Electrical properties of HEMT

1. High Electron Mobility Transistor
Arpan Deyasi
RCCIIT, India
5/23/2021 1
Arpan Deyasi, India

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Problems in conventional transistor
D
Scattering between donors/acceptors and mobile carriers
Impurity scattering
High noise
M.C

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Solution of problem
Separate the two
How?
Doping is done in one region and
Mobile carriers will subsequently migrate
into another region
Process is known as modulation doping

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Carrier separation
Metal
GaAs buffer
AlGaAs
donor layer
Undoped
AlGaAs
Metal Ionized donors
Free
electrons

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Structure
Semi-insulating substrate
GaAs
2DEG
Undoped AlGaAs spacer layer
n+ GaAs n+ GaAs
n+ AlGaAs
n+ AlGaAs
Source Drain
Gate

6. Characteristics of HEMT
mobility of free carriers are very high due to suppressed
ionized impurity scattering which makes very low
gate-to-source resistance
carrier freezeout problem is not present at extremely
low temperature because of electrons presence in a
region of energy below donor levels in high bandgap
material. So the device is treated as high-gain, low-noise one
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7. Characteristics of HEMT
Using materials with higher conduction band discontinuity,
large device transconductance can be obtained
Because of smaller active channel, it can be operated at
lower temperature
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Materials used in HEMTs
InP: used in some of the most advanced HEMTs
GaAs: used in the first HEMTs
GaN: an improvement upon the GaAs based HEMTs

9. Band Diagram
qφb
qVG
z
z=0
z=-ds
z=-d
ΔEC
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10. Φb: barrier height of Schottky barrier gate
ds: spacer layer distance
d: gate-to-channel distance
ξ: electric field at the interface region of barrier
ns: 2-DEG density
Sheet charge Density
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11. Sheet charge Density
From Gauss law
b s
qn
  =
dielectric of barrier region
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12. Sheet charge Density
Poisson’s equation is barrier region
2 ( )
b
qN z


 = −
where ( ) D
N z N
= s
d z d
−  
( ) 0
N z = 0
s
d z
−  
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13. Sheet charge Density
Integrating
0 0
( ') '
' '
z
z z b
d d q
N z dz
dz dz
 

=
− = − 
0
( ') '
'
z
z b
d q
N z dz
dz



= − − 
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14. Sheet charge Density
Further integrating
( ) ( 0) ( ') '
d d
b ds ds
q
z d z d dz N z dz
  

− −
− −
= − − = = −  
2
( ) 0 ( )
D s
b
q
V z d d N d d


− = − − = − −
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15. Sheet charge Density
2
( ) ( )
D
s
b
qN
V z d d d d


= − = − −
Let’s define 2
( )
D
p s
b
qN
V d d

= −
This is the necessary voltage to pinch-off the doped
AlxGa1-xAs layer
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16. Sheet charge Density
( )
b b
s p
n V V z d
q qd
  
 
= = − = −
 
From band diagram
( ) C
F
b G
E
E
V z d V
q q


= − = − + −
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17. Sheet charge Density
where
VG: gate voltage
EF: Fermi level
ΔEC: conduction band discontinuity
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18. Sheet charge Density
b C
s G p b
E
n V V
qd q


 

= + + −
 
 
Let’s define
C
off b p
E
V V
q


= − −
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19. Sheet charge Density
b
s G off
n V V
qd

 
= −
 
cut-off potential
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I-V characteristics
Current in active region
( ) 2
0
0.5
n
D G Th D D
W C
I V V V V
L

 
= − −
 

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I-V characteristics
In active region ( )
D G Th
V V V
 −
( )
0
n
D G Th D
W C
I V V V
L

 −
Transconductance
0
n
D
m D
G
W C
dI
g V
dV L

= =

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I-V characteristics
In saturation region ( )
D G Th
sat
V V V
 −
( )
2
0
n
D G Th D
sat sat
W C
I V V V
L

= −
2
0
n
m D
sat sat
W C
g V
L

=

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Differences between MOSFETs and HEMTs
MOSFETs
Operation in the UHF
range (300 MHz-3 GHz)
Doped region is used as
the channel
HEMTs
Operate in the microwave
range (300 MHz - 300 GHz)
Heterojunction is used as the
channel

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Applications
Power electronics
Precision sensors
Next generation wired/wireless communication
Advanced radars

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Areas to be covered in future
New structures need to be developed to reduce parasitic
capacitance and address the failure mechanisms
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