Abstract:
Embodiments of the invention relate to a CMOS image sensor. In detail, a CMOS image sensor can have improved sensitivity. The CMOS image sensor includes a photodiode on a semiconductor substrate, a drive transistor including a gate connected to the photodiode, a first grounded electrode and a second electrode connected to a current detector, a transfer transistor connected between the photodiode and the gate to apply voltage or charges generated in the photodiode to the gate, an optional select transistor between the second electrode and the current detector, and an optional reset transistor connected to a power line, configured to reset the photodiode. Accordingly, the CMOS image sensor can read the output of a photodetector without substantial attenuation.

Description:
[0001]    The present application claims priority under 35 U.S.C. 119 and 35 U.S.C. 365 to Korean Patent Application No. 10-2006-0135584 (filed on Dec. 27, 2006), which is hereby incorporated by reference in its entirety. 
       BACKGROUND 
       [0002]    In general, an image sensor is a semiconductor device that converts an optical image into electrical signals. Image sensors may be classified into Charge Coupled Device (CCD) image sensors, in which individual Metal Oxide Silicon (MOS) capacitors are located closely to each other such that charge carriers are stored in or discharged from the capacitors, and CMOS image sensors, employing a switching mode to sequentially detect pixel outputs by providing a predetermined number of MOS transistors to each pixel, manufactured using a CMOS technology and using peripheral devices, such as a control circuit and a signal processing circuit. 
         [0003]    A CMOS image sensor that converts information on a subject into electrical signals includes signal processing chips having photodiodes, an amplifier, an A/D converter, an internal voltage generator, a timing generator and digital logic on one chip. Accordingly, the CMOS image sensor is particularly advantageous in terms of space, power and cost reduction. A CCD is manufactured through a special process. However, the CMOS image sensor can be manufactured in large quantities using CMOS processing on a low-priced silicon wafer, and is advantageous in terms of the integration degree. 
         [0004]    In the CMOS image sensor, light is converted into an electric signal from electric charges stored in a photodiode. When the amount of incident light is insufficient due to darkness, a reduced amount of charge is stored in the photodiode, SO the output signal may not be distinguished from noise. 
       SUMMARY 
       [0005]    An exemplary CMOS image sensor may comprise a photodiode on a semiconductor substrate; a drive transistor including a gate connected to the photodiode, a first grounded electrode and a second electrode connected to a current detector; a transfer transistor connected between the photodiode and the gate, configured to apply a voltage or charge(s) generated in the photodiode to the gate; an optional select transistor between the second electrode and the current detector; and an optional reset transistor connected to a power line, configured to reset the photodiode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a circuit diagram showing a unit pixel of a voltage detection type CMOS image sensor having four transistors; and 
           [0007]      FIG. 2  is an equivalent circuit diagram showing a unit pixel of a voltage detection type CMOS image sensor according to embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0008]      FIG. 1  is a circuit diagram showing a unit pixel of a voltage detection type CMOS image sensor having four transistors according to an embodiment. 
         [0009]    As shown in  FIG. 1 , a voltage detection type CMOS image sensor includes a photodetector  100 , a transfer transistor  101 , a reset transistor  103 , a drive transistor  104 , and a select transistor  105 . The photodetector  100  includes a photodiode that generates electrical charges from optical energy (e.g., light), and the transfer transistor Tx  101  receives an enable or read signal at the gate thereof and carries the charges collected in the photodetector  100  to a floating diffusion area FD  102  when the enable or read signal is active. The reset transistor  103  resets the FD area  102  by receiving a reset signal at the gate thereof, setting the voltage of the FD area  102  to a desired level (e.g., VDD), and discharging the charges of the FD area  102 . The drive transistor  104  receives the voltage of the FD area  102  at the gate thereof, and therefore, serves as a source follower (and optionally, a buffer amplifier), and the select transistor  105  outputs a voltage from the drive transistor Dx  104  in response to addressing functions (which may be generated and/or executed elsewhere on the CMOS image sensor). 
