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The characteristics of GaN-based light-emitting diodes (LEDs) with a hybrid structure incorporating a microhole array, 45 sidewalls, and an appropriate SiO2 nanoparticle (NP)/microsphere (MSs) passivation layer are studied and reported. The use of a SiO2 NP/MSs passivation layer causes a remarkable reduction in reverse-biased leakage current. The employment of this hybrid structure leads to substantial enhancements in optical properties without any degradation in electrical performance. In addition, a lower content of SiO2 NP in the mixed SiO2 NP/MSs solution leads to enhanced optical behavior due to the improved transmittance. Experimentally, as compared with a conventional LED (Device A), the studied Device E shows 50.6%, 50.9%, 48.4%, and 49.9% enhancements in light output power, luminous flux, luminous efficacy, and wall-plug efficiency, respectively. These advantages are mainly attributed to the increased scattering probability and the opportunity to find photon escape cones as well as the reduced total internal reflection and Fresnel reflection effects. Therefore, the studied hybrid structure provides a promise for high-performance GaN-based LED applications.

A new and simple Al/aluminum-doped zinc oxide (AZO) composite structure is proposed to act as a transparent conductive layer (TCL) for GaN-based LEDs. The Al/AZO composite layers effectively improve the current spreading performance, compared with an AZO TCL. Experimentally, the specific contact resistance of the studied Al/AZO LED is reduced by about 27.6%, compared with an AZO LED. In addition, under an operating current of 20 mA, the forward voltage and light output power (LOP) of the studied Al/AZO LED are decreased by 0.15 V and increased by 19.5%, respectively. Uniform and enhanced intensities are also found in light emission mapping images of the studied Al/AZO LED. Consequently, based on benefits including simple structure and improved electrical and optical properties, the studied Al/AZO TCL is suitable for GaN-based LED applications.

The Characteristics of GaN-based LEDs with hybrid microhole arrays and SiO2 microspheres (MSs)/ nanoparticles (NPs) are comprehensively studied. The SiO2 MSs/NPs antireflection coating, deposited by a rapid convection deposition, acts as a passivation layer of GaN-based LEDs. Since the critical angle could be enlarged by antireflection coating, Fresnel reflection could be reduced. In addition, due to the roughened surface of SiO2 MSs/NPs antireflection coating, the scattering effect could also be increased. Thus, the light extraction efficiency could be further enhanced. As compared with a conventional LED with a planar aluminum-doped zinc oxide current spreading layer (Device A), the studied device with the proposed hybrid structure and a sputtered SiO2 passivation layer (Device E) causes a suppressed leakage current and % enhancements on light output power, external quantum efficiency, and wall-plug efficiency performance.

A high-power GaN-based light-emitting diode (LED) with an inductively coupled plasma (ICP)-transferred nanohemispherical hybrid backside reflector is studied. A self-assembled 100 ± 5 nm SiO2 nanosphere monolayer is drop-coated on the backside of a sapphire substrate as a mask to transfer nanohemispherical patterns onto the backside of the sapphire substrate by ICP. Nanohemispherical patterns could be transferred to the deposited backside reflector. Thus, reflected photons could be redirected and scattered into arbitrary directions for light extraction. As compared with a conventional LED without a backside reflector, at 350 mA, the studied device exhibits a 118.2% enhancement in light output power without the degradation of electrical properties. Note that the adhesion between an ICP-transferred sapphire substrate and the hybrid backside reflector is better than when directly inserting an SiO2 nanosphere monolayer in the device. Thus, the process yield could be enhanced for applying in the solid-state lighting.

Enhanced light extraction efficiency (LEE) of high-power GaN-based light-emitting diodes (LEDs) is achieved by inserting a self-assembled SiO2 nanosphere monolayer between the substrate and backside reflectors. Due to the presence of concave surfaces and photonic crystal-like air voids, downward photons emitted from multiple quantum well toward 3-D backside reflectors, could be reflected, scattered, and redirected into arbitrary directions for light extraction. These textured 3-D backside reflectors with an SiO2 nanosphere monolayer could also extract the lateral light inside device into the normal direction and improve LEE. As compared with a conventional LED without a backside reflector and an LED with a planar hybrid backside reflector, at 350 mA, the studied device with a 3-D hybrid backside reflector exhibits 136.4% (165%) and 23.6% (27.4%) enhancements in light output power (luminous flux) without the degradation of electrical properties. Higher light intensities in light emission mapping image and far-field pattern are also obtained. These results show that a textured 3-D backside reflector could be easily formed by inserting an SiO2 nanosphere monolayer to significantly enhance the performance of high-power GaN-based LEDs.

