KR100421121B1 - Method for operating a pixel circuit of a cmos image sensor - Google Patents

Abstract

The present invention relates to a method of driving a pixel circuit in a CMOS image sensor. The pixel circuit driving method according to an embodiment of the present invention uses an on-chip voltage generator to apply a Vpp voltage higher than the driving power supply voltage Vcc to a unit pixel. Increasing the variation of the output voltage has the advantage that the characteristics of the image quality is improved.

Description

METHODS FOR OPERATING A PIXEL CIRCUIT OF A CMOS IMAGE SENSOR}

The present invention relates to a method of driving a pixel circuit in a CMOS image sensor which drives the source follower circuit of the pixel to any voltage generated by the on-chip voltage generator to obtain a high quality image for the CMOS image sensor pixel.

In general, a CMOS image sensor is a device that converts an optical image into an electrical signal using a CMOS fabrication technology, and employs a switching method in which MOS transistors are made by the number of pixels and the output is sequentially detected using the same. Compared to the CCD (Charge Coupled Device) image sensor, which is widely used as an image sensor, CMOS image sensor can be easily driven, various scanning methods can be implemented, and the signal processing circuit can be integrated on a single chip, making the product smaller. Not only is this possible, but it also uses compatible CMOS technology to lower manufacturing terminals and significantly lower power consumption.

There are many types of pixels used in CMOS image sensors, but among them, a typical commercially available pixel type is a pixel of 3-T (3-transistor) structure composed of three basic transistors and one photodiode as shown in FIG. As shown in FIG. 12, there are four-transistor (4-T) pixels including four basic transistors and one photodiode.

11 is a pixel circuit diagram of a CMOS image sensor according to a first embodiment of the prior art. As shown in the drawing, the pixel circuit is composed of one photodiode PD and three NMOS transistors that convert light into electrons. The NMOS transistor changes the current of a source follower circuit composed of Dx, Sel, and DC gate according to the change of electrode voltage of reset transistor Rx and floating diffusion to reset the potential of the photodiode PD. The drive transistor Dx changes the output voltage of the pixel, and the select transistor Sel selects a row address of the pixel array.

The DC gate is a load transistor that applies a gate voltage of the transistor to a constant voltage at all times so that only a constant current flows, Vcc is a driving power supply voltage, Vss is a ground voltage, and Output is an output voltage of a unit pixel.

The operation of the pixel circuit constructed as described above is as follows.

First, when the photodiode PD is reset by the Vcc power supply by the reset transistor Rx, and light is irradiated to the reset photodiode PD, electrons and holes are formed in the junction region of the photodiode PD and the hole is formed. The electrons diffuse into the silicon substrate and accumulate in the junction region, and the accumulated charge is applied to the gate electrode of the drive transistor Dx of the source follower circuit to turn on the drive transistor Dx, and the select transistor Sel is selected. In this case, the output voltage of the unit pixel is generated according to the voltage change of the FD electrode to output the pixel information analogously.

To summarize the above operation process as follows.

If the FD voltage is referred to as Vfd, the output voltage is referred to as Vout, and the threshold voltage of the transistor is referred to as Vth, the voltage of the FD in the reset state may be expressed by Equation 1.

Vfd = Vcc-Vthr

Here, Vthr represents the threshold voltage when Vbs = Vfd due to the body effect of the transistor generated when the potential of Vfd increases to the transistor threshold voltage when Vbs = 0 of the reset transistor.

The voltage of this Vfd is transferred to the voltage of the photodiode as it is, and this voltage causes the charge depletion region to be generated by the reverse voltage between the P-type junction of the silicon substrate and the N-type junction of the photodiode.

In this state, when the reset transistor is turned off and light is irradiated for a predetermined time (optical integration time), the photodiode diffuses holes into the P-type substrate by electron-hole generation generated in the charge depletion region of the photodiode, and the electrons are photodiode. Accumulate at the N-type junction of. Therefore, the voltage Vfd is reduced below the potential of the reset state by the accumulated charge of the photodiode.

If the voltage of Vfd at this time is defined as Vfd ', Equation 2 is obtained.

Vfd '= Vfd-Q / C

Where Q is the total amount of electrons generated by the light, and C is the sum of the junction capacitance of the photodiode and the junction capacitance of the FD electrode, the gate oxide capacitance of the drive transistor, and the overlap capacitance of the gate-source electrode of the reset transistor.

The charge generated by the light can lower the potential of the photodiode to a minimum of about Vss-0.6V. If it becomes less than this, the junction is automatically forwarded, and electrons escape to the ground voltage of the substrate.

After a certain period of optical integration, the select transistor is selected to output optical information of a unit pixel. The drive transistor is turned on by the voltage of Vfd ', and a voltage as shown in Equation 3 below is transferred to the output.

