Two-dimensional Electron Gas Channel Research Articles

NiO/AlGaN interface reconstruction and transport manipulation of p-NiO gated AlGaN/GaN HEMTs

Normally off AlGaN/GaN high electron mobility transistors with a p-type gate are promising for power switching applications, with advantages of low energy consumption and safe operation. In this work, p-NiO is employed as a gate stack, and the interfacial reconstruction and band structure modification at the p-NiO/AlGaN interface have been demonstrated to manipulate channel transport of AlGaN/GaN high electron mobility transistors by post-annealing. In addition to achieving a positive threshold voltage of 0.6 V and a large saturation output current of 520 mA/mm, we found that the gate leakage and On/Off drain current ratio can be improved significantly by more than 104 due to the p-NiO/AlGaN interfacial reconstruction. However, high annealing temperature also results in an increasing ON-resistance and a dramatically increased knee voltage (VK), which can be attributed to the formation of an ultra-thin γ-Al2O3 layer and the substitution of O on N site as a shallow donor at the p-NiO/AlGaN interface confirmed by experimental analyses. Theoretical calculations indicate that such interface reconstruction facilitates an additional potential well at the p-NiO/AlGaN interface to which electrons are spilled out from a two-dimensional electron gas channel under high forward gate voltage, resulting in the increased VK. Finally, an optimized annealing condition was confirmed that can eliminate this increased VK phenomenon and simultaneously remain these significantly improved device performances. These findings provide deep understanding of the performance manipulation of AlGaN high electron mobility transistors, which is very important for engineering the p-NiO/AlGaN interface toward high-performance and stable devices.

(Invited) Multi-Channel AlGaN/GaN Power Rectifiers: Breakthrough Performance up to 10 kV

High-voltage power rectifiers are widely used in renewable energy processing, electric grids, industrial motor drives, pulse power systems, among other applications. Today’s high-voltage rectifier market is dominated by bipolar Si diodes up to 6.5 kV, which suffer from slow reverse recovery. Wide-bandgap SiC unipolar diodes have been pre-commercialized up to 10 kV, which allows for a much higher switching speed. Recently, we have developed a new generation of high-voltage rectifiers based on the multi-channel AlGaN/GaN platform, which highlight a series of novel device designs incorporating the stacked two-dimensional electron gas (2DEG) channels, p-n junctions, and 3-D fin structures. With these innovations, the performances of our unipolar 1.2-10 kV multi-channel AlGaN/GaN Schottky rectifiers well exceed the Si and SiC 1-D limit, at the same time possessing a lower cost as compared to SiC counterparts. This paper reviews our efforts in the design, fabrication and characterization of these GaN devices. Our results show the tremendous promise of GaN for medium-voltage and high-voltage power electronics applications.

Current oscillation in GaN-HEMTs with p-GaN islands buried layer for terahertz applications

A GaN-based high electron mobility transistor (HEMT) with p-GaN islands buried layer (PIBL) for terahertz applications is proposed. The introduction of a p-GaN island redistributes the electric field in the gate–drain channel region, thereby promoting the formation of electronic domains in the two-dimensional electron gas (2DEG) channel. The formation and regulation mechanism of the electronic domains in the device are investigated using Silvaco-TCAD software. Simulation results show that the 0.2 μm gate HEMT with a PIBL structure having a p-GaN island doping concentration (N p) of 2.5 × 1018 cm−3–3 × 1018 cm−3 can generate stable oscillations up to 344 GHz–400 GHz under the gate–source voltage (V gs) of 0.6 V. As the distance (D p) between the p-GaN island and the heterojunction interface increases from 5 nm to 15 nm, the fundamental frequency decreases from 377 GHz to 344 GHz, as well as the ratio of oscillation current amplitude of the fundamental component to the average component I f/I avg ranging from 2.4% to 3.84%.

Surface sensibility and stability of AlGaN/GaN ion-sensitive field-effect transistors with high Al-content AlGaN barrier layer

As pH sensors, AlGaN/GaN ion-sensitive field-effect transistors (ISFETs) with stacked and high Al-content AlGaN barrier have been fabricated to evaluate the sensing ability and surface reliability. It demonstrates that the stacked barrier shows mild influence on the two-dimensional electron gas channel. In addition, the AlGaN/GaN ISFETs present good pinch-off characteristics in three kinds of solutions with different pH values, showing a sensitivity of approximately 55.5 mV/pH. However, threshold voltage shifting as well as the output current decreasing were observed after worked in alkaline solution. The surface analysis results confirm that the surface was electrochemical etched into many voids, resulting in a positive shift on the threshold voltage. This etching process is related to the surface oxide and dislocation on the high Al-content AlGaN barrier layer. Therefore, the degradation in electrical performances can be attributed to the transformation of AlGaN into surface oxide as well as the followed dissolution in alkaline solution.

