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Summary: We have demonstrated the gallium nitride (GaN)‐based betavoltaic (BV) cells with PN and PIN junction to investigate the relationship between the intrinsic GaN (i‐GaN) layer and power performance of BV cells. A short‐circuit current (ISC) and an open‐circuit voltage (VOC) of the fabricated BV cells were evaluated by using an electron‐beam (e‐beam) irradiation. The cell with PIN junction exhibited an improved ISC and VOC compared to the cell with PN junction. This is because the additional 200 nm‐thick i‐GaN layer extends the depletion region of BV cell, resulting in improved charge collection. When a 17 kV e‐beam irradiated into the fabricated GaN PIN BV cell, the device exhibited an ISC of 1.86 μA, a VOC of 2.23 V, a maximum output power (Pmax.out) of 2.74 μW, and a power conversion efficiency (PCE) of 4.5%, respectively. This PCE is the best value among GaN BV cell researches using 17 keV e‐beam irradiation. To study a role of i‐GaN layer in PIN BV cell, the technology computer‐aided design (TCAD) simulator was implemented. The effect of layer thickness and native defects in GaN material on the power performance of BV cell was evaluated. The power performance of PIN BV cell was degraded by introducing native defects in layers due to an increase of recombination rate. The BV cell with 500 nm‐thick i‐GaN layer exhibited better power performance when the electrons with average energy of Ni‐63 irradiated into device because the maximum absorption rate of electrons was well positioned in the depletion region. The experimental and simulated results showed that the introduction of i‐GaN layer and the optimization of parameters such as thickness and crystalline quality were important to improve the power performance of BV cells. [ABSTRACT FROM AUTHOR]

To improve the stability and sensitivity of pH sensors based on AlGaN/GaN high electron mobility transistors (HEMTs), we proposed a modified sensing structure by integrating a reference HEMT device. This structural pH sensor exhibits a typical Nernstian behavior with a sensitivity of 54.38 mV/pH, which is higher than the value of 49.43mV/pH derived from the AlGaN/GaN HEMT-based sensor without integrating the reference HEMTs device. Furthermore, the stability of the new structural sensor is enhanced by approximately 19.2% in comparison with that of its traditional counterpart. The improved performances are analyzed using an electrical double-layer model together with an equivalent circuit model, and we find that the new sensor structure has a larger capacitor and exhibits a better rectification effect, hence decreasing the electrical noise and enhancing the stability of the testing signal. Meanwhile, the new sensor structure displays a smaller equivalent resistance and results in a larger available output current, hence exhibiting a higher sensitivity. [ABSTRACT FROM AUTHOR]

We have successfully grown AlGaN/GaN high electron mobility transistor (HEMT) structure on the high-quality undoped thick AlN buffer layer with large band offset to replace the conventional high-resistivity GaN buffer layer. The AlGaN/GaN HEMT fabricated on this AlN buffer layer exhibits low OFF-state leakage current with high ION/IOFF of ~106 due to enhanced confinement of the electrons in the 2-D electron gas (2-DEG) channel. The undoped AlN buffer layer is responsible for suppressing the trapping effects to greatly reduce the current dispersion in pulsed ID – VD characteristics, which is hardly avoided in conventional deep acceptor-doped GaN buffer layer. The device also demonstrates high breakdown voltage of 2154 V with very high figure of merit (FOM) of ~1.8 GV2-1 cm−2, one of the highest ever reported, suggesting that the AlGaN/GaN-based Hemts WITH AlN buffer layer are promising for high-performance RF and power applications. [ABSTRACT FROM AUTHOR]

Summary: In this work, we designed and optimized a gallium nitride (GaN)‐based betavoltaic (BV) cell using an AlGaN back‐barrier layer and finger structure for improving the output power density. A short‐circuit current density (JSC) and an open‐circuit voltage (VOC) of the BV cells associated with an output power density were investigated by using electron‐beam (e‐beam) irradiation. The device with the Al0.25Ga0.75N back‐barrier layer exhibited an enhanced JSC because the potential barrier with a high height reduced excess carriers moving to the substrate region. The finger structure of the proposed BV cells was optimized by changing parameters such as the width of the intrinsic GaN region (Wi‐GaN) and heights of the p‐GaN and n‐GaN regions (Hp‐GaN and Hn‐GaN). The optimized BV cell with a Wi‐GaN of 100 nm, a Hn‐GaN of 100 nm, and a Hp‐GaN of 200 nm obtained a higher JSC compared to that of the conventional p‐i‐n BV cell because an optimum structure resulted in a wide depletion area, which was involved in the improved charge collection. As a result, the output power density of the proposed BV cell was enhanced by 14.8% than that of the conventional BV because of the improved JSC. The proposed structure shows a high potential for BV cells with a high‐power conversion efficiency. [ABSTRACT FROM AUTHOR]

The effects of isoelectronic Al doping into epitaxial GaN films grown by metal organic chemical vapor deposition on sapphire substrates were investigated. It was found, based on the measured electron mobility and x-ray analysis, that there is a limiting point of the incorporation of Al into GaN in improving the crystal quality. The electron mobility of the undoped GaN film was 178 cm2/V s and the value greatly increased to 524 cm2/V s by doping a small amount of Al (up to 0.45% in concentration) into the GaN layer. A further increase in the Al concentration resulted in a degradation of the electron mobility, which decreased to 138 cm2/V s when the Al concentration was 0.82%. The output power of a side view light emitting diode (LED) with the Al-doped GaN layer was estimated to be 15.76 mW at a forward current of 20 mA, which improved by 19% compared to that of a conventional LED. These results show that a small amount of Al incorporation into a GaN layer improves the electrical and optical properties of the layer, which are attributed to the reduction of Ga vacancy and associated defects, such as dislocations. [ABSTRACT FROM AUTHOR]

