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TCAD SIMULATION OF ALGAN/INALGAN/GAN HEMTS (HIGH ELECTRON MOBILITY TRANSISTORS) N. Ramkumar*1, Kotha S V Madhav*2, K. Karthik*3, S. Ruby*4, D. Mahesh*5 *1
Assistant Professor, Department of Electronics and Communication Engineering, Anil Neerukonda Institute of Technology & Sciences, Visakhapatnam, India. *2,3,4,5
Students, Department of Electronics and Communication Engineering, Anil Neerukonda Institute of Technology & Sciences, Visakhapatnam, India
ABSTRACT We report microwave and DC performance of a novel 50 nm Quaternary based AlGaN/InAlGaN/GaN (HEMTs) High Electron Mobility Transistor with Al2O3 passivation and T-gate on SiC substrate. TCAD is used for simulating the proposed HEMT structure. A peak drain current density(Ids) is shown at the regrown n++ GaN source/drain ohmic contacts of 2.9 A/mm with low on-resistance 0.49 Ω.mm. A record power gain (fmax) and current gain cut-off frequencies (ft) obtained are 425GHz and 310GHz respectively. These are achieved by substantial reduction in the device extrinsic and intrinsic parasitic capacitance and resistances. Here AlGaN is used as back barrier-structure to the 7nm thin In0.13Al0.83Ga0.04N (Quaternary barrier) layer in order to compensate the short channel effects with 38V improved breakdown voltage. For next generation, the prominent DC characteristic along with microwave characteristic of proposed HEMT device is appropriate candidate for electronic high power millimeter wave applications. Keywords: Quaternary barrier; double hetero-junction; millimeter wave; cut-off frequency; breakdown voltage
I.
INTRODUCTION
The expedient performance of GaN based HEMTs such as low on resistance, high breakdown field, high current density; high electron velocity, high power amplification and high thermal stability empowered the progress of high power and high speed millimeter wave electronics and photonic applications [1-34]. Over the past two decades, extensive research works has been carried out for significant improvements in operating frequency of the GaN-based HEMT. Conventional AlGaN/GaN HEMT with 0.25 µm gate length shown its microwave performance f t/fmax of 82/103 GHz [15]. T. Palacio et. al. fabricated 100 nm AlGaN/GaN HEMT with InGaN back-barrier and the device shown excellent ft/fmax of 153/230 GHz [16]. 150 nm recessed gate InAlN/GaN HEMT recorded f t/fmax of 70/105 GHz with 29 V breakdown voltage [17]. Dong Seup Lee et. al. reported f t/fmax of 245/13 GHz for 30 nm InAlN/GaN HEMT [18]. Fully passivtaed InAlN/GaN HEMT significantly improves the microwave performance f t/fmax of 205/220 GHz [19]. Jinwook W et. al. demonstrated high transconductance results from recessed gate InAlN/GaN with Al 2O3 passivation layer [20]. In spite of short channel effects, 30 nm gate length InAlN/GaN HEMT recorded ft/fmax of 373/28 GHz [21]. Lattice matched In0.17Al0.83N/GaN HEMTs demonstrated excellent high frequency performance than conventional AlGaN/GaN HEMTs [17-21]. However, due to interface roughness scattering, improvement in 2-D electron gas mobility in InAlN/GaN based HEMTs remains challenging [30] by immiscibility between AlN and InN. Existence of narrower immiscibility, Quaternary barrier In 0.16Al0.74Ga0.10N has been demonstrated high carrier mobility (μ> 1800 cm2/V · s) and high electron density (ns ~ 1.8 × 1013 cm−2) [22,23,24,25,29,32,33]. In recent years, the effort of nitride researchers are directed towards lattice matched In0.16Al0.74Ga0.10N/AlN/GaN heterostructures. To obtain high ft/fmax with simultaneous improvement in breakdown voltage for next generation high power millimeter wave electronics, it is necessary to optimize the device structure for low gate resistance, parasitic capacitances and minimum gate leakage current. In this research work, a novel 50 nm T-gate lattice matched quaternary barrier In0.16Al0.74Ga0.10N/GaN HEMT is studied and it’s DC and microwave characteristics are presented. Lg 50 nm InAlGaN/GaN HEMTs on SiC substrate is exhibited a record ft/fmax of 310/425 GHz with simultaneous high output current density of (I ds) of 2.9 A/mm and breakdown voltage of 48 V. AlGaN back barrier-structure along with a very thin 7nm InAlGaN barrier effectively mitigates the short channel effect (DIBL= 80 mV/V) with improved breakdown voltage of 48 V
II.
DEVICE STRUCTURE AND BANDGAP DIAGRAM
In0.16Al0.74Ga0.10N/AlN/GaN/AlGaN double heterostructures on SiC schematic diagram is displayed in Fig.1 (a). The proposed device made up of 7 nm In 0.16Al0.74Ga0.10N quaternary barrier material, 1 nm wide bandgap AlN spacer layer (6.02 eV), GaN channel and Al0.08Ga0.92N back-barrier. The drain and source regions are formed by Si doped n++ GaN www.irjmets.com
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(>1X1019cm-3), which has the direct contact with ohmic Ti/Al/Ni/Au metal stack for low source/drain contact resistances. 600nm is the distance between source and drain,400 nm is gate-drain separation (Lgd) and 150 nm is gatesource separation (Lgs). The region of gate is made by 50 nm foot print T - gate structure with 100 nm stem height for low gate resistance (Ni/Au metal stack is formed for Schottky contact). In order to ovoid current collapse at high V ds and low Cgs & Cgd (gate-source and gate-drain parasitic capacitances), 20 nm Si3N4 passivation layer is deposited over the device surface. Al0.08Ga0.92N back-barrier structure is used in order to enhancee the electron confinement in the channel as well as to reduce buffer leakage current . To avoid excess stress at the interface of channel and back-barrier, the Al mole fraction is less than 10% in the back-barrier. High frequency performance of the HEMT majorly affected by device output and access resistance parasitic capacitances and resistances. The major source of the parasitic resistances are source/drain contact resistances. The ohmic contact with n++ GaN source/drain arrangement result in significant reduction in parasitic resistances. Using T gate structure, the gate access resistance (Rg) is reduced by maintaining smaller gate length (Lg) with wide gate area. And also it reduces the extrinsic gate capacitances. The distance between gate to drain (Lgd) is greater than distance between the gate to source (L gs) for breakdown voltage improvement by maintaining low electrostatic field in the gate-drain space channel region. A 1 nm wide bandgap AlN spacer layer helps the device to confine more electrons in 2DEG and also mitigates the gate leakage current is effectively mitigated. For good thermal conductivity, SiC is used as substrate material in this work.
