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 EngineeringUniversity of California at Santa Barbara

Supported by ONR

Outline

Motivation Introduce two field-plated device

structures and their analysis DC and Small-signal measurements Power characterization Noise characterization Conclusion

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.

Motivation

Field-plated device structures (I)

GaN2DEG

DrainSourceGate

SiNField Plate

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.

Field-plate length

AlGaN

Field-plated device structures (II)

GaN

DrainSource

Gate & Field Plate

2DEG

SiN

Field-plate length

Gate and field-plate are intimately connected. RIE etching of SiN may damage the AlGaN surface.

AlGaN

Field-plate length

GaN

AlGaN

2DEG

DrainSource

SiNx

Gate Field-plate

Field-plated device structure

Field-plate length

DrainSource

SiNx

Gate Field-plate

Normal FP GaN HEMTs Intimately connected FP GaN HEMTs

Possible affected parameters: Rg, Cgd

Gate resistance Rg

1

3g g

WR R

L

j g

First order approximation:

Rg modeling

∆Rg

∆Rfp

Simulated by EM model:Rg = 1.6 Ω for normal FP structure.Rg = 1.33 Ω for intimately connected FP structure.

Rfp

RgSimplified Model

Distributed EM Model

Simulation on a 75 µm gate finger with 0.7 µm field-plate length.

EM simulation at 4 GHz

Field-plate

Gate finger

Field-plate

Gate finger

Normal FP device Intimately connected FP device

Simulation on a 75 µm gate finger.

0.7 µm field-plate length.

DC measurement

0 5 10 15 20

0.0

0.2

0.4

0.6

0.8

1.0

1.2

intimately connected field-plated device normal field-plated device

Vds

(V)

Cur

rent

Den

sity

(A

/mm

)

Intimately connected FP device has higher pinch-off voltage.

Both devices have similar Idss at Vgs=0 V.

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 layern0 = 9.96x1012 cm-2;Hall mobility ~1450 cm2/Vs

SP measurement (ft, fmax)

1 10 1000

10

20

30

40

50

h21,

U (

dB)

Frequency (GHz)

h21 of intimately connected field-plate device U of intimately connected field-plate device h21 of normal Field-plate device U of normal Field-plate device

Small-signal characterization of two field-plated GaN HEMTs

ft = 20 GHzfmax = 40 GHz

ft = 21 GHzfmax = 51 GHz

Normal FP device

Intimately connected FP device

Small signal model and simulation

Lg

Ls

LdRd

Rs

Rg Rgd

Cgs

Cds

Ri

Rds

gmVe-jωτ

+

V

-

Cgd

Gate Drain

Source

-0.5 0.0 0.5-1.0 1.0

S12

S21

S11

S22

Freq. (50 MHz to 30 GHz)

Measurement vs. Simulation

ADS-based parameter extraction routines. Models incorporate dominant parasitics and losses.

Intrinsic small signal parameters

Ri(Ω) 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)

Normal FP device 270.7 0.040 6.531 0.924 5.503

Intimately connected FP device 246.0 0.040 4.830 0.782 5.423

Power Characterization

0 5 10 15 2010

20

30

40

50

Peak PAE=70.9%

PSAT

=10.1W/mm

Po

ut(d

Bm

),G

t(dB

)

Pin(dBm)

10

20

30

40

50

60

70

80

PA

E(%

)

0 5 10 15 20 25

10

20

30

40

50

Peak PAE=60%

PSAT

=12.9 W/mm

Pou

t(dB

m),

Gt(

dB)

Pin(dBm)

10

20

30

40

50

60

70

PA

E(%

)

Normal FP device Intimately connected FP device

Single-tone class B power measurement at 4 GHz with Vds=40 V

Noise characterization (I)

Noise performance of non-field-plated devices, normal field-plated devices and Intimately connected field-plated devices.

4 6 8 10 12

0.8

1.2

1.6

2.0

2.4

2.8

3.2

3.6N

Fm

in (

dB)

Frequency (GHz)

non-FP device normal FP device intimately connected FP device

-15

-10

-5

0

5

10

15

20G

a (dB)

NFmin and Ga were found by sweeping Ids and Vds.

Noise characterization (II)

0.5 0.6 0.7 0.8 0.9 1.0 1.10

3

6

9

12

15

5 GHz

12 GHz

Ga

(dB

)Field-plate length (m)

0.5 0.6 0.7 0.8 0.9 1.0 1.10.4

0.8

1.2

1.6

2.0

2.4

12 GHz

5 GHz

NF

min (

dB)

Field-plate length (m)

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.)

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.