Threshold Voltage Modulation in a Transistor with a Two-Dimensional Electron Gas Channel at the Interface between Al2O3 and Sub-5 nm ZnO Films

Recent reports on the emergence of two-dimensional electron gas at the heterointerface of amorphous and polycrystalline oxide thin films, such as amorphous-Al2O3/polycrystalline-TiO2 (a-AO/TO) and a-AO/polycrystalline-ZnO on a Si wafer, open a venue for oxide-based thin-film-transistor research. Due to the higher Hall mobility of ZnO, the device performance was considerably improved in a-AO/ZnO compared with a-AO/TO. However, an excessively high negative threshold voltage (Vth) issue still remains, resulting in a static power consumption problem in the channel-off state. In this study, the Vth value of the a-AO/ZnO device was shifted positively by decreasing the thickness of the ZnO thin film (tZnO) to less than 10 nm, accompanied by a grain size reduction of the polycrystalline ZnO film. The large density of grain boundaries in the ZnO film with thinner thickness trapped the 2D electrons, leading to a decrease in effective carrier density (ns). As ns decreased by decreasing the tZnO from 11 to 4.6 nm, the Vth increased from −2.6 to −0.63 V. With an optimized tZnO of 5.6 nm, the transistor exhibited a Vth of −0.86 V, a saturated field-effect mobility of ∼4.8 cm2/(V s), and a subthreshold swing of 194 mV/dec.

Role of energy-band offset in photo-electrochemical etching mechanism of p-GaN heterostructures

For developing p-GaN gate-based enhancement-mode AlGaN/GaN high-electron-mobility transistors, the removal of the p-GaN layer around the gate region is demonstrated by photo-electrochemical etching. The etching behavior of p-GaN/AlGaN/GaN heterostructures along with p-GaN/u-GaN reference samples is investigated in acidic H2SO4/H3PO4 electrolyte solution. The strong oxidant sulfate radical SO4−∗ formed by H2SO4 stimulates the etching process by either extracting excess electrons from the valence band or providing supplementary holes in the valence band. By optimizing the etching parameters, an average etching rate of 0.27 nm/min with a surface roughness of 2.3 nm is achieved for removing p-GaN from the AlGaN/GaN structure. The etching dynamics of the heterostructure are attributed to the valence-band offset ΔEv caused by band discontinuity at the p-GaN/AlGaN interface. Under positive anodic bias, the etching process is also controlled by local charge transport between p-GaN and the two-dimensional electron gas channel due to a reduced barrier height.

Abnormal phenomenon of source-drain current of AlGaN/GaN heterostructure device under UV/visible light irradiation* *Project supported by Key-Area Research and Development Program of Guangdong Province, China (Grant Nos. 2019B010132001 and 2019B010132003), the Joint Funding of the National Natural Science Foundation of China (NSFC) & the Macao Science and Technology Development Fund (FDCT) of China (Grant No. 62061160368), the National Key Research

We report an abnormal phenomenon that the source-drain current (I D) of AlGaN/GaN heterostructure devices decreases under visible light irradiation. When the incident light wavelength is 390 nm, the photon energy is less than the band gaps of GaN and AlGaN whereas it can causes an increase of I D. Based on the UV light irradiation, a decrease of I D can still be observed when turning on the visible light. We speculate that this abnormal phenomenon is related to the surface barrier height, the unionized donor-like surface states below the surface Fermi level and the ionized donor-like surface states above the surface Fermi level. For visible light, its photon energy is less than the surface barrier height of the AlGaN layer. The electrons bound in the donor-like surface states below the Fermi level are excited and trapped by the ionized donor-like surface states between the Fermi level and the conduction band of AlGaN. The electrons trapped in ionized donor-like surface states show a long relaxation time, and the newly ionized donor-like surface states below the surface Fermi level are filled with electrons from the two-dimensional electron gas (2DEG) channel at AlGaN/GaN interface, which causes the decrease of I D. For the UV light, when its photon energy is larger than the surface barrier height of the AlGaN layer, electrons in the donor-like surface states below the Fermi level are excited to the conduction band and then drift into the 2DEG channel quickly, which cause the increase of I D.