Vout = Vfd'-Vthd

Here, Vthd is a threshold voltage due to the body effect when Vbs = Vout of the drive transistor.

Therefore, the output voltage at this time can be expressed as Equation 4 again.

Vout = (Vfd-Q / C) -Vthd

= (Vcc-Vthr-Q / C) -Vthd

= Vcc-Vthr-Vthd-Q / C

Therefore, as shown in Equation 4, the output voltage of the CMOS image sensor pixel is determined by Vcc, the threshold voltage caused by the body effect of the transistor, and the change in the total amount of charge and photodiode parasitic capacitance generated by light.

12 is a pixel circuit diagram of a CMOS image sensor according to a second embodiment of the prior art. As shown in the drawing, the pixel circuit is composed of one photodiode (PD) and four NMOS transistors that convert light into electrons. Four NMOS transistors have a transfer transistor (Tx) for inputting and outputting the potential of the photodiode (PD) to the FD electrode, a reset transistor (Rx) for resetting the potential of the photodiode (PD), and a floating diffusion electrode. Drive transistor (Dx) to change the output voltage of the pixel by changing the current of the source follower circuit consisting of Dx, Sel, and DC gate according to the voltage change, and select transistor (Sel) to select the row address of the pixel array It consists of.

Compared to the pixel of the 3-T structure, the 4-T pixel circuit allows the transfer transistor to be connected to the photodiode so as to separately control the optical integration time until reset and data output. Since the voltage applying method of the 4-T structure pixel is almost similar to the case of 3-T described above, a detailed description thereof will be omitted.

On the other hand, as the density increases, the value of the operating power is also gradually scaled. As described above, the output voltage of the pixel decreases linearly with the decrease of the operating voltage. In this case, the image quality of the pixel gradually decreases. Therefore, as the scaling becomes more difficult.

In other words, according to the prior art, the pixels obtain the output of the pixel by applying a power supply voltage composed of Vcc and Vss to drive the source follow circuit. However, as the density increases, conventional CMOS image sensor products are gradually scaled down from the 5V supply used as the supply voltage to lower to 3.3V, 2.5V, 1.8V, and so on. Since the image quality performance is drastically degraded, there was a problem of reaching the limit of scaling.

The present invention has been proposed to solve such a conventional problem, by using the on-chip voltage generator to apply a Vpp voltage higher than Vcc to the driving power supply voltage of the unit pixel to increase the variation of the output voltage, or to increase the Vss ground voltage. The purpose is to apply a lower negative voltage on-chip voltage Vbb to further increase the change in pixel output voltage so that the quality of image quality is improved.

A pixel circuit driving method of a CMOS image sensor according to the present invention for realizing the above object is a pixel circuit driving method of a CMOS image sensor comprising a reset transistor and a source follower circuit using a photodiode, a negative voltage, An on-chip voltage generator is used to apply a Vpp voltage higher than the driving power supply voltage Vcc to the unit pixel, or to apply a Vbb negative voltage lower than the ground voltage Vss to the unit pixel to increase the variation of the output voltage.

1 to 10 are pixel circuit diagrams of a CMOS image sensor according to the first to tenth embodiments of the present invention;

11 and 12 are pixel circuit diagrams of CMOS image sensors according to the first and second embodiments of the prior art.

The present invention utilizes an internal voltage generator generated by using a Vcc power supply in a CMOS image sensor on-chip to improve the image quality of a pixel expected as the operating voltage of the CMOS image sensor decreases. Despite the reduction in voltage, the image quality of the CMOS image sensor can be improved.

There may be a plurality of embodiments of the present invention. Hereinafter, various embodiments will be described in detail with reference to FIGS. 1 to 10.

FIG. 1 is a pixel circuit diagram of a CMOS image sensor according to a first exemplary embodiment of the present invention, in which Vpp generated from an on-chip higher than a Vcc power source is applied instead of Vcc.

By applying Vpp instead of the operating voltage Vcc, the output voltage of the pixel can be expressed by Equation 5 below.

Vout = Vpp-Vthr-Vthd-Q / C

Therefore, the output voltage of the pixel can be increased in accordance with the voltage of Vpp.

When Vpp is used as the source voltage of the source follower circuit, the gate voltage of the reset transistor also applies the Vpp voltage to maximize the voltage transfer of the reset transistor.

The Vpp value is about 1.5 times the Vcc value, that is, about 4.85V when Vcc is 3.3V, and about 3.75V when Vcc is 2.5V. Therefore, the pixel output also generates this voltage gain, and the image quality is improved according to the voltage gain, so that the image quality is improved even though the operating voltage decreases as the integration density increases.

2 is a pixel circuit diagram of a CMOS image sensor according to a second exemplary embodiment of the present invention, in which a Vbb negative voltage power source generated on-chip lower than Vss is used instead of Vss.

