technology gallium oxide
as new transparent transistor materials for display applications. However, unlike Ga2
O3 O3
, these crystal
structures are amorphous and/or polycrystalline, and are consequently unsuitable for the fabrication of high-power devices. To produce our Ga2 tin-doped n-Ga2
O3
MBE. Conventional Knudsen cells provided the gallium and tin sources, and the oxygen source comprised 5 percent ozone and 95 percent oxygen.
Device isolation techniques have not been developed, so we employed a circular FET pattern (see Figure 3). To form ohmic contacts, we used a mixture of BCl3
transistor, we deposited a channel layer on a native substrate by
and
argon to perform reactive-ion etching, followed by evaporation of Ti/Au. We have found that this etching process substantially reduces contact resistance. After this, we fabricated Schottky gates by Pt/Ti/Au deposition and lift off. MESFETs that did not feature surface dielectric passivation were created that had a 4 µm gate length and a source-drain spacing of 20 µm. The diameter of the inner circular electrodes for the drain was 200 µm.
DC output characteristics for one of our circular Ga2 O3
MESFETs include a maximum drain current density of 25 mA/mm and perfect pinch-off (see Figure 4). A destructive measurement of the three-terminal breakdown voltage in the off-state, which resulted in burned gate electrodes, produced a value of more than 250 V. Transconductance peaked at 2.3 mS/mm for a 40 V drain bias.
Other promising characteristics of our MESFET include an off-state drain leakage current of just 5 µA/mm and an on/off drain current ratio that could hit 10,000. A high proportion of this leakage current will be associated with the large gate pad (see Figure 3(b)), and the leakage from the gate finger will be at least one order of magnitude below this value. The off-state drain current is comparable to the gate leakage current, indicating that there is a negligibly small leakage through the semi-insulating Ga2
O3 substrate. It is possible to drive
down the off-state current even further by simply adjusting the device configuration.
A benchmark to judge these devices by is that of the performance of the GaN MESFETs of the early 1990s: Our oxide transistors deliver comparable or better performance. It is no surprise that our devices are inferior to the far more mature, state-of-the-art SiC and GaN devices of today, but their combination of high breakdown voltage and low leakage current shows that they have great potential as power devices.
To improve the performance of our fledgling devices, we must address a slew of technological challenges. These include the production of substrates with diameters of more than 4-inches, device processing, and epitaxial growth that includes doping.
Figure 4.DC characteristics of the n-Ga2
MESFET June 2012
www.compoundsemiconductor.net 23
So that our devices can find practical application fast, we want to develop normally off transistors that can serve in switching equipment. The best structures for this purpose are MOSFETs, which are likely to use either Al2
O3 or SiO2
for the gate dielectric film. There is
good reason to believe that it will be possible to produce such structures with a high-quality, low-defect- density interface, because both materials are oxides. We believe that our recent development of Ga2
O3
transistors could herald a new era for high-performance power electronics. Such devices will not only help to reduce global energy use; they will also trim energy consumption in the semiconductor industry.
This work is partially supported by the New Energy and Industrial Technology Development Organization (NEDO) and the Japan Science and Technology Agency (JST), Japan.
© 2012 Angel Business Communications. Permission required.
Figure 3. A cross- sectional schematic illustration of the n-Ga2
O3
MESFET structure (a) and an optical micrograph of the fabricated device (b)
O3
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