The document discusses the history and operation of high electron mobility transistors (HEMTs). Key points include: 1) HEMTs were first developed in 1979 and commercialized in the 1980s-1990s, using materials like AlGaAs/GaAs and AlGaN/GaN. 2) HEMTs form a 2D electron gas at the heterojunction between two semiconductor materials with different bandgap energies, allowing for high electron mobility and low noise. 3) The AlGaN/GaN HEMT has advantages like higher speed, frequency, and power efficiency compared to other transistors, making it useful for applications like wireless communications and power switching.

1. 05-April-2021
High Electron Mobility Transistor
Pusan National University
Dr. Sekhar Reddy P.R
05-Apr-21 Pusan National University 1

2. Content:-
History
HEMT Introduction
Formation of 2DEG
AlGaN/GaN HEMT and Its operation
Trapping mechanism of HEMT
Advantageous of HEMT and applications
Summary
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3. Brief History
➢ In the 1979 “Takashi Mimura”, and collogues developed the first HEMT (AlGaAs/GaAs system) at
Fujitsu in Japan.
➢ The HEMT was based on the concept of modulation doping first demonstrated by Ray Dingle and his
collaborators at bell labs in 1978.
➢ 1980 “Takashi Mimura”, Fujitstu laboratories designed the feature of the first HEMT.
➢ 1985 HEMT was announced the lowers noise device.
➢ 1987 Commercialization began for satellite broadcasting receivers.
➢ 1993 Asif Kahn demonstrated the first AlGaN/GaN HEMT
➢ Commercial production took off in the 90’s.
❑ In the last 30 years, HEMTs have been demonstrated in several material systems,
most notably AlGaAs/GaAs and AlGaN/GaN.
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First commercial HEMT

4. Limitations of Conventional Transistors
❖ Short channel effects
❖ Gate leakage current
❖ Gate power dissipation
❖ Ionizing impurities
❖ Lattice and impurities scattering
❖ Less mobility
❖ Less transconductance
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Growth Techniques
1. Metal organic chemical vapor deposition (MOCVD)
2. Molecular beam epitaxy (MBE)
3. Metal organic vapor phase epitaxy (MOVPE).

5. What are they ?
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▪ 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 minimized the degradation in mobility
and peak velocity
▪ The 2-D electron gas =less electron collisions =less noise
Types of HEMT
1. AlGaAs/GaAs
2. Pseudomorphic HEMT (pHEMT)
3. Metamorphic HEMT (mHEMT)
4. InAlAs/InGaAs
5. AlGaN/GaN
Properties of various semiconductors

6. Basic of HEMT
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• A high electron mobility transistor (HEMT) is a kind of
a FET, where the current flow between two ohmic
contacts, source and drain, is controlled by a third
contact, the gate which may be a Schottky barrier contact
in most cases.
• HEMT incorporates a junction between two different
semiconductor materials (i.e. a heterojunction) as the
channel.
HEMT advantages:
• High speed,
• High frequency,
• Digital circuits
• Microwave circuits with low noise.

7. Formation of the 2DEG
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GaN
AlGaN
• By splitting the heterojunction into two separated parts
we can easily understand the formation of the 2DEG.
• AlGaN is grown on a GaN. AlGaN grown layer
induces a polarization effect resulting in a positive
charge on the AlGaN/GaN interface and a negative
charge on the top of the AlGaN layer.
• This differently charged areas result in forming an electric field inside the AlGaN layer.

8. Formation of the 2DEG
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2DEG in n-doped AlGaN
• The polarization described before makes the energy band, including the Fermi level, bending towards the AlGaN/GaN
interface in (b).
• The same effect of energy band bending can be obtained by making a simple planar capacitor using an n-doped
AlGaN as the dielectric and applying voltage on it.
• Because of applied voltage an electric force is acting on the free electrons in the layer forcing them to move towards
the positive electrode, leaving positive space charges by the negative electrode (c).
(a) (b) (c)

9. Formation of the 2DEG
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▪ The built in electric field between the accumulated negative electrons and fixed positive space charges reduces the
polarizing electric field making the inclined energy band flatter.
▪ Because the fermi level of GaN is lower of than of AlGaN, accumulated electrons will flow from the lower AlGaN layer
to the top of GaN layer forming the resulting 2DEG.

10. Structure details of HEMT
Field plates:
• Provides the necessary electric field for the drift between the gate
and drain.
Passivation layers:
• Alleviate the surface trap related degradation mechanism.
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Substrate
Buffer
GaN
AlxGa1-xN
GaN- capping layer
D
S
G
Field plate
2DEG
Passivation-3
Passivation-2
Passivation-1
Source field plate Drain field plate
GaN –cap layer:
• Reduce the reverse leakage current
• Increase in the electric field strength in the AlGaN layer
• Increase in power efficiency by increasing the thickness.
Substrate:
• Provides high thermal conductivity.
Buffer:
• It affects the structural and optical properties of the GaN layer
grown above, thus the on and off state characteristics of the device.
• Increase in buffer thickness yields a reduced substrate leakage.
AGaN/GaN
• High electron density in the channel