We report the drastic enhancement pattern of Shubnikov-de Haas (SdH) oscillations observed in an AlGaN/GaN heterostructure by microwave modulation. The dependence of the SdH pattern on microwave power and temperature is investigated. The underlying mechanism is attributed to the effect of carrier heating. This technique helps study the transport properties of two-dimensional electrons in many wide-band-gap heterostructures, in which moderate mobilities and heavier electron effective mass (rapidly damping SdH amplitudes) are frequently encountered. In addition, this method has the advantage of keeping the carrier concentration fixed and not requiring expensive high-energy laser facilities compared with carrier-modulated SdH measurements. [ABSTRACT FROM AUTHOR]

Temperature-based organic–inorganic photodiodes have recently become attractive applications in branches of science and technology with eco-friendly and hybrid concepts. Here, we describe the use of salmon DNA (SDNA) biomolecules as temperature and light sensors. We demonstrate the temperature- and light-sensitive mechanism of polarity switching in metal/organic/n-GaN bio-hybrid photodiodes based on salmon DNA-cetyltrimethylammonium chloride (SDNA-surfactant). The SDNA-surfactant/n-GaN bio-hybrid temperature photodiode (Bio-HTPD) shows negative bias shift of current (I)–voltage (V) plots by 0.70 and 0.42 V compared to zero-bias at temperatures of 275 and 300 K, respectively, under light illumination. However, the I–V plots of the Bio-HTPD moved towards positive bias by 0.08 V compared to zero-bias at 325 K under light irradiation. This phenomenon resulted in electrically negative photocurrents up to room temperature, which remarkably switched to positive photocurrents at above room temperature. The temperature variations are closely associated with charge activation and unidirectional transport in the SDNA-surfactant biomolecule. Moreover, the change from negative to positive photocurrent could be related to high electron–hole pair generation at higher transition temperature. The formation of an energy band model with thermal hopping is proposed, which explains the reasonable charge transport mechanism. [ABSTRACT FROM AUTHOR]

AlGaN/GaN FinMISHFETs with m-plane sidewall surface channel and various fin widths ($\text{W}_{\textsf {fin}}$) were fabricated and characterized. The investigated devices have much higher current drivability due to the uniform and smooth surface of m-plane than those with the a-plane sidewall surface channel. The AlGaN/GaN FinMISHFETs with $\text{W}_{\textsf {fin}}$ smaller than 36 nm exhibit normally-off operation, high Ion/Ioff ratio of 108, and remarkable subthreshold swing (SS) smaller than 40 mV/decade in the wide current range of at least three orders. Combined with a positive threshold voltage, SS values smaller than 60 mV/decade in a wide current rage of at least three orders are among the world’s best subthreshold characteristics. Furthermore, when $\text{W}_{\textsf {fin}}$ is 31 nm, the off-state drain current is as low as 10–12 A. We show that this sharp switch is due to the simultaneous turn-on of the 2-D electron gas and the m-plane sidewall surface channel. The simulation results are carried out to show the gate-induced variation of the electron concentration within the fin structure, and the assumption of considering gate width as a function of gate bias is also developed to explain the reason for deep sub-60mV/decade in the demonstrated devices. [ABSTRACT FROM AUTHOR]

Two different lateral GaN-based nanowire gate-all-around transistors with and without 2-D electron gas (2-DEG) channel were fabricated using top-down approach, and their noise characteristics were investigated. The nanowire transistor with 2-DEG channel had a relatively larger channel cross section, which consists of regrown AlGaN/GaN plateau on the trapezoidal GaN layer, and exhibited negative threshold voltages (${V} _{\textsf {th}}$). The transistor without 2-DEG channel consisted only GaN layer with triangular-shaped smaller channel cross section and exhibited a positive ${V} _{\textsf {th}}$. Both nanowire transistors clearly demonstrated typical $1/{f}$ noise characteristics, but the AlGaN/GaN nanowire transistor with 2-DEG channel showed larger noise magnitude. The noise characteristics of both devices are well explained by the carrier number fluctuation with correlated mobility fluctuation model. Using this model, the interface trap densities and the remote Coulomb scattering parameters were extracted, revealing a worse interface quality for the AlGaN/GaN device on the one hand, but stronger scattering for the narrow GaN transistor on the other hand. [ABSTRACT FROM AUTHOR]

In0.08Al0.92N/GaN fin-type high-electron mobility transistors (fin-HEMTs) with different gate-to-drain lengths (${L} _{\text {gd}}$) are fabricated and characterized by dc and low-frequency noise (LFN) measurements. The fabricated device with the largest ${L} _{\text {gd}}$ exhibits the degradation of the maximum drain current and transconductance with a positive shift of the threshold voltage. LFN measurements of the In0.08Al0.92N/GaN fin-HEMTs reveal clear 1/ ${f}$ behavior of the noise spectra, and the minimum value is observed in the device at ${L} _{\text {gd}} = {20}\,\,\mu \text{m}$. The devices with smaller ${L} _{\text {gd}}$ follow a carrier number fluctuation noise model owing to electron trapping/detrapping into the In0.08Al0.92N barrier layer from the 2-D electron gas (2DEG) channel. In contrast, the device with the largest ${L} _{\text {gd}}$ shows correlated mobility fluctuations due to the large 2DEG mobility fluctuations in the large access area. [ABSTRACT FROM AUTHOR]

