A new gate design combined MIS and p-GaN gate structures for normally-off and high on-current operation

This study proposes a new gate architecture that integrates both a p-GaN gate and a metal–insulator–semiconductor (MIS) structure for a normally-off AlGaN/GaN high electron mobility transistor. Silvaco TCAD simulation software is used to assess the performance of the proposed design. A comprehensive analysis of the device’s transfer, output, and breakdown characteristics is carried out and compared with the conventional p-GaN gate AlGaN/GaN HEMT. The findings indicate that incorporating MIS in conjunction with the p-GaN gate leads to an augmentation in the on-state current density and a reduction in on-resistance. The proposed HEMT exhibits superior attributes, with an 80% increase in drain current compared to the conventional p-GaN gate HEMT, but remains similar to threshold voltage and breakdown voltage. Consequently, the proposed HEMT demonstrates elevated current density and enhances gate control over the channel without modifying the threshold voltage compared to the conventional p-GaN gate HEMT.


Introduction
][3][4][5] Notably, the AlGaN/GaN heterostructure's inclusion of a two-dimensional electron gas (2DEG) leads to the device naturally operating in a normally-on mode, rendering it exceptionally suitable for low-voltage and HF applications. 6,7)o achieve normally-off operation in AlGaN/GaN HEMTs, a key principle involves preventing the generation of 2DEG while maintaining the gate bias at 0 V. Various methodologies have been explored in research to shift the threshold voltage (V TH ) to positive values by depleting the 2DEG beneath the gate.[10][11] Another method includes the formation of a PN-junction, achieved through the use of a p-type doped semiconductor and a p-(Al) GaN cap layer as the gate, for effectively depleting the transistor channel when the gate is set to 0 V. [12][13][14][15] Alternatively, methods such as plasma etching or epitaxial process changes can efficiently reduce the concentration of 2DEG. Aditionally, the thickness of the AlGaN barrier layer can be reduced through the recessing of the AlGaN barrier layer beneath the metal gate.This is significant because the density of 2DEG is notably influenced by the thickness and aluminum composition of the AlGaN layer.16,17) In contemporary applications, devices featuring p-GaN cap layers have gained widespread usage in power supplies, wireless charging, and various other fields due to their dependable threshold voltage stability and the high repeatability of the fabrication process.Schottky and ohmic p-GaN gate technologies have recently seen extensive adoption in commercial devices, featuring diverse gate electrode connections to the p-GaN layer.18,19) Based on TCAD simulations and experimental findings concerning the metal/p-GaN/AlGaN/GaN system, it was determined 20,21) that employing a Schottky metal gate on p-GaN should result in a higher threshold voltage (V TH ) and reduced gate leakage compared to an ohmic gate.A wellestablished Schottky barrier in p-GaN gate HEMTs prevents substantial current injections at the gate interface, reducing power consumption.Consequently, the prevailing choice in contemporary applications is to utilize a Schottky gate on p-GaN.
However, compared to other normally-off Si-based devices, p-GaN gate HEMTs exhibit a lower drain current density and reduced gate control capability.Hu et al. attempted to develop a normally-off HEMT by selectively incorporating a p-type GaN layer between the gate and barrier layers.Still, the achieved threshold voltage remained below +1 V, and the on-state saturation current reached only 40 m A mm −1 . 22)Chiu et al. adopted an innovative approach involving an AlN etch stop layer and a unique digital etch process in the epitaxial layer design, resulting in a p-GaN gate normally-off device with a threshold voltage of +1.7 V and an on-state saturation current of 280 mA mm −1 .However, this design yielded a high onresistance of 16.1 Ω mm. 23)Niu et al. introduced a hybrid-gate structure that combined Schottky and metal-insulator-semiconductor (MIS) designs on the p-GaN gate to elevate the threshold voltage. 24)Alternatively, Huang et al. showcased a normally-off p-GaN gate InAlN/GaN HEMT to enhance on-resistance, saturation current, and device stability. 25)In addition, the selected p-GaN gate region etched, which extends towards the drain electrode, is a viable option for improving the efficiency and reliability. 26)[29] Using earlier research findings, this work offers a unique gate design that uses TCAD simulations to combine a p-GaN gate with an MIS structure.The goal is to improve the electrical device's qualities and obtain a more balanced performance.The proposed AlGaN/GaN HEMT's improved characteristics have intriguing promise for applications in power electronics.

Device structure design and simulation
In this investigation, we employed an illustrative model from Silvaco TCAD, which serves as a reference structure and features a Schottky-type metal gate on the AlGaN barrier. 30)his model serves as an example to demonstrate the simulation of an AlGaN/GaN HEMT with a p-type GaN gate and is closely aligned with the principles outlined in a paper. 31)Within the simulation framework, the leakage current is computed by incorporating two types of traps: a donor-like trap with an energy level of 3.2 eV originating from the valence band, characterized by a density of 1.27 × 10 18 cm −3 , and an    In the proposed new gate design, the p-GaN layer is selectively etched in the central gate region to include an MIS gate.In this way, both the PN junction and MIS structure control the channel electrons.In the simulation, the gate length (L G ) is 4 μm, the gate width is 1.0 mm, the source-gate distance (L SG ) is 1 μm, and the gate-drain space (L GD ) is 6 μm.As for the proposed HEMT, the gate length of the MIS structure is 3.2 μm.The carrier concentration of the p-GaN layer is 3 × 10 −17 cm −3 .At the same time, devices with a larger gate-drain spacing of 18 μm were used for the off-state I-V characteristics simulation.Figure 2 represents an aspect of the simple fabrication process.The selective etching of the p-GaN layer is used to define the p-GaN gate region.Following the definition of the p-GaN gate area, the process follows conventional steps like mesa isolation, ohmic contacts, and dielectric layer deposition to form an MIS gate.Schottky gate contacts on p-GaN can also be implemented.

