Tadaaki Hirai

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.

Formation of Thin Polyacrylonitrile Films and Their Electrical Properties

Semi- and photo-conductive thin films of polyacrylonitrile with the thickness of a few microns of less are prepared by the silent electric discharge in acrylonitrile monomer vapor followed by the heat-treatment at temperatures of 150°C to 500°C. The growth rate of the films deposited under the discharge is proportional to the monomer vapor pressure and the discharge voltage. The electrical resistivity and its activation energy decrease and the spectral responses of both the photocurrent and the optical absorption shift toward longer wavelength as the heat-treatment temperature rises. Non-ohmic characteristics of the electrical conduction are observed in the filed higher than ~3×104 V/cm, and agree with the theoretical expression of the modified Pool-Frenkel effect. The energy depth of the trap estimated from the thermally stimulated current measurement is ~0.5 eV.

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.

Ultra-High-Sensitive Image Pickup Tubes Using Avalanche Multiplication in a-Se.

Extremely high-sensitive image pickup tubes with sensitivities 1000 times higher than those of conventional tubes are fabricated using the avalanche phenomenon in a-Se as photoconductive targets. The excess avalanche noise of a video signal is found to be much less than that expected, based on the carrier ionization rates. The frequency spectra of the noise currents of both the pickup tubes and sandwich-type photocells are examined. The results are compared with those of a simulation, and it is found that the excess noise can be reduced by the charge-storage operation of imaging devices.

Excess Noise in Amorphous Selenium Avalanche Photodiodes

Excess noise in amorphous Selenium avalanche photodiodes (a-Se APD) has been measured in a frequency range from 3 kHz to 30 kHz. The deduced excess noise factors, including dependences on photocurrent, frequency, applied electric field and the a-Se layers thickness, agreed with McIntyres theoretical values.

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.

A Contact Type Linear Photodiode Array Using an Amorphous Thin Film

A Contact type linearly arrayed imaging sensor not requiring an optical lens system has been successfully developed. This sensor array consists of 40 amorphous Saticon type photodiodes formed on an optical fiber plate, each of which measures 250 µm by 200 µm and the overall sensor length is 10 mm. This device has an advantage of easy fabrication which permits elongation of the total length without deteriorating the high resolution characteristics. Therefore, it may be useful as an imaging sensor for small size facsimile equipment. In this paper, a series of technical data on the proposed structure, the fabrication procedures and also the static and operational performances are described.

Ultrahigh-sensitivity new super-HARP camera

NHK led the world in developing a high-sensitivity Super-HARP pickup tube using the avalanche multiplication effect. The authors have now developed an improved version (2/3- inch, with electromagnetic focusing and electromagnetic deflection) which is eight times more sensitive and has much better lag characteristics. The handy New Super-HARP color camera which uses this newly-developed pickup tube has higher sensitivity than human vision (2000 lux, F/110 equivalent), negligible low lag, and a limiting resolution of over 700 TV lines. It will be a powerful tool in emergency news gathering at night, the production of scientific programs, and other applications.

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”.