density-dependent electron transport and … electron transport and accurate modeling of gan hemts...

DRC2015 Sanyam Bajaj: [email protected] Prof. Siddharth Rajan: [email protected]

Density-Dependent Electron

Transport and Accurate Modeling of

GaN HEMTs

Sanyam Bajaj1, O. F. Shoron1, P. S. Park1, S. Krishnamoorthy1, F.

Akyol1, T.-H. Hung1, S. Rajan1

1Department of Electrical and Computer Engineering

The Ohio State University, Columbus, OH USA

S. Reza2, E.M. Chumbes2

2Raytheon IDS Microelectronics, Andover, MA USA

J.B. Khurgin3

3Department of Electrical and Computer Engineering

Johns Hopkins University, Baltimore, MD USA

Acknowledgment:

Raytheon IDS Microelectronics, EXEDE MURI, DATE MURI

mailto:[email protected]



Outline

2

• Motivation

• Density dependence of velocity

• Velocity measurement

• Simulation using measurement




Outline

3

• Motivation







GaN HEMTs for RF amplification

4

Space Communications

Mobile/Wireless Communications

Military

Advantages of GaN HEMTs

High Power Density/Unit Area

High Frequency Operation

Harsh Environment Applications

THz Imaging




Figures of Merit

5

• Maximum available gain (MAG)

• Power gain cutoff frequency (fmax)

• Current gain cutoff frequency (fT)

Transistor requirements

- High power gain over output contour

- Gain linearity to minimize signal distortion




Small and Large Signal Issues

6

• Actual device shows reduction

in gm, fT and MAG as the VG

(charge density) increases

• Non-uniformity in power-gain,

SOFT GAIN COMPRESSION

David Meyer, WOCSEMMAD 2015




What causes this reduction?

7

gm,int

VG

• Lower 2DEG velocity as the density increases

• Critical to investigate density dependence of velocity

-5-4

-3-2

-10

0

10

20

30

40

50

60

5

10

15

2025

f T (

GH

z)

V DV

G

Intrinsic fT Intrinsic gm

• Intrinsic fT / gm profiles – NOT just an RS effect




Outline

8

• Motivation







Density dependence of velocity

9

Measured Hall

mobility

Low field: ve = μ*F

• Density dependent mobility

0 2 4 6 8 10 12

400

600

800

1000

1200

1400

1600

H

ALL

(cm

2/V

s)

ns (x10

12 cm

-2)





10

• Density dependent velocity has NOT been measured

• Theoretical studies on density dependent velocity

• Models predict a decrease in vsat as sheet density increases

2 4 6 8 10

1.0

1.5

2.0

2.5

3.0

[ref]

New model

vS

at (

107cm

/s)

nS (10

12

/cm2

)

LO phonon-based model:

D. Jena and S. Rajan, arXiv preprint arXiv:1008.1154 (2010)

Fang et al., IEEE Electron Device Letters 33.5 (2012): 709

0 2 4 6 8 10 12

400

600

800

1000

1200

1400

1600

H

ALL

(cm

2/V

s)

ns (x10

12 cm

-2)

Low field: ve = μ*F

• Density dependent mobility

Measured Hall

mobility

High field: ve = vsat

• Density dependent vsat





11

E (k) E (k)

k k

ky ky

• Simplified “toy” model based

on optical phonon emission

• Energy-band structure of

GaN

• 2DEG occupation in k-space:

Fermi circle

• Comparing lower and higher

sheet charge densities (ns)

• No applied field



No Applied Field

kxkxkF

kF

EF

EF





12

E (k) E (k)

k k



kx

ky ky

APPLIED FIELD

kx

F

(-e)F

ħωop

k0

kF

k0

kF

• Simplified “toy” model based

on optical phonon emission

• Energy-band structure of

GaN

• 2DEG occupation in k-space:

Fermi circle

• Comparing lower and higher

sheet charge densities (ns)

• APPLIED FIELD




Comparison

13

kx

ky

kOP

APPLIED FIELD

F

(-e)F

Lower ns

Higher ns



kx




Comparison

14

ky

kOP

APPLIED FIELD

F

(-e)F

k0

k0

Lower ns

Higher ns

• k0 (centroid) – effective

momentum of 2DEG

• k0 for lower ns GREATER

THAN k0 for higher ns

• Strong density-dependence



kx

kx

2 4 6 8 10

1.0

1.5

2.0

2.5

3.0

[ref]

New model

vS

at (

107cm

/s)

nS (10

12

/cm2

)

LO phonon-based model:




Outline

15

• Motivation







Velocity measurement

16

Khan et al., IEEE Trans. on Electron Devices 47(2), 269 (2000)

Danilchenko et al., Appl. Phys. Lett. 104(7), 072105 (2014)

• Non-gated test structures [ref]

• Isolation used to make thin transport

channels

• Negligible resistance from

access regions (RC=0.4 ohm.mm)

