Low Frequency Noise in GaN-Based Advanced Electronic Devices

A Dissertation for

Doctor of Philosophy

By

Nezih Pala

Thesis Advisors:

Dr. Michael Shur

Dr. Remis Gaska

2

Talk outline

• Motivation• Introduction to low frequency noise• Devices under study• Possible noise sources in FETs• Determination of noise sources in HFETs and MESFETs

• New model for 1/f noise in HFETs• Conclusions and Key Contributions

3

Motivation

•Low frequency noise is a figure of merit and limiting factor for high frequency Field Effect Transistors including HFETs and MESFETs.

•Low frequency noise is upconverted to high frequencies limiting the performance of the transistors in the microwave range[1]. Especially when these devices are used as oscillators or mixers, low frequency noise limits the phase noise characteristics and degenerates the performance of the electronic system.

•Low frequency noise analysis is a powerful tool for examining compound semiconductors and yielding information about crystal defects and interfaces [2].

[1] M. N. Tutt, D. Pavlidis, A. Khatibzadeh, B. Bayraktaroglu, IEEE Trans. Microwave Theory and Tech. 43, 1461-1471, (1995)[2]L. K. J. Vandamme, IEEE Trans. Elec. Devices, 41, 2176, (1994)

4

Types of Low Frequency Noise in FETs

•Thermal Noise SV = 4kTR

SI =4kT / R

•Generation- Recombination noise.

•1/f Noise

fNV

fS

I

fS VI

22 )(

)(

)(

)(Hooge Parameter fN

I

S I2

ec 111

222 1

14

FF

nV

N

I

S

olume

TI

100 101 102 103

10-22

10-21

10-20

10-19

10-18

Lorentzian

1/f Noise

G-R contribution

HFETR= 100

Frequency (Hz)

SI (

f)

(A

2 /Hz)

Nyquisit (thermal) noise(S

I = 4kT/R)

5

Low Frequency Noise Measurement Setup

• Main instrument: FFT Spectrum Analyzer SR770, Stanford Research Systems• Measurement range: 1 mHz – 100 kHz• Measurement sensitive to vibrations air suspended table (optical table)• Measurement sensitive to electromagnetic fields and light shielded

enclosure• Use of batteries less disturbances of power system frequencies and

harmonics

6

Devices Under Study

MOS-HFET HFET

30 nm Si:Al0.2Ga0.8N 2x1018

50 nm AlN

7-13 nm SiO2

SiC/Sapphire

1 m i-GaN50-70 nm Si:GaN (2-5)x1017

HD-MOSFET HD-MESFET

2m GaN

AlN

7-13 nm SiO2

Sapphire

50-70 nm Si:GaN (0.5-1.5) x 1018

7

GaN Research Cycle

Epilayer growth by MOCVDUSC

Layout design by L-EditRPI

Mask and Device FabricationRPI,USC

Characterization:DC, RFLow Frequency NoiseRPI, USC

2D SimulationRPI

8

Fabrication of GaN Based FETs I

•Source/Drain ohmic metal deposition• e-beam evaporation, •Ti/Al/Ti/Au (100Å/300Å/200Å/1000Å) • Annealing at 850 oC for 60 sec. in N2 ambient

•Ion implantation for isolation •He+, 40 keV, 2x1014 cm-2

•Provides planar geometry

9

Fabrication of GaN Based FETs II

•SiO2 deposition for MOS-HFET and MOSFET

type devices• Plasma Enhanced Chemical Vapor Deposition (PECVD)

•SiO2 patterning and removal•BOE Etch

10

Fabrication of GaN Based FETs III

•Contact pad metal deposition•e-beam evaporation, •Ti/Au (200Å/5000Å)

•Gate metal deposition•e-beam evaporation,•Ni/Au (300Å/500Å)

11

DC Characteristics of AlGaN/GaN HFET and MOS-HFET

•Several orders of of magnitude reduction in gate leakage current in MOS-HFET•Comparable Drain saturation current about 720 mA/mm.•Larger gate voltage swing for MOS-HFET.•Higher linearity in transconductance of MOS-HFET.•Decreased transconductance for MOS-HFET due to decreased gate capacitance

