Jr-Tai Chen

This work investigates the impact of incorporating an AlGaN back barrier (BB) in buffer-free AlGaN/GaN high electron mobility transistors (HEMTs) on SiC. Both devices use an AlN nucleation layer (NL) for vertical isolation, while the modified structure introduces an Al0.2Ga0.8N BB beneath the GaN channel. The reference device without the BB exhibits strong electrostatic confinement but suffers from pronounced current collapse linked to traps in the AlN nucleation region. In contrast, the device with the AlGaN BB maintains short-channel control while reducing vertical electric-field crowding and mitigating dynamic trapping. Drain current transient spectroscopy confirms the suppression of a slow trap component associated with the AlN NL. At 40 GHz, these improvements result in enhanced power-added efficiency and output power density, reaching 43% and 2.9 W/mm, respectively, compared with 40% and 2.2 W/mm for the reference device. The results demonstrate that the AlGaN BB effectively improves the dynamic and millimeter-wave performance of buffer-free GaN HEMTs and supports further device scaling for high-power RF applications.

Group-III nitrides have transformed solid-state lighting and are strategically positioned to revolutionize high-power and high-frequency electronics. To drive this development forward, a deep understanding of fundamental material properties, such as charge carrier behavior, is essential and can also unveil new and unforeseen applications. This underscores the necessity for novel characterization tools to study group-III nitride materials and devices. The optical Hall effect (OHE) emerges as a contactless method for exploring the transport and electronic properties of semiconductor materials, simultaneously offering insights into their dielectric function. This non-destructive technique employs spectroscopic ellipsometry at long wavelengths in the presence of a magnetic field and provides quantitative information on the charge carrier density, sign, mobility, and effective mass of individual layers in multilayer structures and bulk materials. In this paper, we explore the use of terahertz (THz) OHE to study the charge carrier properties in group-III nitride heterostructures and bulk material. Examples include graded AlGaN channel high-electron-mobility transistor (HEMT) structures for high-linearity devices, highlighting the different grading profiles and their impact on the two-dimensional electron gas (2DEG) properties. Next, we demonstrate the sensitivity of the THz OHE to distinguish the 2DEG anisotropic mobility parameters in N-polar GaN/AlGaN HEMTs and show that this anisotropy is induced by the step-like surface morphology. Finally, we present the temperature-dependent results on the charge carrier properties of 2DEG and bulk electrons in GaN with a focus on the effective mass parameter and review the effective mass parameters reported in the literature. These studies showcase the capabilities of the THz OHE for advancing the understanding and development of group-III materials and devices.

Very fast interface traps have recently been suggested to be the main cause behind poor channel-carrier mobility in SiC metal–oxide–semiconductor field effect transistors. It has been hypothesized that the NI traps are defects located inside the SiO2 dielectric with energy levels close to the SiC conduction band edge and the observed conductance spectroscopy signal is a result of electron tunneling to and from these defects. Using aluminum nitride and aluminum oxide as gate dielectrics instead of SiO2, we detect NI traps at these SiC/dielectric interfaces as well. A detailed investigation of the NI trap density and behavior as a function of temperature is presented and discussed. Advanced scanning transmission electron microscopy in combination with electron energy loss spectroscopy reveals no SiO2 at the interfaces. This strongly suggests that the NI traps are related to the surface region of the SiC rather than being a property of the gate dielectric.

This study presents a novel approach to forming low-resistance ohmic contacts for AlGaN/GaN HEMTs. The optimized contacts exhibit an outstanding contact resistance of approximately 0.15 Ω·mm. This is achieved by firstly recessing the barrier of the heterostructure to a depth beyond the channel. In this way, the channel region is exposed on the sidewall of the recess. The coverage of the Ti/Al/Ti ohmic metalization on the sidewall is ensured through tilting of the sample during evaporation. The annealing process is performed at a low temperature of 550 °C. The approach does not require precise control of the recess etching. Furthermore, the method is directly applicable to most barrier designs in terms of thickness and Al-concentration. The impact of recessed sidewall angle, thickness and ratio of Ti and Al layers, and the annealing procedure are investigated. Structural and chemical analyses of the interface between the ohmic contacts and epi-structure indicate the formation of ohmic contacts by the extraction of nitrogen from the epi-structure. The approach is demonstrated on HEMT-structures with two different barrier designs in terms of Al-concentration and barrier thickness. The study demonstrate large process window in regard to recess depth and duration of the annealing as well as high uniformity of the contact resistance across the samples, rendering the approach highly suitable for industrial production processes.

