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