Design and analysis of 10 nm T-gate enhancement-mode MOS-HEMT for high power microwave applications

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