In this work, we propose a novel enhancement-mode GaN metal-oxide-semiconductor high electron
mobility transistor (MOS-HEMT) with a 10 nm T-gate length and a high-k TiO2 gate dielectric. The DC and
RF characteristics of the proposed GaN MOS-HEMT structure are analyzed by using a TCAD Software. The
device features are heavily doped (nþþ GaN) source/drain regions for reducing the contact resistances
and gate capacitances, which uplift the microwave characteristics of the MOS-HEMT. The enhancementmode GaN MOS-HEMTs showed an outstanding performance with a threshold voltage of 1.07 V,
maximum extrinsic transconductance of 1438 mS/mm, saturation current at VGS ¼ 2 V of 1.5 A/mm,
maximum current of 2.55 A/mm, unity-gain cut-off frequency of 524 GHz, and with a record maximum
oscillation frequency of 758 GHz. The power performance characterized at 10 GHz to give an output
power of 29.6 dBm, a power gain of 24.2 dB, and a power-added efficiency of 43.1%. Undoubtedly, these
results place the device at the forefront for high power and millimeter wave applications.
© 2019 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.
This is an open access article under the CC BY license

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atou
Bisk
Received in revised form
a n
EM
RF characteristics of the proposed GaN MOS-HEMT structure are analyzed by using a TCAD Software. The
and gate capacitances, which uplift the microwave characteristics of the MOS-HEMT. The enhancement-
y trans
[10,11], Pr2O3 [12,13], SiN [14], SiO2 [14] and NiO [15] as the gate
ntact resistance of
U mm due to the
ce-drain distance.
in ohmic contacts
resistivity in the
duces the gate ac-
le maintaining the
e capacitance [18].
d HEMTs [19] and
zations for two-dimensional electron gas (2DEG) formation [19].
Although these types of devices were used in microwave power
ampliﬁers, low noise and RF switching devices, enhancement-
mode MOS-HEMTs [17,20] have added a more advantage in
simpler circuit design and low power consumption due to the
elimination of negative power supply [17] which is suitable for the
radio frequency integrated circuit (RFIC) design. In this paper, we
* Corresponding author. Laboratory of Semiconducting and Metallic Materials,
University of Mohamed Khider Biskra, Algeria
E-mail addresses: zinouu113@yahoo.fr (T. Zine-eddine), hamaiziaz@gmail.com
(H. Zahra), messaimr@yahoo.fr (M. Zitouni).
Contents lists available at ScienceDirect
Journal of Science: Advanc
journal homepage: www.el
Journal of Science: Advanced Materials and Devices 4 (2019) 180e187Peer review under responsibility of Vietnam National University, Hanoi.insulating dielectric is widely investigated, and excellent perfor-
mance is demonstrated utilizing Al2O3 [4,6], TiO2 [7e9], HfO2
MOS-HEMTs [17] are the depletion type due to their unique ma-
terial properties leading to spontaneous and piezoelectric polari-down voltage, high saturation velocity, low effective mass, high
thermal conductivity and high two-dimensional electron gas (2DEG)
density of the order of 1013 cm2 at the hetero interface [1e3].
However, Schottky gate transistors usually exhibit a high gate
leakage current [4], and a drain current collapse when operating at
high frequencies. These are the major factors that limit the perfor-
mance and reliability of HEMT in radio frequency (RF) power
applications.