Observation and characterization of impact ionization-induced OFF-state breakdown in Schottky-type p-GaN gate HEMTs

In this work, the impact ionization-induced OFF-state breakdown is revealed and systematically investigated in 100 V Schottky-type p-GaN gate high-electron-mobility transistors. Impact ionization is found to occur in the peak electric-field region at the source-terminated field-plate edge and is initiated by electrons injected from the source-side two-dimensional electron gas channel through the buffer layer. Hot electrons generated from impact ionization, when being captured by surface traps, could lead to redistribution and peak value reduction of electric-field. Consequently, the sudden rise in the OFF-state leakage current by impact ionization could be self-clamped temporally to avoid catastrophic rupture of the device. The impact ionization is further verified by the increase in dynamic OFF-state leakage current and a negative shift in threshold voltage, both of which result from positively charged holes (generated from impact ionization) drifting toward the gated channel.

New investigation of an E-mode metal-insulator-semiconductor AlInN/AlN/GaN HEMT with an Au-T-gate

In a high electron mobility transistor (HEMT), the density of the two-dimensional electron gas (2DEG) channel is modulated by the application of a bias to a Schottky metal gate. These devices are depletion mode (D-mode), which means that a negative bias must be applied to the gate to deplete the electron channel and turn. The most challenging aspect in the present research activity on based-GaN devices is the development of a reliable way to achieve an enhancement-mode (E-mode) HEMT. Enhancement-mode GaN HEMTs would offer a simplified circuitry by eliminating the negative power supply. In this work, the aim is to investigate the different techniques which can influence the threshold voltage and shift it to a positive value. A novel E-mode metal-insulator-semiconductor (MIS) AlInN/GaN HEMT with an Au-T-gate has been investigated. The impacts of window-recess and deep-recess have been discussed, it was found that for dp=28 nm and wn=1.8 μm the threshold voltage achieves 0.7 V and the transconductance (Gm) peak value of 523 mS at Vgs=3.5 V. The drain current characteristic has been demonstrated.

Effect of Fixed Charges at Interface Between InP and Bonding Layer on Heterogeneous Integration of InP HEMTs

DC performances of depletion-mode 100 nm In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</sub> As/InP high electron mobility transistor (HEMT) heterogeneously integrated to silicon substrate by SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> bonding were investigated by using numerical simulation. Effects of fixed charges at the InP/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> interface, the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> bonding interface, and the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si interface on HEMT are compared. Among these charges, dc characteristics of HEMT are most sensitive to fixed charges at the InP/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> interface. With the density of negative fixed charges at the InP/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> interface increasing from - 5×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> to - 1×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> , the energy band exhibits an upward bending and leads to a decrease of electron concentration in the channel. The threshold voltage shifts from -1.03 to -0.85 V and the output current exhibits a decrease of 96 mA/mm under the gate bias of 0 V; meanwhile, the breakdown voltage increases from 4.4 to 6.5 V under the gate bias of -1.6 V. A shift of C- V curves in pace with transfer characteristic curves further confirms that the two-dimensional electron gas (2DEG) channel of HEMT is indeed influenced by fixed charges at the InP/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> interface. As a comparison, the other two charges exhibit a relatively weak influence, where the increase of charges at the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> bonding interface results in a 36 mV shift of threshold voltage and 20 mA/mm decrease of output current, and the charges at the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si interface only cause 6 mV shift of threshold that can almost be ignored. We proposed an appropriate increase of InP layer thickness to mitigate the impact of fixed charges on the transfer and output characteristic of HEMTs.

Multi-channel nanowire devices for efficient power conversion

Nanowire-based devices can potentially be of use in a variety of electronic applications, from ultrascaled digital circuits to 5G communication networks. However, the devices are typically restricted to low-power applications due to the relatively low electrical conductivity and limited voltage capability of the nanowires. Here, we show that wide-band-gap AlGaN/GaN nanowires containing multiple two-dimensional electron gas channels can be used to create high-electron-mobility tri-gate transistors for power-conversion applications. The multiple channels lead to improved conductivity in the nanowires, and a three-dimensional field-plate design is used to manage the high electric field. Power devices made with 15-nm-wide nanowires are shown to exhibit low specific on resistances of 0.46 mΩ cm−2, enhancement-mode operation, improved dynamic behaviour and breakdown voltages as high as 1,300 V.