The radiation hardness of AlGaN/GaN high-electron-mobility transistors (HEMTs) is found to improve with increasing GaN channel thickness. Epitaxial MgCaO shows promise as a radiation-tolerant gate dielectric, with only small shifts in operating parameters of metal–oxide–semiconductor HEMTs observed at doses up to 1 Mrad(SiO2). Bias-induced electron trapping and radiation-induced-hole trapping can occur in the MgCaO, depending on the applied bias during stress and/or irradiation. AC transconductance measurements are used to help understand charge trapping in these devices. [ABSTRACT FROM PUBLISHER]

DC and 1/ f noise performances of the AlGaN/GaN fin-shaped field-effect transistor (FinFET) with fin width of 50 nm were analyzed. The FinFET exhibited approximately six times larger normalized drain current and transconductance, compared to those of the AlGaN/GaN planar metal-insulator-semiconductor heterostructure field-effect-transistor (MISHFET) fabricated on the same wafer. It was also observed that the FinFET exhibited improved noise performance with lower noise magnitude of 8.5 \times 10^-15~\textA^2 /Hz when compared to the value of 8.7 \times 10^-14\textA^2 /Hz for the planar MISHFET. An intensive analysis indicated that both devices follow the carrier number fluctuation model, but the FinFET suffers much less charge trapping effect compared to the MISHFET (two orders lower charge trapping was observed). Moreover, the FinFET did not exhibit the Lorentz-like components, which explains that the depleted fin structure effectively prevents the carriers from being trapped into the underlying thick GaN buffer layer. On the other hand, the slope of the noise is 2 irrespective of drain voltage and apparently showed the Lorentz-like components, especially at high drain voltage in MISHFET device. This explains that the carrier trapping/detrapping between the 2-D electron gas channel and the GaN buffer layer is significant in MISHFET. [ABSTRACT FROM PUBLISHER]

A hybrid active pixel sensor (APS) was implemented for ultraviolet (UV) imagers by combining a metal–semiconductor–metal (MSM)-type GaN UV sensor and a standard Si CMOS APS controller. The photodetector region of the APS circuit was replaced by a GaN MSM UV sensor, and it was connected together on the printed circuit board with standard Si CMOS APS circuit chip. The dark and photoresponsive current densities of the fabricated MSM UV sensor were 2.5 \times 10^\mathrm \mathbf -6 and 1.6 \times 10^\mathrm \mathbf -3 A/cm ^\mathrm \mathbf 2 , respectively, at 10 V bias. The fabricated hybrid UV APS has clearly distinguishable ON/OFF operation states under dark and 365-nm UV irradiation conditions. The calculated photoresponsivity of the hybrid-type GaN UV APS was as high as 5.1 V/W. [ABSTRACT FROM PUBLISHER]

We present a new semi-insulating carbon-doped GaN/undoped GaN multi-layered buffer structure for AlGaN/GaN heterojunction field effect transistors, which drastically suppresses current collapse in GaN MISHFET with improving the on-current performance of the device but without degrading the breakdown characteristic. It is believed that spatial compensation between the multi-layered GaN layers not only makes the multi-layered buffer layer very highly resistive, but also prevents the electrons from the 2DEG channel from being captured into deep traps in the buffer layer, which maintains high-high breakdown characteristic and leads to great suppression of current collapse in AlGaN/GaN-based transistors, respectively. [ABSTRACT FROM AUTHOR]

The low-frequency noise (LFN) characteristics of AlGaN/GaN FinFETs with omega-gate and combined two-dimensional electron gas (2DEG) and MOS conduction are investigated. It is found that LFN is dominated by carrier number fluctuations whatever the width of the fin. Charge trapping in narrow devices is one order of magnitude lower than in wide fin device. In narrow devices, the sidewall conduction prevails and the noise mainly stems from the carrier trapping in the sidewall Al2O3 gate dielectric. Instead, in wide fin devices, the top gate AlGaN/GaN HEMT structure dominates and the LFN is mostly governed by the carrier trapping in the GaN layer close to 2DEG channel. [ABSTRACT FROM PUBLISHER]

This study investigated characteristics of the fabricated triangle-type light-emitting diode (T-LED), conventional triangular LED (CT-LED), and conventional square LED (CS-LED). The T-LED is expected to provide uniform current spreading that leads to high output power, low current crowding, and high light extraction in the lateral direction of the LED device, compared to CT-LED and CS-LED. The light extraction of the T-LED in the lateral direction is much higher than that of the CT-LED and the CS-LED due to enhancement of light emission from the sidewalls of the T-LED. [ABSTRACT FROM AUTHOR]

Highlights: [•] A detailed investigation of C–V–f and G/ω–V–f characteristics of Au/DNA/n-GaN SBDs studied. [•] The interface states in Au/DNA/n-GaN SBDs are determined. [•] The effect of series resistance to obtain the real diode capacitance and conductance. [•] C–V–f and G/ω–V–f characteristics strongly affect the electrical properties. [ABSTRACT FROM AUTHOR]

We report on the effect of an annealing temperature on the electrical properties of Au/TaO/n-GaN metal-insulator-semiconductor (MIS) structure by current-voltage ( I- V) and capacitance-voltage ( C- V) measurements. The measured Schottky barrier height ( Φ) and ideality factor n values of the as-deposited Au/TaO/n-GaN MIS structure are 0.93 eV ( I- V) and 1.19. The barrier height (BH) increases to 1.03 eV and ideality factor decreases to 1.13 upon annealing at 500 C for 1 min under nitrogen ambient. When the contact is annealed at 600 C, the barrier height decreases and the ideality factor increases to 0.99 eV and 1.15. The barrier heights obtained from the C- V measurements are higher than those obtained from I- V measurements, and this indicates the existence of spatial inhomogeneity at the interface. Cheung's functions are also used to calculate the barrier height ( Φ), ideality factor ( n), and series resistance ( R) of the Au/TaO/n-GaN MIS structure. Investigations reveal that the Schottky emission is the dominant mechanism and the Poole-Frenkel emission occurs only in the high voltage region. The energy distribution of interface states is determined from the forward bias I- V characteristics by taking into account the bias dependence of the effective barrier height. It is observed that the density value of interface states for the annealed samples with interfacial layer is lower than that of the density value of interface states of the as-deposited sample. [ABSTRACT FROM AUTHOR]

