Comparison of Analog Performance of AlN/β-Ga2O3 HEMT by Variation in Dielectric Materials


 In the present paper, a high electron mobility transistor based on β-Gα2O3 material (BGO-HEMT) with different dielectrics (Si3N4, Al2O3, and HfO2) at the interface of aluminum nitride (AlN) and the gate is demonstrated. The device has a 10nm AlN layer with 50 nm barrier width, 50nm gate-length, and a value of 5 nm as gate-to-barrier thickness.A highly doped n+ material with a wider gap in between ohmic contact-barrier layers reveals the proposed device's novel traits. The performance is computd in terms of transfer characteristic, transconductance, gate capacitance, 2nd, and 3rd order transconductance values. The proposed structure reduced the dynamic & access resistance and provided a high gm value equal to 0.15S/µm,drain current density value of 650 A/mm (maximum) at Vds= 5 V. In the future, the proposed device can be utilized in high power radio frequency and microwave applications.

During the fabrication, a challenging issue is setting δdoping position and concentration, which can be managed effectively with the help of spontaneous piezoelectric polarization charges. For better FETs performance, one can combine the Ga 0 O 2 (for high breakdown) and polar IIInitrides (GaN or AlN; for intense polarization) [9].
Similarly, the FETs at low ohmic contact resistances with adequate channel-giving channel giving a shorter gate length can be developed.Furthermore, the beneficial features include low lattice mismatch, maximum room-temperature carrier mobility, and higher effective electron mass [10][11][12][13].The consideration of the FOM and saturation velocity proved Ga 0 O 2 HEMT better than GaN over low-frequency regions.If the operating temperature exceeds 300°C, the device's heating adversely affects the large signal performance and thus, degrades the device performance [14].
The β-Ga 0 O 2 MOSFETs provide better results against parameters like device heating & scaling [15].A heterostructureof the device can lead to a better improvement related to heating effects. Further, the absence of polarization in the device, delta doping of the device barrier region is very crucial for achieving 2DEG at the interface of ( ? 87B ) 0 2 /Ga 0 O 2 . Here, x can have a value < 0.40 as otherwise doping at the interface becomes very challenging [16][17][18].The β-Ga 0 O 2 HEMTs with non-polar and stable properties appreciably outperform other devices in terms of better transport properties, breakdown voltage, electric field, and DC output performance.The reduction in effective potential is why the decline in cutoff frequency becomes effective at higher charge density. The device's non-polar property allows d-doping in the barrier region for inducing 2DEG in the channel. However, the polarization metric is advantageous for better device performance.
The devices with β-Ga2O3 give theoretical performance by yielding benefits from the device's low defect density.The length of the drift region and the doping density of β-Ga2O3 are generally attainable via the current-film growth methodology. A more comprehensive bandgap energy and small drift regions in β-Ga2O3-based devices shifted the avalanche breakdown towards larger electric field(~8.1MV/cm)values. Consequently, it leads to a challenge in managing the field at the junction interface as well as over device edges.The material β-Ga203 is majorly acceptable for designing power devices with breakdown voltage >100 kV.
In the present paper, the HEMT with β-Ga 0 O 2 (BGO-HEMT)is proposed and simulated in SILVACO.We compared the performance of the transfer characteristic, transconductance, cutoff frequency gate capacitance, 2 nd and 3 rd order transconductance of AlN/ β-Ga 0 O 2 HEMT device for different dielectrics-Si 2 N I , Al 0 O 2 and HfO 0 utilized at the interface of AlN and gate.

II. DEVICE STRUCTURE AND SIMULATION PARAMETERS
The DC and radiofrequency performance of BGO HEMTs are simulated and analyzed using ATLAS TCAD [11][12]. Figure 1(a) and 1(b)show schematics of the proposed device with Si 2 N I , Al 0 O 2 and HfO 0 as a dielectric andcalibrated the Id-Vdcharacteristics as a curative measure to simitate the published results of [20], respectively. The doping is done as the uniform profile of n-type material with a higher concentration of 1 × 10 18 cm -3 underneath the drain and source access area.
The layered sequences of the materials consisted of a 10nm dielectric Si 2 N I , Al 0 O 2 and HfO 0 between AlN and Gate metal and 10 nm AlN barrier entirely relied on a 50 nm semiinsulated β-Ga2O3 buffer layer and 10 nm 2DEG channel. A layer of 25 nm Si 2 N I , Al 0 O 2 and HfO 0 is used for surface passivation. Gate material of Ni/Au with length LG = 50 nm,Schottky barrier height of 0.9 eV created by fixing work function ϕm of the metal as 5.1eV, 1µm source and drain contact (Ti/Au/Ni)with good ohmic-contact (ϕm = 3.15 eV). The source-gate (LSG), source-drain (LSD), and gate-drain (LGD) spacing are 0.2, 1.2, and 0.5µm.The β-Ga 0 O 2 the layer is doped with n-type material as an impurity witha concentration value of 10 16 cm -3 . Fig.2 shows the transfer characteristics of AlN/ β-Ga 0 O 2 HEMT with Si 2 N I , Al 0 O 2 and HfO 0 as dielectric material in log Scale 1 (Figure 2a)and in linear Scale (Figure 2(b)) at Vds=5V.From the graph, the Ids of AlN/β-Ga 0 O 2 HEMT with dielectric HfO2 is slightly larger than other dielectric i.eSi 2 N I and Al 0 O 2 . Because of the lower electron affinity of HfO2, the device will become ON at a higher threshold voltage,and the depth of formation of 2DEG becomes larger. So, the threshold voltage of AlN/ β-Ga 0 O 2 HEMT with HfO 0 as a dielectric is higher than the larger thanSi 2 N I and Al 0 O 2 . The ON-state current of AlN/ β-Ga 0 O 2 HEMT with HfO 0 is 0.65A/µm. Fig. 3 shows the graph of threshold voltage variation with dielectric material at Vds and Vgsequal to 5V. The threshold voltage of the proposed device is plotted for different dielectric materials-HfO2, Al2O3, Si3N4. Due to the right shifting of transfer characteristics in HfO2, the Vth of the proposed device is more significant than others, and the Vth is close to 0V.  shows the proposed device Ion/Ioff ratios acquired for different dielectric and passivation layers. For all scenarios, a more significant Ion/Ioff value of 10 9 is obtainedthat is reasonably required for various applications related to power devices.