• Gated-structures (Moll’s method)

cause abrupt peaks in field profile

• Non-uniform ns profile

Velocity directly from 2-terminal pulsed I-V:

J = qnsveve = J / qns




To vary 2DEG sheet density

17

• AlGaN recess to vary 2DEG sheet density

(ns)

• Extremely low-power Cl2-based plasma etch

• On-wafer hall measurement (Van der Pauw)

to confirm ns

• Multiple devices measured to confirm

uniformity

S

D

AlGaN GaNS

heet

density

AlGaN thickness

EF

EC




Hall measurement

18

• Standard AlGaN/GaN HEMT

• ns = 7.4 x 1012 cm-2

• μ = 1423 cm2/Vs

• RSH = 590 ohm/sq

• On-wafer hall measurements for different charge density values

ve = J / qns

0.7 μm GaN

20 nm Al0.2Ga0.8N

2DEG




Measurement setup

19

• Pulsed-iV measurements with 2-Terminal G-S probe

• Accent DiVA (Dynamic i-V Analyzer)

• Pulse width 500 ns; Duty Cycle ~ 0.01%

• SEM to confirm dimensions:

• Error < ~2%

2 μm x 2 μm




Density-dependent velocity

20

• Velocity-field as a function of varying density

ve = J / qns

• ns-dependent saturation velocity:

• strong dependence

• vsat decreases as ns increases




Outline

21

• Motivation







Simulation using measurement

22

• To validate velocity measurement results

• Simulation of DC and RF characteristics using measured

2DEG density-dependent velocity

• Multiple devices simulated with varying gate-lengths

• 2-D TCAD simulator Silvaco ATLAS

GaN

1 nm AlN21 nm AlGaN

2DEG

Lg=60nm




Simulated device characteristics

23

• Simulated DC and RF characteristics for

GaN HEMT with LG = 1 μm [ref]

• All device parameters used from the

report

• Simulation results show excellent

agreement with experimental data

-8 -6 -4 -2 00.0

0.3

0.6

0.9

1.2

1.5

gm (

S/m

m)

I D (

A/m

m)

VG (V)

Experiment

Simulation

VD = 8 V

LG = 1 m

0.0

0.1

0.2

0.3

1 10 1000

5

10

15

20

25

Curr

ent

Gain

(dB

)

Frequency (GHz)

Experiment [19]

Simulation

Experiment: ft = 15 GHz

Simulation: ft = 14 GHz

GaN

33 nm AlGaN

2DEG

Lg=1μm

Ping et al., IEEE Electron Device Lett. 19(2),

54-56 (1998)




Simulated device characteristics

24

• Simulated DC and RF characteristics for

GaN HEMT with LG = 60 nm [ref]

• Simulation results show excellent

agreement with experimental data

• First report of velocity-field-density

curves which can be used for physics based

modeling of GaN HEMTs

-5 -4 -3 -2 -1 0 10.0

0.2

0.4

0.6

0.8

1.0

1.2

gm (

S/m

m)

I D (

A/m

m)

VG (V)

Experiment

Simulation

VD = 5 V

LG = 60 nm

0.0

0.1

0.2

0.3

0.4

0.5

0.6

1 10 1000

10

20

30

40

50

Experiment

Simulation

Curr

ent

Gain

(dB

)

Frequency (GHz)



0 2 4 6 8 100.0

0.5

1.0

1.5 Experiment

Simulation

I D (

A/m

m)

VD (V)

VG = +1V

VG = -1V

GaN

1 nm AlN21 nm AlGaN

2DEG

Lg=60nm

Chung et al., IEEE Electron Device

Lett. 31(3), 195-197 (2010)




Summary

25

• Investigated density and field dependence of 2DEG

velocity in GaN HEMTs

• Pulsed I-V measurement on non-gated test structures

• Electron velocity strongly dependent on density-

decreases with increasing density

• Simulation of DC and RF characteristics using

measured velocity characteristics

• Excellent agreement with experimental results

• Validation of measurement

• Accurate model for GaN HEMTs

1 10 1000

10

20

30

40

50

Experiment

Simulation

Cu

rre

nt

Ga

in (

dB

)

Frequency (GHz)






BACKUP SLIDES

26




Pulsed iV with 2-T RF probes

27

• Pulsed measurements with 2-Terminal G-S probe

• Measurements done using Accent DiVA (Dynamic i-V Analyzer)

• Pulse width 500 ns; Duty Cycle ~ 0.01%

• Multiple devices measured for each unique charge density

• Adjacent isolated structure measured and subtracted




Simulation of test structure

28

• We assume uniform field in

the test structures; hence

uniform sheet charge density

• Simulated potential profile:

• Linear profile showing

uniform potential drop

• Simulated electric field

profile:

• No abrupt field peaks

• Field fairly uniform

across the active region

Simulated Potential Profile

Simulated Electric Field Profile



density-dependent electron transport and … electron transport and accurate modeling of gan hemts...

Documents