My
research aims to use the unique properties of Nitride-based semiconductors
to engineer devices with new functionality and improved performance. My work
involves crystal growth of III-nitrides by molecular beam epitaxy, material
characterization, device processing and device
measurements.
I
try to present a brief overview of my research on this page. Please follow the
links in each section to explore our work in more detail.
If
you have any questions, please contact
me.
Polarization-graded transistors
Home
N-polar Gallium Nitride Electronics
While
most work on GaN devices in the past has been on the Ga-polar direction, the N-polar
face has not received much attention. Wurtzite GaN
has very different properties depending on the crystal orientation, and we are
interested in exploiting the unique properties of N-face or –c oriented GaN.
(ball and stick figures from M.
J. Murphy et al, MRS Internet J. Nitride Semicond.
Res. 4S1, G8.4(1999))
Since
the polarization charges in N-polar devices are reversed with respect to
Ga-polar devices, the band diagrams look quite different. The band diagram
(cartoon) for an GaN/AlGaN/GaN
structure on N-polar GaN is shown below. A Ga-polar structure is also shown for
comparison.
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We
developed molecular beam epitaxial (MBE) growth of N-polar AlGaN/GaN
HEMTs and were able to achieve excellent device
performance. Shown below is an AFM image of a N-polar
GaN film grown by molecular beam epitaxy. The height scale of the AFM image is
5 nm, and the RMS roughness was approximately 1 nm.
Below
are Reflected High Energy Electron Diffraction (RHEED)
patterns from the 3x3, 6x6 reconstructions of Ga atoms on the N-polar GaN
surface.
We
have been able to achieve high electronic charge and mobility in AlGaN/GaN heterostructures grown on N-polar GaN. Shown
below are device characteristics for a N-polar AlGaN/GaN HEMT.
To
read more about N-polar GaN research, please look at the papers below
Growth
and Electrical Characterization of N-face AlGaN/GaN
Heterostructures, S. Rajan et al,
Japanese Journal of Applied Physics
N-polar GaN/AlGaN/GaN High Electron Mobility Transistors, S. Rajan et al, Journal of Applied Physics
N-face High Electron
Mobility Transistors with a GaN-spacer,
M. Wong et al, Physica
Status Solidi
MBE-grown Ga-polar AlGaN/GaN HEMTs
AlGaN/GaN
HEMTs are the most promising technology today for
high-frequency high-power applications. The high breakdown field and excellent
electron transport properties of GaN make it superior to conventional GaAs or Si technology.
We
worked to develop high-performance MBE-grown AlGaN/GaN using a
combination of growth, device fabrication, and advanced characterization
techniques.
The
charge and band diagram (cartoon) of an AlGaN/GaN
HEMT are shown below.
Below
is an RF-IV curve of an AlGaN/GaN microwave
transistor with large-signal dispersion. The main problems
with this device was the large dispersion due to buffer traps. We were
able to solve the large-signal dispersion problems, and achieve excellent power
performance, as shown in the next figure. To read more, please read the papers
linked below.
The
figure below show the excellent power performance achieved by MBE-grown HEMTs.
Today, the technology developed has been used to make transistors operating at
mm-wave frequencies, and with very high power outputs.
To
read more about our work on developing MBE devices please click below to see
our papers on the topic.
Power
Performance of AlGaN/GaN HEMTs
grown on SiC by Plasma-Assisted MBE, S. Rajan et al, (Electron Device Letters)
MBE-Grown
AlGaN/GaN HEMTs on SiC,
S. Rajan et al, (IEEE Lester Eastman Conference Proceedings)
Polarization-graded Field Effect Transistors
While
the abrupt polarization charge at the AlGaN/GaN
interface is used extensively in high electron-moblility
transistors, the same fixed charge can also be used to create a spread out
charge, as shown below. The grading of the Al composition at the nanometer
scale enables many exciting device ideas.
We
have used polarization grading to create a new kind of transistor, the PolFET. Among the advantages of a PolFET
are the high mobility compared to a MESFET, high breakdown field in the AlGaN, and the ability to tailor the transconductance
profile.
