Design and analysis of 10 nm t gate enhancement mode mos hemt for high power microwave applications

Introduction GaN-based high electron mobility transistors HEMTs are the most preferred devices for high-power and high frequency applica-tions, due to their suitable material properties

Original Article

Design and analysis of 10 nm T-gate enhancement-mode MOS-HEMT

for high power microwave applications

Touati Zine-eddinea,*, Hamaizia Zahraa, Messai Zitounib,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

Article history:

Received 17 December 2018

Received in revised form

30 December 2018

Accepted 2 January 2019

Available online 7 January 2019

Keywords:

Enhancement-mode

MOS-HEMT

High-k

TiO 2

Regrown source/drain

TCAD

a b s t r a c t

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 TiO2gate 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 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 VGS¼ 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 (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

GaN-based high electron mobility transistors (HEMTs) are the

most preferred devices for high-power and high frequency

applica-tions, due to their suitable material properties such as high

break-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

insulating dielectric is widely investigated, and excellent

perfor-mance is demonstrated utilizing Al2O3 [4,6], TiO2 [7e9], HfO2

[10,11], Pr2O3[12,13], SiN[14], SiO2[14]and NiO[15]as the gate

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 threshold voltage (Vth) The dielectric with high permittivity (high k) can effectively alleviate these problems

All these devices suffered from the high contact resistance of

>0.3 Umm and the high on-resistance of >1U mm due to the alloyed ohmic contacts and the large source-drain distance Recently, the heavily doped nþ GaN source/drain ohmic contacts allowed a significant reduction of the contact resistivity in the proposed device[16,17] The T-gate structure reduces the gate ac-cess resistance by providing a large gate area while maintaining the smaller gate length and reduces the extrinsic gate capacitance[18] Also, most of the developed AlGaN/GaN based HEMTs[19]and MOS-HEMTs[17]are the depletion type due to their unique ma-terial properties leading to spontaneous and piezoelectric polari-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).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

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

Journal of Science: Advanced Materials and Devices 4 (2019) 180e187

propose a novel enhancement-mode GaN MOS-HEMT with a 10 nm

T-gate length and a high-k TiO2gate 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 TiO2is our choice of the high-k dielectric gate material The

other high-k materials are shown inTable 1with their properties

[21] Among the gate dielectric materials, TiO2is considered as the

most suitable candidate because of its large static dielectric

con-stant (k¼ 80e170) TiO2can 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 TiO2as 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 1shows 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

lattice mismatch The undoped GaN channel is 800 nm thick Doped

with 2.5 1018cm3donors, 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 Iofand a 5-nm GaN cap layer Next, two graded nþ 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,

which have been shown to give a low contact resistance

In a real device, charges exist in all the three interfaces as shown

inFig 2 In the simulation, the polarization charge densities were

modelled asfixed 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 inFig 2 Also, the TiO2/GaN interface

is full of dislocations and traps [27] A donor concentration of

8.7 1012cm2at 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:

jsðxÞj ¼

PPEðAlxGa1xNÞ þ PSPðAlxGa1xNÞ

PSPðGaNÞ

jsðxÞj ¼

2að0Þ  aðxÞ aðxÞ



e13ðxÞ þ e33ðxÞC13ðxÞ

C33ðxÞ

þPSPðxÞ  PSPð0Þ

where a(x) is lattice constant:

and c13, c33are the elastic constants, e33and e31are the piezo-electric constants given as follows:

The spontaneous polarization of AlxGa1-xN is also a function of the Al mole fraction x and is given by:

Table 1

High-k dielectric materials and their properties [21] TiO 2 is the material choice in this research.

Gate dielectric Material Dielectric constant (k) Energy bandgap Eg (eV) Conduction band offsetDEc (eV) Valence band offsetDEc (eV)

Fig 1 Cross-section structure of the proposed GaN MOS-HEMT.

Fig 2 Interface charges and interface traps in GaN MOS-HEMT.

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

!

J

!

where n and p are electron and hole concentrations respectively,mn

andmpare the electron and hole mobility respectively,FnandFp

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

whereε is the permittivity,Jis the electrostatic potential andris

the space charge density

dn

dx¼1

dp

dx¼1

qVJ!n

The continuity equations for electrons and holes are defined by

equations(13) and (14), respectively, J!

n and J!

pare the current densities for electrons and holes, Gnand Gpare the electron and

hole generation rates, Rnand Rpare 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 TCAD modules) 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  103T2

EgðAlNÞ ¼ 6:23 1:799  103T2

Then, 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:

EgðAlxGa1xNÞ ¼ xEgðAlNÞ þ ð1  xÞEgðGaNÞ  bxð1  xÞ (17)

We consider: Eg(A1N)¼ 6.08 eV, Eg(GaN)¼ 3.55eV[31]and the bowing parameter b¼ 1.3 eV[32]at 300K

The electron affinity is calculated such that the band edge offset ratio is given by[33]:

DEc

The electron affinity as a function of composition fraction x is expressed as:

cðAlGaNÞ ¼cðGaNÞ  1:89x þ 0:91xð1  xÞ (19)