Metal oxide semiconductor HEMTs (MOS-HEMTs) with an
All these devices suffered from the high co
>0.3 U mm and the high on-resistance of >1
alloyed ohmic contacts and the large sour
Recently, the heavily doped n þ GaN source/dra
allowed a signiﬁcant reduction of the contact
proposed device [16,17]. The T-gate structure re
cess resistance by providing a large gate areawhi
smaller gate length and reduces the extrinsic gat
Also, most of the developed AlGaN/GaN basemost preferred devices for high-power and high frequency applica-
tions, due to their suitable material properties such as high break-
voltage (Vth). The dielectric with high permittivity (high k) can
effectively alleviate these problems.Keywords:
Enhancement-mode
MOS-HEMT
High-k
TiO2
Regrown source/drain
TCAD
1. Introduction
GaN-based high electron mobilithttps://doi.org/10.1016/j.jsamd.2019.01.001
2468-2179/© 2019 The Authors. Publishing services b
( extrinsic transconductance of 1438 mS/mm, saturation current at VGS ¼ 2 V of 1.5 A/mm,
maximum current of 2.55 A/mm, unity-gain cut-off frequency of 524 GHz, and with a record maximum
oscillation frequency of 758 GHz. The power performance characterized at 10 GHz to give an output
power of 29.6 dBm, a power gain of 24.2 dB, and a power-added efﬁciency of 43.1%. Undoubtedly, these
results place the device at the forefront for high power and millimeter wave applications.
© 2019 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.
This is an open access article under the CC BY license (
istors (HEMTs) are the
dielectric to overcome the aforementioned limitation. These solu-
tions, however, were performed at the expense of a decrease in the
device transconductance (gm) and large shift in the thresholdAccepted 2 January 2019
Available online 7 January 2019mode GaN MOS-HEMTs showed an outstanding performance with a threshold voltage of 1.07 V,30 December 2018 device features are heavily doped (nþþ GaN) source/drain regions for reducing the contact resistancesOriginal Article
Design and analysis of 10 nm T-gate enh
for high power microwave applications
Touati Zine-eddine a, *, Hamaizia Zahra a, Messai Zi
a Laboratory of Semiconducting and Metallic Materials, University of Mohamed Khider
b Electronics Department, Faculty of Sciences and Technology, University of BBA, Algeria
c Laboratory of Optoelectronics and Components, UFAS 19000, Algeria
a r t i c l e i n f o
Article history:
Received 17 December 2018
a b s t r a c t
In this work, we propose
mobility transistor (MOS-Hy Elsevier B.V. on behalf of Vietnamncement-mode MOS-HEMT
ni b, c
ra, Algeria
ovel enhancement-mode GaN metal-oxide-semiconductor high electron
T) with a 10 nm T-gate length and a high-k TiO2 gate dielectric. The DC and
ed Materials and Devices
sevier .com/locate/ jsamdNational University, Hanoi. This is an open access article under the CC BY license
propose a novel enhancement-mode GaNMOS-HEMTwith a 10 nm
T-gate length and a high-k TiO2 gate dielectric, This device could be
placed at the forefront for high power and millimeter wave
applications.
2. Device description and simulation models
2.1. The oxide choice
which have been shown to give a low contact resistance.
að0Þ ¼ aGaN (4)
and c13, c33 are the elastic constants, e33 and e31 are the piezo-
electric constants given as follows:
c13ðxÞ ¼ ð5xþ 103Þ (5)
c33ðxÞ ¼ ð32xþ 405Þ (6)
e13ðxÞ ¼ ð0:11x 0:49Þ (7)
e33ðxÞ ¼ ð0:73xþ 0:73Þ (8)
The spontaneous polarization of AlxGa1-xN is also a function of
the Al mole fraction x and is given by:
his research.
Gate dielectric Material Dielectric constant (k) Energy bandgap Eg (eV) Conduction band offset DEc (eV) Valence band offset DEc (eV)
3.5 4.4
3 4.7
1.1 1.3
1.4 3.3
1.4 3.3
T. Zine-eddine et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 180e187 181SiO2 3.9 9
Al2O3 8 8.8
TiO2 80 3.5
ZrO2 25 5.8
HfO2 25 5.8In a real device, charges exist in all the three interfaces as shown
in Fig. 2. In the simulation, the polarization charge densities were
modelled as ﬁxed interface charge densities. The spontaneous and
piezoelectric polarization charges of AlGaN and GaN layers were
calculated using equations (1)e(9), [25,26]. The calculated polari-
zation charge densities at the TiO2/GaN, GaN/AlGaN and AlGaN/
GaN interfaces are displaying in Fig. 2. Also, the TiO2/GaN interface
is full of dislocations and traps [27]. A donor concentration of
8.7 1012 cm2 at the TiO2/GaN interface is considered.