Critical process modules for the fabrication of metal–insulator–semiconductor high‐electron‐mobility transistors (MISHEMTs) based on a novel ‘buffer‐free’ AlGaN/GaN heterostructure grown with metal–organic chemical vapor deposition (MOCVD) are presented. The methods of isolation and passivation for this type of heterostructure are investigated. Utilizing nitrogen implantation, it is possible to achieve off‐state destructive breakdown voltages (BVs) of 2496 V for gate–drain distances up to 25 μm, whereas mesa isolation techniques limit the BV below 1284 V. The stoichiometry of the SiN_xpassivation layer displays a small impact on the static and dynamic on‐resistance. However, MISHEMTs with Si‐rich passivation show off‐state gate currents in the range of 1–100 μA mm⁻¹at voltages above 1000 V, which is reduced below 10 nA mm⁻¹using a stoichiometric SiN_xpassivation layer. Destructive BVs of 1532 and 1742 V can be achieved using gate‐integrated and source‐connected field plates for MIHEMTs with stoichiometric and Si–rich passivation layers, respectively. By decreasing the field plate lengths, it is possible to achieve BVs of 2200 V. This demonstrates the implementation of MISHEMTs with high‐voltage operation and low leakage currents on a novel “buffer‐free” heterostructure by optimizing the SiN_xstoichiometry.

The 2D electron gas (2DEG) confinement on high electron mobility transistor (HEMT) heterostructures with a thin undoped GaN channel layer on the top of a grain‐boundary‐free AlN nucleation layer is studied. This is the first time demonstration of a buffer‐free epi‐structure grown with metal–organic chemical vapor deposition with thin GaN channel thicknesses, ranging from 250 to 150 nm, without any degradation of the structural quality and 2DEG properties. The HEMTs with a gate length of 70 nm exhibit good DC characteristics with peak transconductances of 500 mS mm⁻¹and maximum saturated drain currents above 1 A mm⁻¹. A thinner GaN channel layer improves 2DEG confinement because of the enhanced effectiveness of the AlN nucleation layer acting as a back‐barrier. An excellent drain‐induced barrier lowering of only 20 mV V⁻¹at aV_DSof 25 V and an outstanding critical electric field of 0.95 MV cm⁻¹are demonstrated. Good large‐signal performance at 28 GHz with output power levels of 2.0 and 3.2 W mm⁻¹and associated power‐added efficiencies of 56% and 40% are obtained at aV_DSof 15 and 25 V, respectively. These results demonstrate the potential of sub‐100 nm gate length HEMTs on a buffer‐free GaN‐on‐SiC heterostructure.

The impact on the performance of GaN high electron mobility transistors (HEMTs) of in situ ammonia (NH₃) pre-treatment prior to the deposition of silicon nitride (SiN) passivation with low-pressure chemical vapor deposition (LPCVD ) is investigated. Three different NH₃ pre-treatment durations (0, 3, and 10 min) were compared in terms of interface properties and device performance. A reduction of oxygen (O) at the interface between SiN and epi-structure is detected by scanning transmission electron microscopy (STEM )-electron energy loss spectroscopy (EELS) measurements in the sample subjected to 10 min of pre-treatment. The samples subjected to NH₃ pre-treatment show a reduced surface-related current dispersion of 9% (compared to 16% for the untreated sample), which is attributed to the reduction of O at the SiN/epi interface. Furthermore, NH₃ pre-treatment for 10 min significantly improves the current dispersion uniformity from 14.5% to 1.9%. The reduced trapping effects result in a high output power of 3.4 W mm⁻¹ at 3 GHz (compared to 2.6 W mm⁻¹ for the untreated sample). These results demonstrate that the in situ NH₃ pre-treatment before LPCVD of SiN passivation is critical and can effectively improves the large-signal microwave performance of GaN HEMTs.

Interfaces containing misfit dislocations deteriorate electronic properties of heteroepitaxial wide bandgap III-nitride semiconductors grown on foreign substrates, as a result of lattice and thermal expansion mismatches and incompatible chemical bonding. We report grain-boundary-free AlN nucleation layers (NLs) grown by metalorganic chemical vapor deposition on SiC (0001) substrates mediated by an interface extending over two atomic layers L1 and L2 with composition (Al_1/3Si_2/3)_2/3N and (Al_2/3Si_1/3)N, respectively. It is remarkable that the interfaces have ordered vacancies on one-third of the Al/Si position in L1, as shown here by analytical scanning transmission electron microscopy and ab initio calculations. This unique interface is coined the out-of-plane compositional-gradient with in-plane vacancy-ordering and can perfectly transform the in-plane lattice atomic configuration from the SiC substrate to the AlN NL within 1 nm thick transition. This transmorphic epitaxial scheme enables a critical breakdown field of ∼2 MV/cm achieved in thin GaN-based transistor heterostructures grown on top. Lateral breakdown voltages of 900 V and 1800 V are demonstrated at contact distances of 5 and 20 μm, respectively, and the vertical breakdown voltage is ≥3 kV. These results suggest that the transmorphic epitaxially grown AlN layer on SiC may become the next paradigm for GaN power electronics.