The permittivity of the nitrides as a function of composition fraction x is given by[25]:

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]:

The recombination rate is given by the following expression

[34,35]:

tp



nþ niexph

E trap

KT L

i

þtn



pþ niexph

E trap

KT L

where Etrapis the difference between the trap energy level and the intrinsic Fermi level, TLis 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 ¼mmin

 T 300

b1

þ ðmmaxmminÞ T

300

b2

1þhNrefT

300

b3iaðT

300Þb 4 (24)

where T is the temperature, Nrefis the total doping density, anda,

b1,b2,b3,b4,mminandmmaxare parameters that are determined from Monte Carlo simulation[36]

Another model used for highfield 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 þysat

n EEn11n1 c

1þaE

E c

n2

þE

E c

The parameters used in the simulation are shown inTable 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 GaN falls below the Fermi level (E) and forms a well at the interface

T Zine-eddine et al / Journal of Science: Advanced Materials and Devices 4 (2019) 180e187 182

[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 polarizationfields 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 ¼sðxÞ



ε0εðxÞ

dAlGaNe2



½e4bðxÞ þ EFðxÞ DECðxÞ (26)

The meaning of parameters used in this equation is described

and listed inTable 3 It is understood that the sheet carrier

con-centration is mainly controlled by the total polarization induced

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:

Schottky Barrier:

Fermi Energy:

EFðxÞ ¼ E0ðxÞ þph2

whereE0ðxÞis the ground state sub band level of the 2DEG, which is given by:

E0ðxÞ ¼

(

9phe2nsðxÞ 8ε0

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 8mðxÞεðxÞ p

)2=3

(30)

where the effective electron mass,ðxÞx0:22me Band Offset:

From the simulation, the 2DEG density at the AlGaN/GaN interface is 9.21 1012cm2 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

Table 2

Electrical and thermal parameters used in this work at 300 K [29,37]

Band Parameters

Effective Richardson Constants

Thermal Velocities

Saturation Velocities

Mobility parameters

Fig 3 Energy band of GaN MOS-HEMT under the gate electrode.

0,0 0,5 1,0 1,5 2,0 2,5

VGS=-1V

VGS=0V

VGS=1V

VGS=2V

Drain voltage (V)

VGS=3V

Table 3

Parameters of equation (26) [25]

Parameters Definition

εðxÞ Relative Dielectric Constant of Al x Ga 1-x N

d AlGaN Thickness of AlGaN layer

f b ðxÞ Schottky Barrier Height of gate contact on top of AlGaN

E F ðxÞ Fermi level w.r.t the conduction band energy level

DE C ðxÞ Conduction band offset at the AlGaN/GaN interface

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

VGS¼ 2 V and 2.5 A/mm at VGS¼ 3 V

The MOS-HEMT is pinched-off completely at VGS¼ 1V InFig 5

(a)the threshold voltage VTHis about 1.07 V The transconductance

gmshown inFig 5 (b)is calculated from the derivative of IDS-VGS

curves atfixed VDSand is expressed in Siemens The peak extrinsic transconductance was ~1438 mS/mm

Fig 6illustrates the transconductance verses gate length char-acteristics of the GaN MOS-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 7displays the reference of gmversus Lgof 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 8shows 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

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

Gate Voltage(V)

VDS=5V

VDS=3.5V

VDS=2.5V (a)

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

Gate voltage (V)

VDS=5V

VDS=3.5V

VDS=2.5V

(b)

Fig 5 (a) Transfer characteristic, (b) transconductance at V DS ¼ 2.5 V, 3.5 V and5 V.

1000

1100

1200

1300

1400

Gate lenth (nm)

V DS =5V

400 600 800 1000 1200 1400 1600 1800

[46]

[44]

[43]

Gate length (nm)

) This work

Fig 7 Comparison of extrinsic peak gm VS Lg with the state-of-the-art results re-ported for GaN-HEMT technology [42e46]

1E-12 1E-11 1E-10 1E-9 1E-8 1E-7 1E-6 1E-5 1E-4 1E-3 0,01 0,1 1

Gate voltage (V)

Without TiO2 With TiO2

T Zine-eddine et al / Journal of Science: Advanced Materials and Devices 4 (2019) 180e187 184

voltage (about 30%) This confirms that the TiO2dielectric thinfilm

acts as an efficient gate insulator

3.4 Microwave results

The high frequency performance of microwave devices can be

evaluated through S-parameters simulations The simulations of

this type are referred to as small signal due to the relatively small

input signal level used for characterization There are several useful

pieces of information that can be extracted about the device

char-acteristics from S-parameters simulations

Cut off frequency fTand maximum oscillation frequency fmax

represent important figures of merit concerning the frequency

limits of the device fTis defined as the frequency where forward

current gain (H21) from hybrid parameters becomes unity and fmax

is defined as the frequency where unilateral gain (Ug) or maximum

stable gain (MSG) becomes unity[4] The gains h21, Ugand MSG

were extracted directly from simulated S-parameters by the

following equations

H21¼

ð1  S11Þ  ð2  S22Þ  S12 S21



1 jS11j2

MSG¼jS21j2

jS12j2



K±qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiK2 1



(34)

