A Readout Integrated Circuit (ROIC) for GaN ultraviolet (UV) focal plane array (FPA) working in “solar-blind” band is studied in this paper. It has a format of 320×256 and a pixel pitch of 30μm. This circuit can operate both in integrating-while-reading (IWR) and integrating-then-reading (ITR) mode with the frame rate higher than 100fps. It is common that trade-offs always exist between chip power consumption and performances in integrated circuits design. In order to get high injection efficiency with small area and low power, A novel low-power capacitive-feedback trans-impedance amplifier (CTIA) with snapshot mode is designed for the proposed circuit. The smallest operational current of CTIA is only 10nA for 5V power supply. The total power consumption of ROIC is reduced significantly to 45mW with the ultra-low-power pixel. By adopting the 0.35μm 2P4M mixed signal process, the high-performance CTIA architecture can make two gain selections which charge capacities are 3.4Me - and 0.16Me - per pixel with 2.5 V output range. According to the experimental results, this circuit works well under 5V power supply and achieves 8MHz pixel-data-transmission rate.
Corona discharges occur in high voltage electrical equipment in case of defects and damage, while ultraviolet(UV) light generated during discharge. High resolution imaging in the solar-blind UV bands has a lot of applications in corona discharge detection. A ultraviolet imager based on 320×256 solar-blind AlGaN focal plane arrays (FPA) was designed that work even in the sunlight, because the Cut-off wavelength of the AlGaN FPA is 280nm. The UV image signal processing system based on FPGA is composed of various function modules include the voltage bias, sequence drive, A/D data acquisition, non-uniformity correction, video transformation. Due to FPGA-based data acquisition and realtime image processing technology, the UV imager can operate at a rate up to 100 frame/s. The results show that the simulation high voltage ultraviolet image can be obtained by the UV imager. The image non-uniformity correction performed is one-points correction method to realize background subtraction. And the images show good uniformity and contrast. The UV image of the alcohol burner flame can be detected by the Ultraviolet Imager. Imaging quality was discussed which can be determined by signal-to-noise ratio (SNR), the integration time, the optics f/number and so on. The best imaging conditions were analyzed and the imaging system was designed and setup. The conclusion is proved that the ultraviolet imager based on solar-blind AlGaN FPA provides a new method for corona discharge detection of high voltage power transmission and distribution system.
A novel column-stage structure of readout integrated circuit (ROIC) for GaN ultraviolet (UV) focal plane array (FPA) working in “solar-blind” band is proposed. The column stage has better drive capability, higher dynamic range, stable bias current and low impedance. The noise voltage of the column readout stage is discussed, which has been reduced by small-current driving, column-stage sample and hold and the technology of divided-output-bus. This research on low-noise ROIC is designed for weak-current UV FPA. It is designed, simulated and laid out using the 0.35um 2P4M CMOS 5V process. The clock rate operates at 8MHz. The simulation input current sets 0.01nA. The output swing is 2.6V and power consumption is 40 mW according to the measurement results.
GaN-based avalanche photodiodes (APDs) have become of increased interest in the UV detection arenas. However,
numerous material-, fabrication-, and design-related problems are exactly settled before GaN-based APDs can be
commercialized. In this study, we, first, discussed recent development of the GaN-based APDs. Then front- and
back-illumination (respectively realizing electron and hole initial impact-ionization) p-i-n heterostructure devices with
various mesa diameters were fabricated. The device with a diameter of 40 μm exhibited a multiplication gain of ~680, at
reverse bias of ~76 V corresponding to the magnitude of the electric field of ~ 3 MV/cm by experiment indicating and
simulation verifying. To confirm the origin of dark current under different reverse bias, the dark current-voltage
characteristic of various sized mesa devices were performed. The dark current could be linearly fitted to the device
diameter (or circumference) implied that the surface leakage along the mesa sidewall was the dominant component of the
dark current. At zero bias, the spectral peak responsivity reached ~ 0.14A/W for front illumination, and ~ 0.152A/W for
back illumination at a wavelength of 358 nm. The positive breakdown voltage coefficient from the
temperature-dependent current-voltage characteristics was 0.02 V/K.
Recently, high-Al-content AlGaN alloy systems have attracted increasing attention, and it is urgent and
important to achieve excellent Ohmic contacts with low specific contact resistivity, good thermally stability, clear
borderline and smooth surface morphology of this alloy systems to optimize the performance of photoelectric
devices. In the experiment, we found that surface disordered layer and oxides including native oxide could be
removed by boiling KOH solution. The surface status of both samples was evaluated with scanning electron
microscope (SEM) and X-ray photoelectron spectra (XPS). For comparison, then A Ti/Al/Ti/Au multilayer was
deposited on the samples with and without wet chemical etching to observe their electric properties. After annealing,
I-Vcharacteristics via Keyley236 electric analyzer was measured. Ohmic contacts with the contact specific resistivity
of 6.55×10<sup>-4</sup>Ωcm<sup>2</sup> were obtained between treated samples and the multi-metals. However, nonlinear I-V curves
indicated that the contact on the untreated sample was still the Schottky contact.
We have investigated wet chemical etching process of n<sup>+</sup>-type Al<sub>0.33</sub>Ga<sub>0.67</sub>N in 20% (by weight) aqueous KOH solutions
at 106°C after Ar<sup>+</sup> ion beam dry etching treatment. Scanning electron microscope and Auger electron spectroscopy were
employed to characterize the surface morphology and stoichiometry with and without wet chemical etching. It is obvious
that dry etching damages were reduced after wet chemical etching. We also fabricated two sets of visible-blind p-i-n
detectors for comparison. I-V characterization indicated that the average leakage current of the wet etching treated
detectors was lower than that of the detectors without treatment by about one order of magnitude. When the reverse bias
was -5 V, the leakage currents of wet etching treated devices varied from -2.16×10<sup>-9</sup> to -6.26×10<sup>-9</sup> A and those of
untreated detectors varied from -2.68×10<sup>-8</sup> to -3.49×10<sup>-8</sup> A. The peak responsivity at 365 nm was also tremendously
enhanced by means of wet chemical etching treatment. It was 0.10 and 0.03 A/W under back illumination, with and
without wet chemical treatment, respectively. When the detector was under front illumination, the result was 0.05 and
0.02 A/W, respectively.