title: Advanced GaN HEMT technology for millimetre-wave amplifiers
Creators: name: Dhongde, Aniket
Publisher: name: University of Glasgow
Year: 2023

description: Gallium Nitride (GaN)-based High-Electron-Mobility Transistor (HEMT) technology is a breakthrough innovation in the semiconductor industry, offering high-frequency and high-power performance capabilities. GaN HEMTs are widely used in power electronics, wireless communication systems, and radar applications over the past two decades. The key advantages of GaN HEMTs to produce heterojunctions to larger bandgap materials Aluminium Gallium Nitride (AlGaN) and the heterostructure results in the formation of the 2- dimensional electron gas (2DEG) which exhibits high electron mobilities of upto 2000 cm²/V.s and high saturation velocity of 2×10⁷ cm/s, resulting in high switching speeds and power densities. Due to its wide bandgap of 3.4 eV, it also allows exceptionally high breakdown fields of 3.3 MV/cm. In this thesis, the focus is on the major challenges in the development of GaN HEMT technology including achieving a low resistance ohmic contact, reducing self-heating, and improving device high frequency performance. Due to the wide bandgap of III-nitride semiconductors, achieving low-resistance Ohmic contact resistance is difficult. Recessing the Ohmic region prior to metallization is a typical approach to lowering the contact resistance. The contact resistance is often minimised by optimising factors such as recess depth, anneal temperature, and metal stack design. In this work, the three approaches involving the recessing of the ohmic region were evaluated. The Ohmic contact area was recessed in patterns similar to a chess board, vertical recessed stripes, and horizontal recessed strips. The two different recess etch depths, shallow and deep etch depths of 9 nm and 30 nm, respectively, were investigate. The lowest contact resistance of 0.32 Ω.mm (compared to 0.59 Ω.mm for a conventional non-recessed Ohmic contact) was observed for a deep horizontal patterned structure. The results also indicate that a highly reproducible process. The other major issue to address was to reduce the impact of device self-heating by effective heat distribution and dissipation. A novel thermal management technique was proposed, and the preliminary results are promising. It exploits the very thin epitaxial layer stack of a buffer-less GaN-on-SiC HEMT structure. III-V nitride material is etched and removed from around the active device area and the Au bond pad electrodes sit directly on the SiC substrate, providing a route for thermal dissipation from the active device to the substrate. This approach was demonstrated to reduce device self-heating and to improve the current density of the device. We fabricated and compared the performance of devices fabricated on the buffer-free and conventional GaN HEMTs. For identically sized 2-μm gate long, two-finger 2 × 50 μm gate width device with a gate to drain spacing of 3 m, the conventional devices broke down at 186 V while for the buffer-free structure, it was over 200 V (above the measurement capability of our equipment). The maximum drain current density of ~631 mA/mm and ~ 686 mA/mm biased at VGS = 1 V for the two-finger 2 × 50 μm gate wide for buffer free and conventional GaN structure, respectively. The buffer free and conventional GaN structure devices were measured to determine their maximum cut-off frequency (fT) and maximum oscillation frequency (fmax) when biased at VDS = 15V. The lower gate leakage currents were observed for the fabricated buffer-free AlGaN/GaN HEMT device as compared to conventional GaN HEMTs 197μA and 260μA, respectively. Also, the buffer free device, which had two fingers each measuring 2x200 μm, yielded measurements of 4.6 GHz for fT and 9.8 GHz for fmax. The conventional GaN device, also with two fingers each measuring 2x200 μm, was tested and resulted in measurements of 6.3 GHz for fT and 14.7 GHz for fmax. These results demonstrate the high quality of the buffer-free GaN heterostructure despite the absence of thick transition layers as currently used in the conventional GaN HEMTs. This indicates that the "buffer-free" design has the potential to be useful for millimetre wave applications in the future. This thesis also describes the fabrication and characterisation of a 100 nm footprint Ni/Au-based T-gate HEMT, 2x25 μm gate width, 1.5 μm drain source spacing, 100nm Si₃N₄ passivation layer thickness and device exhibit quite high peak currents of 805mA/mm and peak transconductance value of 246 mS/mm due to the low thermal boundary resistance on this buffer free epilayer wafer. The breakdown voltage was measured 47 Volts. Yielding a cut-off frequency fT of 87 GHz and maximum oscillation frequency fₘₐₓ of 143 GHz. We have developed a method for fabricating a T-shaped gate for sub 100nm gate foot length. The 100 nm length results in robustness, repeatable and has a high yield. Our findings indicate that this gate design could be beneficial for AlGaN/GaN buffer-free HEMTs used in millimetre wave frequency applications.