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Design, Fabrication, and Characterization of GaN High Power Rectifiers

Design, Fabrication, and Characterization of GaN High Power Rectifiers. Ph.D. Dissertation Defense. Kwang H. Baik Materials Science and Engineering, Univ. of Florida, Gainesville, FL November 2 , 2004. Outline. Motivation Theoretical Calculations GaN Materials Parameters

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Design, Fabrication, and Characterization of GaN High Power Rectifiers

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  1. Design, Fabrication, and Characterization of GaN High Power Rectifiers Ph.D. Dissertation Defense Kwang H. Baik Materials Science and Engineering, Univ. of Florida, Gainesville, FL November 2, 2004

  2. Outline • Motivation • Theoretical Calculations • GaN Materials Parameters • Intrinsic carrier concentration • Breakdown Voltage (VB) • On-state Resistance (RON) • Forward Voltage Drop (VF) & Leakage Current (IR) • Device Modeling • Breakdown Study with Edge Termination Techniques • Electrical Characteristics of GaN Rectifiers • Experimental Data • GaN High Voltage Diodes with Field Plate Termination • High Power Schottky Diode Array (GaN & SiC)

  3. Motivation • High Temperature, High Power and High Frequency Applications The figure-of-merit for power microwave applications • Intrinsic wide bandgap energy • High breakdown field for power applications • Excellent electron transport properties • Heterostructure available and strong piezoelectric polarization effect Johnson’s FOM (vsat EC)2/2 Baliga’s FOM (EC2)

  4. High Power Rectifiers SiC high power rectifier product Current ratings of 1A to 20A at 600V, and 5A to 10A at 1200V http://www.cree.com The applications of IGBT modules - UPS Power Supply,Servo Drive, Medical Power Supply, Motor Drives, Inverters http://www.pwrx.com Objective Develop the technology base for GaN-based rectifiers at power levels above 1MW

  5. GaN Material Parameters Temperature dependence of bandgap energy Density of states for GaN Incomplete ionization of impurity atoms Temperature dependence of bandgap energy of GaN and SiC. Ref. H. Teisseyre et al., 1994

  6. Mobility & Recombination Models Field-Dependent Mobility Model Analytical Mobility Model • Shockley-Read-Hall Recombination • Auger Recombination

  7. Intrinsic Carrier Concentration The small intrinsic carrier concentration in GaN at room temperature enables the high power and temperature applications. Intrinsic carrier concentration in SiC and GaN as a function of temperature. Ref. R. Kolessar et al., 2001.

  8. Impact Ionization Coefficient Fulop’s form (Power law expression) Impact ionization coefficient Simplified breakdown condition Impact ionization integral

  9. Ec & VB 1-D Poisson’s equation

  10. n- n n+ Breakdown Voltage GaN punchthrough diode where EC is critical electric field, WP drift region thickness NA doping concentration, and  permittivity 3 µm GaN epi can give more than 900V of reverse breakdown voltage with the doping concentration of 1016 cm-3 The calculated reverse breakdown voltage of punch-through diode as a function of doping concentration and standoff region thickness

  11. On-state Resistance On-state resistance (RON)

  12. VF & IR Forward Voltage Drop (VF) Reverse Leakage Current (IR)

  13. Mobility Models Bandgap (Eg) Density of States Device design Edge termination Breakdown analysis I-V characteristics Reverse recovery Thermal analysis Incomplete Ionization Medici Impact Ionization coefficients Recombination Models Device Modeling Scheme

  14. Electrode Oxide Potential contour Depletion contour Depletion region contour n Edge Termination • Edge termination is critical for obtaining high breakdown voltage and reduced on-state resistance. • Severe electric field crowding around metal contact periphery. • High leakage current and breakdown at the highest electric field

  15. n Depletion region contour Field Plate Termination • Oxide breakdown up to 0.7 µm thick • Metal contact corner breakdown more than 0.7 µm thick oxide • No further improvement in VB beyond 10 µm overlap

  16. Field Plate Termination

  17. Guard Ring Termination • Junction spacing and doping should be optimized. • Maximum E-field should be induced at the outside of the junction.

