GaN-based High Electron Mobility Transistors Research Articles

Threshold voltage mapping at the nanoscale of GaN-based high electron mobility transistor structures using hyperspectral scanning capacitance microscopy

Threshold voltage mapping at the nanoscale of GaN-based high electron mobility transistor structures using hyperspectral scanning capacitance microscopy

High-speed independent polarization modulation based on dual channel 2DEG anisotropic terahertz metasurfaces

In communication and imaging systems, anisotropic metasurfaces bring new degrees of freedom for the independent control of electromagnetic waves with different polarizations. This paper proposes a dual-polarization modulation metasurface (DPMM) operating in the terahertz (THz) frequency range, which can effectively and independently modulate two orthogonally linearly polarized electromagnetic waves. The metasurface unit adopts an anisotropic structure, and by integrating multiple GaN-based high electron mobility transistors (GaN-HEMTs) into the anisotropic structure, independent control of the two-dimension electron gas (2DEG) carrier concentration in the dual-polarization channel is achieved, enabling selective modulation of x-polarization and y-polarization waves. Simulation and static experimental results demonstrate the excellent modulation isolation performance of the DPMM, with a maximum isolation exceeding 45 dB. Dynamic experiments further highlight the high-speed modulation effect, with modulation speeds exceeding 1 GHz for a single channel and an overall modulation speed of 2 GHz for dual polarization, highlighting its potential for enhancing channel capacity and information throughput in THz communication and imaging systems.

Comparison of performance in GaN-HEMTs on thin SiC substrate and sapphire substrates

We have investigated the electrical properties of GaN-based high electron mobility transistors (HEMT) on N+-type 4H SiC substrates and on sapphire substrate as a reference. The three-terminal off-state breakdown voltage (BV) as a function of gate-drain spacing (LGD) was measured. Both devices possess superior Baliga figure of merit parameters (BFOM= BV3/Ron). For GaN/Sapphire with LGD of 10 μm, Ron is 14.65 mΩ·cm2 and the corresponding BFOM yields a remarkably high value of 9.88 × 108V2·Ω−1·cm−2. Conversely, GaN/N+ SiC with 20 μm LGD exhibited an Ron of 43.23 mΩ·cm2 and a BFOM value of 1.55 × 107V2·Ω−1·cm−2. In addition, we perform AFM, XRD, SEM and TEM analysis to further explore the differences in the DC-current properties for these two distinct substrates. The XRD FWHM scales with surface roughness, moreover grain boundaries for GaN-based HEMTs grown on SiC are more distinct compared to Sapphire substrate. This study provide valuable insight into the structural and morphological disparities that may contribute to the observed variations in device performance.

Temperature rise detection in GaN high-electron-mobility transistors via gate-drain Schottky junction forward-conduction voltages

An electrical measurement method for the junction temperature and thermal resistance of GaN high-electron-mobility transistors (HEMTs) is discussed. It is based on the forward voltage drop across the gate-drain Schottky diode. During measurement of this temperature-sensitive parameter, the source is left floating while the drain is grounded. Previous studies have shown that the hot spot in GaN HEMTs is near the gate side, close to the drain. The forward voltage drop across the gate-drain diode yields higher and more consistent temperatures relative to those using the forward voltage drop across the gate-source Schottky diode. This method could potentially provide more accurate data for thermal management and reliability analyses of GaN-based HEMTs. The maximum temperature of the sample was 44.8 °C on the grate-drain side and approximately 40.5 °C on the gate-source side when the sample exhibited 3.44W thermal power consumption for 1 min. Hence, the proposed method is practical and instructive.

Effect of annealing on the electrical performance of N-polarity GaN Schottky barrier diodes

A nitrogen-polarity (N-polarity) GaN-based high electron mobility transistor (HEMT) shows great potential for high-frequency solid-state power amplifier applications because its two-dimensional electron gas (2DEG) density and mobility are minimally affected by device scaling. However, the Schottky barrier height (SBH) of N-polarity GaN is low. This leads to a large gate leakage in N-polarity GaN-based HEMTs. In this work, we investigate the effect of annealing on the electrical characteristics of N-polarity GaN-based Schottky barrier diodes (SBDs) with Ni/Au electrodes. Our results show that the annealing time and temperature have a large influence on the electrical properties of N-polarity GaN SBDs. Compared to the N-polarity SBD without annealing, the SBH and rectification ratio at ±5 V of the SBD are increased from 0.51 eV and 30 to 0.77 eV and 7700, respectively, and the ideal factor of the SBD is decreased from 1.66 to 1.54 after an optimized annealing process. Our analysis results suggest that the improvement of the electrical properties of SBDs after annealing is mainly due to the reduction of the interface state density between Schottky contact metals and N-polarity GaN and the increase of barrier height for the electron emission from the trap state at low reverse bias.