We
were able to achieve excellent DC and RF performance in this PolFET as shown in the figure below. We achieved 4 GHz
power of 4.3 W/mm with a power-added efficiency of 63 % at a drain bias of 30
V.
We
have been able to measure local Al
mobility for a range of Al compositions using these channels. Shown below is a
plot showing the measured and calculated Al mobility.
To
read more about device performance of PolFETs, and
about the physics of graded AlGaN channels, please
follow the links below
An
AlGaN/GaN Polarization-Doped Field-Effect Transistor
for Microwave Power Applications, S. Rajan et al, Applied Physics Letters
Electron
Mobility in Graded AlGaN Alloys, S. Rajan et al, Applied Physics Letters
Publications
Please
write to me for copies or with questions.
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JOURNAL
PAPERS (PUBLISHED) Electron Mobility in N-polar GaN/AlGaN/GaN
Heterostructures David F. Brown,
Siddharth Rajan, Stacia Keller, Steven P. DenBaars,
Umesh K. Mishra, accepted for publication, Applied Physics Letters. Study of Interface barrier of SiNx/GaN
interface for Nitrogen-polar GaN based High Electron Mobility Transistors Nidhi, Siddharth
Rajan, Stacia Keller, Feng Wu, Steven P. Denbaars, James S. Speck. and Umesh K. Mishra, accepted
for publication, Journal of Applied Physics Properties of oxide deposited on c-plane AlGaN/GaN heterostructure Vert, A.V.; Rajan, S., "," Electronics Letters , vol.44,
no.12, pp.773-774, June 5 2008 AlGaN/GaN HEMTs: Recent Developments and Future Directions Siddharth
Rajan, Umesh K. Mishra, and Tomas Palacios, to appear in
International Journal of High Speed Electronics and Systems N-polar GaN/AlGaN/GaN High
Electron Mobility Transistors, S. Rajan, A. Chini, M. Wong , J.S. Speck, and Electron Mobility in Graded AlGaN
Alloys Siddharth Rajan, Huili Xing, Debdeep
Jena, Steve P. Denbaars and Umesh K. Mishra,
Applied Physics Letters 88, 042109 (2006). Growth and Electrical Characterization of N-face AlGaN/GaN Heterostructures S. Rajan, M. Wong, Y. Fu, F. Wu, J.S.
Speck, and U.K. Mishra, Japanese Journal of Applied Physics, Vol. 44, No. 49,
, pp. L1478-L1480, 2005. MBE-Grown AlGaN/GaN HEMTs on SiC Siddharth Rajan, Christiane Poblenz,
Patrick Waltereir, Arpan Chakraborty, James S.
Speck and Umesh K. Mishra, International Journal of High Speed
Electronics and Systems, 14 (3), pp. 732-737 (2004). An AlGaN/GaN
Polarization-Doped Field-Effect Transistor for Microwave Power Applications Siddharth Rajan, Huili Xing, Debdeep
Jena, Steve Denbaars and Umesh Mishra Appl. Phy.
Lett. 84 (9), pp. 1591-1593, March 1,
2004. Power Performance of AlGaN/GaN
HEMTs grown on SiC by
Plasma-Assisted MBE Siddharth Rajan, Patrick Waltereit,
Christiane Poblenz, Sten J. Heikman,
Daniel S. Green, James S. Speck and Umesh K. Mishra IEEE Electron Device Letters, 25 (5), pp.