Two types of fin-shaped field-effect transistors (FinFETs), one with AlGaN/GaN heterojunction and the other with heavily doped heterojunction-free GaN layer operating in junctionless mode, have been fabricated and characterized. The threshold voltages of both devices shift toward positive direction from large negative value as the fin width decreases. Both devices exhibit high ON-state performance. The heterojunction-free GaN FinFETs show superior OFF-state performance because the current flows through the volume of the GaN channel layer, which can be fully depleted. The proposed GaN nanochannel FinFETs are very promising candidates not only for high performance, but also for high power applications. [ABSTRACT FROM AUTHOR]

We have demonstrated the lateral multifinger-type Schottky barrier diode (SBD) with bonding pad over active structure fabricated on the AlGaN/GaN heterostructure prepared on sapphire substrate. The fabricated GaN-SBD with size of 9 mm^2 exhibited excellent device characteristics such as forward current of 4.5 A at 1.5 V, leakage current of 6 \muA at 600 V, and high breakdown voltage of 747 V. The temperature variations of GaN-SBD for the reverse recovery characteristics are negligible and the value of reverse-recovery charge (Q)rr of GaN-SBD is one twentieth of Si-diode at 175^\circC. [ABSTRACT FROM AUTHOR]

High-quality \SiO2 is deposited on GaN by radio frequency (RF) magnetron sputtering at room temperature. Adding oxygen to the sputtering gas effectively compensated for the oxygen vacancies and resulted in a breakdown field of 9.6 MV/cm for the sputtered- \SiO2 film on GaN. The reduced electron concentration and mobility of the 2-D electron gas due to the sputtering-induced surface damage were effectively removed by an optimized postannealing treatment. A sputtered-\SiO2\/ \AlGaN/GaN metal–oxide–semiconductor high-electron-mobility transistor (HEMT) (MOSHEMT) was demonstrated with the lowest thermal energy requirement among all the dielectric deposition techniques, which exhibited a saturation drain current of 621 mA/mm and a breakdown voltage of 205 V at the gate–drain distance of 2 \mu\m. More than four orders of magnitude lower gate leakage current than conventional HEMT of the same dimension was achieved. These characteristics demonstrate excellent potential of using RF magnetron sputtering to produce gate insulators for GaN-based MOSHEMTs. [ABSTRACT FROM PUBLISHER]

We demonstrate high-performance InGaN–based light-emitting diodes (LEDs) with tunneling-junction-induced 2-D electron gas (2-DEG) at n-AlGaN/GaN heterostructure, inserted in the middle of the \p^++-GaN contact layer of a conventional LED structure. The LED with a 2-DEG layer exhibits about 20% enhancement in output power, as compared with that of the conventional LED at 350 mA, which is believed to be due to enhanced hole-injection efficiency and better lateral current spreading by the presence of 2-DEG at the AlGaN/GaN heterostructure. [ABSTRACT FROM PUBLISHER]

AlGaN/GaN-based fin-shaped field-effect transistors with very steep side-wall have been fabricated by utilizing electron-beam lithography and subsequent anisotropic side-wall wet etch in tetramethyl ammonium hydroxide solution. The investigate device demonstrated extremely broad transconductance ( gm) ranging from \sim 0 to \sim 8 V at VD = 10 V, which is essential for high linearity device performance. Pulse measurement showed that the device exhibits negligible gate lag, but still suffers from drain lag. The device with L\mathrm {GD} of 17~\mu \textm exhibited excellent OFF-state characteristic with subthreshold swing of \sim 58 mV/decade, low OFF-state leakage current of \sim 10^{-12} A, and breakdown voltage of \sim 400$ V at V_{G}=- 9 V. [ABSTRACT FROM PUBLISHER]

Normally off Al2O3/GaN MOSFETs are fabricated with a tetramethylammonium hydroxide (TMAH) treatment as a postgate recess etch. The effects of the surface treatment on the etched GaN surface are investigated using low-frequency (1/ f ) noise and capacitance–voltage ( C – V ) measurements. For a quantitative comparison with conventional devices, the oxide trap density ( ) is extracted using the unified 1/ f noise model, whereas the interface trap density ( D\mathbf {it} ) is extracted using the high–low-frequency C – V method. After the TMAH treatment, N_{\mathbf {ot}} is found to have decreased from 5.40 \times 10^{\mathrm {\mathbf {19}}} to 2.50 \times 10^\mathrm \mathbf 19~\mathrmeV^\mathrm \mathbf -1 \mathrmcm^\mathrm \mathbf -3 , whereas D\mathbf {it}^{\vphantom {(}} is decreased from 2.8 \times 10^\mathrm \mathbf 12 to 1.1 \times 10^\mathrm \mathbf 11~\mathrmeV^\mathrm \mathbf -1 \mathrmcm^\mathrm \mathbf -2 , as compared with conventional devices. The surface treatment is thus shown to lower trap density in the Al2O3/GaN MOSFETs by smoothing the surface and suppressing plasma damage in the recessed GaN surfaces. [ABSTRACT FROM AUTHOR]

