T-Gate shaped AlN/ β -Ga 2 O 3 HEMT for RF and High Power Nanoelectronics

—In this paper, we report record DC and RF performance in β-Ga 2 O 3 High Electron Mobility Transistor (HEMT) with field-plate T-gate using 2-D simulations. The T gate with head-length L HL of 180 nm and foot-length L FL of 120 nm is used in the highly scaled device with an aspect ratio (L G /t barrier ) of ~ 5. The proposed device takes advantage of a highly polarized Aluminum Nitride (AlN) barrier layer to achieve high Two-Dimensional Electron Gas (2DEG) density in the order of 2.3 × 10 13 cm -2 , due to spontaneous as well as piezoelectric polarization components. In the depletion mode operation, maximum drain current I D,MAX of 1.32 A/mm, and relatively flat transconductance characteristics with a maximum value of 0.32 S/mm are measured. The device with source-drain distance L SD of 1.9 µm exhibits record low specific-on resistance R ON,sp of 0.136 mΩ-cm –2 , and off-state breakdown voltage of 403 V, which correspond to the record power figure-of-merit (PFoM) of ~ 1194 MW/cm 2 . Additionally, current gain cut-off frequency f T and maximum oscillation frequency f MAX of 48 and 142 GHz are estimated. The obtained results show the potential of Ga 2 O 3 HEMT for futuristic power devices.


INTRODUCTION
Despite challenges on the front of high-quality native substrates, GaN-based HEMTs have been in use for over a decade and probably surpassed their life-cycle, for various reasons [1]. Currently, gallium-oxide (Ga2O3) is being thoroughly explored for its possible applications in certain areas of power electronics due to its interesting material properties such as large bandgap (4.5 -5.3 eV), estimated high critical field (8 MV/cm), a wide variety of n-type dopants with controllable doping, and availability of single-crystal substrate grown using melt-based systems [2]- [4]. Out of its five crystalline structures, the β-phase of Ga2O3 has been reported as the most stable and looks most suited to high-voltage applications. The constant growth and development of β-Ga2O3 single crystal technology are further fuelling the search for a suitable wide-bandgap (WBG) material, having the potential to supplement existing technologies as well as capable to address new emerging power applications.
The β-Ga2O3 device technology has a footprint in almost all power devices including Schottky barrier diodes [5], [6], MESFETs [7], [8], MOSFETs: depletion-mode (D-mode) [9]- [12] and enhancement-mode (E-mode) [13]- [17], and HEMTs [18], [19]. Specifically, a high breakdown field of 3.8 and 5.2 MV/cm is reported in β-Ga2O3 lateral MOSFET [10] and β-Ga2O3 vertical heterostructure [20]. In addition, a power of 11 and 192.5 MW/cm 2 are reported in [10], and [17] respectively. However, these devices have used relatively thick epi-channel of 200 nm and large gate length of > 1µm together make them less relevant for RF applications. The high frequency applications demand aggressive device scaling, both lateral as well as vertical. On the other hand sub-micron gate led to poor control as well as deteriorated transconductance and current gain due to increased gate-resistance [21]. The T-gate technology enables use of short gate-length while keeping the gate-resistance low simultaneously [21]. It is worth to note that, the switching performance of a power switch critically depends on OFFstate leakage and ON-state conduction loss due to finite ONresistance. Furthermore, the ON-resistance (RON) of the device is proportional to gate-drain length (LGD) and sheet-resistance (RSH) of the 2DEG channel.
In this paper, 2-D simulations of AlN/β-Ga2O3 HEMT are performed to access its switching performance using a physics-based device simulator. The DC and RF characteristics of the proposed device are thoroughly investigated. The following section describes the proposed device architecture and simulation settings, followed by results and discussion in Section III. Results are also benchmarked against similar device structure presented recently. Section IV concludes the paper.

