We report on a transparent gate silicon MOS photo-impedance sensor, where a gated light sensitive semiconductor layer connects fixed capacitances. The resistance of the semiconductor and the capacitance of the MOS structure change with illumination. The frequency dispersion makes the coupling of these capacitances sensitive to light intensity extending the sensor dynamic range and tuning the sensitivity of the sensor. Our modeling results demonstrate advantages of this novel sensor in terms of sensitivity and dynamic range. The design and concept of this device could be extended to many other semiconductor materials, where frequency dispersion is related either to traps, or embedded nanoparticles or carrier generation processes.
We report on the low-frequency current and light noise in 515 nm green GaInN/GaN quantum well LEDs. The current noise was the superposition of the 1/f and the generation-recombination (GR) noise. The characteristic time of the GR process was found to be proportional to the reciprocal current for the entire current range. This dependence is the characteristic for the monomolecular non-radiative recombination. The dominance of the nonradiative recombination is in agreement with the measured low external quantum efficiency (EQE) <10%. Hence, the noise measurements point out that a low EQE is caused by the low internal quantum efficiency and not by an inefficient light extraction. The noise spectra of light intensity fluctuations were close to the 1/f noise and correlated with the LED quantum efficiency and with the recombination current. Higher noise corresponded to a smaller quantum efficiency and to a higher non-radiative recombination current. The relative spectral noise densities of the light intensity fluctuations within the LED spectral line increase with the wavelength decrease. Fluctuations at different wavelengths are found to be correlated.
The current and frequency dependencies of the low frequency noise have been investigated in 4H-SiC p+-n junctions in the frequency range 100-104 Hz and at current densities from 10-4 to 101 A/cm2. Good quality of the p+-n diode under investigation has been ascertained by high value of the recombination time in the space charge region, &tgr;R ≈ 70 ns, extracted from current voltage characteristic. At small current densities j ⩽ 10-3 A/cm2, the spectral noise density SI ∝1/f3/2. At 10-3 A/cm2 < j < 10-2 A/cm2, the generation-recombination (GR) noise predominates. The amplitude of this GR noise non-monotonically depends on current. At j ⩾ 10-2 A/cm2, the 1/f (flicker noise) is dominant. A new model of GR noise of the recombination current in forward biased
p-n junction has been proposed. The model assumes that a trap level located relatively close to the conduction band is responsible for the observed GR noise. The main contribution to the GR noise comes from the fluctuations of the charge state of the trap. The model describes well both current and frequency dependencies of the observed GR noise.
Light emitting diodes (LEDs) are excellent candidates for the applications requiring low noise light sources with wavelengths ranging from 200 nm to 900 nm. These applications include the detection of fluorescence from protein molecules excited with the ultraviolet (UV) light (200-300nm) for identifying miniscule amounts of hazardous biological pathogens. The detection system including the light source must exhibit low noise and high stability over tens of minutes. In comparison with xenon, tungsten halogen lamps, lasers, and other conventional UV sources, UV LEDs are more stable, have lower noise, are smaller, cheaper, and easier to use. We report on the low frequency fluctuations of the current and light intensity of LEDs (fabricated by SET, Inc.) with wavelengths ranging from 265nm to 340nm. The results are compared with the noise properties of the halogen lamps and other commercially available LEDs with the wavelengths of 375nm, 505nm and 740nm. We show that the LEDs fabricated by Sensor Electronic technology, Inc. are suitable for studying steady state and time-varying UV fluorescence of biological materials. The correlation coefficient between the current and light intensity fluctuations varies with the LED current and load resistance. This dependence is explained in terms of the contributions to the 1/f noise from the active region and from the LED series resistance. The noise level could be reduced by operating the LEDs at a certain optimum current level and by using a large external series resistance (in the current source driving mode).
