JPH06133227A - Charge transfer device - Google Patents

Abstract

PURPOSE:To automatically make a depth of a static potential under a reset gate proper by providing a potential detection transistor(TR) generating a voltage with correlation to the static potential under the reset gate in the vicinity of a reset TR and connecting its output terminal to a reset drain of the reset TR. CONSTITUTION:A potential detection TR 9 is formed in the vicinity of a reset TR 7 by the same process as the TR 7 and with almost the same structure thereof. A source 10 of the TR 9 is connected to a reset drain 5 of the TR 7 via an impedance conversion buffer 13. Thus, when a static potential under the reset gate 6 of the TR 7 gets deep/shallow, the potential of the source 10 of the TR9 is high/low and then the potential of the drain 5 naturally gets high/low. Thus, when the depth of the static potential under the gate 6 is deep, the dynamic range of a floating diffusion 4 is automatically secured when the TR 7 is turned off.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charge transfer device, and more particularly to a charge transfer device having a reset transistor formed of a floating diffusion, a reset gate and a reset drain adjacent to the final stage of a charge transfer section.

[0002]

2. Description of the Related Art Charge-coupled charge transfer devices are often used for solid-state image pickup devices and delay devices. In many cases, a floating diffusion amplifier (FDA) method is adopted for detecting the signal charge of the charge transfer device. FIG. 4 is a sectional view showing a signal charge detection unit of a conventional charge transfer device.

In the drawings, 1 is a semiconductor substrate, 2 is a CC
The D-type charge transfer section 3 is biased to a predetermined potential by an output gate provided adjacent to the final stage of the CCD-type charge transfer section 2. Reference numeral 4 is an n ⁺ type floating diffusion formed adjacent to the output gate portion, 5 is a reset drain spaced apart from the floating diffusion 4, and 6 is a portion between the floating diffusion 4 and the reset drain 5. A reset gate formed above the gate insulating film, the reset gate 6, the floating diffusion 4, and the reset drain 5 constitute a reset transistor 7. The reset drain 5 is biased to a predetermined potential by the voltage generating means E.

Reference numeral 8 is a buffer composed of a detection transistor for detecting the potential of the floating diffusion 4. The FDA type charge transfer device detects a signal by periodically reading the potential of the floating diffusion 4 via a buffer composed of a detection transistor, but sweeps away the signal charge of the floating diffusion 4 each time the reading is completed. It is necessary to perform (reset), and the reset transistor does this.

FIGS. 5A and 5B are electrostatic potential profile diagrams showing the operation of the charge transfer device, and the reset operation will be described accordingly. A predetermined constant potential is applied to the reset drain 5, and a reset pulse RP is applied to the reset gate 6, so that the electrostatic potential under the reset gate 6 is changed.
It becomes deeper as shown in FIG. 5A or becomes shallower as shown in FIG. The charges of the floating diffusion 4 are swept out when the electrostatic potential under the reset gate 6 becomes deep. When the electrostatic charges become shallow, the signal charges from the charge transfer unit 2 reach the floating diffusion 4 through the output gate 3 and the potentials prevent the signal charges from flowing from the floating diffusion 4 to the reset gate side. Create a barrier. At this time, the signal is read out via the buffer 8.
That is, the larger the amount of signal charges existing in the floating diffusion 4, the lower the potential of the floating diffusion 4. Therefore, the amount of signal charges is detected by detecting the potential of the floating diffusion 4.

[0006]

By the way, a reset pulse R under the reset gate 6 of the reset transistor 7 is used.
The electrostatic potential when P is "high" and "low" varies depending on the impurity concentration of the substrate 1, the oxide film thickness, and the like. In the case of a charge transfer device applied to a complicated IC such as a vertical overflow drain type CCD solid-state imaging device, the electrostatic potential changes depending on the well concentration. Although it is becoming smaller due to the remarkable development of the semiconductor manufacturing technology, it is inevitable that the impurity concentration of the substrate and the well and the oxide film thickness vary. Therefore, variations occur in the depth of the electrostatic potential under the reset gate 6 for the same reset pulse RP. Especially, an IC with a MONOS gate structure
In that case, the tendency is strong.

When the electrostatic potential becomes shallow due to variations, when the reset pulse RP as shown in FIG. 5A becomes "high" and the electrostatic potential under the reset gate 4 becomes deep, That is, the on resistance when the reset transistor 7 is turned on increases,
There is a risk that the signal charges of the floating diffusion (FD) 4 are not completely discharged (reset) to the reset drain 5. And if this is discharged (reset)
If it is incomplete, the so-called remaining signal charges that could not be completely discharged and the next signal charges are mixed, and accurate signal output cannot be performed.