II. DEVICE STRUCTURE AND SIMULATION FRAMEWORK
The proposed device schematic cross-section is shown in Fig. 1. The epitaxial layer sequence is arranged as follows. On a semi-insulating β-Ga2O3 substrate, 0.275 µm β-Ga2O3 buffer layer exists, which is doped with acceptor-like traps to account for unintentional Fe-dopants, followed by a 10 nm thick AlN material as a barrier layer on which Schottky gate contact with a barrier height of 0.8 eV is fixed. The source/drain contacts are assumed to be ohmic, and contact resistance of 0.4 Ω-mm is assumed as measured in [22]. The low contact resistance is achieved using a heavily doped n-type Gaussian profile with a peak concentration of 6 × 10 19 cm -3 . The β-Ga2O3 material parameters and user-defined model parameters are mentioned at places where used, whereas the default physical models are used as given in [23]. The gate length, LG equal to T-gate foot length (LFL) of 120 nm and T-gate head length (LHL) of 180 nm. The gate-source (LGS) and gate-drain distance (LGD) are equal to 0.32 and 1.4 µm respectively. Spontaneous and piezoelectric polarization models are evoked for the AlN barrier layer with default settings given in [23]. Apart from  Shockley-Read-Hall (SRH) recombination, Fermi-Dirac for carrier statistics, electric field dependent mobility modelnegative differential conductivity (NDC) is used to capture electron velocity saturation effect. To analyze breakdown characteristics, the impact ionization model-Selberherr is used.
The band bending and electron concentration at the heterointerface along the cut-line a -a' are shown in Fig. 2. The 2DEG density is estimated to be 2.3 × 10 13 cm -2 . This high 2DEG density is attributed to high polarization charges confined in large conduction band offset (ΔEC). The β-Ga2O3 material parameters such as energy bandgap EG of 4.9 eV, static dielectric constant (ε s ) of 10.2 are taken from [24]. Using electron effective mass for conduction and valence band, total densities NC and NV of 3.6 × 10 18 and 2.86 × 10 20 cm -3 respectively, are used in the simulation deck. The β-Ga2O3 substrate is doped with acceptor as well as the donorlike trap of density 1 × 10 18 cm -3 and at energy level 0.82, 4.4 eV respectively. Different β-Ga2O3 impact ionization coefficients for the Selberherr model [23] are taken from [25]. Electric field dependent mobility model NDC [26] is given as follows: where = 1.5 × 10 7 cm/s is the saturation velocity, = 200 kV/cm is the breakdown electric field, 0 = 140 cm 2 /V s is the low-field electron mobility, and = 2.47 is the constant.

III. RESULTS AND DISCUSSION
This section summarizes the numerical calculation of 2DEG density using polarization models, and simulation results of the T-gate AlN/β-Ga2O3 HEMT.
A. 2DEG DENSITY It is widely reported that a higher value of charge density (ns) is critical for HEMTs operations since the current density is ∝ ns. Since β-Ga2O3 does not possess any polarization property, here only AlN barrier layer polarization is considered to calculate total sheet charge density. Total polarization PT = PSP + PPI, where PSP = -0.09 C/m 2 [23] is the spontaneous polarization of the AlN material. Due to tensile strain between AlN epitaxial layer and β-Ga2O3 buffer, piezoelectric polarization PPI is given as: where as = 3.112, a0 = 3.04 are the lattice constants of the AlN and β-Ga2O3 materials respectively, and piezoelectric constants are e31 = -0.53, e33 = -1.5 C/m 2 , and elastic constants are C13 = 127, C33 = 382 for AlN are used from [23]. So the total polarization PT = -0.612 C/m 2 , which corresponds to sheet charge density ns ≈ 3.8 × 10 14 cm -2 . However, this value is roughly one order greater than what is estimated through simulation. This can be attributed to the thickness-dependent piezoelectric polarization of the AlN barrier.

