Variation Of Dark Current Research Articles

Characteristics of a-Si Pixel Arrays for Radiation Imaging

ABSTRACTTwo-dimensional a-Si photodiode arrays with the a-Si TFT readout scheme were fabricated. Some performance characteristics were investigated for detecting visible light, X-ray and γ ray. Arrays of 64×40 1mm2-pixels have good uniformity with ± 1 % in linearity variation, ± 10 % dark current variation and ± 5 % sensitivity variation. The cross-talk between adjacent pixels was found to be negligible so that the intrinsic spatial resolution was determined entirely by the pixel size. Due to the high resistivity of a-Si photodiodes and TFTs, charges generated by pulsed light illumination were retained in photodiodes for a long period of time. This enables the accumulation-mode operation for detecting low-intensity light, where only one image is obtained over a long integration time. An encapsulated Co-57 source (3 μ.Ci) was successfully imaged with 5 sec integration. Alternatively, the array can be operated by the cine-mode for detecting high-intensity light, where a number of images are obtained successively.

Thermionic emission from negative electron affinity silicon

Thermionic emission from negative electron affinity surfaces on silicon has been studied. The value is dependent on the state of activation of the surface, and dark currents in the range 10−11−10−13 A/cm2 may be obtained. These values agree with those measured on sealed−off tubes. The dark current is thought to originate from surface states acting as traps for the generation of electrons. The density of surface states is estimated from the measurement of the variation of dark current during oxygenation. Field enhancement of dark current and photoemission has been observed.

Characteristics of the New Vidicon-Type Camera Tube Using CdSe as a Target Material

A new television camera tube has been developed under the name of “CdSe-vidicon” using the CdSe photoconductive layer as a target material. To explain high sensitivity (quantum yield greater than unity) of this vidicon, a new model is proposed for the n-type photoconductive layer of the target. Main difference between an n-type photoconductive target and a usual p-type is whether electrons of the beam can flow into the layer or not. The variation of dark current and of photocurrent with voltages, and spectral response as a function of layer thickness can be understood by considering the behavior of a non-uniformly illuminated n-type photoconductor with ohmic contacts, in which drift and trapping of photoexcited holes play an important role.

Opto-Electronic Properties of Mercuric Iodide

The optical and photoelectrical properties of mercuric iodide crystals and layers have been investigated, particularly as affected by the phase transformation from tetragonal (low-temperature form) to ortho-rhombic (high-temperature form) which occurs at 400\ifmmode^\circ\else\textdegree\fi{}K. The largest temperature coefficient for band-gap variation of any known material is found for the orthorhombic form: -24\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ ev/degree. The following phenomena are discussed: (1) location and temperature dependence of absorption edge by measurements of transmission, reflectivity, and photoconductivity response spectra; (2) temperature dependence of dark current; (3) electrode effects in photoconductivity; (4) variation of dark current and photocurrent through the phase transformation; (5) thermally-stimulated-currents; (6) variation of photocurrent with voltage, light intensity, and temperature. Temperature-quenching of photoconductivity in Hg${\mathrm{I}}_{2}$ crystals can be described by means of the same analysis as has been applied to various ${A}^{\mathrm{II}}{B}^{\mathrm{VI}}$ photoconductors, giving a location of 0.5 ev above the valence band for the sensitizing centers in Hg${\mathrm{I}}_{2}$.