On the contrary, when the electrostatic potential becomes deep due to the variation, as shown in FIG. 5B, between the lower part of the reset gate 6 and the floating diffusion (FD) 4 when the reset transistor 7 is turned off. The height of the electrostatic potential barrier becomes low and the signal charges in the floating diffusion 4 may flow under the reset gate 6 and overflow into the reset drain 5. Therefore, in order to prevent such a problem from occurring, it is considered to increase the amplitude of the reset pulse applied to the reset gate 6. However, this goes against the low power consumption that is strongly required for CCD type solid-state image pickup devices to which the charge transfer device is applied, and is not preferable. The reason for this is that increasing the pulse amplitude causes more power consumption than increasing the DC bias voltage. Further, the power supply voltage of the IC is changed from 5V to 3.
There is a flow of technology for reducing the voltage to 3V, but if the power supply voltage is lowered to 3.3V along the flow, the amplitude of the reset pulse cannot be increased.

Therefore, without changing the amplitude of the reset pulse RP, the "high" level and the "low" level are integrally adjusted and used so as to correspond to the variation in the electrostatic potential under the reset gate 6. Is being done. Specifically, the level of the reset pulse suitable for each individual charge transfer device is measured, and the “high” and “reset” of the reset pulse are applied so that the reset pulse generated according to the measurement result is applied to the reset gate 6 at the time of use. It is shipped by specifying the "low" level. However, this is not preferable because it requires a great deal of labor for measurement and the like and causes an increase in cost.

The present invention has been made to solve such a problem, and in a charge transfer device having a reset transistor composed of a floating diffusion, a reset gate and a reset drain, the amplitude of a reset pulse applied to the reset gate is changed. The purpose is to make the depth of the electrostatic potential under the reset gate deep and the depth when the electrostatic potential under the reset gate is proper without increasing the level and without obtaining the level of the reset pulse suitable for each device by measurement. To do.

[0011]

According to another aspect of the charge transfer device of the present invention, a potential detection transistor for generating a voltage having a correlation with an electrostatic potential under a reset gate is provided in the vicinity of the reset transistor, and an output of the potential detection transistor. The terminal is connected to the reset drain of the reset transistor. A charge transfer device according to a second aspect is the charge transfer device according to the first aspect, wherein the reset drain of the reset transistor is a source of the potential detection transistor. A charge transfer device according to a third aspect is the charge transfer device according to the first or second aspect, wherein the gate of the potential detection transistor is grounded.

[0012]

According to the charge transfer device of the first aspect, the output of the potential detection transistor for generating a voltage having a correlation with the electrostatic potential under the reset gate in the vicinity of the reset transistor is given to the reset drain of the reset transistor. The level of the reset drain of the transistor changes automatically according to the variation of the electrostatic potential caused by the manufacturing, and therefore the level of the reset pulse is automatically adjusted without reducing the amplitude applied to the reset gate. The level is adjusted and proper operation is performed.

According to the charge transfer device of the second aspect, since the reset drain of the electrostatic potential is used as the source of the potential detection transistor, the area occupied by the reset transistor and the potential detection transistor can be made small, and by extension. This can contribute to low power consumption and elimination of reset gate level adjustment without unnecessarily increasing the occupied area of the charge transfer device. According to the charge transfer device of claim 3, since the gate of the potential detection transistor is grounded, it is not necessary to provide a voltage generating means for setting the potential of the reset drain to a predetermined value, and the circuit configuration of the charge transfer device is simple. become. Even if the gate is grounded, the signal charge from the reset drain can be discharged to the ground side by doping impurities under the gate to form depletion.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The charge transfer device of the present invention will be described in detail below with reference to the illustrated embodiments. FIGS. 3A and 3B show one embodiment of the charge transfer device of the present invention. FIG. 3A is a sectional view and FIG. 3B is an electrostatic potential profile diagram.
This charge transfer device is significantly different from the charge transfer device shown in FIG. 4 in that it has a potential detection transistor, but is substantially the same in other points, and the common points have already been described, so description thereof will be omitted. However, only the differences will be described. In addition, common symbols are used for common parts throughout the drawings.