Effect of Fe Doping Profile on Current Collapse in GaN-based RF HEMTs.

Using computer-aided design (TCAD) simulation, the impact of the Fe doping profile, including concentration, decay rate, and depth of the doping region on current-collapse magnitude (▵CC) in 0.5-μm gated GaN-based high electron mobility transistors (HEMTs) is systematically investigated. Accurate simulation models are established and developed to facilitate the fabrication of electronics. It is elucidated that the intricate interplay between trapping and de-trapping of Fe-related traps at the gate-drain edge is responsible for current collapse. The concentration and decay rate of the doping region have a more significant impact on current collapse than the depth. Increased trap state density near two-dimensional electron gas (2DEG) channel caused by deep-level acceptors would boost ▵CC. However, a minor dynamic reduction in 2DEG density (nT) induces a relatively small ▵CC. By adjusting the concentration, decay rate, and depth of the doping region, ▵CC of GaN-based Radio Frequency (RF) HEMTs can be reduced by approximately 50.3 %. The optimized distribution of Fe doping discussed in this work helps to prepare GaN-based RF HEMTs with a limited current collapse effect.

Performance improvement of enhancement-mode GaN-based HEMT power devices by employing a vertical gate structure and composite interlayers*

In this work, p-n junction vertical gate (JVG) and polarization junction vertical gate (PVG) structures are for the first time proposed to improve the performance of GaN-based enhancement-mode (E-mode) high electron mobility transistor (HEMT) devices. Compared with the control group featuring the vertical gate structure, a highly improved threshold voltage (V th) and breakdown voltage (BV) are achieved with the assistance of the extended depletion regions formed by inserting single or composite interlayers. The structure dimensions and physical parameters for device interlayers are optimized by TCAD simulation to adjust the spatial electric field distribution and hence improve the device off-state characteristics. The optimal JVG-HEMT device can reach a V th of 3.4 V, a low on-state resistance (R on) of 0.64 mΩ cm2, and a BV of 1245 V, while the PVG-HEMT device exhibits a V th of 3.7 V, an R on of 0.65 mΩ cm2, and a BV of 1184 V, which could be further boosted when an additional field plate design is employed. Thus, the figure-of-merit value of JVG- and PVG-HEMT devices rise to 2.4 and 2.2 GW cm−2, respectively, much higher than that for the VG-HEMT control group (1.0 GW cm−2). This work provides a novel technical approach to realize higher-performance E-mode HEMTs.

Insights into dynamic switching behavior and electric field distribution of depletion-mode GaN HEMT with bonding pad over active layout

Bonding pad over active (BPOA) layout, which stacks traditional horizontal structure pad electrodes vertically above the active area, is an area-effective device architecture and packaging-enabled solution for GaN-based high electron mobility transistors (HEMTs). In this work, the dynamic switching and electric field distribution of such layout, as well as associated capacitance–voltage and trapping characteristics, are comprehensively studied on D-mode GaN-on-sapphire HEMT by performing numerical simulations. In terms of different pad electrodes covering the active area, BPOA is composed of various structures such as gate-related and drain-related BPOAs (i.e. G-BPOA and D-BPOA), among which G-BPOA exhibits inferior switching characteristics due to the additional introduction of Miller capacitance that prolongs the device switching, while breakdown voltage of D-BPOA is 400–900 V lower than other BPOA counterparts due to the interplay between D-BPOA and the drain electrode. Furthermore, the effects of trap capture cross section and trap density on switching characteristics are evaluated. These results highlight the differences in electrical characteristics of various structures within the BPOA layout, and provide valuable insights into BPOA device design and performance improvement.

