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It is very important to monitor pH values. In this paper, we fabricated an all-solid-state AlGaN/GaN-based ion-sensitive heterostructure field effect transistor (ISHFET) pH sensor microprobe. The microprobe could facilitate measurement of pH values. The microprobe was fabricated by packaging the pH sensor microprobe chip into a 21 G medical needle. The performance of the integrated pH sensor microprobe was evaluated in pH range of 4–9.18. A sensitivity of 143.57 µA/pH can be achieved using the pH sensor microprobe with a width to length ratio of 4.2 at a drain–source voltage of 0.5 V. The pH sensor microprobe keeps a similar current when the pH values of solutions are the same. A response time of 1 s can be achieved when no neutralization reaction occurs. The pH sensor microprobe can accurately measure pH of solutions under 0.1M interfering ions. This probe-type AlGaN/GaN-based ISHFET pH sensor may be used in medicine or biology, especially in the detection of pH of biological fluid, blood, and some precious biological samples.

High conductivity on the N-face of GaN has long been observed. For many years, sapphire was the basic substrate used for growing GaN layers. Hence, such conductivity was associated with a high concentration of defects at the GaN/sapphire interface. Due to recent advances in GaN growth techniques, bulk GaN in polar and non-polar orientations has become available. In this study, we compare GaN grown on sapphire with bulk GaN. It is found by numerical fitting that the sheet resistance of GaN layers on sapphire substrates is determined by both bulk GaN conduction and polar-induced screening-charge conduction. The same high conductivity on the N-faces of GaN layers is observed even without the interface with sapphire substrates, i.e., when the latter are peeled off and the GaN layers are chemically mechanically polished. A further comparison of three samples with different orientations (c-, a-, and m-planes) suggests that the observed high conductivity is caused by a mobile screening charge attracted to the N-face by the polar charge present on it. This conclusion follows from the fact that no high conductivity is observed in the samples with non-polar orientations.

The capacitance and the series resistance are two main factors which determine the cut-off frequency of Schottky barrier diodes (SBDs) for their application in millimeter-wave and terahertz regions. The junction capacitance is closely related to the anode dimension of a SBD. Reducing the anode size can effectively decrease the junction capacitance, but it will increase the series resistance and the difficulty of the device manufacturing process is also increased. In this paper, an AlGaN/GaN based air-bridge structure planar SBD with a half through-hole is investigated. The half through-hole was formed on the center of a circular anode by inductively coupled plasma etching to the unintentional doping-GaN channel layer. The capacitance formed by the anode metal and the two-dimensional electron gas at the AlGaN/GaN interface is effectively reduced under the condition of holding the metal area of anode. The total capacitance of the 20 μm-radius anode SBD with a 19.95 μm-radius half through-hole dramatically decreases from 2.32 pF of the device without the half through-hole to 21.5 fF. In addition, since the current is mainly distributed at the edge of the circular anode, the series resistance is only slightly increased. The cut-off frequency of the air-bridge planar SBD with a 20 μm-radius anode and a 19.95 μm-radius half through-hole was 114.1 GHz. To reduce the size of the anode and optimize the ohmic contact, the cut-off frequency could be further improved.

Ion-sensitive field effect transistor (ISFET) has been widely used as a pH sensor. Its sensitivity can be expressed in terms of current or voltage which were defined by us as SA and SV in this article. Using current to represent sensitivity is more convenient for it can simplify the subsequent circuits. What’s more, SA contains more information including the geometry of the device, the parameter of the epitaxial material and the property of the sensitive membrane while SV only contains the information of the sensitive membrane, which helps us to analyze the device performance. In this paper, the sensitivity SA was deduced theoretically based on the previous research and was verified experimentally. On this basis, we put forward a method that can separate the effect of the sensitive membrane on the sensitivity from the material completely by designing a series of AlGaN/GaN based heterostructure ISFET pH sensors with different sizes of the channel area (width/length parameter). Experimental result shows the importance of this method in the future research.Ion-sensitive field effect transistor (ISFET) has been widely used as a pH sensor. Its sensitivity can be expressed in terms of current or voltage which were defined by us as SA and SV in this article. Using current to represent sensitivity is more convenient for it can simplify the subsequent circuits. What’s more, SA contains more information including the geometry of the device, the parameter of the epitaxial material and the property of the sensitive membrane while SV only contains the information of the sensitive membrane, which helps us to analyze the device performance. In this paper, the sensitivity SA was deduced theoretically based on the previous research and was verified experimentally. On this basis, we put forward a method that can separate the effect of the sensitive membrane on the sensitivity from the material completely by designing a series of AlGaN/GaN based heterostructure ISFET pH sensors with different sizes of the channel area (width/length parameter). Experimental result shows the...

