1. High Electron Mobility Transistors (HEMT)
BY: Aaron Buehler
&
Jason Vanderlinde

2. Outline Brief History What are they? How they Work Different Types
Band Structure and Diagrams
Applications
Key Points
References

3. Brief History
Developed by Takashi Mimura and colleagues at Fujitsu in Japan in
Faced several issues along the way
Early Applications:
Low noise amplifiers
Installed in radio telescope
Other space and military applications
Commercialization began in 1987 for satellite broadcasting receivers
Commercial production took off in the 90’s
Faced multiple device failure(GaAs MESFET, depletion type MOSFET) until final product was produced by controlling the electrons in the superlattice by introducing a Schottky barrier over the single heterojunction
HEMT’s replaced GaAs MESFET’s because of the shrink in necessary size of the antenna by .5 or more
In the 90’s these entered into the satellite receivers and mobile phone applications, improvements in cost and processes

4. What are they?
Referred to as heterojunction field-effect transistor (FET)
Abrupt discontinuities
Two layers of different semiconductor with two different band gap energies
Separating majority carriers and ionized impurities minimizes the degradation in mobility and peak velocity
The 2-D electron gas = less electron collisions = less noise

5. Different Types Material: AlGaAs-GaAs Pseudomorphic HEMT (pHEMT)
Metamorphic HEMT (mHEMT)
Indium Phosphide (InP)
Galium Nitride (GaN)

6. HEMT structure

7. pHEMT GaAs pHEMT < .5 µm gate length Low noise: 1dB at 12GHz
High gain: 10 dB at 12GHz
Range up to 26GHz
Thin layer so the crystal lattice stretches to fit the other material.
Larger bandgap differences = better performance
Fit together like a two combs.
Used in wireless communications and satellite applications because of high power and extremely low noise capabilities.

8. mHEMT .15 µm gate length Low noise High gain Range up to 100GHz
Large lattice mismatch between the channel and substrate is accommodated by formation of dislocations within a metamorphic buffer.
Modern day can get up to 1THz

9. Band Structure
On the left is a band structure of two different semiconducting materials and on the right is them forming a heterojunction when they come into close contact.

10. AlGaAs-GaAs HEMT band diagrams

11. InP HEMT
Cross section using a scanning electron micrograph

12. GaN HEMT Based on GaN/AlGaN heterojunctions
Uses a Sapphire (Al3O2)/Silicon Carbonide(SiC) substrate because of the wide energy gap of 3.4 eV and 3.3 eV
Applicable to high power supply voltages because of the wide energy gaps
Can withstand high operating temperatures
In comparison to InP with only a band gap of 1.9 eV

13. Applications Originally for high speed applications
High power/ high temperature microwave applications
Power amplifiers
Oscillators
Cell Phones
Radar
Most MMIC’s radio frequency applications
MMIC = Monolithic Microwave Integrated Circuit
RF applications with combination of low noise and very high frequency

14. Key Points
Its two main features are low noise and high frequency capability
A heterojunction is two layers different semiconductors with different band gap energies
The 2-D electron gas is essential to the low noise feature
AlGaAs and GaAs are the most common materials for heterojunction
Used in MMIC’s and radio frequency applications for high performance

15. Sources
"GaAs Pseudomorphic HEMT Transistor." Mimix Broadband, Inc. N.p., 19 July Web. 30 Apr < pdf>.
Grunenputt, Erik. "Pseudomorphic and Metamorphic HEMT-technologies for Industrial W-band Low-noise and Power Applications.” Youscribe. N.p., Dec Web. 30 Apr < metamorphic-hemt-technologies-for-industrial-w-band >.
Poole, Ian. "HEMT, High Electron Mobility Transistor." Radio-Electronics.com. Adrio Communications, June Web. 30 Apr < electronics.com/info/data/semicond/fet-field-effect-transistor/hemt-phemt- transistor.php>.

16. Sources continued
"0.15-um LN MHEMT 3MI." TriQuint.com. N.p., 29 Nov Web. <
Göran, Andersson, ed. "High Electron Mobility Transistors (HEMT)." Laboratory for Millimeter-Wave
Electronics. ETH Zurich, 2 Mar Web. 30 Apr <
mwe-group/research/vision-and-aim/high-electron-mobility-transistors-hemt.html>.
Neamen, Donald. Semiconductor Physics and Devices Basic Principles. 4th ed. New York: McGraw-Hill,
Print.
Mimura, Takashi. "The Early History of the High Electron Mobility Transistor (HEMT)." Early History of
the High Electron Mobility Transistor (HEMT) (2002): Web. 30 Apr < 1&tag=1>.