High Electron Mobility Transistor (HEMT) -...
Transcript of High Electron Mobility Transistor (HEMT) -...
High Electron Mobility Transistor
(HEMT)
Low dimensional
systems and nanostructures
Master in Nanoscience
2008-30-01
Student: Giuseppe Foti
TOPICS
• Advantages
• Types of HEMTs
• Structure 2 DEG formation• Structure
• Working principle
• Figures of merit
• Conclusions
2 DEG formation
Electrons at the interface
Charge control
Advantages
Same functional structure of traditional MESFET but
compositionally different layers are grown in order to
optimize and extend its performance. In the MESFET
charge transport takes place in a highly doped
material. As a consequence, energy losses due to
scattering are high.
MESFET
Highly doped material
In the HEMT the conduction channel is a
bidimensional electron gas (2DEG) confined at the
interface between two materials with different bandgap
instead of a threedimensional structure like in
conventional FETs. The 2DEG takes place in a slightly
doped material. As a result, it has significantly less
Coulomb scattering, resulting in a very high mobility
device structure.
Slightly doped material
Advantages
Same functional structure of traditional MESFET but
compositionally different layers are grown in order to
optimize and extend its performance. In the MESFET
charge transport takes place in a highly doped
material. As a consequence, energy losses due to
scattering are high.
MESFET
Highly doped material
In the HEMT the conduction channel is a
bidimensional electron gas (2DEG) confined at the
interface between two materials with different bandgap
instead of a threedimensional structure like in
conventional FETs. The 2DEG takes place in a slightly
doped material. As a result, it has significantly less
Coulomb scattering, resulting in a very high mobility
device structure.
High speed
High gain
Low noise
High power density
Lattice-matched HEMTs: same lattice
constant
Non-lattice matched or pseudomorphic
HEMT (pHEMT): slightly different lattice
constants
metamorphic HEMT (mHEMT): a buffer
layer is grown between materials with different
Types of HEMTs
layer is grown between materials with different
lattice costant
Examples
Lattice matched: AlGaAs/GaAs, AlInAs/InGaAs/InP
Metamorphic: AlInAs/InGaAs/GaAs, AlInSb/InSb
Non-lattice matched: AlGaAs/InGaAs/GaAs,
SiGe/Si
low-resistance ohmic contacts
source of electrons
thin Schottky layer
diffused or deposited
AlGaAs/GaAs HEMT structure
Epitaxially grown
insulating substrate
isolates defects from the substrate and
creates a smooth surface
high mobility layer
separates the 2DEG from the ionized
donors of the n+ active layer
source of electrons
Charge transfer takes place across the interface to equalize
the Fermi energy on both sides. Electrons from the donor
impurities of the highly doped n-type Ga1-xAlxAs are
transferred to the conduction band of the nearly intrinsic
p-type GaAs.
Positively charged donor ions are therefore left near the
interface on the n-type side and negatively charged
acceptor ions are left near the interface on the p-type side.
2 DEG formation
Working principle
acceptor ions are left near the interface on the p-type side.
Under suitable conditions the conduction band edge on
the p-type side can dip below the Fermi energy and produce
a region whose states are occupied by conduction electrons
that form a 2DEG. Since this region is on the side with
nearly intrinsic material, there are very few ionized
impurities to scatter the electrons, and consequently very
high mobilities can be achieved in the 2DEG.
Triangular quantum well
Electrons at the interface
Working principle
)()(2 2
2
*
2
zzzFezm
n χεχ =
+
∂
∂−
h
for 0≤z ( ) 0=zχ No penetration
in the barrier
≤∞
>=
0
0)(
z
zzFezV
( ) ( )
−
⋅= εχ zFe
Fe
mAiconstz
3/1
222
*2
h
( )ε−
= zFe
Fe
mp
3/1
222
*2
h
Electrons at the interface
Working principle
We obtain the following quantization
of the energy:
Fe
nn pm
Fe3/1
*
222
2
−=
hε
Where pn are the zeros of the Airy function
Penetration in the barrierLow potential barrier at the
AlGaAs/GaAs interface (0.2-0.4eV)
More accurate analisys
Interaction between electrons
Impurity potential (ns comparable with NA )
Electrons at the interface
Working principle
V0(z) = heterojunction potential energy
Vimp(z) = ionized impurity potential energy
Vee = electrons interaction potential energy
)()()()()(
20
2
2
*
2
zzzVzVzV
zmnnneeimp χεχ =
+++
∂
∂−
h
( ) 0≤= zforeAzzkbχ 2
2
h
bbb
Vmk ≈
Ground state wavefunction
If the concentration ND of donor impurities in the
barrier material is increased, the charge transfer is
increased, since the surface concentration of carriers
in the channel is:
An increased transfer of carriers leads to more
( ) 2/1
Ds Nn α
Electrons at the interface
Working principle
effective screening of channel impurities, but
produces at the same time increased scattering
by the ionized impurities in the barrier.
The scattering can be reduced by
inserting an undoped spacer layer
Charge control
Normally-offNormally-on
Working principle
The depletion region extends through both a thin
AlGaAs layer and the junction. The bottom of the
quantum well shifts up. The Fermi level lies under
the lowest energy subband. Thus there are no
electrons inside the channel and the conductivity
along the heterostructure is almost zero. To turn on
the conductivity of the device, it is necessary to
apply a positive voltage to the metal gate.
Normally-offNormally-on
Charge control
Normally-on
Working principle
In this case the built-in voltage drops
across a thick AlGaAs layer so that the
Fermi level lies above the lowest subband
and electrons populate the channel without
an external voltage bias. This channel has
a finite conductivity under normal conditions.
To turn off the conductivity of the device, it
is necessary to apply a negative voltage to
the metal gate.
Normally-on
Figures of merit
constDS
VGS
DSm
V
Ig
=∂
∂=
CC
εr relative dielectric constant
Wg gate width
deff effective gate-to-channel separation
nchannel sheet charge density
nc reference sheet concentration
sat
G
tr
G
satm
vL
C
t
Cg
/
==
vsat saturation velocity
L channel length
Figures of merit
constDS
VGS
DSm
V
Ig
=∂
∂=
CC
2
1
1
+
⋅=
channel
c
eff
gsatr
m
n
nd
Wvg
ε
εr relative dielectric constant
Wg gate width
deff effective gate-to-channel separation
nchannel sheet charge density
nc reference sheet concentration
sat
G
tr
G
satm
vL
C
t
Cg
/
==
vsat saturation velocity
L channel length
Figures of merit
( )gdgs
m
IIT
CC
gff
inout
+
===
π21/
( )m
sg
gsg
RRCfNF
+= π2
Conclusions• High power density
• Very good high frequency characteristics
• Low on-resistance
• High temperature stability (wide bandgap materials)
• Surface defects
• Electromigration
High costs
Gate, drain, and
source contacts
Leakage current,
thermal generation
• Epitaxial growth