In the drawing, reference numeral 9 denotes a potential detecting transistor, which has substantially the same structure as the reset transistor 7 in the same step and is formed close to it, and therefore has substantially the same characteristics. Therefore, the electrostatic potential under the gate due to the same gate voltage varies similarly between the potential detection transistor 9 and the reset transistor 7. Therefore, the potential detection transistor 9 can play a role of detecting the potential under the reset gate of the reset transistor 7.

Reference numeral 10 is a source of the potential detection transistor 9, 11 is a drain, and 12 is a gate. The source 10 of the potential detection transistor 9 is connected to the reset transistor 5 via an impedance conversion buffer (comprising a source follower transistor) 13. It is connected to the reset drain 5. A predetermined positive potential is applied to the gate and drain of the potential detection transistor 9, and the signal charge swept out to the reset drain 5 of the reset transistor 7 is further transferred from the reset drain 5 to the buffer 13 and the potential detection transistor 9 It is designed to be discarded via.

According to this charge transfer device, even if the depth of the electrostatic potential below the reset gate 6 of the reset transistor 7 varies due to the impurity concentration of the substrate and the oxide film thickness, the charge transfer device is provided close to the reset transistor 7. The depth of the electrostatic potential under the gate 12 of the potential detection transistor 9 varies almost in the same manner, and if the electrostatic potential under the reset gate of the reset transistor 7 becomes deeper, the electrostatic potential under the gate 12 of the potential detection transistor 9 will increase. The potential also becomes deep as well.
Then, the gate 12 of the potential detection transistor 9
The depth of the lower electrostatic potential appears as the potential of the source 10, and this is given to the reset drain 5 of the reset transistor 7 via the buffer 13.

Specifically, when the electrostatic potential under the gates 12 and 6 becomes deep, the potential detection transistor 9
Source 10 has a higher potential, and therefore the reset drain 5 of the reset transistor 7 also naturally has a higher potential.
On the contrary, when the electrostatic potential under the gates 12 and 6 becomes shallow, the source 1 of the potential detection transistor 9
The potential of 0 and the reset drain 5 becomes low. That is, the potential detection transistor 9 has a positive correlation with the electrostatic potential under the reset gate 6 of the reset transistor 7. Therefore, the potential applied to the reset drain 5 can be adjusted by the potential detection transistor 9 according to the variation in the electrostatic potential, and the dynamic range of the floating diffusion and the incomplete reset can be eliminated.

More specifically, when the electrostatic potential under the gates 12 and 6 becomes deep due to variations, the potential of the reset drain 5 becomes high as described above, so that when the reset transistor 7 is off. The dynamic range of the floating diffusion 4 can be automatically secured. On the contrary, when the electrostatic potential under the gates 12 and 6 becomes shallow, the potential of the reset drain 5 becomes low, so that incomplete resetting when the reset transistor 7 is on can be prevented.

Therefore, even if the amplitude of the reset pulse applied to the reset gate 6 is smaller than that of the conventional one, the reset pulse can be reset without any trouble, and the power consumption can be reduced.
Then, there is no need to adjust the level of the reset pulse according to the electrostatic potential under the reset gate 6,
The device is easy to use.

FIG. 2 is a sectional view showing another embodiment of the charge transfer device of the present invention. In this embodiment, the drain 5 of the reset transistor 7 is shared by the potential detection transistor 12 as the source 5 (10), and thus the area of the reset transistor 7 and the potential detection transistor 12 can be reduced. .

Reference numeral 14 is a capacitor having one end connected to the reset drain 5 (10) and the other end grounded. This capacitor 14 lowers the impedance of the reset drain 5 to reset the reset drain 5 at the time of reset.
The voltage fluctuation can be reduced. The capacitor 14 may be externally attached, or a capacitive element formed inside may be used. Further, the junction capacitance of the reset drain 5 with the substrate 1 may be appropriately increased and replaced with the capacitor 14. Increasing the junction capacitance can be realized by increasing the area of the diffusion layer of the drain 5.

FIG. 3 is a sectional view showing still another embodiment of the charge transfer device according to the present invention. In this embodiment, the potential detection transistor 9 of the charge transfer device shown in FIG. 2 is depleted by, for example, doping the channel portion with an n-type impurity, and the gate 12 of the potential detection transistor 9 is grounded. It is something that is done. In this way, it is not necessary to create a special circuit for generating a DC voltage applied to the gate. Incidentally, the impurity doping for such depletion does not hinder the electrostatic potential detecting function of the potential detecting transistor 9 if only the doping amount is accurately controlled.