When the voltage Vbb lower than Vss is generated on-chip using Vcc of the operating power supply and the voltage is used as the ground electrode of the DC gate load transistor, the output voltage of the pixel may be expressed as in Equation 6 below.

Vout = (Vcc + Vbb) -Vthr-Vthd-Q / C

At this time, the potential of Vfd can move to a minimum of Vss-0.6V because the substrate potential of the photodiode is at ground. Therefore, even if the ground potential of the source follower circuit is Vbb, the voltage at the output is not effectively reduced to a sufficient negative voltage.

On-chip Vbb value uses 1/3 times of Vcc value, that is, when operating voltage is 3.3V, Vbb voltage uses about 0.99V of negative voltage and when operating voltage is about 2.5V, about 0.75V use.

3 is a pixel circuit diagram in the CMOS image sensor according to the third embodiment of the present invention, in which Vbb is used instead of Vss and the substrate potential of the photodiode is used as Vbb instead of Vss.

The output voltage of the pixel is effectively effectively negative because the minimum potential of the photodiode can drop to Vbb-0.6V by applying Vbb to the substrate potential of the photodiode instead of Vss. Since the voltage can be dropped, the output can be sufficiently dropped to the negative voltage while increasing the saturation state of the photodiode, thereby increasing the output of the pixel by the applied voltage of Vbb more effectively.

4 is a pixel circuit diagram in a CMOS image sensor according to a fourth embodiment of the present invention, in which Vpp is used instead of Vcc and Vbb is used instead of Vss.

If you use Vpp instead of Vcc and Vbb instead of Vss, the output of the pixel can be doubled. The pixel output voltage at this time may be expressed as in Equation 7 below.

Vout = Vpp + Vbb-Vthr-Vthd-Q / C

As can be seen from Equation 7, the output voltage of the final unit pixel is further expanded by the Vpp and Vbb voltages by using the Vpp and Vbb voltages generated inside the CMOS image sensor instead of the pixels Vcc and Vss even though the Vcc operating power is used. Can be increased to the rated voltage.

Of course, in this case, since the substrate voltage of the photodiode is set to Vss, the output voltage also has a disadvantage that it cannot effectively drop a sufficient negative voltage.

5 is a pixel circuit diagram of a CMOS image sensor according to a fifth embodiment of the present invention, in which Vpp is used instead of Vcc and Vbb is used instead of Vss, and the substrate potential of the photodiode is used as Vbb.

Vpp instead of Vcc is used instead of Vss, and Vbb is applied to the substrate voltage of the photodiode to reduce the output of the pixel to the negative voltage more effectively.

FIG. 6 is a pixel circuit diagram of a CMOS image sensor according to a sixth exemplary embodiment of the present invention, in which a pixel output is increased by applying Vpp instead of Vcc in a 4-T pixel.

FIG. 7 is a pixel circuit diagram of a CMOS image sensor according to a seventh exemplary embodiment of the present invention, in which an output voltage of a pixel is increased by applying Vbb instead of Vss ground.

FIG. 8 is a pixel circuit diagram of a CMOS image sensor according to an eighth embodiment of the present invention, in which Vbb is applied instead of Vss and a photo diode is applied to Vbb instead of Vss.

FIG. 9 is a pixel circuit diagram of a CMOS image sensor according to a ninth embodiment of the present invention in which Vpp is applied instead of Vcc and Vbb is applied instead of Vss.

FIG. 10 is a pixel circuit diagram of a CMOS image sensor according to a tenth exemplary embodiment of the present invention in which Vpp instead of Vcc is applied, Vbb instead of Vss, and Vbb instead of Vss of a photodiode.

Since the voltage application method according to the sixth to tenth embodiments of the present invention described with reference to FIGS. 6 to 10 is almost similar to the case of 3-T described with reference to FIGS. 1 to 5, a detailed description thereof will be omitted.

As described above, the present invention increases the variation of the output voltage by applying a Vpp voltage higher than Vcc to the driving power supply voltage of a unit pixel, or by applying an on-chip voltage Vbb of a negative voltage lower than Vss ground voltage. By further increasing the change of the image quality is an effect that is improved.

Claims (4)

In a pixel circuit driving method of a CMOS image sensor composed of a reset diode and a source follower circuit using a photodiode and a negative voltage, CMOS image, characterized by increasing the variation of output voltage by applying Vpp voltage higher than driving power supply voltage Vcc to unit pixel or applying Vbb negative voltage lower than ground voltage Vss to unit pixel using on-chip voltage generator Method for driving pixel circuit of sensor. The method of claim 1, And using the Vpp voltage as a driving power source for a reset transistor and a source follower circuit. The method of claim 1, And using the Vbb voltage as the ground electrode of the source follower circuit. The method of claim 1, And using the Vpp voltage as a driving power source for the reset transistor and the source follower, and using the Vbb voltage as a ground electrode of the source follower circuit.