Advancing GaN HEMT Technology for Microwave Applications: Investigations of Ohmic Contacts, Passivation, and Buffer-Free Concepts
Doktorsavhandling, 2025
The formation of deeply recessed Ti/Al/Ti and Ta/Al/Ta sidewall ohmic contacts, using a low annealing temperature and achieving minimal contact resistance of 0.14 and 0.24 Ωꞏmm, respectively, were explored. The reduced annealing temperature mitigates Al metal melting risk, enabling sharper edges and improved surface morphology. Deep recessing beyond the barrier layers makes the process less susceptible to variations in etching depth since ohmic contacts are formed on the recessed sidewall. The bottom Ti and Ta layer, Al thicknesses, and recessed sidewall angle, were optimized, successfully demonstrating on epitaxial structures with varied barrier designs.
Passivation utilizing low pressure chemical vapor deposition (LPCVD) silicon nitride (SiN) has emerged as an effective method for mitigating surface-related trapping effects. However, surface traps could not be entirely eliminated with passivation, owing to the persistence of defects, dangling bonds and a native oxide layer at the interface between the passivation and epi-structure. Consequently, an in-situ NH3 pretreatment method preceding LPCVD SiN deposition was investigated. The pretreated sample exhibited a 38% reduction in surface-related current collapse compared to the un-pretreated sample, culminating in a 30% augmentation in output power (3.4 vs. 2.6 W/mm) and an enhanced power-added efficiency (44% vs. 39%) at 3 GHz. Additionally, the pretreated samples demonstrated improved uniformity in device performance.
Traditionally, adequate buffer insulation and 2DEG confinement have been achieved through the intentional acceptor-like dopants (iron (Fe) and carbon (C)) or the AlGaN back-barrier in the GaN buffer. In this thesis, the impact of different carbon concentrations in AlGaN back-barrier and GaN buffer is studied. The results highlight that the back-barrier effectively screens the trapping effects underneath the back-barrier and the importance of optimization C-doping level in GaN channel, back-barrier and GaN buffer. However, solutions involving acceptor dopants, and a back-barrier have been reported to increase trapping effects and thermal resistivity, respectively. Therefore, a novel buffer-free epitaxial scheme, QuanFINE, was proposed. It removes thick Fe-/C-doped GaN buffer, enabling a GaN channel thickness of 250 nm to be directly grown on an AlN nucleation layer. Consequently, the AlN nucleation layer serves as a back-barrier. This approach results in a lower buffer-related current collapse (15% vs. 18%) compared to a conventional epi-structure with a thick Fe-doped GaN buffer. Furthermore, the reduction of GaN channel thickness from 250 nm to 150 nm is explored to facilitate the development of highly scaled devices. No degradation of 2DEG properties was observed in the epitaxial structure with the GaN channel thickness reduced to 150 nm. An exemplary drain-induced barrier lowering (DIBL) of 20 mV/V was measured on a device with a Lg of 70 nm. While the sought-after 2DEG confinement and buffer insulation can be achieved, QuanFINE is not devoid of traps. This thesis also investigates an epitaxial structure with a band structure engineering at the interface of GaN channel and AlN nucleation layer using Si delta doping, which exhibited a lower buffer-related current collapse (19.8% vs. 26.8%), a more rapid current recovery speed, and a mitigation of long time constant as compared to the standard QuanFINE structure.
GaN HEMT
Ohmic contact
Passivation
QuanFINE
Pre-treatment
Författare
Ding-Yuan Chen
Chalmers, Mikroteknologi och nanovetenskap, Mikrovågselektronik
Structural investigation of ultra-low resistance deeply recessed sidewall ohmic contacts for AlGaN/GaN HEMTs based on Ti/Al/Ti-metallization
Semiconductor Science and Technology,;Vol. 38(2023)
Artikel i vetenskaplig tidskrift
A versatile low low-resistance ohmic contact process with ohmic recess and low low-temperature annealing for GaN HEMTs
Semiconductor Science and Technology,;Vol. 33(2018)
Artikel i vetenskaplig tidskrift
Impact of in situ NH3 pre-treatment of LPCVD SiN passivation on GaN HEMT performance
Semiconductor Science and Technology,;Vol. 37(2022)
Artikel i vetenskaplig tidskrift
Characterization of Trapping Effects Related to Carbon Doping Level in AlGaN Back-Barriers for AlGaN/GaN HEMTs
IEEE Transactions on Electron Devices,;Vol. 71(2024)p. 3596-3602
Artikel i vetenskaplig tidskrift
Microwave Performance of ‘Buffer-Free’ GaN-on-SiC High Electron Mobility Transistors
IEEE Electron Device Letters,;Vol. 41(2020)
Artikel i vetenskaplig tidskrift
Impact of the Channel Thickness on Electron Confinement in MOCVD-Grown High Breakdown Buffer-Free AlGaN/GaN Heterostructures
Physica Status Solidi (A) Applications and Materials Science,;Vol. 220(2023)
Artikel i vetenskaplig tidskrift
D.Y. Chen, K.H. Wen, M. Thorsell, J.T. Chen, and N. Rorsman, “Investigation and Mitigation of Trapping Mechanisms in Buffer-Free AlGaN/GaN HEMTs”
Gallium nitride (GaN) high electron mobility transistor (HEMT) is revolutionizing electronics by enabling high-power and high-frequency devices that are smaller, faster, and more efficient. Its unique properties—like exceptional electron mobility and robust performance under high voltage—make it ideal for applications like 5G communication and energy-efficient power systems. However, optimizing GaN devices faces hurdles such as contact resistance, trapping effects, and challenges in confining the channel of electrons. This research tackles these challenges with the following methods.
A novel process for forming low-resistance electrical contacts was developed, improving device performance. By precisely etching and fine-tuning metal layers, outstanding ohmic contacts was achieved with ultra-low contact resistance. Additionally, advanced NH3 surface treatments reduced high frequency performance loss, boosting device power output by 30% and efficiency by 13% at high frequencies.
To improve trapping effects, a groundbreaking design called QuanFINE was proposed. Unlike conventional approaches that rely on thick insulating layers, QuanFINE replaces them with a 10 times thinner structure, reducing electron traps and enabling even smaller device sizes without performance loss. Cutting-edge adjustments to the material structure further enhanced electron confinement and high frequency performance, paving the way for next-generation wireless electronics.
These advancements bring us closer to a future where electronics deliver more power with less energy waste, opening doors for faster wireless networks and greener technologies.
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Styrkeområden
Nanovetenskap och nanoteknik
Ämneskategorier (SSIF 2011)
Nanoteknik
Infrastruktur
Nanotekniklaboratoriet
ISBN
978-91-8103-159-1
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: Technical Report MC2-5617
Utgivare
Chalmers
Kollektorn at Kemivägen 9, Chalmers
Opponent: Farid Medjdoub, Dr., CNRS senior scientist, Group leader, IEMN