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.
Optrodes, which are single shaft neural probes integrated with microelectrodes and optical light sources, offer a remarkable opportunity to simultaneously record and modulate neural activities using light within an animal’s brain; however, a common problem with optrodes is that stimulation artifacts can be observed in the neural recordings of microelectrodes when the light source on the optrode is activated. These stimulation artifacts are undesirable contaminants, and they cause interpretation complexity when analyzing the recorded neural activities. In this paper, we tried to mitigate the effects of the stimulation artifacts by developing a low-noise, double-sided optrode integrated with multiple Electromagnetic Shielding (EMS) layers. The LED and microelectrodes were constructed separately on the top epitaxial and bottom substrate layers, and EMS layers were used to separate the microelectrodes and LED to reduce signal cross-talks. Compared with conventional single-sided designs, in which the LED and microelectrodes are constructed on the same side, our results indicate that double-sided optrodes can significantly reduce the presence of stimulation artifacts. In addition, the presence of stimulation artifacts can further be reduced by decreasing the voltage difference and increasing the rise/fall time of the driving LED pulsed voltage. With all these strategies, the presence of stimulation artifacts was significantly reduced by ~76%. As well as stimulation suppression, the sapphire substrate also provided strong mechanical stiffness and support to the optrodes, as well as improved electronic stability, thus making the double-sided sapphire optrodes highly suitable for optogenetic neuroscience research on animal models.
Traditional GaAs-based frequency multipliers still exhibit great challenges to meet the demand for solid-state high-power THz sources due to low breakdown voltage and heat dissipation of the Schottky barrier diode (SBD). In this study, a GaN SBD chain was fabricated with n−/n+-GaN structure. As a consequence, the breakdown voltage of 54.9 V at 1 μA and cut-off frequency of 587.5 GHz at zero bias were obtained. A 120 GHz frequency-doubler module based on the GaN SBD chain was designed and fabricated. When driven with 500 mW input power in a continuous wave, the output power of the frequency-doubler module was 15.1 mW at 120 GHz. Moreover, the experiments show that the frequency-doubler module can endure an input power of 2 W. In addition, it is worth noting that the SBD chain works well at an anode temperature of 337.2 °C.
The AlGaN/GaN Schottky barrier diodes (SBDs) working as high-power mixer and multiplier show great potential in millimeter wave (MMW) field owing to their high breakdown voltage. Nevertheless, its further application is severely limited by large reverse leakage current (Jr) since the two-dimensional electron gas (2DEG) channel is hard to be pinched off at low voltage. To address this limitation, a graded AlGaN/GaN heterostructure is introduced to extend the 2DEG channel into a quasi-three-dimensional electron slab. By comparing the fixed Al composition AlGaN/GaN SBD, Jr of the graded AlGaN/GaN SBD is significantly reduced due to the extension of channel carriers, confirming the effective Jr suppression effect of this structure. Furthermore, on this basis, a recessed anode structure is utilized to expect a smaller Jr. The results indicated that the graded AlGaN/GaN SBDs with air-bridge structure have achieved a pretty low Jr value (1.6 × 10−13 A at -15 V), and its cutoff frequency is as high as 60.6 GHz. It is expected that such SBDs with low Jr have significant advantages in future applications.
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