-16 -12 -8 -4 0 4 8 12

10-1210-1110-1010-910-810-710-610-510-4 HFET

MOS-HFET

Gat

e Le

akag

e C

urre

nt

I G

(A

)

Gate Bias VGS

(V)-20 -15 -10 -5 0 5

0.0

0.2

0.4

0.6

0.8

0

20

40

60

80

100

120

140

Dra

in S

atur

atio

n C

urre

nt

I DS

at

(A

/mm

)

Gate Bias VGS

(V)

HFET MOS-HFET

Tra

nsco

nduc

tanc

e G

(m

S/m

m)

12

DC Characteristics of GaN HD-MESFET

Lg= 1.5 m =100 cm2/Vs Ne=1.5x1018 cm-3

•High saturation current density.•RC 0.3 mm ( C ~ 1x10-6 cm2 ).•Low gate leakage current (<10 nA) indicates the quality of Schottky contacts.•Good agreement with the simulation results is encouraging for submicron devices.

-10 -8 -6 -4 -2 00

50

100

150

200

250

300

350

0

15

30

45

60

75

Tra

nsco

nduc

tanc

e (m

S/m

m)

Dra

in S

atur

atio

n C

urre

nt I D

sat (

mA

/mm

)

Gate Bias VGS

(V)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0

200

400

600

800

1000

=100 cm2V/s

=400 cm2V/s

=1200 cm2V/s

Dra

in S

atur

atio

n C

urre

nt I D

sat (

mA

/mm

)

Gate Length (m)

HFET

MESFET

13

DC Characteristics of GaN HD-MESFET

Lg= 1.5 m =100 cm2/Vs Ne=1.5x1018 cm-3

•High saturation current density.•RC 0.3 mm ( C ~ 1x10-6 cm2 ).•Low gate leakage current (<10 nA) indicates the quality of Schottky contacts.

-6 -5 -4 -3 -2 -1 0

10-11

10-10

10-9

10-8

VD=12 V

VD=0.05 V

Gat

e Le

akag

e C

urre

nt

I G

Gate Bias VGS

(V)0 2 4 6 8

0

50

100

150

200

250

300

VG=-5V

VG=-4V

VG=-3V

VG=-2V

VG=-1V

VG=0V

Dra

in c

urr

en

t

I d

(m

A/m

m)

Drain voltage Vd (V)

14

Noise spectra for HFETs and MESFETs

The noise spectra SId/Id2 have the form of 1/f noise with close to unity

(= 1.0-1.15) for both types of devices.

fNI

S

d

I 2

310)32(

1 10 100 1000

-150

-140

-130

-120

-110

-100

-90

-80HD-MESFET, V

G=0V

1/f

HFET, VG=0V

MOS-HFET, VG=0V

SN

D

SI /

I2 (d

B/H

z)

Frequency (Hz)

35 1010

MESFET

HFET

15

Possible Noise Sources in FETs

•1/f noise•Contact Noise •Gate Leakage Current•Surface Noise•Channel (under the gate and source-gate, gate-drain regions) Noise•Fluctuations of the Schottky barrier space charge region (SCR) in MESFETs

•Generation-Recombination Noise

RGaN=RS1+RCh+RS2

Lg

Source Gate

RS1RCh RS2

Drain

RCRC

L

16

Contribution of Contact Noise Sources TLM Measurements

1 10- 135

- 130

- 125

- 120

1/L 2

1/L

S I /

I2,

dB

/Hz

Spacing L , um

22GaN

RRcI

R

SS

I

SGaN

222

1

LR

S

I

S

GaN

RcI

LR

S

I

S

GaN

RI GaN1

22

•SRc>>SGaN

•SRc<<SGaN

Contact noise

Channel or Surface noise

Noise form the channel is dominant.

(RC<<RGaN)

tW

LRGaN

NfR

SR 2

2

2

22

2

222 )()( GaNC

GaN

GaN

R

GaNC

C

C

RcRI

RR

R

R

S

RR

R

R

S

R

S

I

SGaN

L

W

t

17

Calculation of Current Noise Density for Series Resistors

    

  

 

)( SC RRIV )()(0 SCSC RRIRRIV

)()( SCSC RRIRRI 22 )}({)}({ SCSC RRIRRI

)()( 22

sC RRSCI SSIRRS

2

2

22

2

22 )()( CS

S

S

R

SC

C

C

RI

RR

R

R

S

RR

R

R

S

I

SsC

RC RS

V

0CS RR

18

The contribution of the gate leakage current fluctuations to the output drain current noise of AlGaN/GaN HFETs was studied by three different methods:

1. The low frequency noise in the AlGaN/GaN HFETs and Metal-Oxide-Semiconductor Heterostructure Field Effect Transistors (MOS-HFETs) have been compared.

