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

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

Brief History Developed by Takashi Mimura and colleagues at Fujitsu in Japan in 1979 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

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

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

HEMT structure

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.

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

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.

AlGaAs-GaAs HEMT band diagrams

InP HEMT Cross section using a scanning electron micrograph

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

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

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

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Sources continued "0.15-um LN MHEMT 3MI." TriQuint.com. N.p., 29 Nov. 2007. Web. <http://www.triquint.com/prodserv/foundry/docs/0.15.LN.mHEMT.3MI.pdf>. Göran, Andersson, ed. "High Electron Mobility Transistors (HEMT)." Laboratory for Millimeter-Wave Electronics. ETH Zurich, 2 Mar 2010. Web. 30 Apr 2013. <http://www.mwe.ee.ethz.ch/en/about- 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, 2012. 602-9. Print. Mimura, Takashi. "The Early History of the High Electron Mobility Transistor (HEMT)." Early History of the High Electron Mobility Transistor (HEMT). 50.3 (2002): 780-82. Web. 30 Apr. 2013. <http://ieeexplore.ieee.org.libpdb.d.umn.edu:2048/stamp/stamp.jsp?tp=&arnumber=98996 1&tag=1>.