CMOS active pixel sensor

• US 6,365,950 B1
• Filed: 05/27/1999
• Issued: 04/02/2002
• Est. Priority Date: 06/02/1998
• Status: Expired due to Fees

• Current Assignee: Samsung Electronics Co. Ltd.
• Original Assignee: Samsung Electronics Co. Ltd.
• Inventors: Sohn, Il-Young
• Primary Examiner(s): Williams, Alexander O.
• Application Number:
  US09/320,873
• Time in Patent Office: 1,041 Days
• Field of Search: 257/431, 257/233-239, 257/292, 257/461, 257/294, 257/369, 257/382, 257/435, 348/155, 348/300, 348/152, 348/308, 348/294, 348/309, 365/185.03, 365/185.33, 365/115, 250/208.1, 250/214.R
• US Class Current: 257/431
• CPC Class Codes: H01L 27/14643 Photodiode arrays; MOS imagers
• CPC Class Codes: H04N 25/63 applied to dark current

Abstract

The present invention relates to a CMOS active pixel sensor which includes a compensation circuit capable of compensating a lowered pixel voltage output due to leakage current of a photodiode. The CMOS active pixel sensor having a light sensing unit for generating an output voltage when light is incident thereupon, the sensing unit having an amount of leakage current before the incidence of light. A reset unit resets the output voltage of the light sensing unit to an initial reset voltage in response to a reset signal. A sense transistor has a source, a drain coupled to a power source voltage, and a gate coupled to the output of the light sensing unit. A select transistor has a drain connected to a source of the sense transistor, and provides the voltage of the sense transistor to a bit line, in response to a select signal. A compensation unit supplies a voltage corresponding to the output voltage of the light sensing unit lowered by the leakage current.

Claims

1. A CMOS active pixel sensor comprising:

light sensing means for generating an output voltage at an output when light is incident thereupon, said light sensing means having an amount of leakage current before said light incidence;

reset means for resetting the output voltage of the light sensing means to an initial reset voltage in response to a reset signal;

a sense transistor having a source, a drain coupled to a power source, and a gate coupled to the output of the light sensing means;

a select transistor having a drain connected to the source of the sense transistor, for providing a voltage at a source of the sense transistor to a bit line, in response to a select signal; and

compensation means for supplying a voltage to the select transistor substantially corresponding to the output voltage of the light sensing means lowered by the leakage current.

2. The CMOS active pixel sensor according to claim 1, wherein the compensation means comprises:

shielded light sensing means shielded from incident light,

a first transistor for resetting an output voltage of the shielded light sensing means to the initial reset voltage in response to the reset signal; and

a second transistor having a source, a drain coupled to the power source and a gate coupled to the output of the shielded light sensing means, for increasing current flow to the drain of the select transistor in an amount proportional to the leakage current of the shielded light sensing means.

3. The CMOS active pixel sensor according to claim 2, wherein the first transistor is of the same type as the reset transistor.

4. The CMOS active pixel sensor according to claim 2, wherein the second transistor is of a complementary type as compared to the sense transistor.

5. A circuit for compensating voltage readout loss in a CMOS active pixel sensor, said sensor having a first reset transistor, a first sense transistor, and a first photodiode having characteristic leakage current flow, said circuit comprising:

a second sense transistor commonly connected at drains and sources to the first sense transistor, said second sense transistor is of a type complementary to said first sense transistor;

a second reset transistor commonly connected with said first reset transistor at a voltage source and a reset signal line; and

a second photodiode having the same characteristic leakage current flow as the first photodiode, said second photodiode being shielded from light incident upon said first photodiode, said second photodiode connected to said second reset transistor and said second sense transistor in the same configuration as the connection of said first photodiode to said first reset and sense transistors.

6. The circuit according to claim 5, wherein said first sense transistor is a NMOS transistor and the second transistor is a PMOS transistor.