Image pick-up device and camera system comprising an image pick-up device
The invention relates to an image pick up device ( 401 ) with pixels arranged in rows and columns. Every pixel ( 301 ) comprises a photosensitive element ( 302 ), a floating diffusion ( 304 ), a transfer transistor ( 303 ), an amplifying transistor ( 305 ) having its control electrode connected to the floating diffusion ( 304 ), and a reset transistor ( 306 ). An external node ( 310 ) is coupled to a selection switch via a row selection bus ( 406 ). The selection switch ( 411 ) provides either a first bias voltage, generated by a first voltage source ( 409 ), or a second bias voltage, generated by a second volt age source ( 410 ), to the row selection bus ( 406 ). Applying the first bias voltage and simultaneously turning on the reset transistor ( 306 ) programs the floating diffusion ( 304 ) to the first bias voltage which biases the amplifying transistor ( 305 ) in an on-mode, thereby selecting the pixel ( 301 ). Likewise, applying the second bias voltage biases, the amplifying transistor ( 305 ) in an off-mode, thereby deselecting the pixel ( 301 ). This way of selecting and deselecting pixels avoids the need for a separate selection transistor in every pixel, thereby increasing the fill factor of the pixels. This approach is especially useful for image pick up devices applying correlated double sampling (CDS).
1. An image pick up device ( 401 ) comprising a plurality of pixels, at least one pixel ( 301 ) comprising a photosensitive element ( 302 ) for converting light into an electrical charge, a charge-to-voltage conversion node ( 304 ) for converting said electrical charge into a voltage level, a transfer transistor ( 303 ) having its main conductive channel connected between said photosensitive element ( 302 ) and said charge-to-voltage conversion node ( 304 ), an amplifying transistor ( 305 ) having its control electrode connected to said charge-to-voltage conversion node ( 304 ) and its main conductive channel connected to a first external node ( 309 ) of said pixel and coupled to a second external node ( 310 ) of said pixel, and a reset transistor ( 306 ) having its main conductive channel connected between said charge-to-voltage conversion node ( 304 ) and said second external node ( 310 ), characterized in that said second external node ( 310 ) is coupled to biasing means ( 406 , 411 , 409 , 410 ) for providing at least two different voltage levels to said charge-to-voltage conversion node ( 304 ) in order to bias said amplifying transistor ( 305 ) in an off-mode and in an on-mode, respectively.
2. An image pick up device according to claim 1 , characterized in that said pixels ( 301 ) are arranged in rows and columns which constitute a matrix, the second external nodes ( 310 ) of pixels within a row being connected to a row selection bus ( 406 ).
3. An image pick-up device according to claim 2 , characterized in that said biasing means comprise a first voltage source ( 409 ) for providing a first bias voltage, a second voltage source ( 410 ) for providing a second bias voltage source, and a selection switch ( 411 ) for coupling either said first voltage source ( 409 ) or said second voltage source ( 410 ) to said row selection bus ( 406 ).
4. An image pick-up device according to claim 1 , characterized in that said photosensitive element ( 302 ) comprises a photodiode.
5. An image pick-up device according to claim 1 , characterized in that said photosensitive element comprises a photogate.
6. An image pick-up device according to claim 1 , characterized in that said amplifying transistor ( 305 ) is an insulated gate field effect transistor.
7. An image pick-up device according to claim 1 , characterized in that said reset transistor ( 306 ) is an insulated gate field effect transistor.
8. An image pick-up device according to claim 1 , characterized in that said transfer transistor ( 303 ) is an insulated gate field effect transistor.
9. A camera system comprising optical means for focusing an image on to an image section ( 505 ) of an image pick-up device ( 502 ), characterized in that said image pick up device ( 502 ) is an image pick-up device according to claim 1 .