Figure 1: (a) In0.16Al0.74Ga0.10N/AlN/GaN/AlGaN heterostructure
(b) Bandgap diagram
The bandgap structure of In0.16Al0.74Ga0.10N/AlN/GaN with AlGaN back-barrier heterostructure is shown in Fig.1 (b). The In0.16Al0.74Ga0.10N barrier with Al0.08Ga0.92N back-barrier results in high electron density in the channel (1.81x10 13 cm-2) and high electron mobility (~1800 cm 2/V-s) associated with sheet resistance of 220 Ω/□. Low Al content AlGaN back barrier induces negative polarization at the interface of GaN/AlGaN channel rather than conduction band discontinuity, which act as high barrier for buffer leakage and results in high 2DEG density.
III.
RESULT AND DISCUSSION
Fig. 3.1. Displays the drain current characteristics of the HEMT. The V gs (gate-source voltage) varied from -4 V to +2 V and Vds (drain-source voltage) was swept from 0 to 8 V. The output current density I d,max of 2.9 A/mm is reached at Vgs=2 V and the extracted on-resistance (Ron) is 0.49 Ω.mm. Reduced roughness and alloy scattering and the lattice matched In0.16Al0.74Ga0.10N/GaN heterojuntion associated with a very thin spacer layer greatly helped the device to reach the output current density of 2.9 A/mm for 50 nm gate length devices. Fig.3.2. shows the proposed device’s three terminal breakdown characteristics. The transistor’s breakdown voltage (VBR) is a key factor for high power millimeter wave application. The asymmetric gate position along with back barrier structure for reducing the buffer leakage current contributed for this higher VBR in the proposed structure. Lg 50 nm HEMT recorded off-state breakdown voltage of 38 V. Transfer characteristics of the HEMT is shown in Fig.3.3. Threshold voltage (Vth) of -2.8 is extracted for Vds = 6 V. www.irjmets.com
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The transconductance variation with Vgs is displayed in Fig.3.4, the maximum transconductance (g m) of 0.9 S/mm is reached at Vds = 6 V and Vgs = -2 V. The obtained result is the highest gm for Lg 50 nm GaN based HEMT from the literature research. Fig. 3.5 shows the log-scale plot of leakage current characteristics of Lg 50 nm Quaternary barrier HEMT. FowlerNordheim (FN) tunnelling gate leakage current model is used for the proposed device and the gate leakage current (I g) is reached 1x10-9 at zero gate bias. The microwave performance of the HEMT is depicted in Fig. 3.6. The cut-off frequencies are extracted for different gate-source bias.Current gain cut-off frequency (ft) of 310 GHz is obtained with a record power gain cut-off frequency (fmax) of 425 GHz.The obtained results are best of GaN-based HEMT with simultaneous achievement of high output current density (2.9 A/mm) and high breakdown voltage (38 V) for 50 nm gate length device. The obtained results are the best outstanding balanced DC as well as microwave characteristics (f t & fmax) for 50 nm gate length among existing research work from the author’s knowledge. The higher cut-off frequencies are achieved by reduction the gate resistance (Rg), source/drain contact resistances (Rs and Rd), gate-source (Cgs) and gate to drain (Cgd) parasitic capacitances mainly due to the T-gate structure, SiN passivation layer, and heavily doped source/drain region.
Figure 3.1: Common-source I-V’s for gate length (50 nm) depletion mode HEMT.
Figure 3.2: Breakdown characteristics of gate length (50 nm) depletion mode HEMT. www.irjmets.com
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Figure 3.3: Transfer characteristics of gate length (50 nm) depletion mode HEMT.
Figure 3.4: Transconductance variation of gate length (50 nm) depletion mode HEMT.
Figure 3.5: Leakage current characteristics of gate length(50 nm) depletion mode HEMT. www.irjmets.com
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Figure 3.6: Microwave characteristics of gate length (50 nm) depletion mode HEMT. The performances of proposed Quaternary barrier GaN-HEMT comparison with various experimental results of GaNbased HEMT are presented in below graph. The proposed device shows the better performance than existing work.
IV.
CONCLUSION
A novel Quaternary barrier (InAlGaN) HEMT is designed and investigated its DC and microwave performance. T-gate structure with Lg 50 nm InAlGaN/GaN HEMTs on SiC substrate is exhibited a record ft/fmax of 310/425 GHz with simultaneous high output current density of (I ds) of 2.9 A/mm. AlGaN back barrier-structure along with a very thin 7 nm InAlGaN barrier effectively mitigates the buffer leakage currents with improved breakdown voltage of 38 V. The prominent microwave and DC characteristic of proposed HEMT is appropriate potential transistors for (next generation) millimetre wave electronics.
V.
REFERENCES
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