Ganesh Mainali

Reliable engineering of low-resistance metal semiconductor (M/S) contacts is critical for advancing indium oxide (In2O3) based semiconductor technologies. As device dimensions shrink to the micrometer regime, minimizing contact resistance becomes essential to ensure channel limited operation rather than contact dominated. In this study, we systematically optimized Ni/In2O3 contacts using mild rapid thermal annealing (RTA) at 250 °C in nitrogen ambient conditions compatible with back-end-of-line (BEOL) processing and evaluated their performance using the transfer line method (TLM). Ni/In2O3 TLM structures were fabricated via photolithography and liftoff, followed by RTA at varying durations. Quantitative TLM analysis demonstrated a substantial reduction in contact resistance (RC) to 6.24 Ω, normalized contact resistance (N · RC) to 1.25×10−1 Ω cm, specific contact resistivity (ρC) to 1.53×10−6 Ω cm2, and a short transfer length (LT) of 123 nm, all achieved without intentional doping or complex metallization. This process driven, BEOL compatible approach provides a robust route to low-resistance contacts in In2O3 thin films, enabling a reliable foundation for next generation low power, high performance oxide electronics.

Beta Gallium Oxide (β-Ga2O3) Junction Barrier Schottky diodes possess excellent breakdown characteristics and efficiency, but the lack of p-type β-Ga2O3 has led to the use of alternative materials, such as NiOx, which pose challenges regarding lattice mismatch and temperature stability. This work reports a unipolar β-Ga2O3 diode enabled by the magnesium (Mg) diffusion process to create high-resistance electron-blocking regions. The Mg diffusion process developed in this work uses a room temperature compatible chemical process followed by an 800 °C annealing step, which is lower compared to already reported methods in the literature. Therefore, the proposed method is thought to reduce the device processing temperature budget and hence cost. The resulting device achieves an Ion/Ioff ratio of ∼109, a knee voltage of ∼0.9 V, and a breakdown voltage of 596 V. In addition, it demonstrates stable operation from 100 to 500 K, highlighting its suitability for electronics operating across a broad temperature range.

This study demonstrates the high-temperature operation of AlGaN/GaN metal–insulator–semiconductor high electron mobility transistors (MIS-HEMTs) based on nickel oxide (NiOx) as an intermediate gate oxide, achieving stable performance up to 400 °C. Compared to the control sample with only an SiNx gate dielectric, the proposed device exhibited significant improvements: (1) enhanced thermally induced VTH stability, (2) a flat transconductance (gm) curve indicating improved linearity, and (3) lower and stable drain-to-source saturation voltage (VDS,sat). Notably, the ΔVTH shift for D-mode MIS-HEMT with NiOx was effectively reduced to ∼+0.4 V, compared to ∼−1.4 V in the control sample, over a temperature range from 25 to 400 °C. This improvement is attributed to hole carrier generation in the NiOx layer, which increases the depletion region and stabilizes the stored charge underneath the gate at high temperatures. This work demonstrates that the NiOx gate oxide layer significantly enhances VTH stability and linearity in GaN MIS-HEMT, ensuring reliable and stable device operation at high temperatures.

Normally off beta-phase gallium oxide (β-Ga2O3) metal-oxide field-effect transistors (MOSFETs) on GaN-on-Si substrates were fabricated with a threshold voltage (VTH) of 3 V. β-Ga2O3 thin films were deposited using pulsed laser deposition. The device demonstrated an excellent breakdown voltage of ∼540 V at VGS = 0 V and an extremely low gate leakage current of ∼10−7 mA/mm. Additionally, it exhibited a low subthreshold swing (SS) of ∼167 mV/dec and a current on/off ratio of ∼106. The growth and fabrication of β-Ga2O3 MOSFETs on GaN-on-Si substrates are significant for high performance and their monolithic integration with GaN devices in future power-integrated circuits.