247-249, May 2004. Study of Interface barrier of SiNx/GaN
interface for Nitrogen-polar GaN based High Electron Mobility Transistors Nidhi, Siddharth
Rajan, Stacia Keller, and Umesh Mishra, accepted (2008) Journal of
Applied Physics Properties of N-polar AlGaN/GaN
heterostructures and field effect transistors grown by metal-organic chemical
vapor deposition Stacia Keller, Chang Soo Suh, Zhen Cheng,
Rongming Chu, Siddharth Rajan,
Nicholas Fichtenbaum, Motoko Furukawa, Steven P. DenBaars, James S. Speck, and Umesh K Mishra, Journal of Applied Physics 103, 033708 (2008). AlGaN Channel High Electron Mobility Transistors Ajay Raman, Sansaptak Dasgupta,
David Brown, Siddharth Rajan,
James S. Speck and Umesh K. Mishra, accepted for publication, Japanese
Journal of Applied Physics. N-face
High Electron Mobility Transistors with a GaN-spacer M.H.
Wong, S. Rajan, R.M. Chu, T. Palacios, C.S. Suh, L.S. McCarthy, S.
Keller, J.S. Speck and U.K. Mishra, Phys. Stat. Sol. (a) 204, 2049 (2007). Carrier transport and confinement in
polarization-induced 3D electron slabs: Importance of alloy scattering in AlGaN John Simon, Albert Wang, Siddharth Rajan, Huili Xing and Debdeep Jena, Applied Physics
Letters, Appl. Phys. Lett.
88, 042109 (2006). Characterisation of
multiple carrier transport in indium nitride, grown by molecular beam epitaxy T. B. Fehlberg, G. A. Umana-Membreno, B. D. Nener, G.
Parish, C. S. Gallinat, G. Koblmueller,
S. Rajan, S. Bernardis,
and J. S. Speck, Japanese Journal of Applied Physics Part 2-Letters &
Express Letters 45 (37-41):
L1090-L1092 Oct 2006 Review of Recent Developments in Growth of AlGaN/GaN High-Electron Mobility Transistors on 4H-SiC by
Plasma-Assisted Molecular Beam Epitaxy Andrea Corrion, Christiane Poblenz, Patrick
Waltereit, Tomas Palacios, Siddharth
Rajan, Umesh K. Mishra and Jim S.
Speck, IEICE Transactions, E89-C (7),
pp. 906-912. Ion Implanted GaN/AlGaN HEMTs with non-alloyed Ohmic Contacts Haijiang Yu, L.
McCarthy, S. Rajan, S. Keller, S. Denbaars,
J. Speck, U. Mishra, IEEE Electron
Device Letters, 26 (5), pp. 283-285, May 2005. Effect of AlN nucleation
layer growth conditions on buffer leakage in AlGaN/GaN
high electron mobility transistors grown by molecular beam epitaxy (MBE) C. Poblenz, P. Waltereit, S. Rajan, U. K.
Mishra, J. S. Speck, P. Chin, I. Smorchkova, and B.
Heying, J. Vac. Sci. Technol. B 23, 1562
(2005) Influence of the Dynamic Access Resistance in the gm
and fT Linearity of AlGaN/GaN
HEMTs T. Palacios , S. Rajan, A. Chakraborty, S. Heikman, S. Keller, S. P. DenBaars,
and U. K. Mishra, IEEE Trans. Elec. Dev., 52 (10), pp. 2117-2123
(2005). High-Power AlGaN/GaN HEMTs for Ka-Band Applications T. Palacios, A. Chakraborty, S. Rajan, C.
Poblenz, S. Keller, S. P. DenBaars, J. S. Speck,
and U. K. Mishra, IEEE Elec. Dev. Lett. 26 (11),
pp. 781-783 (2005) Effect of carbon doping on buffer leakage in AlGaN/GaN high electron mobility transistors Christiane Poblenz, Patrick Waltereit, Siddharth
Rajan, Sten J. Heikman,
Umesh K. Mishra and James S. Speck, Journal of Vacuum Science &
Technology B, 22(3), pp.1145-1149, May 2004. Structural Properties of GaN Buffer Layers on
4H-SiC(0001) Grown by Plasma-Assisted Molecular Beam Epitaxy for High
Electron Mobility Transistors Patrick Waltereit, Christiane Poblenz, Siddharth
Rajan, Feng Wu, Umesh K. Mishra and James S.
