Publication: Physical modeling of non-alloyed ohmic contact towards gallium nitride-based high electron mobility transistor applications
Loading...
Date
2024-10-01
Authors
Tung, Kok Siong
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Advantages of GaN's High Electron Mobility Transistor (GaN HEMT), such as concentrated channel electron, superior electron mobility characteristic and high breakdown voltage bring the opportunity to replace the Silicon-based devices in the near future of modern power conversion systems. Low resistance Ohmic contacts of AlGaN/GaN-based devices are essential to achieve forecasted device performance. There hasn’t been much work done on the device’s software based system (TCAD) to study the contact resistance for GaN and establish a strategy to minimize the contact resistivity. This paper objective was to study the Ohmic characteristic of metal contact on HEMT semiconductor with perform simulation with develop a physical mode reflecting it current transport mechanism and extract the TLM plot from I-V curve go gain the contact resistivity. In this research work, using Silvaco TCAD Atlas, the study started with modeling the contact resistance with vertical structure. The study extended to the lateral structure, which is more feasible for physical manufacturing, whereby different n++ with various doping under metal were studied to obtain the best optimization for the ohmic contact. Increasing the doping of a semiconductor, resulted in higher possibility of tunneling as the width of barrier become narrower, current flow in the form of field emission cause the reduction of resistance. Base on vertical structure simulations, sheet resistance value obtain from TLM plot compare to theoretical calculations using formula of s = 1/qμNd are matching proven that the validity of the model. Base on the lateral structure simulation TLM plot, it can be seen that when doping rises, the slope representing the sheet resistance decreases even the mobility of electron or hole increase infer that increasing of doping overcome the effect of scattering or collision. In n++ under metal structure, the heavily doped layer enable more electron tunneling at the junction with higher doping and higher heavily doped layer thickness which enable higher possibility of tunneling. Hence, higher current density able the cross the metal-semiconductor junction with lower transfer length and thus lower the contact resistivity. Based the the data extracted from the IV curved and TLM plot, reduction in contact resistivity saturated after 18 nm thickness and contact resistivity achieve < ~1E-6 Ω∙cm2. TLM parameters are in good agreement with the theoretical sheet resistance which demonstrate that the validity of the model. It also revealed that with n++ layer under metal Ohmic contact was observed, in contrary Schottky contact was observed without n++ layer under metal. However, simulations indicate a significant of higher contact resistivity than the experimental values. This is because the simulations are performed under perfect conditions, with no surface defects or recombination centres. Also, the flaws, traps and surface contaminants was not consider in the simulations are to be blamed that the differences between simulations and actual devices. It is concluded from this study that a heavily doped metal layer exists in the metal-semiconductor interface which enable the metal contact to form Ohmic contact.