Gallium oxide (Ga2O3), with its ultra-wide bandgap (>4.5 eV), is a key material for the next-generation power electronics due to its high breakdown voltage and efficient power-switching capabilities. Multifinger (MF) Ga2O3 MOSFETS, designed to enhance current handling and thermal management, experience significant self-heating effects that can lead to localized hotspots, thermal runaway, and reduced device reliability. Accurate thermal characterization is therefore critical to ensure the reliable operation and longevity of such devices. Conventional methods, such as thermoreflectance imaging, Raman thermometry, and infrared thermography, are limited by complex setups, slow response times, resolution constraints, and cost, making them less practical for real-time, on-chip applications. On-chip thermal characterization directly at the active regions of the device provides an unparalleled opportunity to overcome these limitations by capturing localized temperature variations during operation. In this study, we demonstrate the integration of micro-thin film thermocouples (micro-TFTCs) onto multifinger Ga2O3 MOSFETs for precise, real-time, and localized thermal monitoring. The sensors captured temperature variations across different gate fingers, with the measured maximum channel temperature reaching 40.5 °C under peak power dissipation. Predicted thermal behavior under high power densities shows temperatures rising to approximately 80 °C at 5 W/mm2, illustrating the thermal challenges faced by Ga2O3 devices. This work demonstrates that micro-TFTCs are not only compatible with complex device architectures but also highly effective for localized thermal characterization, making them a promising tool for improving the thermal management and reliability of Ga2O3-based power electronics.

In this work, we report on the beta-gallium oxide (β-Ga2O3) monolithic bidirectional switch. The as-fabricated switch works on enhancement mode operation with a threshold voltage of ∼4 V. Maximum drain current density of 1.93 mA/mm is obtained with a drain voltage of 5 V. The bidirectional switch in a bidirectional mode has an ON/OFF current ratio of ∼107 with ON-resistance of 1.11 and 1.09 kΩ · mm in forward and reverse direction, respectively. However, in diode mode, the device shows an ON/OFF current ratio of 1.6 × 108 and 1.4 × 108 in forward and reverse conduction modes, respectively. The fabricated β-Ga2O3 monolithic bidirectional switch is then used to chop a 60 Hz Alternating Current signal at a chopping frequency of 1 kHz, indicating its potential applications in a range of converters.

Aluminum nitride (AlN) is a promising ultrawide bandgap material with significant advantages for power electronics and optoelectronic applications due to its high breakdown voltage, mobility, and thermal conductivity. AlN Schottky barrier diodes and metal semiconductor field effect transistors have shown potential but are limited by issues such as high off-state leakage current and complex structures to achieve ohmic contacts. To address these challenges, we report on the fabrication and characterization of an AlN metal oxide semiconductor field effect transistor (MOSFET) with a recessed gate structure. The source and drain contacts were fabricated on n-doped AlN epitaxy using Ti-based contacts with a Ti/Al/Ti/Au metal stack. To evaluate the performance of these contacts, a circular transmission line model was employed, and contacts were annealed at various temperatures ranging from 750 °C to 950 °C in a nitrogen ambient. Our results reveal that unannealed Ti-based contacts on AlN showed no current conduction. However, annealing these contacts at 950 °C for 30 s significantly reduced the specific contact resistance to 0.148 Ω·cm², achieving an ∼80% reduction compared to samples annealed at 750 °C. Utilizing these optimized contact conditions, we fabricated, to the best of our knowledge, the first AlN MOSFET. The fabricated AlN MOSFET exhibits a threshold voltage of −10.91 V, an effective mobility of 2.95 cm² V⁻¹ s⁻¹, an on–off current ratio spanning two orders of magnitude, and a reverse breakdown voltage of approximately ∼250 V in air without a field plate.

This study demonstrates pseudo-source-gated beta-gallium oxide (β-Ga2O3) metal oxide semiconductor field effect transistors (MOSFETs). The proposed pseudo-source gated transistor (pseudo-SGT) architecture has a thin (∼11 nm) recessed channel design, effectively emulating conventional SGT characteristics without significantly compromising on-current. The fabricated devices exhibit remarkable intrinsic gain of 104, low output conductance of 10−8 S/mm, transconductance of 10−3 S/mm, and drain saturation voltage of ∼1.5 V, while maintaining a drain current of 1.3 mA/mm. These enhanced performance metrics significantly expand the potential of β-Ga2O3 MOSFETs for the development of Ga2O3 monolithic power integrated circuits.