The total amount of the polarization induced sheet charge
density for an undoped AlxGa1-xN/heterostructure can then be
calculated by using the following equations:
Table 1
High-k dielectric materials and their properties [21]. TiO2 is the material choice in tThe TiO2 is our choice of the high-k dielectric gate material. The
other high-k materials are shown in Table 1 with their properties
[21]. Among the gate dielectric materials, TiO2 is considered as the
most suitable candidate because of its large static dielectric con-
stant (k ¼ 80e170). TiO2 can increase the physical thickness of the
dielectric while maintaining the same oxide capacitance, conse-
quently reducing the leakage current. Previous research work
[22e24] demonstrated that transistors with TiO2 as gate dielectric
had a high breakdown voltage and very low gate leakage current,
accompanied by a slight decrease in transistor transconductance
and small shift in threshold voltage.
2.2. The structure of device
Fig. 1 shows the cross-sectional schematic of the enhancement
(E)-mode GaN MOS-HEMT device with a 10 nm gate-length and
source/drain regrowth. A 3-inch 4H-SiC is used as a substrate to
achieve the good thermal stability. The source/drain length is
500 nm. The source-gate and the gate-drain spacing are both
645 nm. The oxide thickness is 5 nm with a TiO2 dielectric to
minimize the leakage. Looking at the structure from bottom to top,
an AlN nucleation layer is inserted to reduce the stress and the
latticemismatch. The undoped GaN channel is 800 nm thick. Doped
with 2.5 1018 cm3 donors, the Al0.3Ga0.7N of 20 nm thickness
constitutes the barrier layer which depletes the 2DEG and provides
a strong carrier conﬁnement in the quantum well at the hetero-
interface and minimizes junction leakage and off-state leakage
current Iof and a 5-nm GaN cap layer. Next, two graded n þ GaN
(12 nm), doped with 2 1019 cm3#donors, are created for the
source and drain to reduce the access and contact resistances [16].
Non-alloyed contacts are formed for the source/drain regions,Bold represents TiO2 is the material choice in this research.jsðxÞj ¼
PPEðAlxGa1xNÞ þ PSPðAlxGa1xNÞPSPðGaNÞ
(1)
jsðxÞj ¼
2
að0Þ aðxÞ
aðxÞ
e13ðxÞ þ e33ðxÞ
C13ðxÞ
C33ðxÞ
þPSPðxÞ PSPð0Þ
(2)
where a(x) is lattice constant:
aðxÞ ¼ ð0:077xþ 3:189Þ1010 (3)
Fig. 1. Cross-section structure of the proposed GaN MOS-HEMT.
Fig. 2. Interface charges and interface traps in GaN MOS-HEMT.
T þ 1462
cðAlGaNÞ ¼ cðGaNÞ 1:89xþ 0:91xð1 xÞ (19)
ancThen, the band-gap energy dependence of the AlxGa1-xN ternary
on the composition fraction x using Vegard Law is described, where
b is the bowing parameter:PSPðxÞ ¼ ð 0:052x 0:029Þ (9)
2.3. Physical models
Simulations were performed using Two dimensional (2D) sim-
ulations of Silvaco ATLAS TCAD tool. The Boltzmann transport
theory has shown that the current densities in the continuity
equations may be approximated by a drift-diffusion model (DD).
