1. 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, , (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
Hooge Parameter

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 HD-MOSFET HD-MESFET 7-13 nm SiO2
30 nm Si:Al0.2Ga0.8N 2x1018
50 nm AlN
7-13 nm SiO2
SiC/Sapphire
1 m i-GaN
50-70 nm Si:GaN (2-5)x1017
HD-MOSFET
HD-MESFET
2m GaN
AlN
7-13 nm SiO2
Sapphire
50-70 nm Si:GaN ( ) x 1018

7. GaN Research Cycle Layout design by L-Edit RPI
Epilayer growth by MOCVD
USC
Mask and Device Fabrication
RPI,USC
Characterization:
DC, RF
Low Frequency Noise
RPI, USC
2D Simulation
RPI

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

12. DC Characteristics of GaN HD-MESFET
HFET
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.

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.

14. Noise spectra for HFETs and MESFETs
The noise spectra SId/Id2 have the form of 1/f g noise with g close to unity (g =  ) for both types of devices.

15. Possible Noise Sources in FETs
RGaN=RS1+RCh+RS2 Lg
Source
Gate
RS1
RCh
RS2
Drain RC L
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

16. Contribution of Contact Noise Sources TLM Measurements
(RC<<RGaN)
Channel or Surface noise
SRc>>SGaN
Contact noise
SRc<<SGaN L W t
Noise form the channel is dominant.

17. Calculation of Current Noise Density for Series ResistorsRC RS V

18. Contribution of the Gate Current Fluctuations
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.

19. Contribution of the Gate Current Fluctuations
■Measured
●Calculated from measured gate noise
dominant contribution

20. } Correlation Between Drain and Gate Current Fluctuations Vd Drain SVgRd Rg Vg
SVd
Gate Ig
Source }

21. Location of the Noise Sources in MESFETs
Space charge region fluctuations
noise from surface and channel out of the gate:
Channel Noise a = 10-3

22. Location of the 1/f Noise Sources in HFETs
No fit indicates the concentration dependence of .
channel noise
noise from surface and channel out of the gate:

23. Estimation of Electron Concentration and Mobility

24. Hooge Parameter a as a Function of 2D Electron Concentration
(Typical for tunneling[1,2])
Since we can obtain channel concentration from the C-V measurements, we can extract the dependence a(nch).
The obtained dependence indicates that absolute value of noise originated from the channel does not depend on Vg.
This is a typical situation for the Si MOSFET where the mechanism of the noise is tunneling of the electron to the traps in SiO2.
Similar mechanism might be dominant in HFETs also: tunneling to GaN.
In this case the absolute noise depends only on F which in GaN does not depend on Vg.
The best candidate for the level in GaN is the shallow donor level.
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)

25. New Model for 1/f Noise in Doped Channel HFETs
AlGaN
GaN C F D 1 2 x0 -e q  x
Tunneling into donor states in GaN
Phonon assisted tunneling
Longer jumps correspond to lower frequency noise

26. Equations for the Tunneling Model
: capture rate due to tunneling
: 2D tunneling cross-section
: electron momentum in 2 DEG
Where :
The noise is 1/f !

27. Equations for the Tunneling Model
This is 1/f noise

28. Comparison With the Experiment
Theory
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

29. Temperature Dependencies of Noise Density in HFETs and MOS-HFETs
Such S(T) dependencies are typical for the noise from local levels
Ea= eV,

30. What is the Origin of the G-R Noise?
G-R Process by a level in the channel t F C
Al0.2Ga0.8N
GaN
Surface
Channel d 1 3 2
[*]
where
  310-18 cm : To small to be true!
Nts1026 cm : 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 = eV, sn » ( )cm2,
Nt »5´1016 cm-3 : All reasonable values.
[*] Copeland J. A., IEEE Trans. Elect. Dev. 18, 50, 1971

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

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 , 1991.
5. D. Fleetwood, T.L. Meisenheimer, J. Scofield, IEEE Trans. Elct. Dev. Vol. 41, No 11, p. 1936
6. L.K. J Vandamme, X. Li, D. Rigaud, IEEE Trans. Elct. Dev. Vol. 41, No:11, p. 1936, Nov. 1994
7. Present Work
8. 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 , (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
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, Submitted
12. M. E. Levinshtein and S. L. Rumyantsev, Techn. Phys. Lett. vol. 19, no. 7-8, pp , 1993
13. 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 ,(1998), March

34. Conclusions (HFETs)
Hooge parameter a = 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 ~ 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 a = (2-3)10-3 for both devices.
This value is about one order of magnitude smaller than the value of a 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.