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- Chất lượngDesign and analysis of 10 nm T-gate enhancement-mode MOS-HEMT for high power microwave applications nước biển ven bờ từ dữ liệu các trạm quan trắc môi trường phía nam Việt Nam (2013- 2017)
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- Journal of Science: Advanced Materials and Devices 4 (2019) 180e187
Contents lists available at ScienceDirect
Journal of Science: Advanced Materials and Devices
journal homepage: www.elsevier.com/locate/jsamd
Original Article
Design and analysis of 10 nm T-gate enhancement-mode MOS-HEMT
for high power microwave applications
Touati Zine-eddine a, *, Hamaizia Zahra a, Messai Zitouni b, c
a
Laboratory of Semiconducting and Metallic Materials, University of Mohamed Khider Biskra, Algeria
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 a b s t r a c t
Article history: In this work, we propose a novel enhancement-mode GaN metal-oxide-semiconductor high electron
Received 17 December 2018 mobility transistor (MOS-HEMT) with a 10 nm T-gate length and a high-k TiO2 gate dielectric. The DC and
Received in revised form RF characteristics of the proposed GaN MOS-HEMT structure are analyzed by using a TCAD Software. The
30 December 2018
device features are heavily doped (nþþ GaN) source/drain regions for reducing the contact resistances
Accepted 2 January 2019
Available online 7 January 2019
and gate capacitances, which uplift the microwave characteristics of the MOS-HEMT. The enhancement-
mode 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,
Keywords:
Enhancement-mode
maximum current of 2.55 A/mm, unity-gain cut-off frequency of 524 GHz, and with a record maximum
MOS-HEMT oscillation frequency of 758 GHz. The power performance characterized at 10 GHz to give an output
High-k power of 29.6 dBm, a power gain of 24.2 dB, and a power-added efficiency of 43.1%. Undoubtedly, these
TiO2 results place the device at the forefront for high power and millimeter wave applications.
Regrown source/drain © 2019 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.
TCAD This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
1. Introduction dielectric to overcome the aforementioned limitation. These solu-
tions, however, were performed at the expense of a decrease in the
GaN-based high electron mobility transistors (HEMTs) are the device transconductance (gm) and large shift in the threshold
most preferred devices for high-power and high frequency applica- voltage (Vth). The dielectric with high permittivity (high k) can
tions, due to their suitable material properties such as high break- effectively alleviate these problems.
down voltage, high saturation velocity, low effective mass, high All these devices suffered from the high contact resistance of
thermal conductivity and high two-dimensional electron gas (2DEG) >0.3 U mm and the high on-resistance of >1 U mm due to the
density of the order of 1013 cm2 at the hetero interface [1e3]. alloyed ohmic contacts and the large source-drain distance.
However, Schottky gate transistors usually exhibit a high gate Recently, the heavily doped n þ GaN source/drain ohmic contacts
leakage current [4], and a drain current collapse when operating at allowed a significant reduction of the contact resistivity in the
high frequencies. These are the major factors that limit the perfor- proposed device [16,17]. The T-gate structure reduces the gate ac-
mance and reliability of HEMT in radio frequency (RF) power cess resistance by providing a large gate area while maintaining the
applications. smaller gate length and reduces the extrinsic gate capacitance [18].
Metal oxide semiconductor HEMTs (MOS-HEMTs) with an Also, most of the developed AlGaN/GaN based HEMTs [19] and
insulating dielectric is widely investigated, and excellent perfor- MOS-HEMTs [17] are the depletion type due to their unique ma-
mance is demonstrated utilizing Al2O3 [4,6], TiO2 [7e9], HfO2 terial properties leading to spontaneous and piezoelectric polari-
[10,11], Pr2O3 [12,13], SiN [14], SiO2 [14] and NiO [15] as the gate 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
* Corresponding author. Laboratory of Semiconducting and Metallic Materials,
University of Mohamed Khider Biskra, Algeria simpler circuit design and low power consumption due to the
E-mail addresses: zinouu113@yahoo.fr (T. Zine-eddine), hamaiziaz@gmail.com elimination of negative power supply [17] which is suitable for the
(H. Zahra), messaimr@yahoo.fr (M. Zitouni). radio frequency integrated circuit (RFIC) design. In this paper, we
Peer review under responsibility of Vietnam National University, Hanoi.
https://doi.org/10.1016/j.jsamd.2019.01.001
2468-2179/© 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
(http://creativecommons.org/licenses/by/4.0/).