This model is one of the most basic carrier transport model in
semiconductor physics. In this case, the current densities for elec-
trons and holes under the DD model are expressed by the
equations:
J
!
n ¼ nqmnV∅n (10)
J
!
p ¼ nqmpV∅p (11)
where n and p are electron and hole concentrations respectively, mn
and mp are the electron and hole mobility respectively, Fn and Fp
are the electron and hole quasi-fermi potentials, respectively.
The Poisson equation (12), the electron continuity equation (13)
and the hole continuity equation (14), based on DD model, are
numerically solved [28]. A drift-diffusion model is used to solve the
transport equation.
divðεVJÞ ¼ r (12)
whereε is the permittivity, Jis the electrostatic potential and r is
the space charge density.
dn
dx
¼ 1
q
V J
!
n þ Gn Rn (13)
dp
dx
¼ 1
q
VJn
!þ Gp Rp (14)
The continuity equations for electrons and holes are deﬁned by
equations (13) and (14), respectively, J
!
n and J
!
p are the current
densities for electrons and holes, Gn and Gp are the electron and
hole generation rates, Rn and Rp are the electron and hole recom-
bination rates, respectively, q is the magnitude of electron charge
[29].
The basic band parameters for deﬁning heterojunctions in Blaze
(one of the TCADmodules) are the bandgap parameter, the electron
afﬁnity, the permittivity and the conduction and valence band
density of states [29].
Generally, the bandgap for nitrides is calculated in a two-step
process: First, the bandgap of the relevant binary compounds is
computed as a function of temperature (T) using [30]:
EgðGaNÞ ¼ 3:507 0:909 10
3T2
T þ 830 (15)
EgðAlNÞ ¼ 6:23 1:799 10
3T2
(16)
T. Zine-eddine et al. / Journal of Science: Adv182EgðAlxGa1xNÞ ¼ xEgðAlNÞ þ ð1 xÞEgðGaNÞ bxð1 xÞ (17)The permittivity of the nitrides as a function of composition
fraction x is given by [25]:
εðAlxGa1xNÞ ¼ 8:5xþ 8:9ð1 xÞ (20)
The nitride density of states masses as a function of composition
fraction, x, is given by linear interpolations of the values for the
binary compounds [30]:
meðAlxGa1xNÞ ¼ 0:314xþ 0:2ð1 xÞ (21)
mhðAlxGa1xNÞ ¼ 0:417xþ 1:0ð1 xÞ (22)
The recombination rate is given by the following expression
[34,35]:
USRH ¼
n:p n2i
tp
nþ ni exp
h
Etrap
KTL
i
þ tn
pþ ni exp
hEtrap
KTL
i (23)
where Etrap is the difference between the trap energy level and the
intrinsic Fermi level, TL is the lattice temperature andtn, tpare the
electron and hole lifetimes.
The low-ﬁeld mobility is modeled by an expression similar to
that proposed by CaugheyThomas [36]:
m0ðT;NÞ ¼ mmin
T
300
b1
þ ðmmax mminÞ
T
300
b2
1þ
h
Nref
T
300
b3iað T300Þb4 (24)
where T is the temperature, Nref is the total doping density, and a,
b1, b2, b3, b4, mmin and mmax are parameters that are determined from
Monte Carlo simulation [36].
Another model used for high ﬁeld mobility, it is based on an
adjustment to the Monte Carlo data for bulk nitride, which is
described by the following equation [36]:
mnðEÞ ¼
m0ðT ;NÞ þ ysatn E
n11
E
n1
c
1þ a
E
Ec
n2 þ EEc
n1 (25)
The parameters used in the simulation are shown in Table 2.
3. Simulation results and discussion
3.1. Energy band diagram of MOS-HEMT
Fig. 3 illustrates the conduction bands in the E-mode GaN
MOSHEMT under the gate electrode at zero gate bias. This band
diagram is used to explain the 2DEG channel formation in the GaN
MOS-HEMT. The discontinuity in the bandgap, between the AlGaN
and GaN gives rise to a band bending process at the interface. The
band bending is in such a way that the conduction band of the GaNWe consider: Eg (A1N)¼ 6.08 eV, Eg (GaN)¼ 3.55eV [31] and the
bowing parameter b ¼ 1.3 eV [32] at 300K.
