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Title: Characterization and Numerical simulation of Gallium Nitride-based Metal-oxide-semiconductor High Electron Mobility Transistor with High-K Gate Stack
Keywords: Gallium Nitride, Numerical simulation, HEMT, 2DEG, TCAD, Stress
Issue Date: 17-Jan-2012
Source: LOW KIM FONG EDWIN (2012-01-17). Characterization and Numerical simulation of Gallium Nitride-based Metal-oxide-semiconductor High Electron Mobility Transistor with High-K Gate Stack. ScholarBank@NUS Repository.
Abstract: In this work, the fabrication of the AlGaN/GaN MOSHEMT was performed and the process was preceded by the design of the necessary mask set. In addition to the devices that were required for characterisation, several other support and test structures were designed and included into the mask layout. These structures included the Van Der Pauw structure, Vernier Alignment Scale structure, Transmission Line Model (TLM) structure, Step Coverage structure and devices with long gate widths. In the fabrication of the devices, the splits of the experiments with different configurations were discussed with focus on the objective of these processes and their characterisation. Electrical measurement was the main mode of characterisation in this work. Methods such as the C-V, ID-VGS and ID-VDS measurement were performed on experimental devices. The enhancements in device performance due to certain fabrication processes employed in the experiments were evident from the results of the electrical characterisations. In order to better understand these enhancements, a numerical simulation project was undertaken and the results of these simulations are presented in this thesis. Sentaurus TCAD simulation suite was used for the device simulations. The simulation physics and models were discussed before some preliminary simulation results were presented. The simulation results presented was generally in line with the device physics understood from various literatures and in some cases shed light on the possible mechanism of the performance enhancement of the devices by some of the fabrication techniques introduced in the experiments. Firstly, simulation of the effect of different thickness of AlGaN barrier layer and their effect on device performance was performed and this gave results that were in line with the literature as described by Ibbetson. Next, the effect of interface states on device performance was simulated. The reduction of donor-like states in the interface between the AlGaN and high-k gate dielectric caused the 2DEG density to reduce accordingly, resulting in a lower drive current. However, this is not consistent with the surface passivation technique discussed in the fabrication process. This process was shown in the electrical characterisation to improve the drive current by more than 50%. Thus, a different mechanism was proposed in the simulation. Acceptor-like traps and donor-like traps were postulated to be present at the above-mentioned interface. The hypothesis is that the surface passivation technique would tend to passivate more acceptor-like traps. This effect was simulated and the results of the simulation show a close resemblance of the effect seen in the characterisation of the experimental devices. Finally, the effect of a Diamond-Like Carbon (DLC) stress liner on the MOSHEMT was also simulated and it was shown that this compressive stress liner not only provided a compressive stress in the gate region but also a coupled tensile stress in the region beyond the gate region. This change in carrier distribution resulted in a positive shift in threshold voltage as well as a slight enhancement in drive current. It is noted that in this simulation, the stress was simulated using a separate simulation program and the resulting polarisation charge was input into the Sentaurus TCAD simulation suite separately, thus the effect of change in mobility due to band structure deformation [2] and change in trap concentration [3] were not simulated accordingly. For this reason, this simulation could only be a qualitative description of the actual effect of stress on AlGaN/GaN MOSHEMTs.
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