Improving Optical and Electrical Characteristics of GaN Films via 3D Island to 2D Growth Mode Transition Using Molecular Beam Epitaxy

Molecular beam epitaxy (MBE) is demonstrated as an excellent growth technique for growing a low-defect GaN channel layer, which is crucial for controlling vertical leakage current and improving breakdown voltage (BV) in GaN-based high-electron mobility transistors (HEMTs). The 3D islands to 2D growth mode transition approach was induced by modulating substrate growth temperature (Tsub), displaying an overall improvement in film quality. A comprehensive investigation was conducted into the effects of Tsub on surface morphologies, crystal quality, and the optical and electrical properties of GaN films. Optimal results were achieved with a strain-relaxed GaN film grown at 690 °C, exhibiting significantly improved surface characteristics (root-mean-square roughness, Rq = 0.3 nm) and impressively reduced edge dislocations. However, the film with the smoothest surface roughness, attributed to the effect of the Ga-rich condition, possessed a high surface pit density, negatively affecting optical and electrical properties. A reduction in defect-related yellow emission further confirmed the enhanced crystalline quality of MBE GaN films. The optimized GaN film demonstrated outstanding electrical properties with a BV of ~1450 V, surpassing that of MOCVD GaN (~1180 V). This research significantly contributes to the advancement of MBE GaN-based high electron mobility transistor (HEMT) applications by ensuring outstanding reliability.

Impact of Parasitic Gate Capacitance on RF Performance in GaN-Based HEMTs for X-Band Applications

GaN-based high-electron-mobility transistors (HEMTs) are intensively researched for high-power radio frequency (RF), low noise, and aerospace applications, thanks to their high breakdown electric field, high carrier mobility and density at the hetero-interface, and wide bandgap [1-2]. For the improvement of RF performance, various approaches have been reported, including T-gate structure [3-4], surface passivation [5], n+-regrown source and drain contact [6], thin barrier structure [7], and graded-channel [8]. Among them, the T-gate structure has been extensively employed for the reduction of the gate resistance (Rg ) and thereby improvement of the RF performance. However, the usage of the wide T-gate structure enlarges parasitic gate capacitance components, which deteriorates the RF performance. As a result, one should pay careful attention to the dimension of the wide T-gate structure to reduce Rg as much as possible, not to enlarge the parasitic gate capacitance components. This motivates us to explore the optimum dimension of the wide T-gate structure in GaN-based HEMTs for X-band RF applications.To investigate the detailed impact of the gate resistance and parasitic gate capacitance components, we fabricated LG = 0.15 μm GaN-based HEMTs with various dimensions of the T-gate head from 0.5 to 0.8 μm during an e-beam lithography process step. The unit gate width (WG ) and number of gate finger (NF ) were 100 μm and 2, respectively.For comparison of the device characteristics with respect to the dimension of the gate head, DC and RF measurements were conducted for the fabricated GaN-based HEMTs with various dimensions of T-gate head. The DC characteristics in terms of the threshold voltage (VTH ), the maximum transconductance (gm,max ), and the saturation drain current (Id,sat ) were almost identical, regardless of the dimension of the T-gate head. On the contrary, the current-gain cut-off frequency (fT ) and maximum oscillation frequency (fMAX ) were improved by 26.5% and 21.0%, respectively, when the T-gate head size was reduced from 0.8 to 0.5 μm.In an effort of explore the origin of the dependence of the T-gate head on the RF performance, we have carried out small-signal modeling to extract the parasitic gate capacitance and gate resistance components from the measured RF data. As the T-gate head size was reduced from 0.8 to 0.5 μm, the gate-source parasitic capacitance component (Cgs_par ) was reduced by 19%, while the gate resistance component was increased by 32%. In our device design and fabrication, it turned out that the reduction of Cgs_par was more beneficial than the increase of Rg , leading to the improved RF performance with the gate head size decrease.In this work, our systematic research revealed that, unlike earlier reports [3-4], the reduction of the parasitic gate capacitance was essential for the improvement of the RF performance as opposed to the reduction of the gate resistance in LG = 0.15 μm GaN-based HEMTs for X-band RF applications. Acknowledgement This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MIST) (No. NRF-2021M3C1C3097672).

