Gallium Oxide Power Transistors
Due to the increasing demands on higher dielectric strengths of electronic components in power electronics, FBH is intensively focusing on new semiconductor materials which have a high potential to meet these requirements. The promising semiconductor gallium oxide (β-Ga2O3) is characterized by a very wide bandgap of around 4.8 eV, from which a high dielectric strength with values far above that of gallium nitride or silicon carbide can be derived. This opens up the possibility of a significant reduction in the gate/drain separation in the transistor, so that semiconductor switches can be manufactured even more compactly and efficiently.
Fully processed 2-inch β-Ga2O3 wafer with lateral transistor devices.
Therefore, FBH has been involved in research activities in cooperation with the Leibniz Institute for Crystal Growth in recent years regarding the process development of lateral and vertical power switching transistors based on β-Ga2O3. This includes the development of essential process modules such as improvements in Ohmic contact formation via ion implantation, optimizations in low-damage etching procedures, interface engineering approaches and realization of a suitable gate topology.
Lateral devices
Large-periphery 720 mΩ Ga2O3 MOSFET with 92 mm total gate width.
By optimizing the layer growth and the device processing with regard to the gate topology, it has been possible in previous investigations to produce transistors that are characterized by a high breakdown strength of up to 2.5 MV/cm. In addition, breakdown voltages of 1.8 kV were achieved, resulting in a high power density of 155 MW/cm².
Furthermore, using an optimized ion implantation process, FBH successfully realized large-periphery β-Ga2O3 MOSFET devices on 2-inch semi-insulating β-Ga2O3 wafers, achieving a record drain current of up to 13 A and an on-state resistance as low as 720 mΩ. Moreover, this approach enabled high-voltage operation at 4 A / 300 V, representing the first demonstration of kilowatt-class switching in a Ga2O3-based transistor device.
Vertical devices
Schematic cross section of the vertical Ga2O3 FinFET structures developed at FBH.
SEM cross section image of a fully processed Ga2O3 FinFET device with a pitch of the fins of 1.2 μm.
Devices in a vertical structure offer great advantages to better exploit the potential of β-Ga2O3 with its high breakdown field strength. Due to the ideal separation of the high potentials on the top and bottom of the wafer through the epitaxially grown drift zone, the device is significantly less sensitive to surface effects, so that voltages in the range above 1 kV can be switched reliably. In addition, the chip area can be used more efficiently, which results in a significant potential for device scaling.
However, in contrast to lateral device structures, other requirements are placed on material and process technology. For example, conductive substrates are required on which layers of several micrometers thickness have to be grown epitaxially in order to realize a low-doped drift zone.
In cooperation with the Leibniz Institute for Crystal Growth, FBH is working intensively on the realization of vertical FinFETs based on the semiconductor material β-Ga2O3.The effective control of such device is largely determined by the width of the respective fin structures, which means that using electron beam lithography to generate structures in the sub-micrometer range is absolutely necessary. By means of dry chemical etching processes, fin structures that exhibit a high steepness with a width of 200 nm and a height up to 1 µm can be produced. This is an essential prerequisite for realizing vertical FinFETs.
First vertical Ga2O3 FinFET demonstrators exhibit promising device performance, including enhancement-mode operation and decent voltage blocking capabilities with an average breakdown strength of 2.7 MV/cm.