III. SIMULATION RESULTS AND DISCUSSION
The drain current (Ids) as a function of the drain voltage (Vds) at a constant value of Vgs=5V for DG-CP HEMT and DGdoped HEMT are plotted in Figure 4. The graph clearly shows that Idsof DG-CP HEMT aremore significant in comparison to DG-doped HEMT. It is found steeply raised with lower drain biasing, which signifiedhigher mobility in 2DEG devices. Thus, it means the motive of the proposed HEMT device.  One of the crucial parameters, namely transconductance (gm),which is defined as drain current differential w.r.t gate voltage for consistent drain voltage values, gm is responsible for RF and linearity parameter of the device. Higher gm providesa device to work for a higher frequency range. It decides gain, cutoff-frequency, and other linearity parameters.The maximum gm value increases with a decrease in the gate-source length, although gate-voltage swing reduces due to high resistivity across the drain-access region. This agrees with one of the models, namely the phonon model.
The temporal charging of the device traps is a function of variations in the gate and drain potentials. The trapping of charges under the gate causes a threshold shift, while trapping within the gate-source/drain area causes a reduction in the transconductance.A minor decrease in the transconductance value with a moderate rise in the threshold voltages is expected in MOS-HEMTs with high-k gate dielectrics.The increase in source & drain resistance values due to the imperfect contacts reduces the value transconductance.It minimizes the microwave performance of the device.
Due to the shifting of threshold voltage in the right side direction in HfO2, the peak transconductance is observed at 2V.Infigure 5a, the gm1 increases similarly to ON-state drain current, but OFF-state transconductance is very low for low drain biasing and low gate biasing values.Because of saturation in drain current after 2V, the gm will be reduced. Figure 5(b) shows the transconductance of third-order (gm3) with a variation in gate biasingat various drain voltages.
A variation in gm3 is found more at low drain biasing values with saturated drain current values at increased gate biasing values. A high value of gm3 is high compared to 2nd order transconductance (gm2) and transconductance (gm1). It is found that transconductances rise in direct proportion with higher distortions.
In Fig.5 (c), a graph between transconductance and Vgs is plotted for different dielectrics. For ON-state current values of the proposed HEMT device, a more significant value is derived for HfO 0 over other dielectric materials, namely Si 2 N I and Al 0 O 2 . A higher value of U is depicted for a better response of the device. The U value of the proposed device is found to be 0.15S/µm.   V ds =5v In Figure 6, the gate-to-gate capacitance is reduced for the increasing gate biasing and drain biasing values (Vds = 5 V). Therefore, the gate capacitance is responsible for lower signal analysis and RF performance analysis of the proposed HEMT.   Subthreshold slope (SS) plays a vital role in circuit application, and it decidesthe device's response from OFF state to ON-state condition. For better device performance, the SS should be as low as possible. In conventional MOSFET, the SS is 63mV/Dec. Fig.7 shows the graphs of SS wrtVgs.The lower SS is obtained for HfO2, and its value is equal to 35mV/Dec. The response of HfO2 based device is faster than the other two.
Next parameter, VW is defined as the derivative of drain current and drain voltage.
VW is also called output transconductance. For better performance of device, VW Should be high. It decides the ON-state resistance (Ron) of the device, and it is inversely proposedto Gds. Due tothe larger drain current of HfO 0 -based device, the Ron is low and is shown in Fig. 9.

IV. CONCLUSION
A high electron mobility transistor based on β-Ga 0 O 2 material (BGO-HEMT) device is proposed for different dielectrics-Si3N4, Al2O3, and HfO2 at the interface of AlN and gate of the device. The transconductance is steeply raised at lower drain biasing with higher mobility in the 2DEG device, which signified the motive of the proposed HEMT device. The findings of the paper are very significant as the dynamic & access resistance get reduced by the proposed structure. Furthermore, it provided a high gm value equal to 0.15S/µm, drain current density value of 650 A/mm (maximum) at Vds= 5 V. The paper has added the findings which are inline with the fact that (β-Ga 0 O 2 ) is much preferred choice of the researchers. The lower gate capacitance values ensured more insufficient signal analysis and RF performance analysis of the proposed HEMT in future microwave applications.