- T. Zine-eddine et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 180e187 181
propose a novel enhancement-mode GaN MOS-HEMT with 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
The 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 Fig. 1. Cross-section structure of the proposed GaN MOS-HEMT.
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.
Fig. 2. Interface charges and interface traps in GaN MOS-HEMT.
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
P ðAl Ga NÞ þ PSP ðAlx Ga1x NÞ
source/drain regrowth. A 3-inch 4H-SiC is used as a substrate to jsðxÞj ¼ PE x 1x
(1)
PSP ðGaNÞ
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 að0Þ aðxÞ C13 ðxÞ
2 e ðxÞ þ e ðxÞ
13 33
C33 ðxÞ
minimize the leakage. Looking at the structure from bottom to top, jsðxÞj ¼ aðxÞ (2)
an AlN nucleation layer is inserted to reduce the stress and the
þP ðxÞ P ð0Þ
SP SP
lattice mismatch. The undoped GaN channel is 800 nm thick. Doped
with 2.5 1018 cm3 donors, the Al0.3Ga0.7N of 20 nm thickness
where a(x) is lattice constant:
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 aðxÞ ¼ ð0:077x þ 3:189Þ1010 (3)
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 að0Þ ¼ aGaN (4)
source and drain to reduce the access and contact resistances [16].
Non-alloyed contacts are formed for the source/drain regions, and c13, c33 are the elastic constants, e33 and e31 are the piezo-
which have been shown to give a low contact resistance. electric constants given as follows:
In a real device, charges exist in all the three interfaces as shown
in Fig. 2. In the simulation, the polarization charge densities were c13 ðxÞ ¼ ð5x þ 103Þ (5)
modelled as fixed interface charge densities. The spontaneous and
piezoelectric polarization charges of AlGaN and GaN layers were
c33 ðxÞ ¼ ð32x þ 405Þ (6)
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 e13 ðxÞ ¼ ð0:11x 0:49Þ (7)
is full of dislocations and traps [27]. A donor concentration of
8.7 1012 cm2 at the TiO2/GaN interface is considered.
e33 ðxÞ ¼ ð0:73x þ 0:73Þ (8)
The total amount of the polarization induced sheet charge
density for an undoped AlxGa1-xN/heterostructure can then be The spontaneous polarization of AlxGa1-xN is also a function of
calculated by using the following equations: the Al mole fraction x and is given by:
Table 1
High-k dielectric materials and their properties [21]. TiO2 is the material choice in this research.
Gate dielectric Material Dielectric constant (k) Energy bandgap Eg (eV) Conduction band offset DEc (eV) Valence band offset DEc (eV)
SiO2 3.9 9 3.5 4.4
Al2O3 8 8.8 3 4.7
TiO2 80 3.5 1.1 1.3
ZrO2 25 5.8 1.4 3.3
HfO2 25 5.8 1.4 3.3
Bold represents TiO2 is the material choice in this research.
- 182 T. Zine-eddine et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 180e187
We consider: Eg (A1N) ¼ 6.08 eV, Eg (GaN) ¼ 3.55eV [31] and the
PSP ðxÞ ¼ ð 0:052x 0:029Þ (9) 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 0:7
2.3. Physical models ¼ (18)
DEv 0:3
Simulations were performed using Two dimensional (2D) sim- The electron affinity as a function of composition fraction x is
ulations of Silvaco ATLAS TCAD tool. The Boltzmann transport expressed as:
theory has shown that the current densities in the continuity
equations may be approximated by a drift-diffusion model (DD). cðAlGaNÞ ¼ cðGaNÞ 1:89x þ 0:91xð1 xÞ (19)
This model is one of the most basic carrier transport model in The permittivity of the nitrides as a function of composition
semiconductor physics. In this case, the current densities for elec- fraction x is given by [25]:
trons and holes under the DD model are expressed by the
equations: εðAlxGa1x NÞ ¼ 8:5x þ 8:9ð1 xÞ (20)
! The nitride density of states masses as a function of composition
J n ¼ nqmn V∅n (10)
fraction, x, is given by linear interpolations of the values for the
binary compounds [30]:
!
