltage to positive values. It has already been demonstrated that for 1 μm gate length devices, positive threshold voltage is obtainable, leading to E-mode operation of devices [4]. This is a significant achievement as in this procedure Emode devices can be obtained without recessing the gate region, which is a difficult processing step in terms of controllability. Another prior work has shown that a combination of recessing technology and fluorine plasma treatment can result in high performance E-mode devices [5].
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