with

K¼1 jS11j2 jS22j2þ jS11S22 S12S21j2

where K is the stability factor

Fig 9displays the small signal characteristics of the same

MOS-HEMT device with a bias voltage VGS¼ 1.25 V and VDS¼ 6 V ftand

fmaxcan be determined based on this graph; fTis the frequency

value where h21becomes 0 dB and fmax is the frequency where Ug

or MSG becomes 0 dB [47] fTand fmax were determined to be

524 GHz and a record of maximum oscillation frequency (fmax) of

758 GHz

The small-signal parameters (Table 4) are extracted at the bias of the maximum ft A higher intrinsic transconductance and lower gate parasitic capacitance and resistance are expected to lead to higher RF performance.Table 4compares the small-signal equiv-alent circuit parameters between this work and other experimental works These results shown that the proposed E-mode GaN MOS-HEMT is a promising device for future high speed and high-power millimeter wave RF applications

The relationship between the MOS-HEMT gate length and the frequency is shown inFig 10 It can be seen that ftand fmaxincrease steadily with the decrease of gate length Lg The gate source capacitance and gate-drain capacitance decrease steadily with the decrease of the gate length We can see that the decrease of gate source capacitance Cgsand gate drain capacitance Cgd, ftand fmax will increase steadily from equations(36) and (37) Therefore, we should decrease gate length under permission of technology when designing E-mode GaN MOS-HEMT

2 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðRi þ Rs þ RdÞgds þ ð2pFtÞRgCgd

The comparison of our simulation result with various experi-mental and simulation results for different gate lengths is depicted

inFig 11 GaN MOS-HEMT in[50]exhibited an ftof 405 GHz but the obtained power gain cut-off frequency is 200 GHz only In this work the proposed E-mode GaN MOS-HEMT shows a ft/fmax ¼ 524/

758 GHz These high cut-off frequencies with improved drain cur-rent density and record transconductance (gm) show that the

0

10

20

30

40

50

Fmax=758 Ghz

Frequency (Hz)

H21

Ug

Ft=524 Ghz

Fig 9 Small signal characteristics for GaN MOS-HEMT at the bias point V GS ¼ 1.35 V

¼ 6 V.

Table 4 Small-signal equivalent circuit model parameters.

100 200 300 400 500 600 700 800

Gate length (nm)

Ft /Fma

Fmax

Ft

proposed GaN MOS-HEMT is a promising device for future high

speed and high-power millimeter wave RF applications

The Power performance of the GaN MOS-HEMTs were

charac-terized at 10 GHz.Fig 12presents the typical output power and

Power Added Efficiency (PAE) results of the device

Table 5lists the power characteristics of the simulated GaN

MOS-HEMT for various bias conditions Biasing at VGS¼ 2 V and 3 V can be

classified as class A and AB operation At the bias of VGS¼ 2 V &

VDS¼ 10 V (class AB), a linear gain of 23.3 dB, maximum output

power of 29,4dBm (882 mW/mm) and maximum PAE of 42.7% were

obtained With higher VGS¼ 3 V and VDS¼ 10V (class A), higher

linear gain of 24.2 dB, higher maximum output power of 29.6 dBm

(921 mW/mm) and lower maximum PAE of 41.2% were achieved At

VGS¼ 2V, the maximum output power increased (from 882 mW/mm

to 909 mW/mm) with increased VDS(from 10V to 15V), which is the

same in case for VGS¼ 3V biasing These results show the potential for GaNMOS-HEMT to produce millimeter wavelength power

4 Conclusion The objective of this paper was to design and simulate a new E-mode GaN MOS-HEMT with 10 nm gate-length and with a high-k TiO2 gate dielectric and regrown source/drain The very encour-aging results were obtained compared to other works The high cut-off of 524 GHz and with a record of maximum oscillation fre-quencies of 758 GHz were achieved This is the best E-mode GaN MOS-HEMT high-frequency performance reported to date More-over, the present MOS-HEMT design is superior to other lately published GaN TiO2-dielectric MOS-devices It is suitable for high-power RF circuit applications

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100

200

300

400

500

600

700

800

[50]

[45]

[42] [50]

[44]

Ft

/Fmax

Gate length (nm)

Fmax

Ft

[48]

This work

Fig 11 Comparison of extrinsic peak f t /f max vs Lg with the state-of-the-art results

reported for GaN-HEMT technology [42,44,45,48,50]

0

5

10

15

20

25

30

35

Gain Pout PEA

Pin(dBm)

0 10 20 30 40 50

Fig 12 Powercharacteristics, (Pout, Gain and PAE) of the TiO 2 /AlGaN/GaN MOS-HEMT

at 10 GHz.

Table 5

Power characteristics under various bias conditions.

V GS V DS P out Density (mW/mm) Max PEA (%) Linear gain (dB)

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