         [0010]    The voltage detection type CMOS image sensor can read the voltage of the photodetector  100  when the select transistor  105  and the transfer transistor  101  of the unit pixel to be read are turned on. In such a state, when the reset transistor  103  is turned on, the voltage of the photodetector  100  is initialized. Then, if the initialized voltage is subtracted from the voltage of the photodetector  100  (or the voltage of the photodetector  100  is subtracted from the initialized voltage), the resultant value becomes the voltage value proportional to the amount of light accumulated in the photodetector  100  after the previous reset. Such a scheme of reading and comparing the voltages before and after the reset may effectively remove an adverse influence derived from device variations in the unit pixels across the image sensor. 
         [0011]    Since the drive transistor  104  serves as a source follower, the drain voltage of the drive transistor  104  increases as the output voltage of the photodetector  100  increases (or decreases as the amount of charge in the photodetector  100  decreases). Since the body of the drive transistor  104  is fixed to the ground GND, a body effect increases as the source voltage of the drive transistor  104  increases, so the threshold voltage of the drive transistor  104  increases. Thus, voltage variations in the photodetector  100 , particularly small voltages in the photodetector  100 , may not be directly transferred to the drain of the drive transistor  104 . 
         [0012]    Embodiments of the invention can provide the CMOS image sensor with improved sensitivity by converting the voltage of the photodetector into electric current and outputting the electric current. 
         [0013]    Hereinafter, a CMOS image sensor according to other embodiment(s) will be described in detail with reference to the accompanying drawings. 
         [0014]      FIG. 2  is an equivalent circuit diagram showing a unit pixel of a current detection type CMOS image sensor according to the other embodiment(s). 
         [0015]    As shown in  FIG. 2 , the current detection type CMOS image sensor includes a photodetector  200 , a transfer transistor  201 , a reset transistor  203 , a drive transistor  204 , and a select transistor  205 . The photodetector  200  includes a photodiode that generates electrical charges from optical energy, and the transfer transistor  201  receives signals at the gate thereof and transfers the charges collected in the photodetector  200  to a FD area  202 . The reset transistor  203  resets the FD area  202  by receiving reset signals at the gate thereof, setting the voltage of the FD area  202  to a desired level (e.g., VDD or ground), and discharging the charges of the FD area  102 . Alternatively, the terminal of reset transistor  203  opposite to FD area  202  may be coupled to a different power rail (e.g., VCC) or to a reference voltage, such as VCC/2 (which can put the drive transistor  204  is a relatively linear response range). The drive transistor  204  receives signals from the FD area  202  at the gate thereof and serves as a source follower (and optionally, a buffer amplifier), and the select transistor  205  outputs a voltage from the drive transistor Dx  104  in response to addressing functions (which may be generated and/or executed elsewhere on the CMOS image sensor). 
         [0016]    In order to improve the sensitivity of the CMOS image sensor, the source of the drive transistor  204  is grounded, and a current detector  206  is connected to the drain of the drive transistor  204  to measure electric current output by the unit pixel, so that the body effect can be reduced, avoided, or prevented when reading the output voltage of the photodetector  200 . 
         [0017]    As the CMOS image sensor has the construction as described above, the output signals from the unit pixels are transmitted in the form of electric current, ill instead of voltage. Accordingly, the output signals have strong characteristics against noise in the chip as compared with the case where the output signals are in the form of voltage, so the output signals can be amplified at a high rate in an amplifier, and thus, small differences in photocharges stored in the photodetector  200  (even at low levels) can be relatively easily detected. 
         [0018]    Further, since DC level adjustment is easily achieved through addition or subtraction of a predetermined value in the current detection case, as compared with a case of reading the output signals in the form of voltage, analog circuit design can be easily implemented. For example, the current detector can be a conventional analog or digital current detector (in the latter case, the detector may further comprise an analog-to-digital converter [ADC]). 
