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Solid state image sensing device
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Inventors: Arakawa, Kenichi; Nakahara, Kenji;
Assignee: Kabushiki Kaisha Toshiba (Kanagawa-ken, JP)
Primary Examiner: Prenty; Mark V.
Assistant Examiner:
Attorney, Agent or Firm: Finnegan, Henderson, Farabow, Garrett & Dunner, L.L.P.
A solid state image sensing device, comprises: an n-type semiconductor substrate (11), a p-type well (12) formed on a surface of the semiconductor substrate, and a p.sup.+ -type diffusion layer (13, 21) having an impurity concentration higher than that of the well. In particular, the P.sup.+ -type diffusion layer (13) is formed so as to cover at least a part of circumference of an n-type diffusion layer (17) of a load transistor (N3) formed in the well (12) as a source follower circuit. Instead, the P.sup.+ -type diffusion layer (21) is formed between the n-type diffusion layer (17) of the load transistor (N3) and the semiconductor substrate (11). In both the cases, since the n-type diffusion layer (17) of the load transistor (N3) of the source follower circuit is covered with the second conductivity diffusion layer (13, 21) higher in impurity concentration than that of the well, even when a pulse is applied to the semiconductor substrate during electronic shutter operation, the p-type well (12) is not depleted, so that it is possible to prevent punch through current (I) from flowing from the n-type diffusion layer (17) of the load transistor (N3) to the n-type substrate (11). |
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DETAILED DESCRIPTION Accordingly, the object of the present invention is to provide a solid state image sensing device which can prevent the punch through from being generated during the electronic shutter operation and further improve the S/N ratio of the output signal, even if the pixel dimensions are reduced with the advance of the microminiaturization of the image sensing device.
To achieve the above-mentioned object, the present invention provide a solid state image sensing device, comprising: a first conductive type semiconductor substrate; a second conductive type well formed on a surface of the semiconductor substrate, the first and second conductivity types being opposite to each other; and a second conductive type diffusion layer having an impurity concentration higher than that of the well, the second conductive type diffusion layer being so formed as to cover at least a part of circumference of a first conductive type diffusion layer of a load transistor formed in the well as a source follower circuit.
In the solid state image sensing device according to the present invention, since at least part of the circumference of the first conductivity type diffusion layer of a load transistor formed as a source follower circuit is converted with the second conductivity type diffusion layer having an impurity concentration higher than that of the well, even if a pulse is applied to the semiconductor substrate during the electronic shutter operation, it is possible to prevent the second conductivity type diffusion layer from being depleted, so that the generation of the punch through between the first conductivity type diffusion layer and the semiconductor substrate can be prevented, thus improving the S/N ratio of the output signal.
Further, even if the second conductive type diffusion layer having an impurity concentration higher than that of the well is formed between the first conductivity type diffusion layer of the load transistor formed in the well as a source follower circuit and the semiconductor substrate, the same effect can be obtained
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MEMS 
