Solid state image pickup device capable of draining unnecessary charge and driving method thereof

1. A solid state image pickup device comprising:

a semiconductor substrate;

a plurality of vertical charge transfer elements formed in one surface of said semiconductor substrate, each having (i) a vertical charge transfer channel having a first conductivity type and (ii) a plurality of vertical transfer electrodes formed on a first electrically insulating layer formed on the one surface of said semiconductor substrate, the vertical transfer electrodes traversing the vertical charge transfer channels as viewed in plan;

a plurality of photoelectric conversion elements formed in the one surface of said semiconductor substrate in a matrix shape in rows and columns, each of said photoelectric conversion elements being electrically connectable to corresponding one of said vertical charge transfer elements;

a horizontal charge transfer element formed in the one surface of said substrate and being electrically connectable to each of said vertical charge transfer elements, said horizontal charge transfer element having (i) a horizontal charge transfer channel having the first conductivity type and (ii) a plurality of horizontal transfer electrodes formed on the first electrically insulating layer formed on the one surface of said semiconductor substrate, each horizontal transfer electrode traversing the horizontal charge transfer channel as viewed in plan;

a drain region formed in the one surface of said semiconductor substrate along the horizontal charge transfer channel, said drain region having the first conductivity type; and

a barrier region for charge formed between the horizontal charge transfer channel and said drain region, wherein the horizontal charge transfer channel, said drain region and said barrier region constitute a two-electrode element;

wherein said drain region includes a first drain region formed on a side of said barrier region and a second drain region adjacent to the first drain region, the second drain region has a potential for charge deeper than a potential for charge of the first drain region.

2. A solid state image pickup device according to claim 1 , wherein each of said horizontal transfer electrodes covers said barrier region under the first electrically insulating layer.

3. A solid state image pickup device according to claim 1 , further comprising a second electrically insulating layer formed above the first drain region and being thicker than the first electrically insulating layer, wherein each of the horizontal transfer electrodes extends to an upper surface of said second electrically insulating layer.

4. A solid state image pickup device according to claim 1 , wherein said barrier region has the first conductivity type.

5. A solid state image pickup device according to claim 1 , wherein said barrier region has a second conductivity type opposite to the first conductivity type.

6. A solid state image pickup device according to claim 1 , wherein a plurality of said photoelectric conversion elements is disposed in a pixel shift layout.

7. A solid state image pickup device according to claim 1 , further comprising an output unit being electrically connectable to an output terminal of said horizontal charge transfer element and being able to generate voltage signals based on the charges output from said horizontal charge transfer element.

8. A solid state image pickup device according to claim 1 , wherein:

the horizontal charge transfer channel includes alternately configured potential barrier regions and potential well regions,

the horizontal transfer electrodes include well electrodes formed above said well regions, and barrier electrodes formed above said barrier regions, each pair of one of the well electrodes and an adjacent upstream barrier electrode are coupled to form one stage electrode, and the stage electrodes are classified into two groups for receiving two phase drive signal, one of said two groups of stage electrodes is formed to two comb shape electrodes electrically connected by wiring.

9. A solid state image pickup device according to claim 8 , wherein said two comb shape electrodes have common base portions connecting tooth portions, and the common base portions are disposed on opposite side to said drain region with respect to said horizontal charge transfer channel.

10. A solid state image pickup device according to claim 8 wherein the other of said two groups of stage electrodes is formed of two sets of a plurality of U shaped electrodes, and adjacent pairs of said U shaped electrodes are electrically connected by wiring.