         [0019]    In the case of the above drive transistor  204 , electric current proportional to the square of the gate voltage flows through the drive transistor  204  in a saturation state, so a micro value can be easily read in a dark state (e.g., in a high voltage state of the photodetector  200 ). 
         [0020]    According to the related voltage detection type CMOS image sensor, since constant current flows in the source of the drive transistor  204 , the voltage of the photodetector  200  must be higher than the threshold voltage of the drive transistor  204  in order to ensure the operation of a current source. However, according to the current detection type CMOS image sensor (e.g., as shown in  FIG. 2 ), since the drain current flows even when the gate voltage of the drive transistor  204  is lower than the threshold voltage of the drive transistor  204 , the output value of the photodetector  200  can be read. As a result, substantially all output voltages of the photodetector  200  can be read, and the noise threshold of the pixel is considerably smaller than the voltage detection type image sensor. 
         [0021]    In order to convert the output value from an electric current into voltage, as shown in  FIG. 3 , the CMOS image sensor may comprise a current-to-voltage converter  208  and a voltage detector  210  for outputting the detected voltage (e.g., as a single- or multi-bit digital signal). In an alternative embodiment, a conventional analog-to-digital converter may replace output block  206 ′ (see  FIG. 3 ). As shown in  FIG. 4 , a MOSFET having the same current-voltage characteristics (e.g., as drive transistor  204 ) is fabricated in the form of a diode (e.g., a transistor  208 ′ having a gate connected to a drain thereof), and electric current Vout is supplied to the diode  208 ′, so that the voltage of the photodetector can be obtained (e.g., in voltage detector  210 ). The output of voltage detector  210  may be further processed by signal processing logic in the CMOS image sensor to generate an image data file for subsequent display on an image viewing terminal (e.g., a computer monitor, a cellular phone or PDA display screen, a view screen in a motor vehicle, etc.). 
         [0022]    The current-to-voltage detection type CMOS image sensor can read the output of the photodetector  200  when the select transistor  205  and the transfer transistor  201  of the unit pixel to be read are turned on. In such a state, when the reset transistor  203  is turned on, the output of the photodetector  200  is initialized. Then, if the initialized voltage is subtracted from the output of the photodetector  200  (or vice versa, depending on the voltage at the drain of the reset transistor  203 ), the resultant value becomes a voltage proportional to the amount of light accumulated in the photodetector  200  after the previous reset. 
         [0023]    The output signals before and after the reset are read and compared, so that an adverse influence derived from device variations in the unit pixels can be effectively removed. That is, according to the current-to-voltage detection type CMOS image sensor, an output of the photodetector  200  can be read without attenuation, and is transmitted in the form of electric current, so the output signals have strong characteristics against noise in the chip. 
         [0024]    Further, since the noise component is relatively small, the output signals can be amplified at a high rate when the output signals are small. In addition, since the output signals are transmitted in the form of electric current, analog circuit design of the subsequent stage is relatively easy. 
         [0025]    Furthermore, substantially all values of the photodetector  200  can be transmitted without attenuation, and the output of the photodetector  200  can be precisely read in low luminance. 
         [0026]    Embodiments of the invention may have the following advantages. First, the CMOS image sensor can read the output of the photodetector without attenuation, and transmit the output of the photodetector in the form of electric current, so the output signals have strong characteristics against noise in the chip. Further, since the noise component is relatively small, the output signals can be amplified at a high rate when the output signals are small. In addition, since the output signals are transmitted in the form of electric current, analog circuit design of the subsequent stage is relatively easy. 
         [0027]    Second, substantially all values of the photodetector can be transmitted without significant attenuation, and the output of the photodetector can be precisely read in low luminance. 
         [0028]    Third, the CMOS image sensor outputs the photodetector charges in the form of electric current, thereby improving the sensitivity of the sensor and its dynamic range. 
         [0029]    Any reference in this specification to “one embodiment,” “an embodiment,” “example embodiment,” etc., means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of such phrases in various places in the specification are not necessarily all referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with any embodiment, it is submitted that it is within the purview of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments. 
         [0030]    Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.