This study investigated the combined effects of proton irradiation and surface pre-treatment on the current characteristics of Gallium Nitride (GaN)-based metal-insulator-semiconductor high-electron-mobility-transistors (MIS-HEMTs) to evaluate the radiation hardness involved with the Silicon Nitride (SiN) passivation/GaN cap interface. The impact of proton irradiation on the static and dynamic current characteristics of devices with and without pre-treatment were analyzed with 5 MeV proton irradiation. In terms of transfer characteristics before and after the proton irradiation, the drain current of the devices without and with pre-treatment were reduced by an increase in sheet and contact resistances after the proton irradiation. In contrast with the static current characteristics, the gate-lag characteristics of the device with pre-treatment were significantly degenerated. In the device with pre-treatment, the hydrogen passivation for surface states of the GaN cap was formed by the pre-treatment and SiN deposition processes. Since the hydrogen passivation was removed by the proton irradiation, the newly created vacancies resulted in the degeneration of gate-lag characteristics. After nine months in an ambient atmosphere, the gate-lag characteristics of the device with pre-treatment were recovered because of the hydrogen recombination. These results demonstrated that the radiation hardness of MIS-HEMTs was affected by the SiN/GaN interface quality. [ABSTRACT FROM AUTHOR]

AlGaN/GaN Schottky barrier diodes (SBDs) with high Al composition and high temperature atomic layer deposition (ALD) Al2O3 layers were investigated. Current–voltage (I–V), X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM), and capacitance–voltage (C–V) measurements were conducted in order to find the leakage current mechanism and reduce the reverse leakage current. The fabricated AlGaN/GaN SBDs with high Al composition exhibited two orders' higher leakage current compared to the device with low Al composition (20%) due to large bulk and surface leakage components. The leakage current measured at −60 V for the fabricated SBD with Al2O3 deposited at temperature of 550 °C was decreased to 1.5 μA, compared to the corresponding value of 3.2 mA for SBD with nonpassivation layer. The high quality ALD Al2O3 deposited at high temperature with low interface trap density reduces the donorlike surface states, which effectively decreases surface leakage current of the AlGaN/GaN SBD. [ABSTRACT FROM AUTHOR]

Heavily doped GaN nanochannel fin-shaped field-effect transistors (FinFETs) without heterojunction have been fabricated and characterized for the first time. Simplified pragmatical technology for GaN epitaxial growth and FinFET process was used to achieve nanodevices with a channel width from 40 to 100 nm and a gate length of 1 \mu\m. They exhibit excellent on-state performance, such as maximum drain current of 670 mA/mm and maximum transconductance of 168 mS/mm. Record off-state performance was measured: extremely low leakage current of \sim\!\!\10^-11\ \mA and source–drain breakdown voltage of \sim280 V. The subthreshold slope of 68 mV/decade is close to the theoretical limit (60 mV/decade, so far achieved only in SOI MOSFETs) and leads to very high Ion/Ioff ratio of \10^8 - \10^9. The proposed heterojunction-free nanochannel GaN FinFET is a very promising candidate not only for high-performance and high-speed integrated circuits but also for high-power applications. [ABSTRACT FROM PUBLISHER]

A single-nanoribbon \Al2\O3\/GaN metal–insulator–semiconductor field-effect transistor (MISFET) has been fabricated. The fabricated device exhibits normally off operation with a threshold voltage of 2.1 V. The device also exhibits superior performances such as a maximum drain current density of 1.51 A/mm, a maximum transconductance of 580 mS/mm, and a field-effect mobility of 293 \cm^2\cdot \V^-1\cdot \s^-1. This is because the electron concentration in the GaN channels can be increased due to the enhanced gate controllability, which, thus, effectively screens the field lines from the interface traps or the defects near the channels to improve the electron mobility in the channel. The nanoribbon \Al2\O3\/GaN MISFET is a very promising candidate for high-performance normally off GaN FETs. [ABSTRACT FROM PUBLISHER]

A high-performance normally off GaN-based MOSFET was fabricated. The buffer layer of the MOSFET was grown by varying the growth temperature to control the size of nucleation sites which results in an extremely high buffer resistance (> \10^12\ \Omega/\sq). The fabricated small-area MOSFET exhibited excellent normally off device characteristics, such as a threshold voltage of 2 V, maximum drain current of 253 mA/mm, on–off current ratio of \5.5 \times \10^7, destructive breakdown voltage of 830 V, and leakage current of 0.7 \mu\A/mm at a VDS of 600 V. The corresponding values of the large-area MOSFET with a multifinger pattern were 0.6 V, 6 A, \1.3 \times \10^7, 670 V, and 50 \mu\A (0.28 \mu \A/mm). [ABSTRACT FROM AUTHOR]

Multifinger lateral-type AlGaN/GaN Schottky barrier diode (SBD) with bonding pad over active (BPOA) structure was fabricated and exhibited excellent device performances, such as forward current of 6 A at 1.5 V, leakage current of 16 \mu\A at -600 V, reverse-recovery time (Trr) of 18 ns, breakdown voltage of 840 V, and low specific on-resistance (Ron) of 9 \m\Omega \cdot \cm^2, resulting in the figure-of-merit (VBR^2/Ron) as high as 78 \MW/cm^2, which demonstrates that the AlGaN/GaN SBD with BPOA structure has a great potential application to the high-power electronics. [ABSTRACT FROM AUTHOR]

