Pixel cell with high storage capacitance for a CMOS imager
OwnerHoward E. Rhodesinventor
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Abstract
A pixel sensor cell for use in a CMOS imager exhibiting improved storage capacitance. The source follower transistor is formed with a large gate that has an area from about 0.3 μm2 to about 10 μm2. The large size of the source follower gate enables the photocharge collector area to be kept small, thereby permitting use of the pixel cell in dense arrays, and maintaining low leakage levels. Methods for forming the source follower transistor and pixel cell are also disclosed.
Claims29
A method of forming a source follower transistor for use in a CMOS imaging device, said method comprising the steps of:
- forming a doped layer in a substrate;
- forming a first doped region and a second doped region in the doped layer;
- forming an insulating layer on the doped layer between the first and the second doped regions;
- forming a gate layer on the insulating layer, wherein the gate layer having an active area of from about 0.3 μm2 to about 25 μm2, and wherein the gate layer is adapted to be electrically connected to receive charge from a photocharge collector.
A method of forming a pixel cell for use in a CMOS imaging device, said method comprising the steps of:
- forming a doped layer in a substrate;
- forming a photocharge collector on said doped layer;
- and
- forming a source follower transistor having a gate on said doped layer, wherein the gate having an active area of from about 0.3 μm2 to about 25 μm2.
A CMOS imager device comprising:
- means for establishing a reference voltage;
- means for storing a photocharge in a surface well of said CMOS imager, said photocharge functionally related to an intensity of incident photons impinging on a photogate of said imager;
- means for transferring said photocharge to a gate of a control transistor, said gate having an active area of from about 0.3 μm2 to about 25 μm2, said control transistor adapted to control a photovoltage functionally related to said photocharge;
- means for comparing said photovoltage to said reference voltage to produce a difference signal substantially equal to an arithmetic difference between said photovoltage and said reference voltage.
A processor system for capturing an optical image comprising:
- a central processing unit;
- an imager unit coupled to said central processing unit, said imager unit having a source follower transistor including a source region formed in a substrate, a drain region formed in the substrate, a gate layer formed on the substrate between said source region and said drain region, wherein said gate layer has an active area of from about 0.3 μm2 to about 25 μm2, and wherein said gate layer is adapted to be electrically connected to receive a charge from a photocharge collector;
- a sample and hold circuit coupled to said source follower transistor, said sample and hold circuit adapted to receive a first analog signal controlled by said source follower transistor as input and to output a second analog signal related to said first analog signal;
- and
- an analog to digital converter coupled to said sample and hold circuit and adapted to receive said second analog signal therefrom, said analog to digital converter adapted to output a first digital signal related to said second analog signal.
Dependencies are inferred from the claim text.