Spin transport in polarization induced two-dimensional electron gas channel in c-GaN nano-wedges

A two-dimensional electron gas (2DEG), which has recently been shown to develop in the central vertical plane of a wedge-shaped c-oriented GaN nanowall due to spontaneous polarization effect, offers a unique scenario, where the symmetry between the conduction and valence band is preserved over the entire confining potential. This results in the suppression of Rashba coupling even when the shape of the wedge is not symmetric. Here, for such a 2DEG channel, relaxation time for different spin projections is calculated as a function of donor concentration and gate bias. Our study reveals a strong dependence of the relaxation rate on the spin-orientation and density of carriers in the channel. Most interestingly, relaxation of spin oriented along the direction of confinement has been found to be completely switched off. Upon applying a suitable bias at the gate, the process can be switched on again. Exploiting this fascinating effect, an electrically driven spin-transistor has been proposed.

A Dielectrically Modulated GaN/AlN/AlGaN MOSHEMT with a Nanogap Embedded Cavity for Biosensing Applications

In this work, GaN/AlN/AlGaN MOS-HEMT with a cavity below the gate towards the drain side is studied for its sensitivity analysis and viability as a biosensor. The analysis is done by dielectric modulation of the cavity region to emulate the presence of different dielectric biomolecules and charged biomolecules by interface charge variation. MOSFET-based dielectrically modulated sensors have been a success experimentally and this work extends and demonstrates this concept with GaN HEMT. The device performance is evaluated through the shift in threshold voltage (V th) and drain current (I DS), which are used as performance metrics. The proposed device structure simulations were performed with ATLAS Silvaco device simulation tool which depicts the bio-immobilization in the cavity leads to the changes in electrostatic properties like conduction band offset, two-dimensional electron gas (2DEG) sheet carrier concentration and channel potential. The simulation analysis reveals V th and I DS shift up to 1.1 V and 153.7 mA/mm for the neutral biomolecules, whereas for deoxyribo nucleic acid, the shift is up to 0.30 mV and 65.2 mA/mm, respectively, implying a highly sensitive device. The AlGaN layer thickness and cavity fill height variations on device sensitivity are also reported.

The Characteristics of 6-Inch GaN on Si RF HEMT with High Isolation Composited Buffer Layer Design

In this study, a 50-nm Al0.05Ga0.95N back barrier (BB) layer was used in an AlGaN/GaN high-electron-mobility transistor between the two-dimensional electron gas channel and Fe-doped/C-doped buffer layers. This BB layer can reduce the channel layer. The BB layer is affected by doped carriers in the buffer layer and the conduction energy band between the channel and the buffer layers. The Ion/Ioff ratio of the BB device was 4.66 × 105, and the ratio for the device without BB was 1.91 × 103. Lower leakage currents were obtained in the BB device because of the higher conduction energy band. The 0.25-μm gate length device with the BB exhibited a high current gain cutoff frequency of 24.4 GHz, and power gain cutoff frequency of 73 GHz.

Dual metal gate AlGaN/GaN high electron mobility transistors with improved transconductance and reduced short channel effects

AlGaN/GaN high electron mobility transistors with a range of dual metal gate (DMG) lengths have been fabricated and studied. An improvement in transconductance up to 9% has been measured in the DMG devices in comparison to the conventional single metal gate devices. This is attributed to the distribution of the electric field under the gate region as a result of two gate metals. The drain induced barrier lowering is also suppressed using the sub-µm DMG devices, with a drain induced barrier lowering decrease of around 50% due to a potential shielding effect in the two-dimensional electron gas channel.

(Invited) Ga2O3 Phase Control and Heterojunctions Using Plasma-Enhanced Atomic Layer Epitaxy