A \SiO2/\Al2\O3 double dielectric stack layer was deposited on the surface of a GaN-based light-emitting diode (LED). The double dielectric stack layer increases the optical output power of the LED because the first \Al2\O3 layer plays a role as an effective surface passivation layer and the second \SiO2 layer with lower index increases the critical angle of the emitted light and hence the overall extraction efficiency from the LED. The leakage current of the LED passivated with an \Al2\O3 layer was -\3.46 \times \10^-11\ \A at -5 V, at least two and three orders lower in magnitude compared to that passivated with a \SiO2 layer (-\7.14 \times \10^-9\ \A) and that of the nonpassivated LED(-\1.9 \times \10^-8\ \A), respectively, which indicates that the \Al2\O3 layer is very effective in passivating the exposed GaN surface after dry etching and hence reduces the nonradiative recombination as well as reabsorption of the emitted light near the etched surface. [ABSTRACT FROM AUTHOR]

AlGaN/GaN Schottky barrier diodes (SBDs) with and without the in situ silicon carbon nitride (SiCN) cap layer were investigated. The fabricated SBD with the SiCN cap layer exhibited improved electrical characteristics, such as the forward turn-on voltage of about 0.7 V, the forward current of 4.1 A at 1.5 V, and the reverse breakdown voltage of 630 V, as compared with the corresponding values of 0.8 V, 3.8 A, and 580 V for the reference SBD without the SiCN cap layer. This improvement in the device performance of the SiCN-SBD is because the in situ SiCN cap layer not only lowers the barrier height but also effectively passivates the surface of the device with better surface morphology. [ABSTRACT FROM AUTHOR]

Normally off \Al2\O3\/\GaN MOSFETs are fabricated by utilizing a simple tetramethylammonium hydroxide (TMAH) treatment as a postgate-recess process. The TMAH-treated device with a gate length of 2.5 \mu\m exhibited excellent device performances, such as a threshold voltage of 3.5 V, a maximum drain current of 336 mA/mm, and a breakdown voltage of 725 V, along with extremely small gate leakage current of about \10^-9\ \A/mm at Vgs = \15\ \V, which is approximately six orders lower in magnitude compared to that of the device without TMAH treatment. [ABSTRACT FROM AUTHOR]

A gallium nitride (GaN)-based normally off metal–oxide–semiconductor field-effect transistor (MOSFET) using a dual-metal-gate (DMG) structure was proposed and fabricated to improve current drivability. Normally off operation with a high Vth of 2.3 V was obtained using a Cl2/BCl3-based recess etching process. The DMG structure was employed to improve current characteristics, which can be degraded by recess etching. The ID and gm of a DMG-based device with nickel (Ni)-aluminum (Al) were improved by 42.1% and 30.9%, respectively, in comparison to the performances of a single-metal-gate-based device with Ni because the DMG structure increased electron velocity in the channel region. This demonstrates that the DMG structure with a large work-function difference significantly improves the carrier transport efficiency. GaN-based recessed-gate MOSFETs based on the DMG structure hold promising potentials for high-efficiency power devices. [ABSTRACT FROM AUTHOR]

The fabrication of a single pixel sensor, which is a fundamental element device for the fabrication of an array-type pixel sensor, requires an integration technique of a photodetector and transistor on a wafer. In conventional GaN-based ultraviolet (UV) imaging devices, a hybrid-type integration process is typically utilized, which involves a backside substrate etching and a wafer-to-wafer bonding process. In this work, we developed a GaN-based UV passive pixel sensor (PPS) by integrating a GaN metal-semiconductor-metal (MSM) UV photodetector and a Schottky-barrier (SB) metal-oxide-semiconductor field-effect transistor (MOSFET) on an epitaxially grown GaN layer on silicon substrate. An MSM-type UV sensor had a low dark current density of 3.3 × 10−7 A/cm2 and a high UV/visible rejection ratio of 103. The GaN SB-MOSFET showed a normally-off operation and exhibited a maximum drain current of 0.5 mA/mm and a maximum transconductance of 30 μS/mm with a threshold voltage of 4.5 V. The UV PPS showed good UV response and a high dark-to-photo contrast ratio of 103 under irradiation of 365-nm UV. This integration technique will provide one possible way for a monolithic integration of the GaN-based optoelectronic devices. [ABSTRACT FROM AUTHOR]

The AlInN/GaN high-electron-mobility-transistor (HEMT) indicates better performances compared with the traditional AlGaN/GaN HEMTs. The present work investigated the pH sensor functionality of an analogous HEMT AlInN/GaN device with an open gate. It was shown that the Al0.83In0.17N/GaN device demonstrates excellent pH sense functionality in aqueous solutions, exhibiting higher sensitivity (−30.83 μA/pH for AlInN/GaN and −4.6 μA/pH for AlGaN/GaN) and a faster response time, lower degradation and good stability with respect to the AlGaN/GaN device, which is attributed to higher two-dimensional electron gas (2DEG) density and a thinner barrier layer in Al0.83In0.17N/GaN owning to lattice matching. On the other hand, the open gate geometry was found to affect the pH sensitivity obviously. Properly increasing the width and shortening the length of the open gate area could enhance the sensitivity. However, when the open gate width is too larger or too small, the pH sensitivity would be suppressed conversely. Designing an optimal ratio of the width to the length is important for achieving high sensitivity. This work suggests that the AlInN/GaN-based 2DEG carrier modulated devices would be good candidates for high-performance pH sensors and other related applications. [ABSTRACT FROM AUTHOR]

