technology gallium oxide
bandgap of 4.8-4.9 eV and an n-type conductivity that can be controlled from 1016
-1019 cm-3 through doping
with tin or silicon. However, so far there are no reports of clear hole conduction in p-type Ga2
O3 O3 layers.
Thanks to its wider bandgap than SiC and GaN, the β-polytype of Ga2
promises to enable fabrication of
devices with excellent characteristics, including high breakdown voltages, high power capacity and high efficiency. (See Table 1 for a comparison of the important material properties for power device applications for various popular semiconductors, plus β-Ga2
O3 The great material properties of β-Ga2 O3 ). indicate that
this wide bandgap semiconductor should have a breakdown electric field of 8 MV/cm, three times those of either SiC or GaN. This very high value for the electric field strength is a trump card for Ga2
O3 because
Baliga’s figure of merit – the basic parameter to show how suitable a material is for power devices – is far more dependent on breakdown field than mobility. Values for this figure of merit are proportional to the cube of the breakdown field, but only linearly proportional to mobility.
devices can be an order of magnitude lower than those for SiC and GaN devices at the same breakdown voltage.
It is possible to use values for the material properties of semiconductors to calculate theoretical limits of on-resistance at a range of breakdown voltages (see Figure 1). Such efforts indicate that the on-resistance of Ga2
O3
Preparing a platform To fabricate Ga2
O3 devices that can fulfill their potential,
one must begin with a native substrate. The good news is that large, single-crystal Ga2
O3 substrates can be
fabricated from the melt at low cost using very little energy. This is in stark contrast to the expensive, energy-consuming methods employed for creating GaN and SiC bulk crystals and substrates: Sublimation, vapor phase epitaxy, and high-pressure synthesis.
Two of the members of our team, the Tamura and Koha companies, have already succeeded in developing 2-inch-diameter single-crystal β-Ga2
O3
equipment – scaling substrate size is simply a matter of increasing the size of the growth tool.
It should be possible to make Ga2 O3 substrates very
cheaply. We estimate that a mass-production system could churn out high-quality, 6-inch-diameter Ga2
O3
substrates at a unit cost of $120. Thanks to a relatively efficient process, we estimate that the power dissipated per-unit-area of substrate at the time of production is just one-third of that associated with SiC sublimation, due to a lower growth temperature and a higher growth rate.
Building devices We recently succeeded in fabricating the first field-effect transistors based on a single-crystal Ga2 grown on a β-Ga2
O3 O3 channel (010) substrate. These devices
were metal-semiconductor field-effect transistors (MESFETs), simple structures that are suitable for demonstrating transistor action. Our devices are certainly not the first oxide transistors ever produced. For several years, oxide semiconductors such as InGaZnO4
Figure 1. The β-poltype of Ga2
O3
promises to excel in breakdown voltage and on-resistance
(IGZO) and ZnO have been attracting attention
wafers via a melt
growth technology known as edge-defined film-fed growth (EFG). This growth technology has a good track record for producing large sapphire substrates, and the same system configuration can be used for making Ga2
O3 substrates. Figure 2.
This is not the only reason why melt growth is attractive: In principle, melt growth of Ga2
O3 should never
generate micropipe defects, which have historically plagued SiC substrates produced by the most popular method, sublimation. In addition, melt growth does not need a high-pressure environment, which is required for the production of SiC substrates. And on top of these benefits, the size of substrates produced by the EFG method is just determined by the dimensions of the
A melt-based process,which is similar to that employed for sapphire production, can yield 2- inch Ga2
O3 substrates June 2012
www.compoundsemiconductor.net 21
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