Recently, β-Ga2O3 solar-blind photodetectors (PDs) have been extensively investigated for a wide range of civil and military applications. Among them, the metal-semiconductor-metal (MSM) structure is one of the most popular candidates due to the merits of fabrication simplicity, the need for only one single-dopant active layer, easy integration with readout circuitry, high quantum efficiency, etc. However, there is generally a tradeoff between sensitivity and response speed due to the specific internal gain mechanism. In this work, MSM PDs based on the molecular beam epitaxy-grown β-Ga2O3 film were fabricated, and the metal/semiconductor (M/S) interfacial properties were tailored through the low-energy Ar-plasma pretreatment, resulting in the localized oxygen deficiency and a sharper interface. Accordingly, the PD sensitivity was dramatically improved, and the advantage of internal gain, i.e., high quantum efficiency, was preserved or became even larger. For example, the 60-W pretreated sample exhibited a high responsivity (R) of 8.41 A/W and a large specific detectivity (D*) of 1.24 × 1014 Jones, both increasing by one order of magnitude in comparison with the untreated sample. More interestingly, the response speed was unexpectedly accelerated, which is ascribed to the rapid and direct tunneling of electrons at the M/S interface as well as to the reduction in RC time constant based on the data analysis and the underlying physical principle discussion. The treatment conditions can be further optimized to counterbalance some side effects. These findings reveal an efficient technique for comprehensively improving the performance of β-Ga2O3 solar-blind PDs.Recently, β-Ga2O3 solar-blind photodetectors (PDs) have been extensively investigated for a wide range of civil and military applications. Among them, the metal-semiconductor-metal (MSM) structure is one of the most popular candidates due to the merits of fabrication simplicity, the need for only one single-dopant active layer, easy integration with readout circuitry, high quantum efficiency, etc. However, there is generally a tradeoff between sensitivity and response speed due to the specific internal gain mechanism. In this work, MSM PDs based on the molecular beam epitaxy-grown β-Ga2O3 film were fabricated, and the metal/semiconductor (M/S) interfacial properties were tailored through the low-energy Ar-plasma pretreatment, resulting in the localized oxygen deficiency and a sharper interface. Accordingly, the PD sensitivity was dramatically improved, and the advantage of internal gain, i.e., high quantum efficiency, was preserved or became even larger. For example, the 60-W pretreated sample exhibited a...

An all-solid-state AlGaN/GaN based ion-sensitive heterostructure field effect transistor (ISHFET) pH sensor was fabricated by integrating a noble metal (Au) quasi-reference electrode to improve the device stability when measuring the pH value of a small aqueous volume. In this paper, the influence of the size of the quasi-reference electrode against the stability of the pH readings was investigated. Through optimizing the size of the integrated quasi-reference electrode, the all-solid-state ISHFET pH sensor can sustain stable pH measurements for aqueous solutions of micro-litre size. A sensitivity of 55 mV/pH was achieved by the pH sensor at room temperature. Thus, the device may have potential uses in biomedical applications which require small volume pH measurements.An all-solid-state AlGaN/GaN based ion-sensitive heterostructure field effect transistor (ISHFET) pH sensor was fabricated by integrating a noble metal (Au) quasi-reference electrode to improve the device stability when measuring the pH value of a small aqueous volume. In this paper, the influence of the size of the quasi-reference electrode against the stability of the pH readings was investigated. Through optimizing the size of the integrated quasi-reference electrode, the all-solid-state ISHFET pH sensor can sustain stable pH measurements for aqueous solutions of micro-litre size. A sensitivity of 55 mV/pH was achieved by the pH sensor at room temperature. Thus, the device may have potential uses in biomedical applications which require small volume pH measurements.

We demonstrate the enhancement of emission of InGaN/GaN multiple-quantum-well nanorods by nearly a factor of 2 by coupling them to localized surface plasmons of Au nano-particles (NPs). The Au NPs are fabricated in situ on the nanorods using a Ni/SiO2/Au/SiNx compound functional layer. This layer serves as a combination dry-etch mask for fabricating the nanorods and the Au NPs, as well as providing isolation necessary to prevent fluorescence quenching. Time-resolved photoluminescence measurements confirm that emission enhancement originates from the coupling.