2. The gate current fluctuations were measured directly, in AlGaN/GaN HFETs.

3. The correlation between the gate and drain current fluctuations was measured and analyzed.

Contribution of the Gate Current Fluctuations

19

Contribution of the Gate Current Fluctuations

222dg

Vg

d

Id

IR

S

I

S dominant

contribution

10-4 10-3 10-2-160

-140

-120

-100

-80

=10-3

=0.1

SId/I d2 , d

B/H

z

Drain Current Id, A

10-3 2x10-3 3x10-3 4x10-3

-135

-130

-125

-120

-115

=10-4

=0.6-0.8

SId/I d2 , d

B/H

zDrain Current I

d, A

■Measured ●Calculated from measured gate noise

20

100 1000 100000.0

0.2

0.4

0.6

0.8

1.0f=200Hz

low noise-samples

high noise-samples

Co

rre

latio

n

Gate Series Resistance Rg, W

Correlation Between Drain and Gate Current Fluctuations

IgGate

Vg

Rg

SVg

Vd

Drain

Rd

SVd

Source

4diffsum

gd

SSS

VgVd

gd

SS

S

fVVS dgsum /)( 2

fVVS dgdiff /)( 2 }22

2

gRrR

rRSS g

d

dIgVd

21

Location of the Noise Sources in MESFETs

1x10-5 1x10-4 1x10-3-140

-130

-120

-110

-100

-90

-80

Vd=0.5 V

f=200Hz

S Id

/ I d2

(d

B/H

z)

Drain Current Id, A

IRR

R

R

S

I

S

ChS

S

S

RI S

2

2

22 )(

22

2

2 )( dChS

Ch

d

Id

IRR

BdR

I

S

Channel Noise = 10-3

Space charge region fluctuations

noise from surface and channel out of the gate:

2

2

2

2

2 )()( chS

S

SchS

ch

chd

Id

RR

R

fNRR

R

fNI

S

22

10-6 10-5 10-4 10-3 10-2-140

-130

-120

-110

-100

-90

-80

Id

- 1

SI/I

2 , dB

/Hz

Drain current Id, A

2

2

2

2

2 )()( SCh

S

SSCh

Ch

Chd

Id

tot

Rtot

d

Vd

RR

R

fNRR

R

fNI

S

R

S

V

S

Location of the 1/f Noise Sources in HFETs

sCh RR

112

dChd

Id IRI

S

channel noise

noise from surface and channel out of the gate:

2

2

22 )( chs

s

s

Rs

d

Id

RR

R

R

S

I

S

n

ch

I IRI

S

22

1

No fit indicates the concentration dependence of .

23

-8 -6 -4 -2 0 2 4

0.0

0.5

1.0

1.5

2.0

2.5

VT = - 5.6 V

107 C

G (

F/c

m2 )

Gate Voltage, VG (V)

Estimation of Electron Concentration and Mobility

ChCh

gn WRqn

L gV

Vg gCh CdVq

n1

1

0 2x1012 4x1012 6x1012 8x10120

200

400

600

800

1000

Mob

ility

, mn,

cm

2 /Vs

Concentration nCh

, cm-2

24

Hooge Parameter as a Function of 2D Electron Concentration

fAnI

S

ChCh

I

2Chn

1

22

1

Ch

I

nI

S

)1()( 2 FFn

(Typical for tunneling[1,2])

1.L. K. J. Vandamme, X. Li, and D. Rigaud, “1/f noise in MOS devices, mobility or number fluctuations?”, IEEE Transaction Electron Devices 41 (1994) 1936–1945

2. A. L. McWhorter, “1/f noise and germanium surface properties”, in Semiconductor Surface Physics, R.H. Kingston, ed., Philadelphia PA, Univ. of Pennsylvania Press (1957) 207-228