• Normally-off GaN MOSFET is successfully demonstrated by using the selective regrowth of AlGaN layer on source/drain (S/D) regions. • Wide regrown AlGaN layer is applied to GaN MOSFET to accommodate the poor structural quality of narrow regrown AlGaN on the S/D regions. • The proposed device with wide regrown AlGaN S/D has the potential to serve as a robust and uniform normally-off GaN MOSFET. A normally-off GaN MOSFET is successfully fabricated by using the selective regrowth technique (SRT) with regrown AlGaN layer on source/drain (S/D) region. The GaN MOSFET with regrown AlGaN layer and L g of 10 μm shows enhanced electrical performance such as maximum drain current (I D,max) of 57 mA/mm, maximum transconductance (g m,max) of 11 mS/mm, and field-effect mobility (μ FE) of 59 cm2/V·s, respectively, compared to the GaN MOSFET with n+-GaN selective regrowth in S/D region. This is because of the high 2DEG density formed by AlGaN/GaN heterojunction in S/D region. Moreover, to accommodate the poor structural quality of the narrow region regrowth of AlGaN layer on the S/D region, wide regrown AlGaN layer is applied to the GaN MOSFET. Especially, the off-state breakdown voltage improves from 25 V to 192 V with the improved structural quality of wide regrown AlGaN layer and optimized structure and the application of the 70-nm thick SiO 2 passivation. These result shows that GaN MOSFET with wide regrown AlGaN layer on S/D region is beneficial to achieving high-quality and uniform normally-off GaN MOSFETs with excellent electrical performance. [ABSTRACT FROM AUTHOR]

The effect of tetramethylammonium hydroxide (TMAH) treatment on the electrical properties of Ni/Au/GaN Schottky diodes have been investigated by current–voltage (I–V) and capacitance–voltage (C–V) techniques. The barrier heights and ideality factors measured from I–V characteristics are found to be 0.70 eV and 1.32 for without TMAH treatment, and 0.78 eV and 1.14 for with TMAH treatment, respectively. Cheung method is used to measure the series resistance and barrier height of the Schottky diodes, and the barrier height consistency is checked using the Norde method. The magnitude of interface state density for the diodes without and with TMAH treatment are varied from 7.45 × 1013 eV−1 cm−2 to 6.09 × 1012 eV−1 cm−2 and 4.03 × 1013 eV−1 cm−2 to 1.79 × 1012 eV−1 cm−2 in the below the conduction band from E C-0.19 eV to E C-0.63 eV and E C-0.22 eV to E C-0.73 eV. Based on the results, the TMAH treatment effectively removes of surface oxide (Ga x O y ) layer, formed due to the incorporation of the residual oxygen with Ga atom at the GaN surface during the plasma etching. The decrease in interface state density at the Ni/Au/GaN interface could be the reason for the improvement in the electrical properties. [ABSTRACT FROM AUTHOR]

Highlights • AlGaN/GaN heterostructures were grown with the AlIn(Ga)N back barrier grown at 400 torr was almost lattice-matched to GaN layer. • The AlGaN/GaN heterostructures with 10 and 15 nm-thick AlIn(Ga)N back barrier exhibited improved 2-DEG properties, compared to those of the conventional AlGaN/GaN heterostructure without the back barrier. • The high electron mobility transistors (HEMTs) fabricated on the AlGaN/GaN heterostructure with a 15 nm-thick AlIn(Ga)N back barrier exhibited a very low off-state leakage current of ∼2 × 10−7 A/mm. Abstract AlGaN/GaN heterostructures were successfully grown with the AlIn(Ga)N back barrier at 900 °C. However, the atomic composition of the AlIn(Ga)N layer was strongly dependent on the growth pressure, which resulted in a different lattice constant of the layer. The AlIn(Ga)N back barrier grown at 400 torr was almost lattice-matched to GaN layer. The AlGaN/GaN heterostructures with 10 and 15 nm-thick AlIn(Ga)N back barrier exhibited improved 2-DEG properties, compared to those of the conventional AlGaN/GaN heterostructure without the back barrier. The high electron mobility transistors (HEMTs) fabricated on the AlGaN/GaN heterostructure with a 15 nm-thick AlIn(Ga)N back barrier exhibited a very low off-state leakage current of ∼2 × 10−7 A/mm which is about 1 order lower in magnitude than the value of the device without the back barrier. The AlIn(Ga)N back barrier is a promising candidate as an alternative to conventional AlGaN and InGaN back barrier. [ABSTRACT FROM AUTHOR]

Normally-off AlGaN/GaN-based MOS-HEMT has been fabricated by utilizing damage-free self-terminating tetramethyl ammonium hydroxide (TMAH) recess etching. The device exhibited a threshold voltage of +2.0 V with good uniformity, extremely small hysteresis of ∼20 mV, and maximum drain current of 210 mA/mm. The device also exhibited excellent off-state performances, such as breakdown voltage of ∼800 V with off-state leakage current as low as ∼10 −12  A and high on/off current ratio (I on /I off ) of 10 10 . These excellent device performances are believed to be due to the high quality recessed surface, provided by the simple self-terminating TMAH etching. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high electron mobility transistors (HEMTs) are fabricated using a gate recess process and a surface treatment with tetramethylammonium hydroxide (TMAH) prior to gate metal deposition. Electrical characterizations show improved extrinsic transconductance and saturation current, as well as more uniform off-state behavior with reduced off-current by a factor of 3.5 and gate leakage current by a factor of 4.2 in the devices with TMAH treatment. The analyses based on atomic force microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy show that the TMAH treatment effectively reduces the roughness of the recess-etched AlGaN surface and removes the native oxide layer on the AlGaN surface, suggesting a simple and viable route towards the fabrication of gate-recessed HEMTs based on AlGaN/GaN heterostructure with improved controllability and uniformity. [ABSTRACT FROM AUTHOR]