J p ¼ nqmp V∅p (11)
me ðAlxGa1x NÞ ¼ 0:314x þ 0:2ð1 xÞ (21)
where n and p are electron and hole concentrations respectively, mn
and mp are the electron and hole mobility respectively, Fn and Fp mh ðAlxGa1x NÞ ¼ 0:417x þ 1:0ð1 xÞ (22)
are the electron and hole quasi-fermi potentials, respectively.
The recombination rate is given by the following expression
The Poisson equation (12), the electron continuity equation (13)
[34,35]:
and the hole continuity equation (14), based on DD model, are
numerically solved [28]. A drift-diffusion model is used to solve the
n:p n2i
transport equation. USRH ¼ h i h i (23)
Etrap E
tp n þ ni exp KT þ tn p þ ni exp KTtrap
divðεVJÞ ¼ r
L L
(12)
where Etrap is the difference between the trap energy level and the
whereε is the permittivity, Jis the electrostatic potential and r is
intrinsic Fermi level, TL is the lattice temperature andtn, tpare the
the space charge density.
electron and hole lifetimes.
The low-field mobility is modeled by an expression similar to
dn 1 !
¼ V J n þ Gn R n (13) that proposed by CaugheyThomas [36]:
dx q
T b2
T b1 ðmmax mmin Þ 300
dp 1 ! m0 ðT; NÞ ¼ mmin þ (24)
¼ VJn þ Gp Rp (14) 300 h T b3 iað300
T b4
Þ
dx q 1 þ Nref 300
The continuity equations for electrons and holes are defined by
! !
equations (13) and (14), respectively, J n and J p are the current where T is the temperature, Nref is the total doping density, and a,
densities for electrons and holes, Gn and Gp are the electron and b1, b2, b3, b4, mmin and mmax are parameters that are determined from
hole generation rates, Rn and Rp are the electron and hole recom- Monte Carlo simulation [36].
bination rates, respectively, q is the magnitude of electron charge Another model used for high field mobility, it is based on an
[29]. adjustment to the Monte Carlo data for bulk nitride, which is
The basic band parameters for defining heterojunctions in Blaze described by the following equation [36]:
(one of the TCAD modules) are the bandgap parameter, the electron
n 1
affinity, the permittivity and the conduction and valence band m0 ðT; NÞ þ ysat E 1
n E n1
density of states [29]. mn ðEÞ ¼ n2 cn1 (25)
Generally, the bandgap for nitrides is calculated in a two-step 1þa E
Ec þ E
Ec
process: First, the bandgap of the relevant binary compounds is
computed as a function of temperature (T) using [30]: The parameters used in the simulation are shown in Table 2.
0:909 103 T 2
Eg ðGaNÞ ¼ 3:507 (15) 3. Simulation results and discussion
T þ 830
3.1. Energy band diagram of MOS-HEMT
1:799 103 T 2
Eg ðAlNÞ ¼ 6:23 (16)
T þ 1462 Fig. 3 illustrates the conduction bands in the E-mode GaN
Then, the band-gap energy dependence of the AlxGa1-xN ternary MOSHEMT under the gate electrode at zero gate bias. This band
on the composition fraction x using Vegard Law is described, where diagram is used to explain the 2DEG channel formation in the GaN
b is the bowing parameter: MOS-HEMT. The discontinuity in the bandgap, between the AlGaN
and GaN gives rise to a band bending process at the interface. The
Eg ðAlx Ga1x NÞ ¼ xEg ðAlNÞ þ ð1 xÞEg ðGaNÞ bxð1 xÞ (17) band bending is in such a way that the conduction band of the GaN
falls below the Fermi level (Ef) and forms a well at the interface
- T. Zine-eddine et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 180e187 183
Table 2 sheet charge, which can be controlled by varying the alloy
Electrical and thermal parameters used in this work at 300 K [29,37]. composition in the AlGaN layer. Equation (26) also shows that the
Material GaN AlGaN AlN SiC-4H sheet carrier concentration can be increased if the AlGaN layer
Band Parameters
thickness is reduced and/or the Schottky barrier height is increased
Epsilon 9.5 9.55 8.5 9.7 [25]. The following approximations can be used in equation (26) to
Eg (eV) 3.55 3.87 6.08 3.23 calculate the sheet carrier concentration of the 2DEG at the AlGaN/
Chi (eV) 3.05 2.69 1.01 3.2 GaN interface with varying Al mole composition in the AlGaN layer
Nc(per cc) 1.07e18 2.07e18 2.07e18 1.66e19
(x) [26].
