On a footprint as small as possible, power electronic components should offer low energy consumption and achieve ever higher power densities, thus working more efficiently. This is where conventional devices reach their limits. Scientists all over the world are therefore investigating new materials and components that can meet these requirements. The Ferdinand-Braun-Institut (FBH) has now achieved a breakthrough with transistors based on gallium oxide (ß-Ga2O3).
Pictured above is the gallium oxide chip with transistor structures and for measurement purposes, processed at FBH via projection lithography (©FBH/schurian.com).
The newly-developed ß-Ga2O3-MOSFETs (metal-oxide-semiconductor field-effect transistor) provide a high breakdown voltage combined with high current conductivity. With a breakdown voltage of 1.8kV and a record power figure of merit of 155MW per square centimeter, they achieve unique performance figures close to the theoretical material limit of gallium oxide.
At the same time, the breakdown field strengths achieved are significantly higher than those of established wide bandgap semiconductors such as silicon carbide or gallium nitride.
Optimized layer structure and gate topology
In order to achieve these improvements, the FBH team tackled the layer structure and gate topology. The basis was provided by substrates from the Leibniz Institute for Crystal Growth with an optimized epitaxial layer structure. As a result, the defect density could be reduced and electrical properties improved. This leads to lower on-state resistances.
The gate is the central ‘switching point’ of field effect transistors, controlled by the gate-source voltage. Its topology has been further optimized, allowing to reduce high field strengths at the gate edge. This in turn leads to higher breakdown voltages.
The detailed results were published online on August 26, 2019 in the IEEE Electron Device Letters September issue. The following is the abstract from that article:
Lateral β-Ga2O3 MOSFET for power switching applications with a 1.8kV breakdown voltage and a record power figure of merit of 155MW/cm2 are demonstrated. Sub-μm gate length combined with gate recess was used to achieve low ON-state resistances with reasonable threshold voltages above −24V. The combination of compensation-doped high-quality crystals, implantation-based inter-device isolation, and SiN x -passivation yielded in consistently high average breakdown field strengths of 1.8- to 2.2-MV/cm for gate-drain spacings between 2- and 10-μm. These values outperform the results of more established wide-bandgap device technologies, such as SiC or GaN, and the major Ga2O3 material promise – a higher breakdown strength – is well demonstrated.