Ultra-High-Sensitive Image Pickup Tubes Using Avalanche Multiplication in a-Se.
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ULTRA-HIGH-SENSITIVE IMAGE PICKUP TUBES USING AVALANCHE MULTIPLICATION IN a-Se. K. Tsuji*, T. Ohshima*, T. Hirai*, N. Gotoh**, K. Taniokat, and K. Shidarat *Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185, Japan "**Electron Tube Division, Hitachi Ltd., Mobara, Chiba, 297, Japan tNHK Science and Technical Research Laboratories, Setagaya, Tokyo 157, Japan ABSTRACT 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. INTRODUCTION In amorphous selenium (a-Se), stable avalanche multiplication of photogenerated carriers occurs under electric fields higher than 8x105 V/cm [1,2,3]. The most prominent difference of avalanche phenomenon in a-Se and that in crystalline semiconductors is the temperature dependences of their impact ionization rates [4]. In crystalline semiconductors, the impact ionization rates show negative temperature dependences which are explained by the temperature dependences of the effective phonon energy and the phonon mean free path [5]. Conversely, the temperature dependences of the impact ionization rates in a-Se are positive, and it is indicated that there are characteristic factor(s) which affect the probability of ionization events in addition to phonon scattering. One of the candidates is the gap states. If that's the case, the avalanche phenomenon related to the energy distribution of the density of gap states may also be characteristic for other amorphous semiconductors including a-Si:H. Apart from the temperature dependences of the impact ionization rates, there are some characteristic features of the avalanche phenomenon in a-Se which relate to device performance. (1) The impact ionization rate of holes is much larger than that of electrons [2]. This is advantageous when the a-Se APD (Avalanche Photo-Diode) is used for devices, such as image pickup tubes, in which the hole current is dominant in the primary photocurrent. Furthermore, the large ratio of the impact ionization rates is related to the low excess avalanche noise [6]. (2) The electric field in the a-Se layer is almost uniform in thickness, and avalanche multiplication occurs throughout the a-Se layer, unlike the situation in crystalline APDs with p-n junctions. This fact simplifies the structure and fabrication process of the a-Se APD. (3) Multiplication occurs uniformly over a large area. This is essential for application in image sensors. Mat. Res. Soc. Symp. Proc. Vol. 219. @1991 Materials Research Society
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Using these advantageous features, we first developed avalanche-type,
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