A GaN-based light-emitting diode (LED) with an aluminum (Al) reflecting and an SiO2 insulating layers (RILs) deposited on the naturally textured p-GaN surface is fabricated and studied. The use of RIL could enhance the current spreading performance and reduce the photon absorption by the p-pad metal. The textured surface is used to limit the total internal reflection and increase photon scattering. In this paper, effects of the use of an Al RL and/or an SiO2 insulating layer on the performance of GaN-based LEDs are systematically studied and compared in detail. At 20 mA, as compared with a conventional LED with naturally textured (planar) p-GaN surface, the studied device exhibits 12.2% (55.5%) enhancement in light output power. Additionally, a 28.5% (95%) increment of luminous flux is achieved. The studied device also shows 15.6% light intensity improvement of far-filed pattern. Experimentally, although power consumption and junction temperature are slightly increased because of the insertion of RIL structure, these drawbacks could be surpassed by the mentioned optical improvements. Therefore, for conventional GaN-based LEDs, light extraction efficiency could be further improved by the employment of RIL structure.

Interesting pseudomorphic high electron mobility transistors using an electrophoretic deposition (EPD) approach are fabricated and studied. Due to the low-temperature deposited gate structure, the studied device exhibits enhanced performance with less thermal damages and improved Schottky contact properties by EPD approach. In comparison with a thermal evaporation (TE) device, the higher turn-on voltage, lower gate current, and lower interface state density are observed for the EPD device. For the gate dimension of 1 × 100 μm2 the EPD device shows the higher maximum drain saturation current of 242.2 (231.9) mA/mm and excellent maximum extrinsic transconductance of 151.6 (132.5) mS/mm at 300 (420) K. Besides, the EPD device presents a comparable RF performance as compared with the TE one. Due to the improved device performance and advantages of low cost, simple process, flexible deposition on varied substrate, and adjustable metal grain size, the reported EPD approach shows the promise for high-performance device applications.

New metal-semiconductor-metal hydrogen sensors are fabricated to take advantages of symmetrically bidirectional detection. Unlike commonly used single catalytic metal layers, a mixture of Pd and SiO2 inserted between Pd and GaN was employed as sensing media. There are three sensing regions (i.e., 2-D dipole, transient, and 3-D dipole regions) observed in static response. Room-temperature sensitivity larger than 107 was obtained in 1080-ppm H2/N2 ambient. The barrier-height variation is as high as 422 mV. To our best knowledge, these are the highest values ever reported. According to transient response, a short response time of 70 s is obtained at room temperature. Thus, a newly developed concept of forming 3-D dipoles is introduced to possibly explain experimental results.

A new field-effect transistor using a high-barrier n/sup +/ -GaAs/p/sup +/-InGaP/n-GaAs camel-like gate and GaAs/InGaAs heterostructure-channel has been fabricated successfully and demonstrated. Experimentally, an ultra high gate-drain breakdown voltage of 52 V, a high drain-source operation voltage over 20 V with low leakage currents, and a high drain-source off-state breakdown voltage of 39.7 V are obtained for a 1/spl times/100 /spl mu/m/sup 2/ device. The high breakdown behavior is attributed to the use of high barrier camel-like gate and heterostructure channels to reduce the undesired leakage current. Furthermore, the studied device also shows high breakdown behavior in a high temperature environment and good microwave characteristics. Therefore, based on these characteristics, the studied device is suitable for high-breakdown, low-leakage, and high-temperature applications.

New hydrogen-sensing amplifiers are fabricated by integrating a GaAs Schottky-type hydrogen sensor and an InGaP-GaAs heterojunction bipolar transistor. Sensing collector currents (ICN and ICH) reflecting to N2 and hydrogen-containing gases are employed as output signals in common-emitter characteristics. Gummel-plot sensing characteristics with testing gases as inputs show a high sensing-collector-current gain (ICH/ICN) of > 3000. When operating in standby mode for in situ long-term detection, power consumption is smaller than 0.4 ?W. Furthermore, the room-temperature response time is 85 s for the integrated hydrogen-sensing amplifier fabricated with a bipolar-type structure.

The dc performance of a novel InP/InGaAs p-n-p heterostructure-emitter bipolar transistor is first demonstrated. Though the valence band discontinuity at an InP/InGaAs heterojunction is relatively large, the addition of a heavy doped as well as thin p/sup +/-InGaAs emitter layer between a p-InP confinement and n/sup +/-InGaAs base layers effectively eliminates the potential spike at emitter-base junction, lowers the emitter-collector offset voltage, and increases the barrier for electrons, simultaneously. Experimentally, a high current gain of 88 and a low offset voltage of 54 mV are achieved. To the author's knowledge, the offset voltage of the studied device is the lowest value among of the InP/InGaAs p-n-p heterojunction bipolar transistors.

In this paper, a novel InGaP/GaAs multiple S-shaped negative-differential-resistance (NDR) switch based on a heterostructure-emitter bipolar transistor (HEBT) structure is fabricated and demonstrated. An interesting multiple NDR phenomenon resulting from an avalanche multiplication and successive two-stage barrier lowering process is observed under the inverted operation mode. The three-terminal-controlled and temperature-dependent NDR characteristics are also investigated. In addition, a typical transistor performance is found under the normal operation mode. Consequently, owing to the presented different stable operation points and transistor action, the studied device shows a good potential for multiple-valued logic and analog amplification circuit applications.