Al0.4Ga0.6N/Al0.65Ga0.35N heterojunction phototransistors have been fabricated from the epi-structure grown by low-pressure metal organic chemical vapor deposition on c-plane sapphire substrates. P-type conductivity of the AlGaN base layer was realized by using indium surfactant-assisted Mg-delta doping method. Regrowth technique was used to suppress the Mg memory effect on the n-type emitter. The fabricated devices with a 150-μm-diameter active area exhibited a bandpass spectral response between 235 and 285 nm. Dark current was measured to be less than 10 pA for bias voltages below 2.0 V. A high optical gain of 1.9 × 103 was obtained at 6 V bias.

High hole concentration was achieved in Mg-doped AlxGa1−xN (x ∼ 0.4) by using indium-surfactant-assisted delta doping method. A maximum carrier concentration of 4.75 × 1018 cm−3 was obtained, which is three times higher than that of the conventionally delta-doped sample. Sheet resistivity as low as 2.46 × 104 Ω/sq was realized, benefiting from the high hole concentration (p). Analysis results show that the Mg incorporation is effectively enhanced, while the compensation ratio and acceptor activation energy (EA) are significantly reduced by using In surfactant. It was also found that the In surfactant may induce stronger valence-band modulation, contributing to the decrease of EA and the increase of p.

Hole transport control and carrier injection improvement have been demonstrated in the InGaN/GaN light-emitting diodes (LEDs) with step-stage multiple-quantum-well (MQW) structure and Si-doped hole-blocking barriers. Single-wavelength emission was obtained under electrical pumping in these LEDs by utilizing hole-blocking effect. The light emission around 450 nm showed a substantial increase compared with the reference sample with single or step-stage indium-content MQWs. The droop behavior and wavelength stability were also improved significantly. These improvements were attributed to the enhanced carrier injection to the active region due to the alleviation of the quantum-confined Stark effect and the effective hole-blocking effect of the Si-doped barriers.

GaN-based light emitting diodes (LEDs) with shallow triangular quantum wells (TQW) structure were proposed and investigated. LEDs with shallow TQW demonstrated a lower turn-on voltage and 80% higher lighting efficiency at 20 mA than devices without the shallow QW structure. A more stable emission wavelength and a lower ideality factor were achieved with the proposed structure. X-ray reciprocal space mapping revealed a partial strain relaxation in active region due to the insertion of the TQW structure. The improved performance is ascribed to the weakening of the polarization field in the MQW active region induced by the TQW structure.

Crack-free InGaN multiple quantum wells(MQWs)light-emitting diodes with embedded electrode structures (EE-LEDs) were transferred from Si (111) substrate onto the electroplatingcopper submount. Crystalline quality was investigated by the high resolution x-ray diffraction (HR-XRD) measurement, in which no obvious deteriorations were found in the MQWs structure after the LEDs transferred from silicon substrate onto copper except for a partial residual strain relaxation in the film. The strain relaxation after silicon removal leads to a reduction in quantum confined stark effect (QCSE), which results in the enhancement of internal quantum efficiency (IQE). In comparison to the conventional LEDs on silicon substrate, the light output of the EE-LEDs on copper was enhanced by 122% at an injection current of 350 mA. Besides the enhancement of IQE, the improvement is also attributed to the following factors: the removal of the absorptive substrate, the inserting of the metal reflector between the EE-LEDs structure and the copper submount, the elimination of the electrode-shading, and the rough surface of the exposed AlN buffer layer.

We report the anomalous mobility properties of Si-delta-doped GaN with periodically doping profile. Samples with different delta-doping periods or with varied Si source flow were investigated. It is found that for the short delta-doping-period (

In this letter, a method of using selective area growth (SAG) technique was proposed to fabricate the enhancement-mode (E-mode) AlGaN/GaN heterostructure field effect transistors (HFETs), which can effectively avoid the plasma treatment damage to the active region of HFETs in comparison with the conventional methods. The SAG-HFETs exhibited a good performance of the maximum drain current of 300 mA/mm and peak transconductance of 135 mS/mm with a larger positive threshold voltage of 0.4 V. The results indicate that the SAG technique is a promising method to realize the high performance E-mode GaN based HFETs.