Nv(percc) 1.16e19 1.16e19 1.16e19 3.3e19
Effective Richardson Constants Dielectric Constant:
An** 14.7 22.8 22.8 91.3
Ap** 71.8 71.8 71.8 144 εðxÞ ¼ 0:5x þ 9:5 (27)
Thermal Velocities
vn (cm/s) 3.34e7 2.68e7 2.68e7 1.34e7 Schottky Barrier:
vp (cm/s) 1.51e7 1.51e7 1.51e7 1.07e7
Saturation Velocities e4b ¼ ð1:3x þ 0:84Þ (28)
vsatn (cm/s) 1.9e7 1.1e7 1.4e7 2.2e7
vsatp (cm/s) 6.44e6 6.01e6 6.01e6 1e7 Fermi Energy:
Mobility parameters
me (cm2/V.s) 1350 985.5 1280 460
mh (cm2/V.s) 13 13.3 14 124 ph2
EF ðxÞ ¼ E0 ðxÞ þ ns ðxÞ (29)
mðxÞ
whereE0 ðxÞis the ground state sub band level of the 2DEG, which is
given by:
( )2=3
9phe2 ns ðxÞ
E0 ðxÞ ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi (30)
8ε0 8mðxÞεðxÞ
where the effective electron mass, ðxÞx0:22me .
Band Offset:
-
DEC ¼ 0:7 Eg ðxÞ Eg ð0Þ (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
Fig. 3. Energy band of GaN MOS-HEMT under the gate electrode.
the simulation can be accepted to agree reasonably well with the
experimental values [25,41].
[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 3.2. DC results
GaN and AlGaN allows a large 2DEG concentration to be formed in
the device. The electron scattering associated with the impurities is The IDS-VDS curves of Fig. 4 allowed the evaluation of MOS-
less in this region because of the absence of doping in the GaN HEMT characteristics such as the knee voltage (transition be-
channel [39]. tween the linear and saturation region), the on-resistance, the
The sheet electron concentration can be calculated using [40]: maximum current and self-heating.
sðxÞ ε0 εðxÞ
nðsÞ ðxÞ ¼ 2
½e4b ðxÞ þ EF ðxÞ DEC ðxÞ (26) 2,5 VGS=3V
e dAlGaN e
The meaning of parameters used in this equation is described 2,0
Drain current (A/mm)
and listed in Table 3. It is understood that the sheet carrier con-
centration is mainly controlled by the total polarization induced
1,5 VGS=2V
Table 3
1,0
Parameters of equation (26) [25].
Parameters Definition VGS=1V
0,5
εðxÞ Relative Dielectric Constant of AlxGa1-xN
dAlGaN Thickness of AlGaN layer VGS=0V
fb ðxÞ Schottky Barrier Height of gate contact on top of AlGaN 0,0 VGS=-1V
EF ðxÞ Fermi level w.r.t the conduction band energy level 0 2 4 6 8
Drain voltage (V)
DEC ðxÞ Conduction band offset at the AlGaN/GaN interface
e Electronic charge
Fig. 4. IDS-VDS characteristics of the simulated GaN MOS- HEMT.
- 184 T. Zine-eddine et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 180e187
As can be seen in Fig. 4, for IDS-VDS characteristics, the gate curves at fixed VDS and is expressed in Siemens. The peak extrinsic
voltage varied from 1 V to 3 V and drain voltage varied from 0 V to transconductance was ~1438 mS/mm.