Novel T-Shaped Gate Structure of AlGaN/GaN HEMTs on Si for RF Application

Due to the high electron saturation velocity and breakdown electric field, GaN-based high electron mobility transistors (HEMTs) have been widely used in high frequency and power application, such as telecommunication, satellite communication, and radar system [1]. Since first demonstration of AlGaN/GaN HEMTs in 1993 [2], the RF performance of these device has continuously improved by the progress on epitaxy and device scaling technology [3].The current gain cut-off frequency (fT) and the maximum oscillation frequency (fmax) from the small-signal equivalent circuit of the HEMT are derived by the following equations [4].From these equations, it can be concluded that in order to achieve high frequency operation, HEMTs should be designed with a T-shaped gate structure that simultaneously achieves low gate resistance (Rg) and low parasitic capacitance (Cgs and Cgd). However, there are complex fabrication processes to form the T-shaped gate. Typically, the T-shaped gate structure is implemented through electron beam lithography process using a tri-layer resist such as PMMA/PMGI/PMMA, which requires multiple exposures or developments. Multiple exposures can cause overlay errors that can significantly affect the T-shaped gate with smaller gate lengths [5]. Also, it is difficult to obtain an undercut structure that enables a good lift-off and guarantees a high yield due to the need for precise dose control. In addition, since the T-shaped gate has an unstable structure in which a narrow foot supports the weight of a wide head, the gate may collapse during a subsequent process.In this work, we fabricated a novel T-shaped gate using the negative e-beam resist HSQ, which offers a simple T-shaped gate fabrication process. The HSQ that was exposed to e-beam is located underneath the gate head and on both sides of the gate foot in our novel T-shaped gate design, providing a robust structure. The HSQ-assisted gate HEMTs exhibit comparable DC/RF performance compared to conventional T-shaped gate HEMTs. Acknowledgement This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MIST) (No. NRF-2021M3C13097672).

(Invited) Selective Area Growth, Etching, and Doping of GaN By MOCVD for Power Electronics

Gallium Nitride (GaN) possesses a wide band gap and excellent transport properties, making it a highly promising material for high-power and high-frequency applications. Microwave power amplification for commercial and military purposes has already benefited from GaN-based high electron mobility transistors (HEMTs). However, currently available GaN optoelectronic and electronic devices use planar junctions and heterostructures prepared by epitaxy, typically via metalorganic chemical vapor deposition (MOCVD). To enable more sophisticated device configurations, in-plane lateral junctions are necessary. The most effective method for achieving this is through selective area etching (SAE) followed by selective area growth (SAG). In this talk, we will discuss the progress made at Yale in the selective-area etching, growth, and doping of GaN.Selective-area etching of GaN is the most challenging and critical step in this process. Instead of using conventional ICP etching, we have employed tertiarybutylchloride (TBCl) as an in-situ chemical etchant of GaN in MOCVD to create smooth trenches and remove plasma etching-generated damages on the surface. With electrical and material characterizations, we have confirmed that TBCl etching is a low-damage and clean etching process. We will also describe our recent efforts to bypass the constraint of dielectric masking during in-situ etching and perform maskless selective area etching (SAE) to create low-defect lateral p-n junctions for GaN power electronics. Additionally, we will discuss non-planar selective-area growth and doping, including the principles of growth evolution, non-uniform Mg distribution, and parasitic impurities.This work is supported by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. DOE, under DE-AR0000871 as part of the PNDIODES program managed by Dr. Isik Kizilyalli.

Effects of Thermal Boundary Resistance on Thermal Management of Gallium-Nitride-Based Semiconductor Devices: A Review.

