Ultra-thin double barrier AlGaN/GaN high threshold voltage HEMT with graded AlGaN/Si3N4 gate and p-type buffer layer

An ultra-thin double barrier enhancement mode (E-mode) AlGaN/GaN high-electron mobility transistor (HEMT) with p-type buffer layer and Si3N4/graded p-AlGaN gate is proposed and investigated by Silvaco TCAD. The simulation results show that the designed HEMT can obtain a high threshold voltage over 5.0 V and large gate swing. The maximum gate leakage current is 3.11 × 10–4 A/mm at 30 V gate voltage, which decreases four orders of magnitude compared to the conventional double barrier HEMTs. Due to the p-type buffer layer, the cut-off frequency for the proposed HEMT is raised over three-times compared to the conventional double barrier structure HEMT with n-type buffer layer. Meanwhile the designed HEMT exhibits high breakdown voltage and large current-gain. Moreover, the impacts of Si3N4 layer thickness under gate and GaN channel layer thickness are analyzed. Both layers play significant roles in obtaining high threshold voltage for the device by adjusting the conduction band energy of AlGaN/GaN interface potential well.


Introduction
Because of the excellent properties for (Al)GaN-based IIInitride semiconductor materials such as wide band energy, high-electron saturation velocity, high breakdown electric field and better thermal stability, which have been widely used in opto-electronic devices, high temperature high power devices and high frequency microwave devices [1,2]. In the last few decades, GaN-based high-electron mobility transistors (HEMTs) have attracted more and more attention thanks to their outstanding advantages [3][4][5][6]. As a result of spontaneous and piezoelectric polarization effects, a high density two-dimensional electron gas (2DEG) about 10 13 /cm 2 is formed at the interface of AlGaN/GaN heterojunction. Therefore, GaN HEMTs are intrinsic normally-on devices. However, the enhancement-mode (E-mode) GaNbase HEMTs with large positive threshold voltage (> 3 V) and large gate swing (> 10 V) are desirable for fail-safe operation in power electronic applications [7].
So far, the mainly strategies have been proposed to realize the enhancement mode GaN-based HEMTs including thin barrier [8], gate recess structure [9], the injection of fluorine ion under gate [10], p-GaN/AlGaN gate as well as metal-insulator-semiconductor gate structure, etc. [11][12][13]. GaN-base E-mode HEMTs with thin barrier suffer from the low 2DEG in source-gate area [8]. P-GaN cap layer technology faces the challenge of the effective p-type doping. The gate recess structures have the difficulties to accurately control the remaining barrier layer thickness and overcome serious etching damage. The charge formed by fluorine ion injection has the poor thermal stability [14]. Hence, the E-mode GaN-base HEMTs with high threshold voltage V T (> 3 V), low device leakage current and high breakdown voltage still need to be further explored [15].
In this work, we designed an ultra-thin double barrier E-mode HEMT with polarization-induced p-doping in p-AlGaN gate. It is well known that the polarization doping is often used in high Al content AlGaN avalanche photodiodes [16,17]. Meanwhile, the buffer layer with p-type doping is also used in this structure to improve the threshold voltage and cut-off frequency. The performances of the ultrathin double barrier E-mode HEMT with graded AlGaN gate and p-type buffer layer are investigated numerically using Silvaco Atlas.

