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Uttam Singisetti*, Man Hoi Wong, Jim Speck, and Umesh Mishra ECE and Materials Departments

Vertically scaled 5 nm GaN channel Enhancement-mode N-polar GaN MOS-HFET with 560 mS/mm g m and 0.76 W -mm R on. Uttam Singisetti*, Man Hoi Wong, Jim Speck, and Umesh Mishra ECE and Materials Departments University of California, Santa Barbara, CA 2011 Device Research Conference

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Uttam Singisetti*, Man Hoi Wong, Jim Speck, and Umesh Mishra ECE and Materials Departments

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  1. Vertically scaled 5 nm GaN channel Enhancement-mode N-polar GaN MOS-HFET with 560 mS/mm gm and 0.76 W-mm Ron Uttam Singisetti*, Man Hoi Wong, Jim Speck, and Umesh Mishra ECE and Materials Departments University of California, Santa Barbara, CA 2011 Device Research Conference Santa Barbara, CA, USA *uttam@ece.ucsb.edu

  2. Outline • Next generation GaN electronic devices • N-polar GaN HEMTs • Vertically scaled channel devices • Results and Conclusion

  3. Next-generation mm-wave GaN devices John Albrecht, DARPA 1.3 W at 75 GHz 3 W at 87 GHz Caltech, HRL ISSSTT, 2011 Fujitsu, CSIC 2010 W-band GaN power amplifiers • GaN HEMTs: Power-switching, microwave, W-band power amplifiers • Future GaN devices for beyond mm-wave and to sub-mm-wave bands • Higher operating voltages than traditional III-Vs and Si  robust and rugged mixed signal ICs

  4. Device goals and structure • Aggressive dimensional scaling (Lg and Lsd ) • Vertical scaling with back-barrier and high-k dielectric • Parasitic resistances and capacitances scaling • Maintain high breakdown voltage

  5. Ultra-scaled N-polar HEMTs No electron barrier N-polar inverted HEMT N-polar GaN • No barrier to electron on top of 2-DEG grading to narrowgap InN  low resistance contacts (0.027 W-mm)1 • AlGaN back  confinement of 2-DEG, control short channel effects2 • Record high gm = 1105 mS/mm demonstrated4 in D-mode • E-mode devices 1. S.Dasgupta, APL 2010, 2. S. Rajan, IEEE TED 2011 3. NIdhi, DRC 2011

  6. Under S/D contacts* Under sidewall AlN removed under sidewall * S.Dasgupta, APL 2010 E-mode device structure and design Top AlN depletes 2-DEG under gate Under gate Under gate

  7. Short channel effect and vertical scaling 8 nm GaN channel2 20 nm GaN channel1 Vt roll-off with gate length Poor saturation at sub-100nm Lg • Vth roll off with gate length • Vertical scaling needed to maintain E-mode at sub-50 nm gate lengths • Vertical scaling for high Rds at sub-50-nm gate lengths Need 5 nm GaN channel for sub-50 nm devices 1. U.Singisetti, EDL 2010, 2. U.Singisetti, APEX 2011

  8. Ultra-thin channel challenges: Mobility QW thickness flutuations GaN • Need 5 nm thick GaN channel for sub-50 nm devices • Mobility drops with decreasing GaN channel thickness • Interface roughness, surface roughness scattering increases* * U.Singisetti, ISCS 2011

  9. Mobility dependence on Si doping Si : 5 e18 cm-3 Si : 2 e 19 cm-3 • Low mobility in high-3D Si density samples • High Si density may lead to rougher interface

  10. Ultra-thin channel challenges: surface depletion • Surface depletion increases in thin channels • Lower charge in the access regions lead to higher source resistance

  11. 5nm-GaN channel device design • Graded back-barrier high mobility and t reduce the effect of trap* • 4.5 nm of Al2O3 gate dielectric • 1.6×1013 cm-2 in the sidewall access regions after top-AlN etch * M -H Wong, DRC 2011.

  12. Device fabrication process* * U.Singisetti, EDL 2010.

  13. DC characteristics • Reduced short channel effects due to vertical scaling and graded barrier1 • Peak gm = 560 mS/mm, peak Id = 1.3 A/mm • Positive threshold voltage of 1.3 V * M -H Wong, DRC 2011.

  14. DC characteristics: Ron and Rs Gate InN InN Gate No InN • Record low Ron = 0.61 W-mm* for Lg = 115 nm • InN growth optimization for complete coverage near the gate • Regrowth sheet resistance = 100 W/sq, rc = 5 W-mm

  15. RF performance: peak ft • peak ft = 115 GHz at Vds = 4.5 V and Vgs = 2.5 V • low fmax = 30 GHz due to thin W gate ( ~ 1500 W/sq)

  16. RF performance : small-signal model Measured (circles) Modeled (line) S21/5 S12*3 S11 S22 • Equivalent circuit model

  17. RF performance: bias dependence • Vgs corrosponding to peak ft is 2.5 V • Absence of drain delay

  18. Conclusionsand future work • Demonstrated vertically scaled 5-nm GaN channel MOS-HFET devices • E-mode with Vth = 1.3 V, peak gm = 560 mS/mm, peak Id = 1.3 A/mm • Record low Ron = 0.61 W-mm, for 115 nm E-mode GaN HEMTs • peak ft = 115 GHz for 120 nm gate length device Future work • Scale the gate length to 50 nm • Top gate for fmax • Scale the gate dielectric (HfO2, ZrO2) This work was supported by DARPA NEXT program

  19. New measurements post DRC • peak ft = 122 GHz at Vds = 5.5 V and Vgs = 2.5 V • ft-Lg product of 14 GHz-mm

  20. New measurements post DRC • maximum Ion/Ioff ratio ~ 2×105 • Breakdown voltage 8. 6 V • dielectric breakdown

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