Wide-bandgap gallium nitride (GaN)-based semiconductors offer significant advantages over traditional Si-based semiconductors in terms of high-power and high-frequency operations. As it has superior properties, such as high operating temperatures, high-frequency operation, high breakdown electric field, and enhanced radiation resistance, GaN is applied in various fields, such as power electronic devices, renewable energy systems, light-emitting diodes, and radio frequency (RF) electronic devices. For example, GaN-based high-electron-mobility transistors (HEMTs) are used widely in various applications, such as 5G cellular networks, satellite communication, and radar systems. When a current flows through the transistor channels during operation, the self-heating effect (SHE) deriving from joule heat generation causes a significant increase in the temperature. Increases in the channel temperature reduce the carrier mobility and cause a shift in the threshold voltage, resulting in significant performance degradation. Moreover, temperature increases cause substantial lifetime reductions. Accordingly, GaN-based HEMTs are operated at a low power, although they have demonstrated high RF output power potential. The SHE is expected to be even more important in future advanced technology designs, such as gate-all-around field-effect transistor (GAAFET) and three-dimensional (3D) IC architectures. Materials with high thermal conductivities, such as silicon carbide (SiC) and diamond, are good candidates as substrates for heat dissipation in GaN-based semiconductors. However, the thermal boundary resistance (TBR) of the GaN/substrate interface is a bottleneck for heat dissipation. This bottleneck should be reduced optimally to enable full employment of the high thermal conductivity of the substrates. Here, we comprehensively review the experimental and simulation studies that report TBRs in GaN-on-SiC and GaN-on-diamond devices. The effects of the growth methods, growth conditions, integration methods, and interlayer structures on the TBR are summarized. This study provides guidelines for decreasing the TBR for thermal management in the design and implementation of GaN-based semiconductor devices.

The effect of Si3N4/Al2O3 stacked structure of AlGaN/GaN HEMTs

Excessive gate leakage current and current collapse effects are the critical reliability issues for GaN-based high electron mobility transistors (HEMTs). In this paper, Si3N4 passivation layers was grown separately on p-GaN gate HRCL-HEMTs with or without Al2O3 film, and the gate leakage and current collapse effects of the devices and their intrinsic mechanisms were investigated. It is found that the device with Si3N4/Al2O3 stacked structure has better performance in terms of gate reverse leakage current, breakdown voltage, etc. The Al2O3 film blocking layer can reduce the H plasma damage to the channel, decrease the gate leakage, and the passivation can effectively suppress the increase of gate leakage current. Through variable temperature characteristic test, it is verified that the dominant mechanism of the device gate reverse leakage current is the two-dimensional variable range hopping (2D-VRH) model. The interface state between Si3N4 passivation layer and Al2O3 film increases the trap activation energy of the device, which is the reason for the reduction of gate reverse leakage current, and also causes the current collapse effect of the device.

Trap Characterization Techniques for GaN-Based HEMTs: A Critical Review.

Gallium nitride (GaN) high-electron-mobility transistors (HEMTs) have been considered promising candidates for power devices due to their superior advantages of high current density, high breakdown voltage, high power density, and high-frequency operations. However, the development of GaN HEMTs has been constrained by stability and reliability issues related to traps. In this article, the locations and energy levels of traps in GaN HEMTs are summarized. Moreover, the characterization techniques for bulk traps and interface traps, whose characteristics and scopes are included as well, are reviewed and highlighted. Finally, the challenges in trap characterization techniques for GaN-based HEMTs are discussed to provide insights into the reliability assessment of GaN-based HEMTs.

Design and Simulation of Enhancement-Mode Vertical Superjunction GaN HEMT with Improved Ron and Cut-Off Frequency

GaN-based vertical superjunction high electron mobility transistors (HEMT) are recent avenues for power transistors. In this work, an enhancement mode vertical superjunction GaN-HEMT with gradient doping (GDP-SJ HEMT) is proposed and analyzed. In the GDP-SJ HEMT, the n-pillar doping concentration increases in a gradient manner from top to bottom. Whereas the concentration of the p-pillar is uniform and equivalent to the doping of the middle of the n-pillar. By optimizing the device design and doping profile, the specific-on resistance (Ron,sp) of the device has been reduced from 5.0 mΩ-cm2 to 3.79 mΩ-cm2. To the best of our knowledge, the frequency performance of vertical SJ HEMT has not been documented in the literature. As a result, we also performed a frequency study. The cut-off frequency (f T) of the proposed device is 78 MHz and the maximum frequency (f MAX) is 71 MHz. The device optimization increases the f T by 89% i.e. 148 MHz and f MAX by 67% i.e. 119 MHz.

