Solid state imaging device, imaging system, and manufacturing method of solid state imaging device
A solid state imaging device as an embodiment has a first transfer unit that includes a first gate and transfers charges from a photoelectric conversion portion to a holding portion; a second transfer unit that includes a second gate and transfers charges from the holding portion to a floating diffusion portion; and a third transfer unit that includes a third gate and drains charges from the photoelectric conversion portion to the charge draining portion. The impurity concentration of a second conductivity type in at least a part of a region under the first gate of the first transfer unit is lower than the impurity concentration of the second conductivity type in a region under the second gate of the second transfer unit and the impurity concentration of the second conductivity type in a region under the third gate of the third transfer unit.
1. A solid state imaging device comprising:
a photoelectric conversion portion of a first conductivity type;
a holding portion of the first conductivity type;
a floating diffusion portion of the first conductivity type;
a charge draining portion;
a first transfer transistor that includes a first gate and transfers charges from the photoelectric conversion portion to the holding portion;
a second transfer transistor that includes a second gate and transfers charges from the holding portion to the floating diffusion portion; and
a third transfer transistor that includes a third gate and drains charges from the photoelectric conversion portion to the charge draining portion,
wherein an impurity concentration of a second conductivity type in at least a part of a region under the first gate of the first transfer transistor is lower than an impurity concentration of the second conductivity type in a region under the second gate of the second transfer transistor and an impurity concentration of the second conductivity type in a region under the third gate of the third transfer transistor.
2. The solid state imaging device according to claim 1 , wherein, when the first transfer transistor and the third transfer transistor are in an off-state, a potential barrier of the first transfer transistor is at least temporarily higher than a potential barrier of the third transfer transistor.
3. The solid state imaging device according to claim 1 , wherein a gate width of the first gate of the first transfer transistor is less than a gate width of the third gate of the third transfer transistor.
4. The solid state imaging device according to claim 1 , wherein a gate length of the first gate of the first transfer transistor is greater than a gate length of the third gate of the third transfer transistor.
5. The solid state imaging device according to claim 1 , wherein, during accumulation of charges in the photoelectric conversion portion, a voltage applied to the first gate of the first transfer transistor is lower than a voltage applied to the third gate of the third transfer transistor.
6. The solid state imaging device according to claim 1 , wherein a transfer assist region of the first conductivity type is formed in at least a part of the region under the first gate of the first transfer transistor.
7. The solid state imaging device according to claim 6 , wherein the transfer assist region is formed in a portion on the holding portion side in the region under the first gate of the first transfer transistor.
8. The solid state imaging device according to claim 1 , wherein a first charge barrier region of the second conductivity type is formed in at least a part of the region under the first gate of the first transfer transistor, a second charge barrier region of the second conductivity type is formed in at least a part of the region under the second gate of the second transfer transistor, and a third charge barrier region of the second conductivity type is formed in at least a part of the region under the third gate of the third transfer transistor.
9. The solid state imaging device according to claim 8 , wherein the first charge barrier region is formed on the holding portion side under the first gate of the first transfer transistor, the second charge barrier region is formed on the floating diffusion portion side under the second gate of the second transfer transistor, and the third charge barrier region is formed on the charge draining portion side under the third gate of the third transfer transistor.
10. An imaging system comprising:
the solid state imaging device according to claim 1 ; and
a signal processing device that processes an image signal output from the solid state imaging device.
11. A moving unit comprising:
the solid state imaging device according to claim 1 ;
a distance information acquisition unit adapted to acquire distance information on a distance to an object, from a parallax image based on a signal output from the solid state imaging device; and
a moving unit control unit adapted to control the moving unit based on the distance information.