Division of Electronics and Electrical Engineering
Dongguk University-Seoul Campus
HIGH ELECTRON MOBILITY TRANSISTOR
Presented by: PHAM XUAN HIEN
High voltage electric discharge & Application Lab
ENE6002: Advanced Electronic Physics
1. Brief History
2.2. Modulation Doping
2.3. What is “HEMT”?
3. Structure and Characteristics
Developed by Takashi Mimura and colleagues at Fujitsu in
Japan in 1979
Low noise amplifiers
Other space and military applications
Commercialization began in 1987 for satellite broadcasting
Commercial production took off in the 90’s
2.1. Heterostructure (1)
Hetero-structure is a structure consisting of at least two layers of different
semiconducting materials with distinct band-gaps. The interface two of these
layers is called a heterojunction or heterointerface..
Fig 1. The three types of heteroustructure
2.1. Hetero-structure (2)
The conduction and valance bands of
the two materials cannot be
simultaneously continuous across the
The conduction band and valence
band edges are discontinuous at a
Fig 2. Band diagrams for different
In the type I heterostructure, the sum
of the conduction band and valence band
edge discontinuities is equal to the
energy gap difference ΔEG = ΔEC + ΔEV
2.2. Modulation Doping (1)
Offered an important advantage in device engineering since it provides a
mechanism by which the free carrier concentration within a semiconductor layer
can be increased significantly without the introduction of dopant impurities.
Advantages of modulation doping
Allowed the free carrier concentration to be increased significantly without
compromising the mobility.
The free carriers are spatially separated from the dopants.
Reduced ionized impurity scattering hence higher mobility
Good electron confinement within 2DEG
2.2. Modulation Doping (1)
One constructs a heterostructure formed by an n-type wide-bandgap
semiconductor with an unintentionally doped, relatively narrow gap
When the two materials AlGaAs and GaAs are initially apart, the Fermi level
lies closer to the conduction band edge in the AlGaAs than in the GaAs since the
AlGaAs is doped n-type.
Fig 3. a) Energy band diagrams of the AlGaAs and GaAs layer when
apart and in equilibrium
2.2. Modulation Doping (2)
When the two materials are placed into contact, electron must be transferred
from the AlGaAs layer into the GaAs layer to align the Fermi level.
The energy of the electrons in the conduction band of the AlGaAs layer is higher
than that of the GaAs layer.
Increasing the electron concentration within the GaAs layer because the electron
move from AlGaAs to GaAs.
2.2. Modulation Doping (3)
The ionized donor atoms within the AlGaAs result in a net positive charge,
which balances the net negative charge due to the electrons transferred in the GaAs
Fig. 3 b) Energy band diagrams of the hetero-structure. The dashed
horizontal lines at the heterointerface in the GaAs layer represent energy
subbands arising from spatial quantization effects
2.2. Modulation Doping (4)
Although the ionized donor atoms in the AlGaAs obviously influence the
electrons transferred in the GaAs, the spatial seperation between the two charge
species mitigates the Coulomb interaction between them.
Ionized impurity scattering of the transferred electrons is reduced, the electron
The conduction band edge in the GaAs layer
is strongly bent near the heterointerface.
The band bending is a consequence of the
Electrons within this region of the
semiconductor are then confined by a
triangular well-like potential.
Fig 4. Band diagram of N+ -AlGaAs/ p+ -GaAs
heterojunction as used in HEMTs under thermal
2.3. What is “HEMT”?
Referred to as hetero-junction FET (HFET) or modulation-doped-FET
Can operate at frequencies higher than 10 GHz with ultralow noise
High efficiency communications.
Conventional HEMTs use a AlGaAs/GaAs latest use AlGaN/GaN
3.1. Structure (1)
A charge accumulation layer between the source and drain terminals.
The charge density can be controlled by the gate voltage.
Fig 4. Basic GaAs HEMT Layer Diagram
Fig 5. Cross section of an AlGaN/GaN HEMT
3.1. Structure (2)
Incorporating a junction between two
materials with different band gaps.
Band discontinuities occur at the
interface between the two
Fig 6. Diagram of the band structures of two
InAlAs and InGaAs at the equilibrium
These discontinuities are referred to
as the conduction and valence band
offsets ΔEc and ΔEv.
The conduction band offset can form
a triangular shaped potential well
confining electrons in the horizontal
Fig 7. Semiconductors in contact at the equilibrium.
A 2DEG is formed at the interface.
3.2. Structure (3)
Two-dimensional electron gas:
The electron exhibit wave
Gas of electron free to move
in two dimensions, but tightly
confined in the third.
Fig 8. Charge distribution of GaAs- +nAlGaAs junction
2DEG is confined in a narrow area
Very high electron concentration
in a very thin space.
3.1. Struture (4)
HEMT is a three terminal device
The current between source and drain is controlled by applied gate voltage.
In the vicinity of a semiconductor heterojunction electrons are transferred from the material
with the higher conduction band energy Ec to the material with the lower Ec where they can
occupy a lower energy state
The current between drain and source is flowing right through the two-dimensional
conducting channel created by electron.
Within this region the electrons are able to move freely because there are no other donor
electrons or other items with which electrons will collide and the mobility of the electrons in
the gas is very high. The quality of this channel is depend on substrate used.
The electron flowing through the channel is controlled by the gate.
Maximum VDS = 10V (dashed line) and 20 V
A reduction in drain current occur for VDS
<8V. This reduction in current after the
application of a high drain voltage is referred to
as current collapse.
When the drain voltage is high, electron are
injected into the GaN buffer layer, where they
are trapped. This trapped charge depletes the 2DEG from beneath the active channel and results
in a reduction in drain current.
The trapped charge can be released through
illumination or thermal emission. The gradual
reduction in current for VDS >10V, is due to selfheating
Fig 9. Characteristic for a GaN HEMT
Ion implantation is a materials
engineering process by which ions of a
material are accelerated in an electrical
field and impacted into a solid.
First, ohmic contact formed etching the
AlGaN in the source and drain region.
Next the gate is formed by Schottky
Fig 10. Fabrication of HEMT
Lastly, device fabrication completed
with a deposition of a SiN passivation layer
October 30, 2018
The HEMT was initially developed for a high speed application. However, till
only the first device got fabricated to realize that it display a very low noise
figure. This is due to the nature of 2DEG and less electron collisions.
As a results of its noise performance, it is broadly used in low noise small
signal amplifiers, power amplifiers, oscillators and mixers operating at
frequencies up to 69 GHz and more.
 Characterisation and perormance optimisation of GaN HEMTs and amplifiers
for radar applications, Dr Fornetti's PhD thesis.
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