Ga2O3 has emerged as a promising material for next generation power electronics. While β-Ga2O3 (monoclinic) is the most stable and studied of six Ga2O3 polymorphs, the slightly less energetically favorable α-, ε-, and κ-Ga2O3 phases have unique characteristics that can be exploited such as larger bandgaps, alloying for dopant control, or polarization beneficial to the formation of two-dimensional electron gas (2DEG) channels. Specifically, α-Ga2O3 (rhombohedral, corundum) has the largest bandgap of ~5.3 eV and can be alloyed with α-Al2O3 (8.8 eV) and α-In2O3 (3.7 eV) for bandgap engineering. Both ε-Ga2O3 (hexagonal, P63mc) and κ-Ga2O3 (orthorhombic, Pna21 ) phases are polar, with a predicted spontaneous polarization strength up to 10 times larger than GaN and 3 times larger than AlN. Like the III-N system, polarization induced charges can lead to higher charge densities and mobilities in 2DEGs formed at heterojunctions, which would improve the viability of Ga2O3 electronic devices. Plasma-enhanced atomic layer deposition (PEALD) is a popular, conformal, energy-enhanced synthesis method for thin films due to its many advantages, including: deposition at reduced growth temperatures, access to metastable phases, improved crystallinity, and increased growth rates. In this work, we use PEALD to produce high-quality heteroepitaxial Ga2O3 and (AlxGa1-x)2O3 (AlGO) films and investigate materials properties such as phase selectivity, ternary solubility limits, and electrical and optical performance.All Ga2O3 films were deposited in a Veeco Fiji G2 reactor equipped with a load lock and turbo pump using trimethygallium, trimethylaluminum, and O2 plasma. Initial studies on c-plane sapphire substrates at 350°C and 8 mTorr show the phase could be altered from β to α by a varying the pure O2 flow during plasma pulse from 5-40 sccm [1]. Optical emission spectroscopy indicate that the changes in the relative concentration of atomic oxygen is crucial for phase selectivity while the high ion flux to the surface can contribute to the crystallinity at low Tg [2]. To grow ε(κ)-Ga2O3 on c-plane sapphire required going to a much higher temperature (500°C), pressure (100’s mTorr), and O2 flow (100sccm) [1]. Without modifications to the current ALD system, pure ε(κ)-Ga2O3 on sapphire was not achieved under any conditions. Using optimum growth conditions for the three phases on sapphire, films were deposited on GaN and diamond to determine the effect of substrate structure. Transmission electron microscopy was conducted to determine the specific phases (β, ε, and κ) present in each case, and showed the amount of each phase varied with PEALD parameters. While films on diamond resulted in mixed β/ε(κ) phases, pure ε(κ)-phase films were attained on GaN and the strain varied with pressure and Tg. Vertical breakdown measurements were taken for both β- and ε(κ)-Ga2O3/n+ GaN substrates. Breakdown fields varied between 3.8-7.0 MV/cm dependent on the phase and strained state of the Ga2O3 films. β-Ga2O3 films showed less variability in breakdown field from device to device than ε(κ)-Ga2O3 films, but neither showed a dependence on device size.While PEALD is beneficial for depositing thin films of metastable phases, practical devices often require much thicker barrier and active layers. For this reason, we investigated integrating PEALD metastable Ga2O3 films with traditional semiconductor deposition techniques, such as molecular beam epitaxy (MBE), capable of extending these layers beyond 100 nm in thickness. The same MBE conditions were used to deposit Ga2O3 films on GaN substrates with and without PEALD ε(κ)-Ga2O3 nucleation layers. Those deposited without the PEALD metastable nucleation layer resulted in stable β-phase films, while those with nucleation layers resulted in pure ε(κ)-phase films. This shows importance of PEALD for realizing practical device structures using metastable phases.Finally, to investigate heterojunctions for 2DEG formation, AlxGa1-xO films were developed. While the full stoichiometric range could be reached using a PEALD digital alloying method, crystallinity was lost above x = 0.2 for the β phase, x = 0.35 for the ε(κ) phase, and x=0.6 for the α phase. Initial device structures will be shown in order to establish the feasibility of these films in device applications.[1] Wheeler, et al. Chem. Mater. 2020, 32, 1140-1152[2] Boris, et al. JVST A 2019, 37(6), 060909 Figure 1

Multi-aperture anode based AlGaN/GaN Schottky barrier diodes with low turn-on voltage and high uniformity

In this letter, an anode configuration with multi-aperture structure and fully recessed AlGaN barrier layer is proposed in the AlGaN/GaN Schottky barrier diode. With the Schottky junction formed between the Ni anode metal and the channel of two-dimensional electron gas, as well as the evidently enlarged contact profile by the introduction of multiple apertures, the device’s forward turning-on performances are dominated by the sidewall Schottky contact, achieving a low turn on voltage of 0.35 V with high uniformity. Accompanied with the high breakdown voltage of 2770 V, the diode achieved a power figure-of-merit as high as 1.1 GW cm−2.