A comprehensive review of AlGaN/GaN High electron mobility transistors: Architectures and field plate techniques for high power/ high frequency applications

AlGaN/GaN High Electron Mobility Transistor (HEMT) frequently employs field plate techniques to improve the device's reliability and optimum performance. This literature review investigates the effects of field plate-engineered GaN-based High Electron Mobility Transistors (HEMTs) on RF applications, specifically focusing on elevated breakdown and cut-off frequency. The study comprehensively examines various field plate designs employed in HEMTs, highlighting their significant contributions. The research explores three primary and six secondary types of field plates, each operating on distinct principles to enhance device parameters. For instance, gate, drain, and source-type field plates in GaN-based HEMTs can imprcove breakdown, power and output resistance. These field plate techniques influence device stability, specific resistance and ensure uniform electron flow across terminals. Moreover, the implementation of a drain field plate improves power application performance by reducing the impact ionization rate. The utilization of field plate techniques in GaN-based High Electron Mobility Transistors (HEMTs) offers several benefits, including the minimization of parasitic effects and the reduction of the source-to-gate region without significantly impacting the device's breakdown voltage.

Scanning capacitance microscopy of GaN-based high electron mobility transistor structures: A practical guide

The scanning capacitance microscope (SCM) is a powerful tool to characterise local electrical properties in GaN-based high electron mobility transistor (HEMT) structures with nanoscale resolution. We investigated the experimental setup and the imaging conditions to optimise the SCM contrast. As to the experimental setup, we show that the desired tip should be sharp (e.g., with the tip radius of ≤25nm) and its coating should be made of conductive doped diamond. Most importantly, its spring constant should be large to achieve stable tip-sample contact. The selected tip should be positioned close to both the edge and Ohmic contact of the sample. Regarding the imaging conditions, we also show that a dc bias should be applied in addition to an ac bias because the latter alone is not sufficient to deplete the two-dimensional electron gas (2DEG) in the AlGaN/GaN heterostructure. The approximate range of the effective dc bias values was found by measuring the local dC/dV-V curves, yielding, after further optimisation, two optimised dc bias values which provide strong, but opposite, SCM contrast. In comparison, the selected ac bias value has no significant impact on the SCM contrast. The described methodology could potentially also be applied to other types of HEMT structures, and highly-doped samples.

Light-triggered 2D electron gas in a GaN-based HEMT with sandwiched p-GaN layers.

In this work, a p-n junction-coupled metal-insulator-semiconductor (MIS) normally-off high-electron-mobility transistor (HEMT) UVPD is proposed. A two-dimensional electron gas (2DEG) at the AlN/U-GaN interface is entirely depleted with a dark current of 1.97 × 10-11 A because of the design of the sandwiched p-GaN layers. Under 365 nm illumination, the 2DEG is light triggered at Vds = 1 V with a high light on/off ratio of over 107 at a light power density of 286.39 mW·cm-2. Meanwhile, it exhibits fast rise and decay times of 248.39 and 584.79 µs, respectively. Moreover, a maximum responsivity (R) of 2.33 A/W, a maximum EQE of 793%, and a D* of 1.08 × 1013 Jones are obtained at Vds = 1 V. This can be attributed to the built-in electric fields in the configuration, which accelerate the flow of photogenerated carriers into the AlN/U-GaN channel. Additionally, the device showcases stable durability, repeatability, and a low driving voltage, making it highly suitable for applications in UV communication and space exploration.

Metal-semiconductor direct-current triboelectric nanogenerator based on depletion mode u-GaN/AlGaN/AlN/GaN HEMT

The urgent need for renewable energy source has led to a significant interest in triboelectric nanogenerators (TENGs) as a new energy technology. In contrast to traditional polymer TENGs, semiconductor direct-current TENGs are more suitable for miniaturization and integration with electronic devices. This study proposes a friction material made of depletion mode GaN high electron mobility transistors (HEMTs), which exhibit superior properties such as high two-dimensional electron gas concentration. By sliding a titanium sheet on a depletion mode GaN-based heterostructure, we have designed a metal-semiconductor direct-current triboelectric nanogenerator that achieved voltage up to 45.5 V and a peak power density of 2.32 W/m2. This generator can be used to supply DC power to 14 LEDs in series and drive a digital watch directly. In particular, the generation of direct current is predominantly influenced by the surface states of the undoped GaN cap that produce a large number of electrons and are associated with an additional electric field in the direction of the two-dimensional electron gas created in the u-GaN/AlGaN/AlN/GaN heterostructure of depletion mode GaN-based HEMTs. This research not only introduces a nitride semiconductor material of GaN-based HEMTs for the metal-semiconductor interface friction in the DC TENGs but also elucidates the current generation mechanism of GaN-based HEMT TENGs.