B. DC CHARACTERISTICS
DC electrical transfer characteristics of the proposed device structure are shown in Fig. 3. The maximum value of drain current (ID,MAX), and transconductance (gM,MAX) are found to be 1.32 A/mm, 0.32 S/mm respectively at VGS = 1V and VDS = 12 V. A relatively 'flat' transconductance is obtained here and better device linearity can be expected. The improved gm linearity is mainly due to the 'coupled' channel of the AlN barrier devices [27]. A threshold voltage (VTH) of -3.8 V at ID = ID,MAX/10 3 , and ION/IOFF greater than 10 7 are extracted from the log ID -VGS curve, shown in    On-resistance (RON) of 7.2 Ω-mm is extracted using the minimum of (VDS/IDS) at VGS = 1 V in the simulation deck. The specific on-resistance (RON,sp) of the device is calculated as RON × LSD = 0.136 mΩ-cm 2 . The off-state breakdown voltage of the device is analysed using the Selberherr impact ionization model [23]. The default parameters of the model are replaced by β-Ga2O3 ionization coefficients (an1, an2 = 2.16 × 10 6 , bn1 = bn2 = 1.77 × 10 7 ) reported in [25]. A minimum current density of 1 × 10 -13 A is used in the simulation deck to trigger the breakdown. Since minority carriers are negligible in a wideband semiconductor like β-Ga2O3, numerical method-CLIMIT employed only one carrier-electrons. Compliance parameter is used to set current boundary conditions set at 1 mA/mm. The off-state breakdown voltage (VBR) of the proposed structure with fieldplate T-gate is estimated to be 403 V at VGS = -5V. The threeterminal breakdown characteristics are shown in Fig. 6. The peak electric field distribution at the breakdown is shown in Fig. 7. Moreover, the AlN/β-Ga2O3 HEMT with RON,sp of 0.136 mΩ-cm 2 and VBR of 403 V achieved a record power figure of merit (PFoM = VBR 2 /RON,sp) of 1194 MΩ/cm 2 . This record value of PFoM supports the viability of AlN/β-Ga2O3 HEMT for high voltage Nanoelectronics applications.

C. RF CHARACTERISTICS
The high-frequency RF performance of field-plate T-gate AlN/β-Ga2O3 HEMT is investigated to estimate current gain cutoff frequency (fT) and maximum oscillation frequency (fMAX). This part of the simulation is performed using a small signal analysis with ac frequency varying from 1 GHz to 200 GHz at VGS = -1 V and VDS = 12 V corresponding to gm peak. Post simulation results show fT of 68 GHz and fMAX of 142 GHz and shown in Fig. 8. However, the estimated value of fT is significantly lower than the previously reported fT value by our group for AlN/β-Ga2O3 HEMT with gate-length LG of 50 nm. Here, the lower fT value can be explained based on the relatively large gate capacitance of T-gate as fT ∝ 1/CGG, where CGG is the gate capacitance, and also reported in [21].

D. BENCHMARKING
DC and RF parameters for the simulated AlN/β-Ga2O3 HEMT, along with similar devices reported recently, are provided in Table 1. The improved parameters are mainly due to the one-order higher 2DEG density (ns) as compared to ns ≈ 10 12 cm -2 for delta-doped β-Ga2O3 MESFET [22]. In addition, a higher conduction band offset at the heterointerface ensures highly confined charge carriers in the quantum triangular well. Finally, the device is benchmarked in the RON,sp versus VBR plot along with other suitable reported devices, the respective plot is shown by Fig. 9.

IV. CONCLUSION
In summary, DC and RF characteristics of AlN/β-Ga2O3 HEMT with field-plate T-gate are investigated via 2-D simulations. A high 2DEG density in the order of 10 13 cm -2 is estimated at the heterointerface of AlN/β-Ga2O3 HEMT, mainly because of the highly polarized thin AlN barrier layer. Consequently, a maximum current density of 1.23 A/mm and peak transconductance of 0.32 S/mm are obtained. The proposed device employed a field plate T gate with headlength LHL of 180 nm and foot-length LFL of 120 nm to optimize its DC as well as RF performance. The T-gate effectively controls the channel and a threshold voltage of -3.8 V is estimated. The device has an ON-resistance of 7.2 Ω-mm, and a specific on-resistance of 0.136 mΩ-cm 2 corresponding to LSD of 1.4 µm for the proposed device. Furthermore, using the impact ionization model and threeterminal breakdown characteristics, a breakdown voltage of 403 V is estimated, and combining RON,sp, and VBR, a record PFoM of 1194 is estimated. The proposed device structure investigated here demonstrates the potential of AlN/β-Ga2O3 HEMTs for futuristic high-power Nanoelectronics applications.