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  3. 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)

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)

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. 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/js

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  1. 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/).
  2. 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.
  3. 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
  4. 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:
  5. 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.
  6. 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.
  7. 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.
  8. 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) Fig. 11. Comparison of extrinsic peak ft/fmax vs Lg with the state-of-the-art results References reported for GaN-HEMT technology [42,44,45,48,50]. [1] K. Jena, R. Swain, T. Lenka, Impact of oxide thickness on gate capacitanceeModelling and comparative analysis of GaN-based MOSHEMTs, proposed GaN MOS-HEMT is a promising device for future high Pramana 85 (2015) 1221e1232. [2] S.-i. Takagi, A. Toriumi, Quantitative understanding of inversion-layer capac- speed and high-power millimeter wave RF applications. itance in Si MOSFET's, IEEE Trans. Electron. Dev. 42 (1995) 2125e2130. The Power performance of the GaN MOS-HEMTs were charac- [3] F.M. Yigletu, S. Khandelwal, T.A. Fjeldly, B. In ~ iguez, Compact charge-based terized at 10 GHz. Fig. 12 presents the typical output power and physical models for current and capacitances in AlGaN/GaN HEMTs, IEEE Trans. Electron. Dev. 60 (2013) 3746e3752. Power Added Efficiency (PAE) results of the device. [4] Z. e. Touati, Z. Hamaizia, Z. 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