Structure, parameters and physical models
The schematic of the ultra-thin double barrier E-mode HEMT with polarization-induced p-type doping gate is shown in Fig. 1a.  Fig. 1c is unintentionally doped layer and the residual carrier concentration is 1 × 10 16 cm −3 . The length of gate, the distance from the source to gate and the distance between gate and drain are 1.4 μm, 1 μm and 18 μm, respectively. In simulation, Poisson's equation and carrier continuity equation were applied with Shockley-Read-Hall recombination model, the polarization model including spontaneous and piezoelectric polarization, the nitride specific high field mobility model, the Albrecht low field mobility mode and the impact ionization model. Meanwhile, the Fowler-Nordheim tunneling and the hot carrier injection model are included as the main gate leakage current mechanisms [18]. A donor-like trap with trap energy level of 3.2 eV above valence band and trap density of 1 × 10 18 cm −3 , as well as an acceptor-type trap with energy level of 0.36 eV below the conduction band and trap density of 7 × 10 17 cm −3 in buffer layer are taken into account [5]. The impact ionization parameters are same as the reference [5]. The source and drain are ohmic contact. The gate is schottky contact and work function is 5.2 eV. The other parameters of (Al) GaN come from the Silvaco default material properties.   layer is lower than − 2 V, indicating a normally on operation. In contrast, E-mode is realized for conventional HEMT with p-type buffer layer and proposed HEMT. The threshold voltages for both HEMTs are 4.46 V and 5.02 V, respectively. Here, we define the threshold voltage is the gate voltage when the drain current reaches a constant of 10 μA/ mm as reference [19]. The drain current at V GS = 0 V are 8.82 × 10 −14 A/mm and 2.54 × 10 −13 A/mm correspondingly. With the increase in the gate voltage, the drain currents for both conventional structures enhance to a peak value and then decline. This may be caused by the competition between gate leakage current and drain current at high gate voltage. The drain current for the proposed structure shows a slight upward trend and exceeds that of conventional HEMT with p-type buffer layer when the gate voltage is over 25.2 V, showing a better gate swing. The maximum drain current for proposed HEMT is 0.52816 A/mm at 30 V gate voltage. The maximum gate leakage current for proposed HEMT is 3.11 × 10 -4 A/mm, showing four orders of magnitude lower than that of both conventional structures. Moreover, the proposed structure also exhibits a large on/off current ratio of 10 8 and steep sub-threshold slope of 71.1 mV/dec. The band energy, the electron concentration, the hole concentration and electric field distribution for three HEMTs have been calculated under gate along the vertical channel direction as shown in Fig. 3 to analyze the electric characteristics in Fig. 2. For proposed structure, a high bulk hole concentration near 2.65 × 10 18 cm −3 in graded AlGaN layer caused by polarization-induced effect is shown in Fig. 3b, which obviously increases the build-in electric field in graded p-AlGaN layer and enhances the barrier height for electron transmission from GaN channel layer to gate as displayed in Fig. 3a, d. The enhanced barrier combined with wide-band energy Si 3 N 4 insulator result in a very small gate leakage current as observed in Fig. 2b even at large gate voltage. We also find that there is a shallow potential well at the interface of AlGaN barrier layer/GaN channel layer for three structures. The conduction band diagrams in the potential well are all above the Fermi level, which produce the maximum electron density less than 8 × 10 6 cm −3 at the interface (Fig. 3c). Hence, the three structures should be the E-mode HEMTs in theory. However, the conventional HEMT with n-type buffer layer is a depletion-mode (D-mode) and the reason will be analyzed later. An additional potential well can be noticed at Si 3 N 4 /graded p-AlGaN interface in proposed HEMT. With the increased forward gate voltage, the electron will inject from AlGaN/GaN interface potential well into this additional potential well. The electron in additional potential well has no contribution to drain current. Meanwhile, the conduction band energy of AlGaN/ GaN interface potential well drops very slowly thanks to the small forward voltage applied at this area because this potential well is connected in series with a large resistor Si 3 N 4 layer. Thus, a higher threshold voltage is needed to collect the electron in AlGaN/GaN interface potential well and turn on the HEMT [20].