The I-V characteristics of GaN/AlGaN HFET and 1/f noise at 4K have been measured in strong magnetic fields, where the electron mobility is affected by geometric magnetoresistance. The magnetic field dependence of the 1/f noise shows that the number of electrons fluctuations is the dominant mechanism of the 1/f noise and precludes the mobility fluctuations mechanism. The channel mobility extracted from the magnetoresistance data first increases with gate bias reaching the maximum value of ~(0.9-1.0) m2/Vs at the 2D electron concentration of 5x1012 cm-2. This maximum value is close to the estimated ballistic mobility limit of 1.2 m2/Vs determined by the electron transit time with the Fermi velocity.
Low frequency noise in 4H-SiC BJTs with the current gain β ≈ 10-15 and unity current gain frequency fT of about 1.5 GHz has been investigated. The corner frequency fc was found to be fc = 2×104 Hz. The value of the coefficient KB,
which is the figure of merit for the low frequency noise in the region of noise proportional to squared current, was found
to be 6×10-7 μm2. This value is only an order of magnitude higher than the typical values for high-frequency Si-BJTs.
We report on fabrication, characterization, and properties of nanocrystalline semiconductor films and thin-film devices chemically deposited on fibers, cloth, and large area flexible substrates at low temperatures (close to room temperature). We also describe the photovoltaic effect in CdS/CuS films deposited on viewfoils and trylene threads. CdS films deposited on viewfoils exhibit unique behavior under stress and UV radiation exposure with reproducible resistance changes of several orders of magnitude with bending up to 10 mm curvature. The measurements of the 1/f noise in these nanocrystalline structures indicate a high quality of nanocrystallites.
Generation-recombination (GR) noise in GaN and AlGaN thin films, GaN based Metal Semiconductor Field Effect Transistors (MESFETs), Heterostructure Field Effect Transistors (HFETs) and Schottky diode photodetectors was investigated. AlGaN thin films, AlGaN/GaN HFETs and Schottky barrier Al0.4Ga0.6N diodes exhibited GR noise with activation energies of 0.8 - 1 eV. AlGaN/GaN HFETs also presented GR noise with activation energies of 1 - 3 meV and 0.24 eV at cryogenic temperatures. No such noise was observed in thin doped GaN films and GaN MESFETs. GR noise with the largest reported activation energy of 1.6 eV was measured in AlGaN/InGaN/GaN Double Heterostructure Field Effect Transistors (DHFETs). We conclude that the local levels responsible for the observed noise in HFETs and DHFETs could be located in AlGaN barrier layers.
We will discuss results of the experimental and theoretical study of the low-frequency noise in GaN/AlGaN 2D structures and examine possible sources of noise, including contacts, surface and 2D channel itself. 2D GaN/AlGaN heterostructures exhibit a much smaller level of 1/f noise than bulk GaN films. In the frame of model linking noise to the tail states, this might be explained by a high degeneracy of the 2D electrons in this structures. Due to the electron degeneracy, the tail states mechanism of the 1/f noise is suppressed in GaN-based 2D structures. Our measurements show that contacts do not contribute much to overall low frequency noise. Concentration dependence of the Hooge parameter points out to the tunneling mechanism of noise in these structures.
The newly developed optical fibers are described designed to transmit intense radiation in the visible and near IR spectral regions. The main characteristic feature of the fibers is a core made of unique purity silica with extremely low OH content not exceeding 20 ppb. As a result, the fiber spectral attenuation in the infrared is substantially reduced: the absorption spectrum is a smooth monotonously rising curve with typical loss levels of less than 0.2 dB/m at 2.1, 10 dB/m--at 2.7 and 27 dB/m--at 2.8 microns. Thin (approx. 10% of the fiber diameter) fluorinated silica cladding together with the high purity core ensure excellent optical damage resistivity of the fibers and greatly facilitate their coupling to high power lasers. The plasma outside deposition technology used for clad fabrication can provide fluorine doping levels sufficient to produce the fibers with numerical aperture NA up to 0.24. The fibers can have either hermetic metal (In, Sn, Pb, Zn, Al) or plastic protective coating. For the Al-coated fibers the mechanical properties and environmental durability are characterized by the following figures: tensile strength--more than 7 GPa, minimal bend radius--40 times the fiber radius (long term), operating environment--from--196 up to 400 degree(s)C, 100% humidity.
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