Yukio Takasaki

Avalanche-mode Amorphous Selenium Photoconductive Target for Camera Tube

Publisher Summary This chapter discusses avalanche-mode amorphous selenium photoconductive target for camera tube. The chapter presents the characteristics of the experimental 18 mm tubes for standard television systems. It is well known that there are two types of photoconductive target layers: the injection type and the blocking type. In the case of injection target layers, quantum efficiency as great as unity or more can be achieved, but it has the disadvantages of time lag and high dark current. In spite of its blocking-type target, the camera tube shows high sensitivity quantum efficiency greatly in excess of unity, the upper limit of the sensitivity of a conventional blocking target layer. The phenomenon of avalanche multiplication in an amorphous selenium layer was found for the first time at an applied field of greater than 0.8 × lo 8 V m –1 . This phenomenon was exploited in the target layer of an experimental camera tube.

Photoconductive imaging using hydrogenated amorphous silicon film

Highly resistive hydrogenated amorphous‐silicon film has been fabricated and examined as blocking‐type photoconductive target of a vidicon‐type image pickup tube. The results indicate that this novel silicon vidicon has many advantages over conventional ones.

Impact ionization process in amorphous selenium

Abstract Temperature and electric field dependences of impact ionization rates α (for electron) and β (for hole) in a-Se have been determined by photo-multiplication measurements on sandwich type photo-cells. It has been found that both α and β decrease with decreasing temperature, in contrast to those of crystalline semiconductors. The results obtained at room temperature have been fitted to those calculated by Baraffs theory taking the geminate recombination process into account. The observed temperature dependences suggest characteristic mechanisms of the impact ionization process in this material.

Single-Tube Color Imager Using Hydrogenated Amorphous Silicon

Reactively sputtered hydrogenated amorphous silicon film is used as the photoconductive target of a vidicon type image pickup tube. It is indicated that hydrogen partial pressure in the discharge gas has to be 40% or more in order to attain suitable image pickup tube characteristics. Photo-electric properties and lag-characteristics are enormously improved by lightly doping boron into amorphous silicon film, and a blocking type target structure is effective for suppressing dark current of the pickup tube. Using doped amorphous silicon film with a color-filter-integrated face plate, a single-tube color imager is fabricated.

High-sensitivity digital radiography using an avalanche-type image pickup tube camera

A medical imaging camera with an avalanche-type image pickup tube (HARPICON) was developed for digital radiography. The camera obtains high quality images even at low doses because its sensitivity is as much as 32 times higher than that of a conventional pickup tube camera. The camera also has a wide dynamic range and a high signal-to-noise ratio because of HARPICONs gamma characteristic.

Analyses of Noise in a Highly Sensitive Image Device

A dipole charge-discharge (i.e., DCD) model was proposed to explain the excess noise factor (i.e., θ) of an amorphous selenium APD target below unity beyond the theoretical limit of shot noise. If the dipole distribution between stored holes and electrons is proper, the electric field in the target is formed between storage periods without interfering with the reading-writing mechanism for the signal, and can control the number of stored carriers so that θ becomes smaller than unity. The theoretical relationship between the signal (i.e., Is) and was obtained and could explain the experimental value well. The theoretical photoelectric conversion characteristics were obtained, and agreed with the experimental values obtained at a high illumination intensity. The magnitude of the negative lag was calculated and was in good agreement with the experimental value.

Molecular doping in amorphous selenium

Abstract The transport properties of vacuum evaporated selenium doped with small quantities of atoms (Li, Ga, In, Cl, etc.), and compounds (LiCl, GaF3, In2O3, etc.) are examined using time-of-flight technique. When doped with certain compounds (for example, GeO2), electrons and holes show comlex transport behavior different from the compensation effect due to co-doping of the component atoms (Ge, O). This suggests that these compounds are taken into selenium in the “molecular form”.

Amorphous Silicon Image Pickup Devices

Reactively sputtered amorphous silicon film is used as a photoconductive target of the vidicon type image pickup tube. Variation of tube characteristics with the hydrogen partial pressure in the discharge gas during sputtering are investigated. The results indicate that hydrogenated amorphous silicon films prepared in hydrogen partial pressures of 1~2×10-3 Torr are suitable for image pickup tubes. This amorphous silicon vidicon has several excellent characteristics: better sensitivity of about twice that of the conventional tubes, high resolution, little after-image, and no blooming.

Optical and Electrical Properties of Nitrogen Doped a-Si: H Films for Solid-State Image Pickup Devices

Hydrogenated amorphous silicon (a-Si: H) films deposited by RF sputtering and doped with nitrogen have been found to have high dark resistivity (1013 Ωcm) and high photoconductivity at a low field. The Fermi level is shifted by 0.2 eV toward the conduction band, compared with undoped film. The spectral response of this film covers the visible region well and a peak quantum efficiency of 1.0 has been obtained at a wavelength of 500 nm. The γ-value of the film is nearly equal to unity for visible radiation. Rise and fall time constants are shorter than 500 µs for a pulsed radiation of 550 nm emitted from an LED. We have found that nitrogen doped n-type a-Si: H films are quite suitable as a photoconductive material for a solid state color imager.

Development of a 4000-scanning-lines high-resolution TV camera for real-time digital radiography system

We have developed a 4200 scanning lines TV camera using a 2-inch diameter, electrostatic focus and electrostatic deflection type SATICONTM image pick-up tube which have been newly developed for real-time digital radiography (DR) system. We obtained 23% in AR value at 2600 TV line and 47.4 dB in S/N value at signal current 1200 nA in 4000 lines mode.