1. High Electron Mobility Transistor
Chad Auginash
April 20, 2015

2. 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.

3. Outline How HEMTs work? Structure History
Differences between MOSFETs and HEMTs
How HEMTs work?
Structure
Specific HEMT example
Applications
Summary

4. History
The invention of the HEMT is credited to Takashi Mimura in 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

5. Differences between MOSFETs and HEMTs
Operate in the microwave range (300MHZ-300GHz)
Heterojunction is used as the channel
MOSFETs
Operation in the UHF range (300MHZ-3Ghz)
Doped region is used as the channel

6. 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

7. 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

8. 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

9. 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

10. 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

11. Fig 6. Cross sectional view of the InP HEMT
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

12. 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

13. 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

14. Applications Precision sensors
Next generation wired/wireless communication
Advanced radars
Power electronics

15. 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

16. 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 220GHz have relatively low atmospheric attenuation
94GHz band is used for this particular millimeter wave image sensor
Fig 9. Electrical performance of LNA using InP HEMTs
Fig 10. Millimeter wave image of human body

17. 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

18. References
Cho, A. Y., and J. R. Arthur. "Molecular beam epitaxy." Progress in solid state chemistry 10 (1975):
Dingle, R., et al. "Electron mobilities in modulation‐doped semiconductor heterojunction superlattices." Applied Physics Letters 33.7 (1978):
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):
Khan, M. Asif, et al. "High electron mobility transistor based on a GaN‐AlxGa1− xN heterojunction." Applied Physics Letters 63.9 (1993):
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):
Mimura, Takashi. "The early history of the high electron mobility transistor (HEMT)." IEEE Transactions on microwave theory and techniques 50.3 (2002):
Neamen, Donald A. "Semiconductor physics and devices." 1992 (1994).

19. Concepts Who is credited with inventing the HEMT?
What material was first to create the HEMT?
Name two applications for HEMT.
Name two materials used in HEMT.
What is used as the channel in the HEMT?