Results and discussion
The transfer characteristics of both the conventional p-GaN gate AlGaN/GaN HEMT and the proposed MIS and p-GaN gate AlGaN/GaN HEMT are illustrated in Fig. 3(a) at V DS = 15 V.It is evident that the threshold voltage for both the conventional p-GaN gate AlGaN/GaN HEMT and the proposed HEMT are identical, +1.3 V at an I D of 1 mA, due to the same control from the p-GaN region.Nonetheless, in the proposed MIS and p-GaN gate AlGaN/GaN HEMT, as the V GS increases, the I D keeps rising and eventually reaches a higher maximum due to the presence of an MIS gate.Consequently, the transconductance (G m ) from the proposed MIS and p-GaN gate HEMT shows a broader range than the conventional p-GaN gate AlGaN/GaN HEMT under the same peak value.
Figure 3(b) shows the output characteristics of the conventional and proposed AlGaN/GaN HEMTs.Notably, the proposed device has a higher I D,MAX of 0.45 A at V GS = 5 V and V DS = 20 V, whereas the traditional HEMT has an I D,MAX of 0.25 A at the same conditions (V GS = 5 V and V DS = 20 V).The I D,MAX has increased by 80% compared to the conventional device, indicating that the MIS and p-GaN gate AlGaN/GaN HEMT can enhance on-state current and improve the gate's ability to control the channel without altering the V TH .The on-resistance for the conventional p-GaN gate AlGaN/GaN HEMT and the proposed AlGaN/ GaN HEMT are 6.45 Ω mm and 6.00 Ω mm, respectively.In Fig. 3(c), the transfer characteristic curves for both HEMTs are presented on a semi-log plot to show the similar turn-on characteristics (subthreshold characteristics) and higher drain current in the proposed MIS and p-GaN gate AlGaN/GaN HEMT. Figure 3(d) shows the I G -V GS indicating a lower gate leakage in the proposed MIS and p-GaN gate AlGaN/GaN HEMT due to the MIS gate under the same gate size.Due to the presence of the insulator layer and no p-GaN layer in the MIS gate AlGaN/GaN HEMT, the drain current is highest whereas threshold voltage for the MIS gate AlGaN/GaN HEMT is −3.1 V at an I D of 1 mA.The corresponding G m is broadest, as shown in Fig. 4.However, the proposed MIS and p-GaN gate AlGaN/GaN HEMT in this study offers improved characteristics compared with the conventional p-GaN gate AlGaN/GaN HEMT, including an E-mode operation, higher drain current, broader G m , and lower gate leakage current.Furthermore, large gate swings and improved gate reliability are possible by reducing gate leakage.
The increase in on-state current density observed in the proposed MIS and p-GaN gate AlGaN/GaN HEMT arises

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© 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd from the presence of an MIS structure, which enhances the electron concentration within the channel.Figures 5(a) and 5(b) show the distribution of electron concentration under onstate bias conditions (V GS = 3 and 7 V) for both the conventional and proposed HEMTs to justify the difference in I D .The higher electron concentration can be attributed to the presence of Al 2 O 3 in the gate region when V GS exceeds V TH .Moreover, a higher electron concentration in the access region between the gate and the drain is found, which is resulted from the lower channel resistance under the same V DS (higher electrons injected from the MIS gate region).Figure 6 compares the off-state I D -V DS characteristics between the conventional p-GaN gate AlGaN/GaN HEMT and the proposed MIS and p-GaN gate HEMT.The blocking voltage (BV) is determined when the drain leakage current reaches 0.1 mA.These results were simulated with a gate-todrain spacing of 18 μm for both structures.The BV for the conventional p-GaN gate AlGaN/GaN HEMT is 861 V, while it is 866 V for the proposed MIS and p-GaN gate HEMT, representing a similar result due to the same p-GaN edge of the gate toward the drain.

Conclusions
In this study, we introduce a novel gate design that incorporates both a p-GaN gate and MIS structure for a normally-off AlGaN/ GaN HEMT, and subsequently, we conduct simulations.Furthermore, a comprehensive analysis of the device's transfer, output, and breakdown characteristics is performed using Silvaco TCAD simulation software.The results reveal that through the MIS combined with the p-GaN gate, there is an increase in onstate current density and a decrease in on-resistance.Notably, the proposed HEMT exhibits the most favorable characteristics, an 80% higher drain current than that of conventional p-GaN gate HEMT.The threshold voltage and breakdown voltage for the proposed HEMT closely resemble those of conventional p-GaN gate HEMT.Consequently, the proposed HEMT demonstrates a high current density and enhances the gate's control over the channel without altering the threshold voltage when compared to conventional p-GaN gate HEMT.

Fig. 2 . 2 ©Fig. 3 .
Fig. 2. Part of the fabrication process steps for the new gate design in an AlGaN/GaN HEMT.

Fig. 5 .
Fig. 5. Electron concentration along the 2DEG channel of the conventional p-GaN gate AlGaN/GaN HEMT and the proposed HEMT at (a) V GS = 3 V and V DS = 15 V, and (b) V GS = 7 V and V DS = 15 V.

Fig. 6 . 5 ©
Fig. 6.Simulated off-state I D -V DS characteristics of the conventional p-GaN gate AlGaN/GaN HEMT and the proposed HEMT.