High Threshold Voltage Normally off Ultra-Thin-Barrier GaN MISHEMT with MOCVD-Regrown Ohmics and Si-Rich LPCVD-SiNx Gate Insulator

A high threshold voltage (VTH) normally off GaN MISHEMTs with a uniform threshold voltage distribution (VTH = 4.25 ± 0.1 V at IDS = 1 μA/mm) were demonstrated by the selective area ohmic regrowth technique together with an Si-rich LPCVD-SiNx gate insulator. In the conventional GaN MOSFET structure, the carriers were induced by the inversion channel at a high positive gate voltage. However, this design sacrifices the channel mobility and reliability because a huge number of carriers are beneath the gate insulator directly during operation. In this study, a 3-nm ultra-thin Al0.25Ga0.75N barrier was adopted to provide a two-dimensional electron gas (2DEG) channel underneath the gate terminal and selective area MOCVD-regrowth layer to improve the ohmic contact resistivity. An Si-rich LPCVD-SiNx gate insulator was employed to absorb trace oxygen contamination on the GaN surface and to improve the insulator/GaN interface quality. Based on the breakdown voltage, current density, and dynamic RON measured results, the proposed LPCVD-MISHEMT provides a potential candidate solution for switching power electronics.

(Invited) Radiation Effects on the Self-Heating of AlGaN/GaN High Electron Mobility Transistors

Gallium nitride (GaN) has emerged as one of the most attractive wide bandgap materials for near-future electronics technology, specifically for applications that require high-power and high-frequency devices. The spontaneous and piezoelectric polarization of GaN utilized in AlGaN/GaN high electron mobility transistors (HEMT), provides significantly low ON-resistance due to the formation of a two-dimensional electron gas (2-DEG) channel at the AlGaN/GaN heterointerface without any intentional doping. The fact that these devices do not rely upon dopants to carry the current, in conjunction with superior material property of GaN, makes AlGaN/GaN HEMTs perfect candidates for deep space applications. However, the high voltage operation and the inherently low ON-resistance in AlGaN/GaN HEMTs result in extremely high power densities. Consequently, intense self-heating deteriorates device performance and more importantly, reliability.In order to counter the potential thermal problems associated with these device technologies for deep space applications, it is necessary to advance the knowledge of device irradiation and its effect on self-heating, material interfaces, and properties of constituent materials. To achieve this goal, a set of AlGaN/GaN HEMTs were irradiated with a proton flux at an energy level of 1 MeV at a fluence of 2x1015/cm2. Diverse optical thermography techniques including infrared thermography, thermoreflectance thermal imaging, and micro-Raman spectroscopy were used to quantify the effect of radiation on the device temperature distribution. It was observed that, as compared to pre-irradiated devices, irradiated devices show a 40% higher device peak temperature under identical power dissipation (5 W/mm). To understand the contributing factors behind the observations (and the resulting deterioration of device mean-time-to-failure), the device material stack and transmission line measurement (TLM) structures were analyzed further. Steady-state thermoreflectance (SSTR) measurement, standard electrical characterization, and 3D coupled electro-thermal modeling were performed to capture changes in the thermal conductivity, interfacial thermal boundary resistance, ohmic contact resistance, and sheet resistivity of the devices. This comprehensive study will support the exploitation of the full potential of GaN electronics for deep space and other applications that require radiation hardness.

Modulation of the two-dimensional electron gas channel in flexible AlGaN/GaN high-electron-mobility transistors by mechanical bending

We investigate the effect of strain on the two-dimensional electron gas (2DEG) channel in a flexible Al0.25Ga0.75N/GaN high-electron-mobility transistor (HEMT) by mechanical bending to prove the concept of active polarization engineering to create multifunctional electronic and photonic devices made of flexible group III-nitride thin films. The flexible HEMTs are fabricated by a layer-transfer process and integrated with a 150-μm-thick Cu film. The strain values are estimated from high-resolution x-ray diffraction and Raman spectroscopy in 4-cm bend-down and −4-cm bend-up test conditions. The strain-induced piezoelectric polarization can alter the charge density of the 2DEG in the channel at the AlGaN/GaN interface and thus modify the output characteristics of the flexible HEMTs. Accordingly, output characteristics show an increase in output current by 3.4% in the bend-down condition and a decrease by 4.3% in the bend-up condition. Transfer characteristics show a shift of threshold voltage, which also supports the 2DEG channel modulation during bending. Computational simulation based on the same structure confirms the same current modulation effect and threshold voltage shift. Furthermore, the electrical characteristics of the flexible HEMTs show a repeatable dependence on the strain effect, which offers potential for electro-mechanical device applications.