CMOS image sensor capable of increasing punch-through voltage and charge integration of photodiode, and method for forming the same

DETAILED DESCRIPTION The present invention is intended to overcome the above described disadvantages of the conventional technique.
Therefore it is an object of the present invention to provide a CMOS image sensor and a fabricating method therefor, in which the punch-through voltage of a transfer transistor is increased, the potential barrier is heightened between the photodiode and the floating diffusion region during the turn-off of the transfer transistor so as to increase the charge accumulation amount of the photodiode, and the photodiode is formed without resorting to a self-aligning ion-implantation.
In achieving the above object, the CMOS image sensor having a transfer transistor for transferring charges from a photodiode to a floating diffusion region according to the present invention includes a semiconductor substrate; a gate electrode of the transfer transistor, the gate electrode being formed on the semiconductor substrate; a photodiode including a first conduction type first impurity region and a second conduction type second impurity region, the two regions being formed at one end of the gate electrode and within the semiconductor substrate; a floating diffusion region including a second conduction type third impurity region, the third impurity region being formed at another end of the gate electrode and within the semiconductor substrate; and a first conduction type fourth impurity region formed under the gate electrode and within the semiconductor substrate, and isolated from the photodiode and the floating diffusion region.
In another aspect, the present invention includes a method for forming a CMOS image sensor with a transfer transistor included therein for transferring the charges from a photodiode to a floating diffusion region, the method including steps of forming a first conduction type first impurity region in a photodiode-forming region and within a semiconductor substrate; forming a second conduction type second impurity region in a transfer transistor region and within the semiconductor substrate; forming a gate insulating film and a gate electrode of the transfer transistor on the semiconductor substrate, a portion of the gate electrode being overlapped with the second impurity region; forming a second conduction type third impurity region upon the first impurity region and within the semiconductor substrate; and forming a first conduction type floating diffusion region isolated from the photodiode region and across the gate electrode