6 V. The device exhibited a peak current density of ~1.5 A/mm at Fig. 6 illustrates the transconductance verses gate length char-
VGS ¼ 2 V and 2.5 A/mm at VGS ¼ 3 V. acteristics of the GaN MOS-HEMTs. It reduces the transconductance
The MOS-HEMT is pinched-off completely at VGS ¼ 1V. In Fig. 5 from 1430 mS/mm to 1258 mS/mm with the gate length change
(a) the threshold voltage VTH is about 1.07 V. The transconductance from 10 nm to 60 nm.
gm shown in Fig. 5 (b) is calculated from the derivative of IDS-VGS 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
4,0
performance is achieved in our work which is highly desirable not
VDS=5V only for high power applications but for high frequency
3,5
VDS=3.5V applications.
VDS=2.5V
(a)
Drain current (A/mm)
3,0
3.3. Gate leakage performance
2,5
Fig. 8 shows a comparison of the gate leakage performance of
2,0 the HEMTs and E-mode GaN MOS-HEMTs with the same device
dimensions. The leakage current of MOS-HEMTs is found to be
1,5
significantly lower than that of the Schottky gate HEMTs. The gate
1,0
leakage current density of MOS-HEMTs is almost 3e5 orders of
magnitude lower than that of the HEMTs. Such a low gate leakage
0,5 current should be attributed to the large band offsets in the TiO2/
HEMT and a good quality of both the reactive-sputtered TiO2
0,0 dielectric. This leads to an increase of the two-terminal reverse
-1 0 1 2 3 4 breakdown voltage (about 25%) and of the forward breakdown
Gate Voltage(V)
1,6 (b) 1800
VDS=5V
1,4 [44]
VDS=3.5V 1600
Transconductance (S/mm)
This work
Transconductance (mS/mm)
1,2 VDS=2.5V 1400
[46]
1,0 1200
0,8 1000
0,6 800
[42]
[45]
0,4 600
[43]
0,2 400
0 20 40 60 80 100
0,0 Gate length (nm)
-1 0 1 2 3 4
Gate voltage (V)
Fig. 7. Comparison of extrinsic peak gm VS Lg with the state-of-the-art results re-
Fig. 5. (a) Transfer characteristic, (b) transconductance at VDS ¼ 2.5 V, 3.5 V and5 V. ported for GaN-HEMT technology [42e46].
1
VDS=5V 0,1 Without TiO2
1400
0,01 With TiO2
Transconductance (mS/mm)
1E-3
Gate current (A/mm)
1300 1E-4
1E-5
1E-6
1200
1E-7
1E-8
1E-9
1100
1E-10
1E-11
1000 1E-12
10 20 30 40 50 60 -8 -6 -4 -2 0 2 4 6 8
Gate lenth (nm) Gate voltage (V)
Fig. 6. Transconductance with respect of gate length of GaN MOS-HEMT. Fig. 8. Gate leakage currents for the E-mode GaN HEMT and MOS-HEMT.
- T. Zine-eddine et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 180e187 185
voltage (about 30%). This confirms that the TiO2 dielectric thin film The small-signal parameters (Table 4) are extracted at the bias of
acts as an efficient gate insulator. the maximum ft. A higher intrinsic transconductance and lower
gate parasitic capacitance and resistance are expected to lead to
higher RF performance. Table 4 compares the small-signal equiv-
3.4. Microwave results alent circuit parameters between this work and other experimental
works. These results shown that the proposed E-mode GaN MOS-
The high frequency performance of microwave devices can be HEMT is a promising device for future high speed and high-
evaluated through S-parameters simulations. The simulations of power millimeter wave RF applications.
this type are referred to as small signal due to the relatively small The relationship between the MOS-HEMT gate length and the
input signal level used for characterization. There are several useful frequency is shown in Fig. 10. It can be seen that ft and fmax increase
pieces of information that can be extracted about the device char- steadily with the decrease of gate length Lg. The gate source
acteristics from S-parameters simulations. capacitance and gate-drain capacitance decrease steadily with the
Cut off frequency fT and maximum oscillation frequency fmax decrease of the gate length. We can see that the decrease of gate
represent important figures of merit concerning the frequency source capacitance Cgs and gate drain capacitance Cgd, ft and fmax
limits of the device. fT is defined as the frequency where forward will increase steadily from equations (36) and (37). Therefore, we
current gain (H21) from hybrid parameters becomes unity and fmax should decrease gate length under permission of technology when
is defined as the frequency where unilateral gain (Ug) or maximum designing E-mode GaN MOS-HEMT.