Results and discussion
To explore the normally-on behavior for n-type conventional device, we calculate the transfer characteristic curves and conduction band energies for n-type convention device at different channel layer thickness with and without considering donor trap in channel layer as shown in Fig. 4. From  Fig. 4a, we can see that the n-type conventional device without donor trap displays a normally-on behavior when channel layer thickness is lower than 25 nm. The device becomes the normally-off when channel layer thickness is over 25 nm, the threshold voltage is 1.9 V for the device with 35 nm thickness channel layer, which is slightly larger than the experiment value 1.5 V for p-GaN HEMT with same channel layer thickness as reported in reference [21]. The threshold voltages are all less than 2.7 V for different structure. The phenomenon of n-type conventional device changing from normally-on to normally-off with increased channel layer can be explained from the conduction band energy as shown in Fig. 4c. The electric field direction in channel layer should be from the channel layer to buffer layer, while the electric field direction of the buffer layer is just opposite judging from the energy band bending. The high concentration electron in source region can enter the buffer layer by overcome the low barrier height and then inject into drain electrode through the thin channel layer, which results the n-type conventional device with thin channel layer is normally-on. The n-type conventional devices with donor trap in channel layer at different channel layer thickness are all normally-on (Fig. 4b), which may be partly caused by the lower barrier for electron from channel layer to buffer layer as displayed in Fig. 4d and the partly by the enter directly of electron from trap to drain under positive drain voltage.
The current gains of frequency dependence for three HEMTs at V DS = 10 V are studied by the AC small-signal analysis as shown in Fig. 5. The cut-off frequencies for proposed HEMT and conventional HEMT with p-type buffer 2 GHz. These two structures raise the cut-off frequency over three-times compared to the conventional structure with n-type buffer layer (3.84 GHz) at V DS = 10 V. . The large cut-off frequencies for both HEMTs may be attributed to the p-type doping in buffer layer, which makes the conduction band of buffer layer bend upward and AlGaN/GaN interface potential well narrower. Narrow potential well can raises the electron velocity in the channel, thereby improving the cut-off frequency [6]. Figure 6a presents the drain current-voltage curves for proposed HEMT and conventional HEMT with p-type buffer at V GS = 0 V. From Fig. 6a, it can be found that the breakdown voltage for proposed HEMT is 860 V, exhibiting a large improvement as compared with the conventional HEMT with p-type buffer (808 V). The improvement of breakdown voltage for proposed HEMT is ascribed to the insertion of Si 3 N 4 layer between gate and graded p-AlGaN layer, which evidently reduces the electric field strength in the region from gate to drain, and makes the electric field distribution more uniform along the channel as shown in Fig. 6b. Figure 7a, b are the I DS -V GS curves for designed HEMT as a function of Si 3 N 4 layer thicknesses under gate and GaN channel layer thicknesses, respectively, where the drain voltage is 10 V. From Fig. 7a, it is observed that the threshold voltages are 3.94 V, 4.79 V and 5.02 V as the Si 3 N 4 thicknesses are 10 nm, 15 nm and 20 nm. The threshold voltages emerge an ascent trend with the increasing Si 3 N 4 thicknesses. The influence of GaN channel layer thickness on the threshold voltage is illustrated in Fig. 7b. When GaN channel layer thickness varies from 5 to 15 nm, the threshold voltage decreases from 5.02 to 4.25 V. As the GaN layer thickness further increases to 25 nm, the threshold voltage is less than 0 V. The device degenerates into the depletion mode. This can be interpreted by the band energy diagram at V GS = 0 V and V DS = 0 V as shown in Fig. 7c. The conduction band energy of AlGaN/GaN interface potential well is closer to Fermi level with increased channel layer thickness, which makes the more electrons accumulate in the channel layer and causes the HEMT to lose its E-mode.

Conclusion
In conclusion, for fail-safe operation in power electronic applications, a high threshold voltage ultra-thin double barrier AlGaN/GaN HEMT with bulk polarizationinduced doping p-AlGaN /Si 3 N 4 gate has been presented. Meanwhile, the p-type buffer layer was also considered in this structure. In this work, two conventional double barrier HEMTs with n-type buffer and p-type buffer layer are investigated as comparison. The device performance including threshold voltage, gate leakage current, on/off current ratio, sub-threshold slope, cut-off frequency and  Fig. 6 a the breakdown characteristic and b the electric field distribution along the channel at V DS = 100 V for designed HEMT and conventional HEMT with p-type buffer layer breakdown voltage show obviously improved, thanks to the high p-type doping density in graded p-AlGaN gate caused by polarization effect, as well as p-type buffer layer and Si 3 N 4 layer under gate. Moreover, the influence of Si 3 N 4 dielectric layer and GaN channel layer thickness on device performance has also been simulated. With the increased Si 3 N 4 thickness, the threshold voltage enhances apparently. In contrast, a drastic decreasing threshold voltage is observed with increased channel layer thickness. The reason is that the conduction band diagram of AlGaN/ GaN interface potential well is closer to Fermi level with increased channel layer thickness. Hence, the more electrons accumulate in the channel layer, leading to a small open voltage.
Funding This work was supported in part by the Natural Science Foundation of the Anhui Higher Education Institutions (2022AH051125, 2022AH051098, KJ2021A1087), in party by the National Natural Science Foundation of China (No. 62241401).

Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.