Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2002-10-07
2003-07-01
Ngô, Ngân V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S291000
Reexamination Certificate
active
06586789
ABSTRACT:
FIELD OF THE TECHNOLOGY
Embodiments of the present invention relate to solid-state image sensors.
BACKGROUND
Image sensors can be used in a variety of applications, such as digital still cameras, PC cameras, digital camcorders and Personal Communication Systems (PCS), as well as analog and digital TV and video systems, video game machines, security cameras and micro cameras for medical treatment. With the development of the telecommunication and computer system, the demand for image sensors will be much more increased.
An image sensor cell typically has a photodiode element, which is capable of converting light (e.g., visible light, infrared light and ultraviolet light) into electric signals. When photons are absorbed, electron-hole pairs are created through photoelectric conversion. A depletion region is formed in a photodiode when the photodiode is reverse-biased. The electric field in the depletion region separates the electron-hole pairs generated from photoelectric conversion.
The electric current generated from the photoelectric conversion can be directly measured to determine the intensity of the light. However, the signal generated from the direct measurement of the current from photoelectric conversion typically has a poor signal to noise (S/N) ratio. Thus, a typical image sensor accumulates the charges generated from photoelectric conversion for a predetermined period; and, the amount of accumulated charges is measured to determine the intensity of the light.
To measure the accumulated photoelectric charges, a CMOS (Complementary Metal-Oxide Semiconductor) Active Pixel Sensor (APS) contains active circuit elements (e.g., transistors) for measuring the signal associated with the accumulated photoelectric charges. Alternatively, the accumulated charges can be moved out of image sensor cell for measurement (e.g., in a CMOS Passive Pixel Sensor (PPS) or in a Charge Coupled Device (CCD) image sensor). In order to prevent noise, a CCD image sensor uses a complicated process to transfer the accumulated charges from the sensor cell to an amplifier for measurement. A CCD device uses complicated driving signals of large voltage swings, and thus, consumes a lot of power. While a CMOS PPS can be fabricated using a standard CMOS process, a typical CMOS PPS has a poor Signal to Noise (SIN) ratio. A typical CCD fabrication process is optimized for charge transfer; and it is not compatible with a standard CMOS process. Thus, a CCD image sensor is difficult to be integrated with signal processing circuitry, which is typically implemented by Complementary Metal-Oxide Semiconductor (CMOS) circuitry, and thus, difficult to be implemented in a wider variety of applications.
A CMOS APS detects (or amplifies) the signal within the sensor cell to greatly reduce the noise in determining the signal. However, the circuit in a typical CMOS APS sensor cell consumes an amount of area, resulting in a reduced fill factor and low sensitivity. Another typical drawback associated with a CMOS APS sensor is high reset noise. A CCD sensor can allocate a large area for the light-sensing element, since the amplifiers and detecting circuits are not in the image sensor cell, when a double correlated sampling circuit is implemented. Thus, a CCD sensor typically has a large fill factor and high sensitivity. However, the transistors for correlated double sampling on a CMOS APS sensor can further reduce the sensor fill factor. Thus, many CMOS APS sensors using none correlated double sampling to balance the need for a large fill factor and reduced reset noise.
Although a CMOS image sensor, fabricated using the related simple CMOS process, typically has low power consumption, single power supply and the capability of on-chip system integration, in contrast with CCD image sensors, CMOS image sensors has not been yet widely used in image capture application because of low sensitivity and high noise.
SUMMARY OF THE DESCRIPTION
Pixel image sensors with lateral photodiode elements and vertical overflow drain systems are described herein.
According to at least one embodiment of the present invention, an image sensor pixel includes a lateral photodiode element and a vertical overflow drain system for draining excessive charges accumulated in the charge collecting region of the lateral photodiode element and for resetting the charge collecting region of the lateral photodiode element.
In one example according to the present invention, the lateral photodiode element has an N-type region and a P-type region separated by an intrinsic (or P- or N-) semiconductive material; the N-type region is surrounded by the intrinsic semiconductive material; and, the P-type region of the lateral photodiode element is shaped to substantially enclose the N-type region to form a P-I-N type lateral photodiode element. The vertical overflow drain system is formed by a layer of the intrinsic (or P- or N-) semiconductive material separating the lateral photodiode element and an N-type substrate. When the lateral photodiode element is reverse biased, the N-type region collects electric charges generated from photoelectric conversion in the lateral photodiode element. When: 1) the P-type region is at a first potential level (e.g., 0 V), 2) the N-type region is at second potential level (e.g. 1 V), and 3) the substrate is at a third potential level (e.g., 0.2 V), a potential barrier formed in the vertical overflow drain system prevents the electric charges accumulated in the N-type region from moving across the intrinsic layer into the substrate until the N-type region approaches a overflow potential level (e.g., 0.2 V). When the N-type region is forced to approach the overflow potential level, the electric charges in the N-type region are capable of moving across the intrinsic layer into the substrate. Thus, the charge collecting region (the N-type region) can be reset by forcing the charge collection region to approach the overflow potential level. In one example, the lateral photodiode element is not forward biased when the N-type region is forced to approach the overflow potential level while the P-type region remains at the first potential level.
In one example according to the present invention, an image sensor pixel includes a capacitor for applying control signals and a transistor for reading out signals, in addition to the lateral photodiode element and the vertical overflow drain system. The gate of the transistor is connected to the charge collecting region (e.g., the N-type region); and, the capacitor has one surface connected to the charge collecting region and the other surface connected to the control signal line. In one example, the other surface of the capacitor is connected to a drain region of the transistor. In one example, the P-type region is maintained at one potential level (e.g., 0 V); and, the substrate is maintained at another potential level (e.g., 2 V). In a reset operation, the control signal line is set to 0 V. The potential level of the N-type region follows the control signal through capacitor coupling. However, the potential level of the N-type region will not go beyond an overflow point (e.g., 0.2 V). When the potential level of the N-type region reach the overflow level, charges accumulated in the N-type region flow to the substrate; and, the potential level of the N-type region remains the overflow point. After the reset operation, the control signal line is set to 2 V to accumulate photoelectric charges. The potential level of the N-type region follows the control signal through capacitor coupling (e.g., to 1 V) to a level below the overflow point so that a potential barrier is formed in the vertical overflow drain system. The potential barrier prevents photoelectric electrons accumulated in the N-type region from moving into the substrate; and, the potential barrier also prevents electrons from moving from the substrate to the N-type region. Thus, the electrons from photoelectric conversion in the lateral photodiode element are collected and accumulated in the N-type region. When the control signal line is ma
Mei Ivy Y.
Ngo Ngan V.
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