Al-rich AlGaN based transistors

Research results for AlGaN-channel transistors are reviewed as they have progressed from low Al-content and long-channel devices to Al-rich and short-channel RF devices. Figure of merit (FOM) analysis shows encouraging comparisons relative to today’s state-of-the-art GaN devices for high Al-content...

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Veröffentlicht in:Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films Surfaces, and Films, 2020-03, Vol.38 (2)
Hauptverfasser: Baca, Albert G., Armstrong, Andrew M., Klein, Brianna A., Allerman, Andrew A., Douglas, Erica A., Kaplar, Robert J.
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container_title Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films
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creator Baca, Albert G.
Armstrong, Andrew M.
Klein, Brianna A.
Allerman, Andrew A.
Douglas, Erica A.
Kaplar, Robert J.
description Research results for AlGaN-channel transistors are reviewed as they have progressed from low Al-content and long-channel devices to Al-rich and short-channel RF devices. Figure of merit (FOM) analysis shows encouraging comparisons relative to today’s state-of-the-art GaN devices for high Al-content and elevated temperatures. Critical electric field (EC), which fuels the AlGaN transistor FOM for high Al-composition, is not measured directly, but average gate-drain electric field at breakdown is substantially better in multiple reported AlGaN-channel devices compared to GaN. Challenges for AlGaN include the constraints arising from relatively low room temperature mobility dominated by ternary alloy scattering and the difficulty of making low-resistivity Ohmic contacts to high Al-content materials. Nevertheless, considerable progress has been made recently in the formation of low-resistivity Ohmic contacts to Al-rich AlGaN by using reverse compositional grading in the semiconductor, whereby a contact to a lower-Al alloy (or even to GaN) is made. Specific contact resistivity (ρc) approaching ρc ∼ 2 × 10−6 Ω cm2 to AlGaN devices with 70% Al-content in the channel has been reported. Along with scaling of the channel length and tailoring of the threshold voltage, this has enabled a dramatic increase in the current density, which has now reached 0.6 A/mm. Excellent ION/IOFF current ratios have been reported for Schottky-gated structures, in some cases exceeding 109. Encouraging RF performance in Al-rich transistors has been reported as well, with fT and fmax demonstrated in the tens of gigahertz range for devices with less than 150 nm gates. Al-rich transistors have also shown lesser current degradation over temperature than GaN in extreme high-temperature environments up to 500 °C, while maintaining ION/IOFF ratios of ∼106 at 500 °C. Finally, enhancement-mode devices along with initial reliability and radiation results have been reported for Al-rich AlGaN transistors. The Al-rich transistors promise to be a very broad and exciting field with much more progress expected in the coming years as this technology matures.
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Nevertheless, considerable progress has been made recently in the formation of low-resistivity Ohmic contacts to Al-rich AlGaN by using reverse compositional grading in the semiconductor, whereby a contact to a lower-Al alloy (or even to GaN) is made. Specific contact resistivity (ρc) approaching ρc ∼ 2 × 10−6 Ω cm2 to AlGaN devices with 70% Al-content in the channel has been reported. Along with scaling of the channel length and tailoring of the threshold voltage, this has enabled a dramatic increase in the current density, which has now reached 0.6 A/mm. Excellent ION/IOFF current ratios have been reported for Schottky-gated structures, in some cases exceeding 109. Encouraging RF performance in Al-rich transistors has been reported as well, with fT and fmax demonstrated in the tens of gigahertz range for devices with less than 150 nm gates. Al-rich transistors have also shown lesser current degradation over temperature than GaN in extreme high-temperature environments up to 500 °C, while maintaining ION/IOFF ratios of ∼106 at 500 °C. Finally, enhancement-mode devices along with initial reliability and radiation results have been reported for Al-rich AlGaN transistors. 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Challenges for AlGaN include the constraints arising from relatively low room temperature mobility dominated by ternary alloy scattering and the difficulty of making low-resistivity Ohmic contacts to high Al-content materials. Nevertheless, considerable progress has been made recently in the formation of low-resistivity Ohmic contacts to Al-rich AlGaN by using reverse compositional grading in the semiconductor, whereby a contact to a lower-Al alloy (or even to GaN) is made. Specific contact resistivity (ρc) approaching ρc ∼ 2 × 10−6 Ω cm2 to AlGaN devices with 70% Al-content in the channel has been reported. Along with scaling of the channel length and tailoring of the threshold voltage, this has enabled a dramatic increase in the current density, which has now reached 0.6 A/mm. Excellent ION/IOFF current ratios have been reported for Schottky-gated structures, in some cases exceeding 109. Encouraging RF performance in Al-rich transistors has been reported as well, with fT and fmax demonstrated in the tens of gigahertz range for devices with less than 150 nm gates. Al-rich transistors have also shown lesser current degradation over temperature than GaN in extreme high-temperature environments up to 500 °C, while maintaining ION/IOFF ratios of ∼106 at 500 °C. Finally, enhancement-mode devices along with initial reliability and radiation results have been reported for Al-rich AlGaN transistors. 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(SNL-NM), Albuquerque, NM (United States)</aucorp><format>journal</format><genre>article</genre><ristype>JOUR</ristype><atitle>Al-rich AlGaN based transistors</atitle><jtitle>Journal of Vacuum Science &amp; Technology A: Vacuum, Surfaces, and Films</jtitle><date>2020-03-01</date><risdate>2020</risdate><volume>38</volume><issue>2</issue><issn>0734-2101</issn><eissn>1520-8559</eissn><coden>JVTAD6</coden><abstract>Research results for AlGaN-channel transistors are reviewed as they have progressed from low Al-content and long-channel devices to Al-rich and short-channel RF devices. Figure of merit (FOM) analysis shows encouraging comparisons relative to today’s state-of-the-art GaN devices for high Al-content and elevated temperatures. Critical electric field (EC), which fuels the AlGaN transistor FOM for high Al-composition, is not measured directly, but average gate-drain electric field at breakdown is substantially better in multiple reported AlGaN-channel devices compared to GaN. Challenges for AlGaN include the constraints arising from relatively low room temperature mobility dominated by ternary alloy scattering and the difficulty of making low-resistivity Ohmic contacts to high Al-content materials. Nevertheless, considerable progress has been made recently in the formation of low-resistivity Ohmic contacts to Al-rich AlGaN by using reverse compositional grading in the semiconductor, whereby a contact to a lower-Al alloy (or even to GaN) is made. Specific contact resistivity (ρc) approaching ρc ∼ 2 × 10−6 Ω cm2 to AlGaN devices with 70% Al-content in the channel has been reported. Along with scaling of the channel length and tailoring of the threshold voltage, this has enabled a dramatic increase in the current density, which has now reached 0.6 A/mm. Excellent ION/IOFF current ratios have been reported for Schottky-gated structures, in some cases exceeding 109. Encouraging RF performance in Al-rich transistors has been reported as well, with fT and fmax demonstrated in the tens of gigahertz range for devices with less than 150 nm gates. Al-rich transistors have also shown lesser current degradation over temperature than GaN in extreme high-temperature environments up to 500 °C, while maintaining ION/IOFF ratios of ∼106 at 500 °C. Finally, enhancement-mode devices along with initial reliability and radiation results have been reported for Al-rich AlGaN transistors. 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title Al-rich AlGaN based transistors
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