The electron afﬁnity is calculated such that the band edge offset
ratio is given by [33]:
DEc
DEv
¼ 0:7
0:3
(18)
The electron afﬁnity as a function of composition fraction x is
expressed as:
ed Materials and Devices 4 (2019) 180e187falls below the Fermi level (Ef) and forms a well at the interface
sheet charge, which can be controlled by varying the alloy
composition in the AlGaN layer. Equation (26) also shows that the
sheet carrier concentration can be increased if the AlGaN layer
thickness is reduced and/or the Schottky barrier height is increased
[25]. The following approximations can be used in equation (26) to
calculate the sheet carrier concentration of the 2DEG at the AlGaN/
GaN interface with varying Al mole composition in the AlGaN layer
(x) [26].
Dielectric Constant:
εðxÞ ¼ 0:5xþ 9:5 (27)
Schottky Barrier:
e4b ¼ ð1:3xþ 0:84Þ (28)
(
2
)2=3
Table 2
Electrical and thermal parameters used in this work at 300 K [29,37].
Material GaN AlGaN AlN SiC-4H
Band Parameters
Epsilon 9.5 9.55 8.5 9.7
Eg (eV) 3.55 3.87 6.08 3.23
Chi (eV) 3.05 2.69 1.01 3.2
Nc(per cc) 1.07e18 2.07e18 2.07e18 1.66e19
Nv(percc) 1.16e19 1.16e19 1.16e19 3.3e19
T. Zine-eddine et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 180e187 183Effective Richardson Constants
An** 14.7 22.8 22.8 91.3
Ap** 71.8 71.8 71.8 144
Thermal Velocities
vn (cm/s) 3.34e7 2.68e7 2.68e7 1.34e7
vp (cm/s) 1.51e7 1.51e7 1.51e7 1.07e7
Saturation Velocities
vsatn (cm/s) 1.9e7 1.1e7 1.4e7 2.2e7
vsatp (cm/s) 6.44e6 6.01e6 6.01e6 1e7
Mobility parameters
me (cm2/V.s) 1350 985.5 1280 460
mh (cm2/V.s) 13 13.3 14 124[26,38]. This well is called the quantum well, and the electron in-
side the well obeys the electron wave characteristics. The large
band discontinuity associated with strong polarization ﬁelds in the
GaN and AlGaN allows a large 2DEG concentration to be formed in
the device. The electron scattering associated with the impurities is
less in this region because of the absence of doping in the GaN
channel [39].
The sheet electron concentration can be calculated using [40]:
nðsÞðxÞ ¼
sðxÞ
e
ε0εðxÞ
dAlGaN e2
½e4bðxÞ þ EFðxÞ DECðxÞ (26)
The meaning of parameters used in this equation is described
and listed in Table 3. It is understood that the sheet carrier con-
centration is mainly controlled by the total polarization induced
Fig. 3. Energy band of GaN MOS-HEMT under the gate electrode.
Table 3
Parameters of equation (26) [25].
Parameters Deﬁnition
εðxÞ Relative Dielectric Constant of AlxGa1-xN
dAlGaN Thickness of AlGaN layer
fbðxÞ Schottky Barrier Height of gate contact on top of AlGaN
EF ðxÞ Fermi level w.r.t the conduction band energy level
DECðxÞ Conduction band offset at the AlGaN/GaN interface
e Electronic chargeE0ðxÞ ¼
9phe nsðxÞ
8ε0
ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
8mðxÞεðxÞp (30)
where the effective electron mass, ðxÞx0:22me.