stable gain (MSG) becomes unity [4]. The gains h21, Ug and MSG
were extracted directly from simulated S-parameters by the gm
ft ¼ (36)
following equations. 2pðCgs þ CgdÞ
2 S21
H21 ¼ (32) ft
ð1 S11 Þ ð2 S22 Þ S12 S21 fmax ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi (37)
2 ðRi þ Rs þ RdÞgds þ ð2pFtÞRgCgd
jS21 j2 The comparison of our simulation result with various experi-
Ug ¼ (33) mental and simulation results for different gate lengths is depicted
1 jS11 j2 1 jS22 j2
in Fig. 11. GaN MOS-HEMT in [50]exhibited an ft of 405 GHz but the
obtained power gain cut-off frequency is 200 GHz only. In this work
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi the proposed E-mode GaN MOS-HEMT shows a ft/fmax ¼ 524/
jS21 j2
MSG ¼ 2
K± K2 1 (34) 758 GHz. These high cut-off frequencies with improved drain cur-
jS12 j rent density and record transconductance (gm) show that the
with
Table 4
1 jS11 j2 jS22 j2 þ jS11 S22 S12 S21 j2 Small-signal equivalent circuit model parameters.
K¼ (35)
2 jS12 j2 jS22 j2 This work [46] [48] [49]
Gate length (nm) 10 20 20 80
where K is the stability factor. gm (mS/mm) 1430 1252 1620 620
Fig. 9 displays the small signal characteristics of the same MOS- gd (S/mm) 0.385 0.245 0.149 60
HEMT device with a bias voltage VGS ¼ 1.25 V and VDS ¼ 6 V. ft and Cgs (fF/mm) 317 312 551 810
Cgd(fF/mm) 121 107 106 361
fmax can be determined based on this graph; fT is the frequency Ri (U.mm) 0.13 0.04 0.04 0.8
value where h21 becomes 0 dB and fmax is the frequency where Ug Rg (U.mm) 0.33 0.37 0.36 e
or MSG becomes 0 dB [47]. fT and fmax were determined to be Rs (U.mm) 0.04 0.05 0.11 0.8
524 GHz and a record of maximum oscillation frequency (fmax) of Rd (U.mm) 0.14 0.12 0.18 1.0
Ft (Ghz) 522 453 354 60
758 GHz.
Fmax (Ghz) 750 487 501 127
50
H21 800
Ug Fmax
40 700
Ft
600
30
Gains (dB)
Ft/Fmax (Ghz)
500
20
400
Fmax=758 Ghz 300
10
Ft=524 Ghz
200
0
1E9 1E10 1E11 1E12 100
Frequency (Hz) 10 20 30 40 50 60
Gate length (nm)
Fig. 9. Small signal characteristics for GaN MOS-HEMT at the bias point VGS ¼ 1.35 V
and VDS ¼ 6 V. Fig. 10. The relationship between GaN MOS-HEMT gate length and Ft/Fmax.
- 186 T. Zine-eddine et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 180e187
800 same in case for VGS ¼ 3V biasing. These results show the potential
for GaNMOS-HEMT to produce millimeter wavelength power.
This work Fmax
700
Ft
600 4. Conclusion
Ft/Fmax (Ghz)
500 The objective of this paper was to design and simulate a new E-
[48] mode GaN MOS-HEMT with 10 nm gate-length and with a high-k
400 TiO2 gate dielectric and regrown source/drain. The very encour-
[45] [50] aging results were obtained compared to other works. The high
300 cut-off of 524 GHz and with a record of maximum oscillation fre-
[44] quencies of 758 GHz were achieved. This is the best E-mode GaN
[42] [50] MOS-HEMT high-frequency performance reported to date. More-
200
over, the present MOS-HEMT design is superior to other lately
100 published GaN TiO2-dielectric MOS-devices. It is suitable for high-
0 20 40 60 80 100 power RF circuit applications.
Gate length (nm)
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