Extremely high potential barrier height and gate turn-on voltage of a novel GaAs field-effect transistor with n/sup +//p/sup +//n/sup +//p/sup +//n double camel-like gate structure are demonstrated. The maximum electric field and potential barrier height of the double camel-like gate are substantially enhanced by the addition of another n/sup +//p/sup +/ layers in gate region, as compared with the conventional n/sup +//p/sup +//n single camel-like gate. For a 1/spl times/100 /spl mu/m/sup 2/ device, a potential barrier height up to 2.741 V is obtained. Experimentally, a high gate turn-on voltage up to +4.9 V is achieved because two reverse-biased junctions of the double camel-like gate absorb part of positive gate voltage. In addition, the transistor action shows a maximum saturation current of 730 mA/mm and an extrinsic transconductance of 166 mS/mm.

We report the performance of GaAs camel-gate FETs and its dependence on device parameters. In particular, the performance dependence on the doping-profile of a channel was investigated. In this study, one-step, bi-step, and tri-step doping channels with the same doping-thickness product are employed in camel-gate FETs, while keeping other parameters unchanged, For a one-step doping channel FET, theoretical analysis reveals that a high doping channel would provide a large transconductance which is suitable for logic applications. Decreasing the channel concentration increases the drain current and the barrier height. For a tri-step doping channel FET, it is found that the output drain current and the barrier height remain large and the relatively voltage-independent transconductance is also increased. These are the requirements for the large input signal power amplifiers. A fabricated camel-gate FET with a tri-step doping channel exhibits a large drain current density larger than 750 mA/mm and a potential barrier greater than 1.0 V. Furthermore, the relatively voltage-independent transconductance is as high as 220 mS/mm and the applied gate voltage is up to +1.5 V. A 1.5/spl times/100 /spl mu/m/sup 2/ device is found to have a f/sub t/ of 30 GHz with a very low input capacitance.

The author reports a novel InGaP/InGaAs/GaAs double delta-doped pseudomorphic high-electron mobility transistor (pHEMT) with n/sup +/-GaAs/p/sup +/-InGaP/n-InGaP camel-like gate structure grown by MOCVD. Due to the p-n depletion from the p/sup +/-InGaP gate to the channel region and the presence of /spl Delta/Ec at the InGaP/InGaAs heterostructure, the turn-on voltage of gate is larger than 1.7 V. For a 1/spl times/100-/spl mu/m/sup 2/ device, the experimental results show an extrinsic transconductance of 107 mS/mm and a saturation current density of 850 mA/mm. Significantly, an extremely broad gate voltage swing larger than 6 V with above 80% maximum g/sub m/ is obtained. Furthermore, the unit current cut-off frequency f/sub T/ and maximum oscillation frequency are up to 20 and 32 GHz, respectively. The excellent device performance provides a promise for linear and large signal amplifiers and high-frequency circuit applications.

In this letter, a new InGaP/GaAs superlattice-emitter resonant tunneling bipolar transistor (SE-RTBT) is fabricated and demonstrated. A 5-period InGaP/GaAs superlattice is used to serve the RT route and the confinement barrier for minority carriers. Due to the large valence band discontinuity (/spl Delta/E/sub v/) at the InGaP/GaAs heterointerface, a high current gain (/spl beta//sub max//spl sime/220) is obtained. Furthermore, the interesting N-shaped negative-differential-resistance (NDR) phenomena resulting from RT effect are found both in the saturation and forward-active region of current-voltage (I-V) characteristics at room temperature.

An interesting multiple negative-differential-resistance (MNDR) device, based on an AlGaAs-InGaAs-GaAs metal-insulator-semiconductor (MIS)-like structure, has been fabricated and demonstrated. Three and six switching phenomena have been observed at room temperature and -105/spl deg/C, respectively. The impressive MNDR behaviors are believed to be caused by the sequential accumulation process of carriers at In/sub x/Ga/sub 1-x/As subwells and the successive barrier lowering and potential redistribution effects. These effects yield the step by step enhancement of tunneling through the "insulated" AlGaAs barrier. It is known that, from experimental results, the temperature variation plays an important role on carriers transport and experimental current-voltage (I-V) characteristics.

An interesting multiple S-shaped negative-differential-resistance (NDR) phenomenon is observed for an InGaP/GaAs heterostructure-emitter bipolar transistor (HEBT) under the inverted operation mode. This behavior results from a sequential avalanche multiplication and two-stage barrier lowering effect. The two-stage barrier lowering effect is assumed to be caused by the high valence-band-discontinuity (/spl Delta/E/sub v/) to conduction-band-discontinuity (/spl Delta/E/sub c/) ratio at InGaP/GaAs heterointerface which gives holes and electrons accumulation effect successively. Under normal operation mode, a typical common-emitter current gain of 60 is obtained at collector current density of 400 A/cm/sup 2/ for the studied HEBT without emitter-edge thinning structure. Consequently, the controlled switching and transistor performances provide a promise of the studied device for circuit applications.