High electron mobility transistor

Gallium nitride (GaN) high-electron-mobility transistors (HEMTs) have become one of the most popular devices for high-frequency and high-power applications in recent years. Compared to
traditional silicon devices, GaN material has several remarkable properties, such as better electron mobility at the high electric field, wider energy bandgap (3.4 eV), higher breakdown electric field and higher
saturation electron drift velocity [1–3]. Such excellent material properties have made AlGaN/GaN
devices the streamlined technology for high-frequency and high-power applications for next-generation
wireless communication systems at millimeter-wave frequencies

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
➢Inthe1979“TakashiMimura”,andcolloguesdevelopedthefirstHEMT(AlGaAs/GaAssystem)at
FujitsuinJapan.
➢TheHEMTwasbasedontheconceptofmodulationdopingfirstdemonstratedbyRayDingleandhis
collaboratorsatbelllabsin1978.
➢1980“TakashiMimura”,FujitstulaboratoriesdesignedthefeatureofthefirstHEMT.
➢1985HEMT was announced the lowers noise device.
➢1987Commercialization began for satellite broadcasting receivers.
➢1993Asif Kahn demonstrated the first AlGaN/GaN HEMT
➢Commercial production took off in the 90’s.
❑In the last 30 years, HEMTshave 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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•Ahighelectronmobilitytransistor(HEMT)isakindof
aFET,wherethecurrentflowbetweentwoohmic
contacts,sourceanddrain,iscontrolledbyathird
contact,thegatewhichmaybeaSchottkybarriercontact
inmostcases.
•HEMTincorporatesajunctionbetweentwodifferent
semiconductormaterials(i.e.aheterojunction)asthe
channel.
HEMT advantages:
•High speed,
•High frequency,
•Digital circuits
•Microwave circuits with low noise.

7. Formation of the 2DEG
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GaNAlGaN
•Bysplittingtheheterojunctionintotwoseparatedparts
wecaneasilyunderstandtheformationofthe2DEG.
•AlGaNisgrownonaGaN.AlGaNgrownlayer
inducesapolarizationeffectresultinginapositive
chargeontheAlGaN/GaNinterfaceandanegative
chargeonthetopoftheAlGaNlayer.
•ThisdifferentlychargedareasresultinforminganelectricfieldinsidetheAlGaNlayer.

8. Formation of the 2DEG
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2DEG in n-doped AlGaN
•Thepolarizationdescribedbeforemakestheenergyband,includingtheFermilevel,bendingtowardstheAlGaN/GaN
interfacein(b).
•Thesameeffectofenergybandbendingcanbeobtainedbymakingasimpleplanarcapacitorusingann-doped
AlGaNasthedielectricandapplyingvoltageonit.
•Becauseofappliedvoltageanelectricforceisactingonthefreeelectronsinthelayerforcingthemtomovetowards
thepositiveelectrode,leavingpositivespacechargesbythenegativeelectrode(c).
(a) (b) (c)

9. Formation of the 2DEG
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▪Thebuiltinelectricfieldbetweentheaccumulatednegativeelectronsandfixedpositivespacechargesreducesthe
polarizingelectricfieldmakingtheinclinedenergybandflatter.
▪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
Al
xGa
1-xN
GaN-capping layer
DS
G
Field plate
2DEG
Passivation-3
Passivation-2
Passivation-1
Source field plate Drain field plate
GaN–caplayer:
•Reducethereverseleakagecurrent
•IncreaseintheelectricfieldstrengthintheAlGaNlayer
•Increaseinpowerefficiencybyincreasingthethickness.
Substrate:
•Provideshighthermalconductivity.
Buffer:
•ItaffectsthestructuralandopticalpropertiesoftheGaNlayer
grownabove,thustheonandoffstatecharacteristicsofthedevice.
•Increaseinbufferthicknessyieldsareducedsubstrateleakage.
AGaN/GaN
•High electron density in the channel

11. ❖Trappingofelectronsinjectedfromthegateelectrode
orfromthe2DEGintosurface-states
❖Thenegativelychargedregionwithtrappedelectrons
actsasavirtualgatedepletingthechannelbeneathit
whenthestressisremoved.
Trapping mechanisms of HEMT
Trapping at the surface:
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GaN
Al
xGa
1-xN
GaN-capping layer
S
G
2DEG
Passivation Layer
D
Trapped Electrons
Lower 2DEG
concentration
Al
xGa
1-xN
GaN-capping layer
S
G
2DEG
Passivation Layer
D
Trapped ElectronsLower 2DEG
concentration
GaN
oTrapping from the 2DEG into the GaN and AlGaN regions.
oTrapped 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
❖Fieldplatedesigntoreleasetheelectricfiledreducesthecreationoftrapsasaconsequenceofthe
inversepiezoelectriceffect.
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 (n
s)
❑Threshold Voltage (V
T)
❑Maximum Drain current (I
D)
❑Transconductance (G
m)
❑Conductivity of the two dimensional channel
Operating parameters ofHEMT
ε
i
dielectric permeability and d
i
thickness of the wide bandgap semiconductor
AlGaN/GaN heterojunction of n
s ~ 10
13
/cm
2
∆dcan be interpreted as the effective thickness of the 2DEG
I
d= drain current
V
d= drain voltage
V
g= gate voltage
Φ
b= Barrier height
∆E
C= Change in the conduction band
N
D= 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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600 km (EPA) range
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11,750 km range
Caltrain
Electric train

16. Key points
❖Its two main features are low noise and high frequency capability
❖A heterojunctionis 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
➢AnewHybridFEchargetrapgatestackofGaNMIS-HEMTwasdesigned.
➢ThedevicealsoshowsimprovedV
thstabilityduringthePositivebiastemperatureinstability(PBTI)test.
➢HighV
th,Highmaximumcurrentdensity,lowR
ONandgoodV
th–temperaturestability.
➢120mmgatewidthdeviceofFEG-HEMTdeviceforhighpowerdeviceapplication.
➢TheMISGaNFEG-HEMTisapromisingtechnologyforfuturepowerswitchingdeviceapplications.
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