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Characterization of two Field-Plated GaN HEMT Structures
Hongtao Xu, Christopher Sanabria, Alessandro Chini, Yun Wei, Sten Heikman, Stacia Keller, Umesh K. Mishra and Robert A. York Electrical and Computer Engineering University of California at Santa Barbara Supported by ONR
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Outline Motivation Introduce two field-plated device structures and their analysis DC and Small-signal measurements Power characterization Noise characterization Conclusion
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Motivation Optimize GaN HEMT performance from the device structure level. Use field-plated GaN HEMT structure for high power microwave circuits. Further improve the power capacity, PAE and breakdown. GaN HEMT for low noise applications.
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Field-plated device structures (I)
GaN 2DEG Drain Source Gate SiN Field Plate AlGaN Field-plate length Field-plate is connected to the gate through the common path of the gate and gate feeder in the extrinsic device region. GaN HEMT devices of 32 W/mm was reported with this structure. Most commonly used structure.
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Field-plated device structures (II)
GaN Drain Source Gate & Field Plate 2DEG SiN AlGaN Field-plate length Gate and field-plate are intimately connected. RIE etching of SiN may damage the AlGaN surface.
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Field-plated device structure
Intimately connected FP GaN HEMTs Normal FP GaN HEMTs Field-plate length GaN AlGaN 2DEG Drain Source SiNx Gate Field-plate Field-plate length Drain Source SiNx Gate Field-plate Possible affected parameters: Rg, Cgd
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Gate resistance Rg jg First order approximation:
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Rg modeling Simulated by EM model: Rg = 1.6 Ω for normal FP structure.
Rfp Simulated by EM model: Rg = 1.6 Ω for normal FP structure. Rg = 1.33 Ω for intimately connected FP structure. Rg Simplified Model Simulation on a 75 µm gate finger with 0.7 µm field-plate length. Distributed EM Model ∆Rg ∆Rfp
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EM simulation at 4 GHz Intimately connected FP device Normal FP device
Field-plate Field-plate Gate finger Gate finger Simulation on a 75 µm gate finger. 0.7 µm field-plate length.
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DC measurement 200 ns pulsed I-V curves of two field-plated GaN HEMTs Specifications: Lg = 0.7 µm; Wg = 2x75 µm; Field-plate length = 0.7 µm; 2000 Å SiN passivation layer n0 = 9.96x1012 cm-2; Hall mobility ~1450 cm2/Vs Intimately connected FP device has higher pinch-off voltage. Both devices have similar Idss at Vgs=0 V.
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SP measurement (ft, fmax)
Small-signal characterization of two field-plated GaN HEMTs Normal FP device ft = 21 GHz fmax = 51 GHz Intimately connected FP device ft = 20 GHz fmax = 40 GHz
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Small signal model and simulation
Measurement vs. Simulation Lg Ls Ld Rd Rs Rg Rgd Cgs Cds Ri Rds gmVe-jωτ + V - Cgd Gate Drain Source S11 -0.5 0.0 0.5 -1.0 1.0 S12 S21 S22 ADS-based parameter extraction routines. Models incorporate dominant parasitics and losses. Freq. (50 MHz to 30 GHz)
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Intrinsic small signal parameters
Rds(Ω) Rgd(Ω) Cgd(fF) Cds(fF) Normal FP device 7.178 1297.6 22.882 45.497 14.842 Intimately connected FP device 8.276 960.0 15.374 70.885 16.340 Cgs(fF) gm(S) Rs(Ω) Rg(Ω) Rd(Ω) 270.7 0.040 6.531 0.924 5.503 246.0 4.830 0.782 5.423
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Power Characterization
Single-tone class B power measurement at 4 GHz with Vds=40 V Intimately connected FP device Normal FP device
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Noise characterization (I)
Noise performance of non-field-plated devices, normal field-plated devices and Intimately connected field-plated devices. NFmin and Ga were found by sweeping Ids and Vds.
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Noise characterization (II)
Noise performance of field-plate devices with different field-plate lengths. Rg decreases as field-plate length increases. (NFmin decreases.) Cgd increases as field-plate length increases. (NFmin increases.)
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Conclusion Two field-plated device structures were characterized and analyzed. The structure with intimately connected field-plate helps to reduce the gate resistance, but the larger Cgd reduces the gain and efficiency. The noise performance of field-plated device is better than non-field-plated device.
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