Speck, Japanese Journal of Applied Physics, 43 (12A), pp. L1520-L1523,
2004 Submitted/In Preparation N-face High Electron Mobility Transistors with AlN back-barrier Man Hoi Wong, Polarization in Semi-Polar InGaN/GaN
and AlGaN/GaN Heterostructures Siddharth
Rajan, Chrisopher Olson, Umesh
K. Mishra, and James S. Speck, submitted to Journal of Applied Physics Migration-enhanced epitaxy of N-polar Indium Nitride Siddharth Rajan, Man Hoi Wong, Feng Wu, James S. Speck and Umesh K. Mishra, in
preparation Transconductance Modification in Polarization-Graded
Field Effect Transistors (PolFETs) Siddharth
Rajan, James S. Speck, Umesh K. Mishra, in preparation Electron Transport in Vicinal N-polar AlGaN/GaN Heterostructures Siddharth
Rajan, Eric Hsieh, Stacia Keller, Steve P. DenBaars and Umesh K. Mishra, in preparation Air breakdown and dispersion in SiN-passivated
AlGaN/GaN HEMTs Siddharth
Rajan and Umesh K. Mishra, in preparation. Wide Bandgap Power Device Figure of Merit Siddharth
Rajan, Debdeep Jena, and Umesh K. Mishra, in preparation. |
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CONFERENCE PRESENTATIONS |
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Surface Passivation
of AlGaN/GaN HEMTs Siddharth Rajan, Yi Pei, Zhen Chen,
Steve P. DenBaars, and Umesh K. Mishra, Accepted, Device Research Conference 2008, Santa
Barbara CA . Electron Transport in N-polar Vicinal AlGaN/GaN Heterostructures Siddharth Rajan, Yun-Hao
Hsieh, Steve P. DenBaars, James S. Speck and Umesh
K. Mishra, Accepted Electronic Materials Conference 2008, Santa
Barbara CA . Electron Transport in N-polar Vicinal AlGaN/GaN Heterostructures Siddharth Rajan, Eric Hsieh, Steve P. DenBaars, James S. Speck and Umesh K. Mishra, WOCSEMMAD
2008, Pal Springs, USA, February 2007. Migration-enhanced epitaxy of N-polar Indium Nitride Siddharth Rajan, Man Hoi Wong, Feng Wu, James S. Speck and Umesh K. Mishra, EMC
2007, Las Vegas, USA, February 2007. Migration-enhanced epitaxy of N-polar Indium Nitride Siddharth Rajan, Man Hoi Wong, Feng Wu, James S. Speck and Umesh K. Mishra, WOCSEMMAD
2007, Savannah, USA, February 2007. Advanced Transistor Structures Based on N-face GaN S. Rajan, A. Chini, M. Wong, C. Suh, Y.
Fu, M. J. Grundmann, F. Wu, J. S. Speck and U. K. Mishra32nd International
Symposium on Compound Semiconductors (ISCS), Sept 18-22 2005, Europa-Park Rust, Germany N-face Modulation Doped Field Effect Transistors S. Rajan, F. Wu, M. Wong, Y. Fu, J. S.