11. ❖ Trapping of electrons injected from the gate electrode
or from the 2DEG in to surface-states
❖ The negatively charged region with trapped electrons
acts as a virtual gate depleting the channel beneath it
when the stress is removed.
Trapping mechanisms of HEMT
Trapping at the surface:
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GaN
AlxGa1-xN
GaN- capping layer
S
G
2DEG
Passivation Layer
D
Trapped Electrons
Lower 2DEG
concentration
AlxGa1-xN
GaN- capping layer
S
G
2DEG
Passivation Layer
D
Trapped Electrons
Lower 2DEG
concentration
GaN
o Trapping from the 2DEG into the GaN and AlGaN regions.
o Trapped electrons deplete the channel above or beneath
them after the stress is removed.
❖ The doping in the structure affects the concentration and
energy level of bulk traps.
Trapping in the bulk regions:
(a)
(b)

12. Origin of Traps
Traps responsible for current degradation mechanism can have two different origins:
1.They are related to the quality of the layers
2.They are generated by inverse piezoelectric effect
When the stress is applied
Created as a consequence of applied stress
❖ Field plate design to release the electric filed reduces the creation of traps as a consequence of the
inverse piezoelectric effect.
How to alleviate trap related degradation mechanisms
1. Material quality improvement
2. Field plates
3. Passivation
4. Improved growing techniques (buffer optimization)
5. Better confinement of electrons in the 2DEG
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13. ❑ 2DEG sheet charge concentration (ns)
❑ Threshold Voltage (VT)
❑ Maximum Drain current (ID)
❑ Transconductance (Gm)
❑ Conductivity of the two dimensional channel
Operating parameters of HEMT
εi dielectric permeability and di thickness of the wide bandgap semiconductor
AlGaN/GaN heterojunction of ns ~ 1013/cm2
∆d can be interpreted as the effective thickness of the 2DEG
Id= drain current
Vd= drain voltage
Vg= gate voltage
Φb= Barrier height
∆EC= Change in the conduction band
ND= Doping of the GaN layer
µ= mobility
C= Capacity
W= gate width
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14. Advantages of AlGaN/GaN
Cheaper technology compared to SiC or Diamond.
Material properties:
❖ Wide bandgap
❖ Piezo polarization nature
Technology properties:
❖ Heterostructure based
Devices
❖ Possibility to grow GaN on Si
Lower intrinsic carrier
concentration
Lower leakage current
High channel concentration
without doping
High electron mobility in the channel
High electron mobility of
electrons in the channel
Lower on state resistance
Low conduction losses
Higher converter efficiency
Fast switching
Possibility to operate at higher
Frequencies reducing the size
Of the passive components
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15. HEMT Applications
▪ Originally for high speed
▪ High power and high temperature
▪ Power amplifiers
▪ Oscillators
▪ Cell Phones
▪ Radar
▪ Most MMIC’s radio frequency applications
Monolithic microwave integrated circuit
Used as electric power switching devices
➢ Inverters
➢ Relay switching devices
➢ High frequency devices
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16. 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 (2DEG) is essential to the low noise feature
❖ AlGaN/GaN are the most common materials for Heterojunction
❖ Used in MMIC’s and Radio frequency applications for high performance
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❖ AlGaN/GaN HEMTs transistor don’t need doping to obtain a high electron density
Normally-ON: Negative voltage must be applied on the gate in order to block the current.
Normally-OFF: Reduce the circuit complexity and eliminated standby power consumption.
• By increasing the thickness of the AlGaN layer, the 2DEG density is enhanced.
• By increasing the thickness of the GaN layer, the 2DEG density is decreased

17. Sources of HEMT
▪ "GaAs Pseudomorphic HEMT Transistor." Mimix Broadband, Inc. N.p., 19 July 2008. Web. 30 Apr. 2013.
▪ Grunenputt, Erik. "Pseudomorphic and Metamorphic HEMT-technologies for Industrial W-band Low-noise and
Power Applications.” Youscribe. N.p., Dec. 2009. Web. 30 Apr. 2013.
▪ Poole, Ian. "HEMT, High Electron Mobility Transistor." Radio-Electronics.com. Adrio Communications, June 2010.
Web. 30 Apr. 2013.
▪ Göran, Andersson, ed. "High Electron Mobility Transistors (HEMT)." Laboratory for Millimeter-Wave. Electronics.
ETH Zurich, 2 Mar 2010. Web. 30 Apr 2013.
▪ Neamen, Donald. Semiconductor Physics and Devices Basic Principles. 4th ed. New York: McGraw-Hill,
2012. 602-9.
▪ 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.
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18. Summary
➢ A new Hybrid FE charge trap gate stack of GaN MIS-HEMT was designed.
➢ The device also shows improved Vth stability during the Positive bias temperature instability (PBTI) test.
➢ High Vth, High maximum current density, low RON and good Vth–temperature stability.
➢ 120 mm gate width device of FEG-HEMT device for high power device application.
➢ The MIS GaN FEG-HEMT is a promising technology for future power switching device applications.
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