Transcript ResearchPresentations\High Electron Mobility Transistors

High Electron Mobility
Transistor
CHAD AUGINASH
APRIL 20, 2015
Abstract
High electron mobility transistor(HEMT) is a transistor that operates at higher
frequencies, typically in the microwave range. They are used in applications that
require high frequency, such as cell phones, RF applications, and some power
applications. HEMTs are transistors that utilize the 2-dimensional electron
gas(2DEG) created by a junction between two materials with different band gaps
called a heterojunction. The two most commonly used materials to create the
heterojunction are a highly doped n-type donor material, typically AlGaAs and
an undoped material, typically GaAs.
Outline
• History
• Specific HEMT example
• Differences between MOSFETs
and HEMTs
• Applications
•How HEMTs work?
• Summary
•Structure
History
•The invention of the HEMT is credited to Takashi Mimura in 1979. Mimura
couldn’t build the HEMT until he heard about a modulation-doping technique
developed at Bell Laboratories
•Dingle et al. at Bell laboratories developed the modulation-doped
heterojunction superlattice, which was instrumental in the development of the
HEMT
•In 1986, a telescope equipped with a HEMT amplifier discovered new
interstellar molecules in the Taurus Molecular Cloud about 400 light years away
Differences between MOSFETs and
HEMTs
HEMTs
MOSFETs
• Operate in the microwave range
(300MHZ-300GHz)
• Operation in the UHF range
(300MHZ-3Ghz)
• Heterojunction is used as the
channel
• Doped region is used as the channel
How do HEMTs work?
•HEMT’s take advantage of 2DEG which
is created at the AlGaAs/GaAs
heterojunction. The 2DEG is confined at
the heterojunction and free to move
parallel to the channel. This results in a
higher electron mobility which is good
for large gain and high frequency
characteristics.
Fig1.Energy band diagram for AlGaAs/GaAs
heterojunction at thermal equilibrium
Fabrication
•Molecular beam epitaxy(MBE) is one of
the growth techniques used to create
the AlGaAs/GaAs heterojunction
•Metal organic chemical vapour
deposition(MOCVD) is another
common process used to create the
heterojunction
•Other typical lift-off techniques are
used to create the contacts
Fig 2. SEM image of a cross-section structure
of alternating 500nm layers of GaAs and
AlGaAs
Molecular beam epitaxy(MBE)
• Materials are grown using MBE are grown in a low
pressure vacuum usually down to 10−11 torr
• Effusion cells are used to direct the material onto
the substrate. Effusion is where individual
molecules travel through a small hole without
collision.
• Effusion cells are turned off and on by shutters and
controlled by the temperature of the cell.
Fig 3. Diagram showing the main
components and rough layout of the
main chamber of the MBE system
Reflection high energy electron
diffraction(RHEED)
• RHEED is an in-situ characterization tool used in
the MBE process
• RHEED can relay information to the user about
the surface structure, roughness, growth rate,
and cleanliness
• RHEED allows high levels of accuracy in the
thickness of the material grown
Fig 4. Diagram showing the main
components and rough layout of the
main chamber of the MBE system
Structure
•Typical cross sectional view of a GaN HEMT
•Structure is similar for GaAs
•The lattice matching layer is used to match
the different lattice structures of the GaN
and the Si substrate.
•Lattice matching is important for the
electron mobility
•GaN has been grown on sapphire because of
the superior lattice structure matching
Fig 5. GaN HEMT structure
Fujitsu’s InP HEMT
• InP HEMTs exhibit a current-gain
cutoff frequency of 500Ghz making
them the fastest transistors to date
(2006)
• Fujitsu introduces a Y-shaped
structure for the gate. The large cross
sectional of the gate electrode is
used to reduce gate resistance. The
small gate contact is used to reduce
gate length
Fig 6. Cross sectional view of the InP HEMT
Fujitsu’s cavity structure in the InP HEMT
• Fujitsu’s cavity structure reduces gate
electrode parasitic capacitance
• Increased capacitance between gate and
drain is almost zero
• Capacitance between gate and source is
nearly halved
• The reduction is parasitic capacitance
leads to a 14% increase in speed in a
static frequency divider
Fig 7. Cavity structure in InP HEMT
Materials used in HEMTs
•GaAs: used in the first HEMTs
•GaN: an improvement upon the GaAs based HEMTs
•InP: used in some of the most advanced HEMTs today
Applications
•Precision sensors
•Next generation wired/wireless communication
•Advanced radars
•Power electronics
Passive millimeter wave image sensor
• Millimeter wave image sensors are
used at TSA checkpoints
• Passive device that detects
electromagnetic waves radiation
• Millimeter waves can pass through
clothing, fire, and thin walls, to
detect a human body, making it
ideal for security and disaster
prevention.
Fig 8. Block diagram of passive imaging sensor
Passive millimeter wave image sensor
• Low noise amplifier(LNA) used in the
passive millimeter wave image sensor
• Millimeter wave image sensor is
limited to short range due to
atmospheric attenuation
• Frequencies at 94Ghz, 140GHz, and
Fig 9. Electrical performance of LNA using InP HEMTs
220GHz have relatively low
atmospheric attenuation
• 94GHz band is used for this particular
millimeter wave image sensor
Fig 10. Millimeter wave image of human body
Summary
•Reliability of GaN and InP HEMT’s are excellent at lower frequencies
•Reliability issues need to be resolved in Gan and InP HEMT’s at higher
frequencies
•Failure mechanisms such as gate sinking, thermal degradation of ohmic contact,
and charge trapping needs further investigation
•New structures need to be developed to reduce parasitic capacitance and
address the failure mechanisms
References
Cho, A. Y., and J. R. Arthur. "Molecular beam epitaxy." Progress in solid state chemistry 10 (1975): 157-191.
Dingle, R., et al. "Electron mobilities in modulation‐doped semiconductor heterojunction superlattices." Applied Physics Letters 33.7 (1978): 665667.
Kawano, V. Yasuhiro Nakasha V. Yoichi, V. Masaru Sato, and V. Tsuyoshi Takahashi V. Kiyoshi Hamaguchi. "Ultra high-speed and ultra low-noise InP
HEMTs." Fujitsu Sci. Tech. J 43.4 (2007): 486-494.
Khan, M. Asif, et al. "High electron mobility transistor based on a GaN‐AlxGa1− xN heterojunction." Applied Physics Letters 63.9 (1993): 1214-1215.
Meneghesso, Gaudenzio, et al. "Reliability of GaN high-electron-mobility transistors: state of the art and perspectives." Device and Materials
Reliability, IEEE Transactions on 8.2 (2008): 332-343.
Mimura, Takashi. "The early history of the high electron mobility transistor (HEMT)." IEEE Transactions on microwave theory and techniques 50.3
(2002): 780-782.
Neamen, Donald A. "Semiconductor physics and devices." 1992 (1994).
http://en.wikipedia.org/wiki/High-electron-mobility_transistor
http://en.wikipedia.org/wiki/Molecular_beam_epitaxy
Concepts
1. Who is credited with inventing the HEMT?
2. What material was first to create the HEMT?
3. Name two applications for HEMT.
4. Name two materials used in HEMT.
5. What is used as the channel in the HEMT?