  18. p- p+ JTE layer Depletion boundary N Junction Termination Extension • VB values are highly sensitive to the charge in the JTE layer. • Multiple JTE termination technique (JTE1 + JTE2).

  19. Comparison of Edge Termination Methods • JTE the highest VB values. (4-fold increase) • The choice of edge termination should be based on the device type, size, and the effectiveness of termination method. • The edge termination designs with a numerical solution technique. Reverse breakdown voltage as a function of edge termination techniques.

  20. Anode (Pt) 5µm n (11016 cm-3) Cathode 1µm n+ (51019 cm-3) Forward I-V characteristics • Temperature dependence of I-V - Mobility degradation effects • Forward turn-on voltage • 1.6 V @ 100 A·cm-2 • Experimental values ~ 3.5 V • Materials issues (defects) • VF for pin diodes (even @573K) > VF for Schottky diodes • The absolute value of VF is also much higher than typical experimentally reported values, which are  5 V.

  21. Pt/Au SiO2 Ti/Al/Pt/Au Ti/Al/Pt/Au 3m undoped GaN 3m n+ GaN Al2O3 substrate Fabrication of GaN Rectifiers • GaN high power rectifiers • GaN Schottky & PiN rectifiers • - GaN epi layer on sapphire • - GaN epi layer on freestanding GaN • (Vertical geometry) • Device processing • Mesa etch (ICP dry etch) • Oxide deposition (PECVD) • p-guard rings (Implantation) • Window opening (RIE) • Ohmic metal formation (RTA) • Schottky metal deposition (E-beam)

  22. GaN Rectifier Processing: Mesa ICP mesa etch: Plasma-Therm ICP Mesa Lithography: AZP 4330 -- 4k RPM, 30 sec. (~3.75 µm straight wall resist) Etch Conditions: 10 sccm Cl2 5.0 sccm Ar 2 mTorr, 25° C ICP: 300 W RF: 150 W

  23. GaN High Voltage Diodes Unterminated diode Diameters of Schottky metal 54/72/98/134 µm Dielectric edge terminated diode Diameters of Schottky metal 44/62/88/124 µm

  24. Diode Forward I-V • RON=~2.2 mcm2 • Very close to simulation results • Device breakdown after JR=10/cm2 • VB=150 – 240 V Emcore substrate # K1645

  25. Breakdown Voltage Sample #: M5221sc4 Sample #: M5217sc3 Breakdown voltage at 10A/cm2

  26. Pulse Measurements of Large-Area Diode • Independent of measurement frequencies • RON=3.4 cm2≥ 3.31 mcm2 • The total defect density determined by TEM is ~106 cm-2.

  27. Electroplated Au (3 µm) Nitride Oxide (1500 Å) Schottky (Pt/Au) m Freestanding GaN (200 µm) 500 500µm m m 500µm 500 m GaN Schottky diode array layout GaN Schottky Diode Array • Schottky diode array with the size of 500 µm×500 µm. • Nitride windows interconnected with electroplated Au (~3µm) The schematic of high power GaN diode

  28. GaN Power Diode Array • Promising results for practical “on-state current” • Very close to simulated RON values (3.3 mΩ·cm2) • 161 A forward output current @ 7.12 V • RON (On-state resistance) = 8 mΩ·cm2 • 1.1 kW for 66 mm2 (active device area)

  29. SiC Power Diode Array • 430 A forward output current @ 5.7 V • RON (On-state resistance) = 5.8 mΩ·cm2 • 2.45 kW for 99 mm2 (active device area)

  30. Acknowledgement • Y. Irokawa, J. R. LaRoche, B. S. Kang, J. Kim, and K.P. Lee • Professors F. Ren and S. J. Pearton • S. S. Park and S. K. Lee for GaN substrates • D. Sheridan and G. Y. Chung about device modeling

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