1012 1013

10-3

10-2

HFET MOS-HFET

1/nCh

Concentration ns, cm-2

25

New Model for 1/f Noise in Doped Channel HFETs

•Tunneling into donor states in GaN•Phonon assisted tunneling•Longer jumps correspond to lower frequency noise

AlGaN GaN

C

F

D

1

2

x0

-e

q

x

26

Equations for the Tunneling Model

)()()( ddc fxx

,)2(

212

2

pd

fm

p

c

x

dxxp0

)(2

exp'

mFxxmxp c 2)(2)(

2/3

'

Fxx

e

3/2

24

3

mF

xF

2

1

0

0

':

':

d

d

e

e

d

d

2/30 )/exp( Fc xx

: 2D tunneling cross-section

: capture rate due to tunneling

: electron momentum in 2 DEG

Where :

27

Equations for the Tunneling Model

dxx

Fxffx

An

N

n

xSd ddddSn22

020 ))((1

)/()1()(4)(

d dd

dddddFdSn d

An

ffxN

n

S

)1())/(ln(3

)](1)[(823/12

020

This is 1/f noise

kTTN

Ng

ggkT dc

c

DdF

exp)(42

ln(2

fn

TC

n

Sn20

20

)(~

0

F

dd Fx

exp0

kT

ddFd

ge

f

1

1)(

28

0 50 100 150 200 250 30010-14

10-13

10-12

10-11

100 Hz

SId/I d2 ,1

/Hz

Temperature T, K

Comparison With the Experiment

•Very good agreement of the noise maximum amplitude and position

•But the shapes of the dependencies are different

•And the temperature dependence of the 1/f noise has to be checked in undoped HFETs

Experiment

Theory

29

Temperature Dependencies of Noise Density in HFETs and MOS-HFETs

300 350 400 450 500 550-150

-145

-140

-135

-130

-125

-120

3200Hz1600Hz

800Hz400Hz200Hz

100Hz

Vg= 0 VVd=0.5 V

SiO2/AlGaN/GaN MOS-HFET

SI /

I2 (

dB/H

z)

Temperature (K)300 350 400 450 500 550

-150

-145

-140

-135

-130

-125

-120

Smax

Tmax

3200Hz

400Hz

1600Hz800Hz

200Hz

110Hz

Vg=0 VVd=0.5 V

AlGaN/GaN HFET

SI /

I2 (

dB/H

z)

Temperature (K)

Such S(T) dependencies are typical for the noise from local levels

Ea= 0.8 - 1.0 eV,

30

What is the Origin of the G-R Noise?

22

22 1

14

FF

LWn

N

I

S C

s

tsI

Fsc vn /1

[*]

[*] Copeland J. A., IEEE Trans. Elect. Dev. 18, 50, 1971

mv FF /2

)/exp(0 kTt FCn vN /10 t

F

C

Al0.2Ga0.8N

GaN

SurfaceChannel

d

1

t

3

2

1) G-R Process by a level in the channel

  310-18 cm : To small to be true!Nts1026 cm-2 : To large to be true!

2) Electrons can be captured via tunneling. Might lead 1/f noise not GR noise.

3) G-R Process by a level in AlGaN barrier layer

Et = 0.8 - 1.0 eV, n (10-12 - 10-13)cm2,

Nt1016 cm-3 : All reasonable values.

where

222 1

14

FF

LWn

dN

I

S

s

tI

31

Temperature Dependencies of Noise in HD-MESFETs and HD-MOSFETs

300 350 400 450 500 550-140

-135

-130

-125

-120

HD-MESFET

3000Hz

1500Hz

800Hz

400Hz

200Hz

100Hz

SI /

I2 (

dB/H

z)

Temperature (oK)300 350 400 450 500 550

-140

-135

-130

-125

-120

HD-MOSFET

3000Hz

800Hz

400Hz

1500Hz

200Hz

100Hz

SI /

I2 (

dB/H

z)Temperature (oK)

Temperature dependence of noise in HD-MESFETs and HD-MOSFETs is weaker than the one in HFETs. Contribution of GR noise is weaker in HD-MESFETs compared to 1/f noise.

