Wide-bandgap semiconductors, such as Gallium Nitride (GaN), offer significant advantages compared to silicon for high-power and high-frequency applications, such as a larger breakdown field.
Among the main lateral device concepts based on GaN, the high electron mobility transistor (HEMT) has become a mature technology, which can operate at higher frequencies and temperatures than its silicon-counterparts, while ensuring larger blocking voltages and substantially higher system efficiency in smaller area footprints. The research at NaMLab focuses on optimization of critical modules of aluminium gallium nitride (AlGaN)/GaN HEMT devices, which determine the overall performance and reliability. The processing steps for integrating high-k dielectrics in the gate stack of metal-insulator-semiconductor (MIS)-HEMT devices are carefully optimized to improve the stability of the threshold voltage. Gold-free ohmic contacts annealed at relatively low temperatures (≤600°C) are developed to enhance integration flexibility. Additionally, the current research focuses on integrating ferroelectric thin films in the gate stack of MIS-HEMT devices to tune the threshold voltage, and realize devices that can switch between depletion- and enhancement-mode.
Vertical GaN power devices are also of great technological interest due to their area-independent breakdown voltage scaling. Vertical GaN MOSFET with trench gate configuration have been developed at NaMLab. Apart from addressing device processing challenges, the research at NaMLab mainly focuses on developing reliability tests to identify referential regions for breakdown onset.
Contact
Dr.-Ing. Roberto Guido
E-Mail: info[at]namlab.com
High Electron Mobility Transistor. A HEMT technology based on contact lithography for 150 mm wafers is utilized for material characterization and device development in co-operation with external partners. The focus is on process modules for improving overall device performance, stability and reliability. In particular, we investigate the integration of a high-k dielectric material underneath the gate electrode to fabricate a Metal-Insulator-Semiconductor (MIS)-HEMT, while considering the process engineering on the AlxGa1-xN surface prior to the deposition of the dielectric and the post-annealing of the gate structures, which impacts the threshold voltage stability. Additional research efforts are spent to optimize ohmic contacts to enable a higher integration flexibility.
Fully Vertical GaN Power Trench MOSFET. The vertical GaN MOSFET with trench-gate configuration has been investigated at NaMLab since 2014. In recent years, fully vertical GaN MOSFETs with trench-gate configuration have been fabricated and characterized, building upon extensive expertise with pseudo-vertical devices. The work focused on demonstrating high-voltage operation and on systematically identifying and mitigating technological and physical limitations that currently prevent these devices from realizing their full potential. In this regard, careful development of a low-leakage surface passivation technique and optimization of edge-termination design were identified as crucial steps for achieving a high breakdown voltage. Channel performance was improved by optimizing the trench-formation process and developing a wet-chemical post-treatment.
Integration of Ferroelectric with High Electron Mobility Transistor. Integrating ferroelectric materials with HEMTs is a promising strategy to achieve enhancement (E)-mode operation (transistor normally-off), and enable switching between depletion (D)-mode (transistor normally-on) and E-mode operation within the same HEMT, which opens fascinating perspectives for realizing reconfigurable integrated circuits, novel energy-efficient power converters, and HEMT-based non-volatile memories with non-destructive reading. We are investigating the integration of ferroelectric materials with HEMT through a floating gate approach, referred to as ferroelectric-metal-HEMT (FeMHEMT), which allows removing the complex interface between ferroelectric and AlxGa1-xN, while controlling the voltage drop across the HEMT gate stack and the metal-ferroelectric-metal (MFM) capacitor connected in series. This is realized through adjusting the area ratio (AR) between the MIS and MFM capacitors, and ferroelectric switching can be achieved without electrical breakdown of the HEMT gate stack.
Electrical characterization and reliability of GaN-based power devices. To benchmark the performance of GaN-based power devices, electrical characterization and reliability testing is routinely performed. In particular, parameters such as effective field-effect channel mobility, threshold voltage, ON-state resistance, current density, and breakdown voltage are monitored.
Running Projects: Ecsel All2GaN.
Closed Projects: Ecsel UltimateGaN, GaNHOCH – VFT, TeleGaN, EFFSIL300, LeistungsSchaltER, F-GaN.