The initially dry-etched GaN layer with trapezoidal cross-section was laterally etched along the 〈11 2 ¯ 0〉 direction in the tetramethyl ammonium hydroxide (TMAH) solution to form a sidewall normal to the direction, which is corresponding to the (11 2 ¯ 0) plane. On the other hand, the etched sidewall still maintains the trapezoidal shape with angle of 58.4° when etched along the 〈1 1 ¯ 00〉 direction, which is corresponding to the (1 1 ¯ 01) plane. The GaN lateral nanowires with two different types of cross-sections, Ω-shape which is connected to underlying thick buffer layer through very narrow neck region and rectangle shape which is completely separated from underlying buffer layer, were realized with second lateral TMAH wet etching along the 〈11 2 ¯ 0〉 direction and by using the atomic layer deposited (ALD) HfO 2 layer as a sidewall spacer. The shape is dependent on both the height of the second dry-etched GaN sidewall below the HfO 2 spacer and the second wet etching time in TMAH solution. It was found that the dangling bond density at the surface of the crystal plane is responsible for the strong lateral anisotropic etching property of the GaN layer in TMAH solution. [ABSTRACT FROM AUTHOR]

AlGaN/GaN FinFETs with various fin widths ( W fin ), which have both a 2DEG channel and two sidewall MOS channels, have been fabricated by using electron-beam lithography and subsequent sidewall wet etch in tetramethyl ammonium hydroxide (TMAH) solution. The devices with wide W fin of 150 nm showed normally-on operation with threshold voltage ( V th ) of −2.5 and −5.0 V, respectively. The devices also exhibited broad transconductance ( g m ), and excellent off-state performance with very low subthreshold swing (SS). On the other hand, narrow device with W fin of 50 nm exhibited normally-off operation with V th of 3.0 V, but degraded SS due to trapping effect at the sidewall of fin. [ABSTRACT FROM AUTHOR]

Abstract: AlGaN/GaN FinFETs, with high quality atomic layer deposited (ALD) Al2O3 gate dielectric, have been fabricated. The devices have a two-dimensional electron gas (2DEG) channel formed at AlGaN/GaN heterointerface and two sidewall GaN MOS channels. Two distinct transconductance peaks can be observed, one for the 2DEG channel and the other for the sidewall GaN MOS channels. On the other hand, we present heterojunction-free GaN FinFETs with junctionless configuration. The current flows through the volume of the heavily doped GaN fin rather than at the surface channel, which leads to superior off-state performance and less drain-induced virtual substrate biasing (DIVSB) effect. [Copyright &y& Elsevier]

Abstract: Several types of GaN MOSFETs with normally-off operation have been fabricated on insulating substrate and evaluated. In recessed-gate GaN MOSFET, the threshold voltage (V th) can be easily controlled, but the current drivability is modest and needs to be improved by adopting appropriate device structure and/or process. We achieved enhanced performance by combining the recessed-gate technology with additional processes, such as: (i) the post-recess tetramethylammonium hydroxide (TMAH) treatment to remove the plasma damage, (ii) the post-deposition annealing of gate oxide to decrease the gate leakage current, (iii) the re-growth of n+-GaN layer for source/drain to improve the access resistance and V th uniformity, (iv) the stress-control technology to secure high 2-D electron–gas density (2DEG) on source/drain and decrease the series resistance, and (v) the use of the p-GaN back-barrier to decrease the buffer leakage current. We also present the characteristics of GaN-based FinFETs with very narrow fin. [Copyright &y& Elsevier]

Abstract: The light output and electrical characteristics of GaN-based vertical light emitting diodes were investigated as a function of n-GaN thickness. The forward voltage increases from 3.34 to 3.42V at an injection current of 350mA as the n-GaN thickness decreases from 5.0 to 2.0μm. Even at a high injection current of 2.0A, LEDs with 2.0μm-thick n-GaN reveal stable forward characteristics which are comparable to those of LEDs with 5.0μm-thick n-GaN. All the samples exhibit almost the same reverse current up to approximately −8V. The output power increases with decreasing n-GaN layer thickness. For example, LEDs with 2.0μm-thick n-GaN yield about 12% higher light output power as compared to LEDs with 5.0μm-thick n-GaN. Their light output power continuously increases without saturation as the injection current increases up to 1A. The n-GaN thickness dependence of the electrical characteristics is described and discussed. [Copyright &y& Elsevier]

Abstract: We have grown high quality GaN layers on (111)-oriented silicon substrate using a two-step growth method and fabricated high-performance normally-off n-channel GaN Schottky-barrier MOSFET (SB-MOSFET). Indium-tin-oxide (ITO) was used as Schottky-barrier contact for source and drain (S/D) because the work function of ITO is close to the electron affinity of GaN. Due to enhanced crystalline quality and reduced surface roughness of GaN layer grown by two-step process, the fabricated device exhibited much improved performances: sufficiently high threshold voltage of 3.75V, subthreshold slope of 171mV/dec, low specific on-resistance of 9.98mΩcm2, and very high field-effect mobility of 271cm2/Vs. This is the highest mobility value among the GaN MOSFETs ever reported so far. [Copyright &y& Elsevier]

Abstract: Magneto-gyrotropic photogalvanic and spin-galvanic effects are observed in (0001)-oriented GaN/AlGaN heterojunctions excited by terahertz radiation. We show that free-carrier absorption of linearly or circularly polarized terahertz radiation in low-dimensional structures causes an electric photocurrent in the presence of an in-plane magnetic field. Microscopic mechanisms of these photocurrents based on spin-related phenomena are discussed. Properties of the magneto-gyrotropic and spin-galvanic effects specific for hexagonal heterostructures are analyzed. [Copyright &y& Elsevier]