Speck and U. K. Mishra, 6th International Conference on Nitride
Semiconductors (ICNS) 2005, Aug 28- Sept 2 2005, Bremen, Germany. Structural and Electrical Characterization of N-face
GaN grown on C-face SiC by MBE Siddharth Rajan; Feng
Wu; Manhoi Wong; Yenyun
Fu; James S. Speck; Umesh K. Mishra, 47th Electronic Materials Conference,
June 22-24 (2005), Santa Barbara, California USA. Electron Mobility in Graded AlGaN
Layers Siddharth Rajan, Tomas Palacios, Steven
P. Denbaars and Umesh K. Mishra, WOCSEMMAD 2005,
Miami, USA, February 2005. Progress in the Development of an all-MBE HEMT Siddharth Rajan, Christiane
Poblenz, Patrick Waltereir, Arpan Chakraborty,
James S. Speck and Umesh K. Mishra, 11th Advanced Heterostructure Workshop,
Hawaii, 2004. MBE-Grown AlGaN/GaN HEMTs on SiC Siddharth Rajan, Christiane
Poblenz, Patrick Waltereir, Arpan Chakraborty,
James S. Speck and Umesh K. Mishra, IEEE Lester Eastman Conference on High
Performance Devices, August 2004, Troy, NY, USA. Tailoring of Transconductance Profile for Improved
Linearity in AlGaN/GaN Polarization- Doped Field
Effect Transistors Siddharth Rajan; Xing, Huili ;
Chakraborty, Arpan ; Chini, Alessandro ; Grundmann, Michael J.; Palacios,
Tomas ; DenBaars, Steven P.; Jena, Debdeep ;
Mishra, Umesh K.., International Workshop on Nitride Semiconductors, July
2004, Pittsburgh, PA, USA. Growth and Power Performance of MBE-grown AlGaN/GaN HEMTs Siddharth Rajan, Patrick Waltereit,
Christiane Poblenz, Sten J. Heikman,
James S. Speck and Umesh K. Mishra, International Workshop for Physics of
Semiconductor Devices (IWPSD), December 2003, Chennai, India. Characterization of MOCVD-Grown N-Polar
GaN/AlGaN Heterostructures: Nidhi Nidhi; Siddharth Rajan; Stacia Keller;
Steven DenBaars; Umesh Mishra. Accepted
Electronic Materials Conference 2008, Santa Barbara CA . AlGaN Channel HEMTs Grown by PAMBE: Structural
and Device Characterizations Sansaptak Dasgupta; Ajay Raman; Siddhath Rajan, Umesh Mishra, Accepted
Electronic Materials Conference 2008, Santa Barbara CA Gate Leakage Reduction in AlGaN/GaN
HEMTs Grown by Plasma- Assisted MBE Rongming Chu; Christiane Poblenz; Man Hoi
Wong; Sansaptak Dasgupta; Siddharth Rajan; Yi Pei; Likun Shen; James Speck; Umesh Mishra,
Accepted Electronic Materials Conference 2008, Santa Barbara CA Polarization-induced 3-Dimensional slabs
in Graded AlGaN layers John Simon, Siddharth Rajan, Kejia Wang, Huili Xing and Debdeep Jena, to be presented
at 2006 MRS Fall Meeting, Boston Dipole Engineering in Nitride-based HEMTs T. Palacios, S. Rajan, A. Chakraborty, S.
Keller, S. P. DenBaars, and U. K. Mishra, 208th
Meeting of the Electrochemical Society, Los Angeles, October 16-21 2005 An Experimental Method to Identify Bulk and Surface
Traps in GaN HEMTs A. Chini, Y. Fu, S. Rajan, J. S. Speck and U.
K. Mishra, 32nd International Symposium on Compound Semiconductors (ISCS),
Sept 18-22 2005, Europa-Park Rust, Germany, Fabrication and Characterization of N-face GaN/AlGaN/GaN HEMTs A. Chini, S.
Rajan., M. Wong, Y. Fu, J. S. Speck, U. K. Mishra, 63rd Device Research
Conference, June 20-22 (2005), Santa Barbara, California USA Ion implantation for unalloyed ohmic contacts to AlGaN/GaN HEMTs Yu, H.; McCarthy, L.; Rajan, S.; Keller, S.; Denbaars, S.P.; Speck, J.S.; Mishra, U.K.; Device
Research Conference, 2004. 62nd DRC. Conference Digest [Late News Papers
volume included], 21-23 June 2004 Page(s):37 - 38 vol.1 Influence
of the access resistance in the rf performance of
mm-wave AlGaN/GaN HEMTs Palacios,
T.; Rajan, S.; Shen, L.; Chakraborty, A.; Heikman,
S.; Keller, S.; DenBaars, S.P.; Mishra, U.K, Device
Research Conference, 2004. 62nd DRC. Conference Digest 21-23 June 2004
Page(s):75 - 76 vol.1 |
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