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

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 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

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

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

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 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

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): 665-667. 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 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?