1. 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

2. Outline
Motivation
Introduce two field-plated device structures and their analysis
DC and Small-signal measurements
Power characterization
Noise characterization
Conclusion

3. 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.

4. 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.

5. 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.

6. 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

7. Gate resistance Rg jg
First order approximation:

8. 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

9. 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.

10. 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.

11. 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

12. 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)

13. 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

14. Power Characterization
Single-tone class B power measurement at 4 GHz with Vds=40 V
Intimately connected FP device
Normal FP device

15. 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.

16. 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.)

17. 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.