32

Comparison of Hooge Parameters

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E+0110

1

10-1

10-2

10-3

10-4

10-5

10-6

10-7

GaN[7,13]

Si[1,2]

GaAs[1,2]

SiC[3,4]

GaN HFETon sapphire[8,9]

GaAs HEMT[5]

GaN HFETon SiC[8,11]

GaN MESFET [7]

GaN MOSHFET[10]

GaAs MESFET[12]

Si NMOS[6]

33

Comparison of Hooge Parameters References

1. R.H.,Clevers, Physica B 154, 214, (1989)2. F.N. Hooge, M. Tacano, Physica B 190, 145, (1993) 3. M. Levinshtein, S. Rumyantsev, J. Plamour, D. Slater, J. Appl. Phys. 81, 1758, (1997) 4. M. Tacano and Y. Sugiyama, Solid State Elect., Vol. 34. No 10, pp.1049-53, 1991. 5. D. Fleetwood, T.L. Meisenheimer, J. Scofield, IEEE Trans. Elct. Dev. Vol. 41, No 11, p. 19366. L.K. J Vandamme, X. Li, D. Rigaud, IEEE Trans. Elct. Dev. Vol. 41, No:11, p. 1936, Nov. 1994 7. Present Work8. A. Balandin, S. Morozov, G. Wijeratne, C. Cai, L. Wang, C. Viswanathan, Appl. Phys. Lett., 75, No. 14, p.2064, (1999)9. S. Rumyantsev, M.E.Levinshtein, R. Gaska, M. S. Shur, J. W. Jang, and M. A. Khan, J. Appl. Phys. 87, N4 pp.1849-1854, (2000)10. N. Pala, R. Gaska, S. Rumyantsev, M. S. Shur M. Asif Khan, X. Hu, G. Simin, and J. Yang, Electronics Letters, vol. 36, No. 3, p. 268, Feb. 2000.11. S. L. Rumyantsev, N. Pala, M. S. Shur, R. Gaska, M. E. Levinshtein, M. Asif Khan, G. Simin, X. Hu, and J. Yang, Electronics Letters, Submitted12. M. E. Levinshtein and S. L. Rumyantsev, Techn. Phys. Lett. vol. 19, no. 7-8, pp. 55-59, 199313. N.V. Dyakonova, M.E. Levinshtein, S. Contreras, W. Knap, B. Beaumont, P. Gibart.,"Low-frequency noise in GaN" Semiconductors. v.32, N 3, pp.257-260,(1998), March

34

Conclusions (HFETs)• Hooge parameter = 10-3 - 10-5 for both HFETs and MOS-HFETs. SiO2 deposited on AlGaN in MOS-HFETs does not contribute much to noise.

• In the devices with low gate leakage current the contribution of the gate leakage current to the low frequency noise of drain current is fully masked by other noise mechanisms.

•In the transistors with no contribution of the gate leakage current to the output noise the noise sources are located in the channel.

•Hooge parameter a is inversely proportional to the channel concentration in GaN/AlGaN HFETs – typical for tunneling mechanism. The model based on this mechanism is in qualitative agreement with our experimental data but the agreement must be checked further for undoped HFETs.

•Generation-recombination noise with activation energy of Ea ~ 0.8 - 1.0 eV has been found in both HFETs and MOS-HFETs. The analysis shows that the trap responsible for the observed generation-recombination noise can be located in the AlGaN barrier layer.

35

Conclusions (MESFETs)

• The noise properties of MESFETs and MOSFETs are similar. Hooge parameter = (2-3)10-3 for both devices. • This value is about one order of magnitude smaller than the value of reported for bulk n-type GaN.

• Drain and source contacts do not contribute much to the low frequency noise.

• The noise originates from the bulk of GaN in the channel and in the source to gate and drain to gate regions.

• The temperature dependence of noise shows a weak contribution of generation-recombination noise at elevated temperatures.

36

Key Contributions

•Design and fabrication of MOS-HFET with SiO2 as gate dielectric:

•Reduction in gate leakage current six orders of magnitude,

•Design and fabrication of HD-MESFET :

•Comparable output characteristics with HFETs with the advantages of simpler epilayer structure.

•Systematic measurement and analysis of low frequency noise to determine:

•Effect of gate leakage current ,

•Location of trap level causing GR noise,

•Origin of 1/f noise,

•Concentration dependence of 1/f noise.

•Development of a new model to explain 1/f noise in doped channel HFETs.