Band Offset:
DEC ¼ 0:7
EgðxÞ Egð0Þ
(31)
From the simulation, the 2DEG density at the AlGaN/GaN
interface is 9.21 1012 cm2. This value is about 15% smaller than
the experimental measurements using room-temperature Hall
measurement. It is reported in the literature that the sheet carrier
concentration between experimental measurement and theoretical
calculation can differ by ±20%. Therefore, the 2DEG densities from
the simulation can be accepted to agree reasonably well with the
experimental values [25,41].
3.2. DC results
The IDS-VDS curves of Fig. 4 allowed the evaluation of MOS-
HEMT characteristics such as the knee voltage (transition be-
tween the linear and saturation region), the on-resistance, the
maximum current and self-heating.
0 2 4 6 8
0,0
0,5
1,0
1,5
2,0
2,5
VGS=-1V
VGS=0V
VGS=1V
VGS=2V
D
ra
in
c
ur
re
nt
(A
/m
m
)
Drain voltage (V)
VGS=3VFermi Energy:
EFðxÞ ¼ E0ðxÞ þ
ph2
mðxÞnsðxÞ (29)
whereE0ðxÞis the ground state sub band level of the 2DEG, which is
given by:Fig. 4. IDS-VDS characteristics of the simulated GaN MOS- HEMT.
As can be seen in Fig. 4, for IDS-VDS characteristics, the gate
voltage varied from1 V to 3 V and drain voltage varied from 0 V to
6 V. The device exhibited a peak current density of ~1.5 A/mm at
VGS ¼ 2 V and 2.5 A/mm at VGS ¼ 3 V.
TheMOS-HEMT is pinched-off completely at VGS¼1V. In Fig. 5
(a) the threshold voltage VTH is about 1.07 V. The transconductance
gm shown in Fig. 5 (b) is calculated from the derivative of IDS-VGS
curves at ﬁxed VDS and is expressed in Siemens. The peak extrinsic
transconductance was ~1438 mS/mm.
Fig. 6 illustrates the transconductance verses gate length char-
acteristics of the GaNMOS-HEMTs. It reduces the transconductance
from 1430 mS/mm to 1258 mS/mm with the gate length change
from 10 nm to 60 nm.
Fig. 7 displays the reference of gm versus Lg of our E-mode de-
vices against some state-of-the-art results reported in the literature
based on various technologies. Obviously, a more balanced, DC
performance is achieved in our work which is highly desirable not
only for high power applications but for high frequency
applications.
3.3. Gate leakage performance
Fig. 8 shows a comparison of the gate leakage performance of
the HEMTs and E-mode GaN MOS-HEMTs with the same device
dimensions. The leakage current of MOS-HEMTs is found to be
signiﬁcantly lower than that of the Schottky gate HEMTs. The gate
leakage current density of MOS-HEMTs is almost 3e5 orders of
magnitude lower than that of the HEMTs. Such a low gate leakage
current should be attributed to the large band offsets in the TiO2/
HEMT and a good quality of both the reactive-sputtered TiO2
dielectric. This leads to an increase of the two-terminal reverse
breakdown voltage (about 25%) and of the forward breakdown-1 0 1 2 3 4
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
D
ra
in
c
ur
re
nt
(A
/m
m
)
Gate Voltage(V)
VDS=5V
VDS=3.5V
VDS=2.5V (a)
-1 0 1 2 3 4
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
Gate voltage (V)
Tr
an
sc
on
du
ct
an
ce
(S
/m
m
)
VDS=5V
VDS=3.5V
VDS=2.5V
(b)
Fig. 5. (a) Transfer characteristic, (b) transconductance at VDS ¼ 2.5 V, 3.5 V and5 V.
1200
1300
1400
du
ct
an
ce
(m
S
/m
m
)
VDS=5V
800
1000
1200
1400
1600
1800
[46]
[44]
sc
on
du
ct
an
ce
(m
S
/m
m
) This work
[45] [42]
T. Zine-eddine et al. / Journal of Science: Advanced Materials an