This nonprovisional application claims priority under 35 U.S.C. xc2xa7119(a) on Patent Application No.  2002-2704 filed in KOREA on Jan. 11, 2002, which is herein incorporated by reference. 
1.  Field of the Invention
The present invention relates to a photodiode of an image sensor; and, more particularly, to a photodiode of an image sensor of which unit pixel area is decreased by forming the photodiode on a semiconductor substrate. 
2.  Description of Related Arts
Generally, an image sensor is a semiconductor device that converts an optical image into an electric signal.  Among the image sensors, a charge coupled device (CCD) is a device wherein each metal-oxide-silicon (hereinafter referred as to MOS) capacitor is closely located and charges are stored into the MOS capacitor and transferred.  A complementary metal oxide semiconductor (hereinafter referred as to CMOS) image sensor employs CMOS technology that uses a control circuit and a signal processing circuit as peripheral circuits and adopts a switching mode that detects outputs sequentially.  MOS transistors formed in the peripheral circuit as the same number of pixels. 
There are several problems in using the CCD due to its complex driving mode, high power dissipation, a complex process having lots of steps for a mask process and a difficulty in one chip realization since the signal processing circuit cannot be constructed on a CCD chip.  Therefore, there has been actively researched on the CMOS image sensor that uses sub-micron CMOS technology to overcome the above problems.  The CMOS image sensor obtains an image by forming a photodiode and a MOS transistor within a unit pixel and then detecting signals sequentially through a switching mode.  The use of the CMOS technology results in less-power dissipation and an enabled one chip process for the signal processing circuit.  Also, compared with the CCD process that requires approximately 30 to 40 masks, the CMOS image sensor implemented with the CMOS technology needs approximately 20 masks because of a simplified process.  Hence, the CMOS image sensor is currently highlighted as a next generation image sensor. 
FIG.  1A is a circuit diagram showing a unit pixel of a conventional CMOS image sensor including a photodiode and four transistors in a unit pixel.  Particularly, the unit pixel includes one photodiode (hereinafter referred as to PD) and four negative-channel-metal-oxide semiconductor (NMOS) transistors.  The NMOS transistors includes a transfer transistor Tx for transferring charges collected at the PD to a floating diffusion area (hereinafter referred as to FD), a reset transistor Rx for resetting the FD by setting an electric potential of the FD to a desired value and discharging charges, a drive transistor Dx that enacts as a source follower buffer amplifier and a select transistor Sx for providing an addressing function.  Also, there exists a load transistor outside of the unit pixel for reading an output signal. 
FIG.  1B is a circuit diagram showing a unit pixel of an image sensor including one PD and three transistors in a unit pixel.  These three transistors are identical transistors of the unit pixel excluding the Tx and the FD. 
In other words, the unit pixel has a structure wherein optical charges generated at the PD are directly inputted to a gate electrode of the Dx instead of inputting to the Tx, and drives the Dx. 
Also, a Rx and a Sx of this unit pixel have similar functions as of the unit pixel having four transistors.  The Rx initializes the PD in accordance with a signal inputted to the gate electrode, and the Sx transmits an output of the Dx to an outer part of the unit pixel in accordance with a switching signal inputted to the gate electrode. 
The PD in the conventional image sensor having the above circuit structure occupies large areas since it is formed on an active area of a semiconductor substrate.  Also, since transistors such as Tx, Rx, Dx and Sx are also formed on the active area, the area that can be used for the PD is actually less than a half of the total unit pixel area.  Therefore, the conventional image sensor has a disadvantage of a decreased fill factor, which is a ratio of an optical sensing part occupied within the unit pixel. 
It is, therefore, an object of the present invention to provide an image sensor improved on a fill-factor by modifying a process for forming a photodiode. 
In accordance with an aspect of the present invention, there is provided an image sensor comprising a photodiode receiving light, a floating diffusion area receiving photo-charges generated in the photodiode, a transfer transistor transferring the photo-charges from the photodiode to the floating diffusion area, a reset transistor controlling a voltage of the floating diffusion area, a drive transistor driven by the photodiode and supplying a source voltage and select transistor for addressing, the photodiode including: a first conductive layer formed on a semiconductor substrate and connected to an impurity area in the semiconductor substrate, wherein the semiconductor substrate has a first conductive type, the first conductive layer and the impurity area have a second conductive type; and a second conductive layer formed on the first conductive layer, wherein the second conductive layer has the first conductive type. 
In accordance with another aspect of the present invention, there is provided an image sensor comprising a photodiode receiving light, a floating diffusion area receiving photo-charges generated in the photodiode, a transfer transistor transferring the photo-charges from the photodiode to the floating diffusion area, a reset transistor controlling a voltage of the floating diffusion area, a drive transistor driven by the photodiode and supplying a source voltage and select transistor for addressing, the image sensor further comprising: a semiconductor substrate, wherein the semiconductor substrate has a first conductive type; a gate electrode of the driver transistor on the semiconductor substrate; and a photodiode including a first conductive layer connected to the gate electrode of the driver transistor and a second conducive layer on the first conductive layer, wherein the first conductive layer has a second conductive type and the second conductive layer has the first conductive type. 
The present invention employs a technology used for forming a capacitor in a dynamic random access memory (DRAM) device so that a fill-factor is enhanced by forming a photodiode at an upper side of a transistor. 
In a conventional unit pixel of an image sensor, a photodiode is typically formed on an active area and stores optical charges into a depletion layer to generate an image.  However, the photodiode has a limitation in decreasing an area in contrary to other components of which size can be decreased due to technological advancement related to processes.  The reason for this limitation is because of a wavelength of light and an interference occurring between photodiodes nearly located. 
In the present invention, dynamic random access memory (DRAM) process technology is used to increase a fill-factor by constructing the photodiode as in an antenna form as to generate an optical charge after receiving an incident light from upper sides of a transistor, and forming other transistors or elements on an active area.