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.
Baca, Albert G., et al. "Al-rich AlGaN based transistors." Journal of Vacuum Science and Technology A, vol. 38, no. 2, Jan. 2020. https://doi.org/10.1116/1.5129803
Baca, Albert G., Armstrong, Andrew M., Klein, Brianna A., Allerman, Andrew A., Douglas, Erica A., & Kaplar, Robert J. (2020). Al-rich AlGaN based transistors. Journal of Vacuum Science and Technology A, 38(2). https://doi.org/10.1116/1.5129803
Baca, Albert G., Armstrong, Andrew M., Klein, Brianna A., et al., "Al-rich AlGaN based transistors," Journal of Vacuum Science and Technology A 38, no. 2 (2020), https://doi.org/10.1116/1.5129803
@article{osti_2561434,
author = {Baca, Albert G. and Armstrong, Andrew M. and Klein, Brianna A. and Allerman, Andrew A. and Douglas, Erica A. and Kaplar, Robert J.},
title = {Al-rich AlGaN based transistors},
annote = {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.},
doi = {10.1116/1.5129803},
url = {https://www.osti.gov/biblio/2561434},
journal = {Journal of Vacuum Science and Technology A},
issn = {ISSN 0734-2101},
number = {2},
volume = {38},
place = {United States},
publisher = {American Vacuum Society},
year = {2020},
month = {01}}
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Vol. 557, Issue 2https://doi.org/10.1016/j.nima.2005.10.128