[0024]

According to the charge transfer device of the present invention, a potential detecting transistor for generating a voltage having a correlation with the electrostatic potential under the reset gate is provided in the vicinity of the reset transistor, and the output terminal of the potential detecting transistor is the above-mentioned. It is characterized by being connected to the reset drain of the reset transistor. Therefore, according to the charge transfer device of the first aspect, the output of the potential detection transistor that generates a voltage having a correlation with the electrostatic potential under the reset gate near the reset transistor is given to the reset drain of the reset transistor. Since the level of the reset drain of the device naturally changes according to the variation of the electrostatic potential caused by manufacturing, proper operation can be performed without reducing the amplitude applied to the reset gate and without adjusting the level of the reset gate. Done.

According to a second aspect of the charge transfer device of the present invention, the reset drain of the reset transistor is used as the source of the potential detection transistor. Therefore, according to the charge transfer device of the second aspect, since the reset drain of the electrostatic potential is used as the source of the potential detection transistor, the area occupied by the reset transistor and the potential detection transistor can be reduced, and the charge can be extended. This can contribute to low power consumption and unnecessary reset gate level adjustment without unnecessarily increasing the occupied area of the transfer device.

A charge transfer device according to a third aspect is characterized in that the gate of the potential detecting transistor is grounded. Therefore, according to the charge transfer device of the third aspect, since the gate of the potential detection transistor is grounded, it is not necessary to provide the voltage generating means for setting the potential of the reset drain to a predetermined value, and the circuit configuration of the charge transfer device. Will be easier.

[Brief description of drawings]

1A and 1B show one embodiment of a charge transfer device of the present invention, FIG. 1A is a sectional view, and FIG. 1B is a potential profile view.

FIG. 2 is a sectional view showing another embodiment of the charge transfer device of the present invention.

FIG. 3 is a sectional view showing still another embodiment of the charge transfer device of the present invention.

FIG. 4 is a sectional view of a conventional example of a charge transfer device.

5A and 5B are change diagrams of electrostatic potential profile diagrams for explaining the operation of the charge transfer device.

[Explanation of symbols]

 2 charge transfer unit 4 floating diffusion 5 reset drain 6 reset gate 7 reset transistor 9 potential detection transistor 10 source 11 drain 12 gate

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[Procedure amendment]

[Submission date] May 27, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0007

[Correction method] Change

[Correction content]

When the electrostatic potential becomes shallow due to variations, when the reset pulse RP as shown in FIG. 5A becomes "high" and the electrostatic potential under the reset gate 6 becomes deep, That is, the on resistance when the reset transistor 7 is turned on increases,
There is a risk that the signal charges of the floating diffusion (FD) 4 are not completely discharged (reset) to the reset drain 5. And if this is discharged (reset)
If it is incomplete, the so-called remaining signal charges that could not be completely discharged and the next signal charges are mixed, and accurate signal output cannot be performed.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0012

[Correction method] Change

[Correction content]

[0012]

According to the charge transfer device of the first aspect, the output of the potential detection transistor for generating a voltage having a correlation with the electrostatic potential under the reset gate in the vicinity of the reset transistor is given to the reset drain of the reset transistor. naturally vary according to the variation of the electrostatic potential level of the reset drain of the transistor caused by manufacturing, therefore, even without large comb amplitudes applied to the reset gate also automatically without level adjustment of the reset pulse The level is adjusted and proper operation is performed.

[Procedure 3]

[Document name to be amended] Statement

[Name of item to be corrected] 0024

[Correction method] Change

[Correction content]

[0024]

According to the charge transfer device of the present invention, a potential detecting transistor for generating a voltage having a correlation with the electrostatic potential under the reset gate is provided in the vicinity of the reset transistor, and the output terminal of the potential detecting transistor is the above-mentioned. It is characterized by being connected to the reset drain of the reset transistor. Therefore, according to the charge transfer device of the first aspect, the output of the potential detection transistor that generates a voltage having a correlation with the electrostatic potential under the reset gate near the reset transistor is given to the reset drain of the reset transistor. Since the level of the reset drain of changes automatically depending on the variation of the electrostatic potential caused by manufacturing, the amplitude applied to the reset gate
Even without large comb and proper operation is performed without the level adjustment of the reset gate.

[Procedure amendment 4]

[Document name to be corrected] Drawing

[Name of item to be corrected] Figure 5

[Correction method] Change

[Correction content]

[Figure 5]

Claims (3)

[Claims] 1. A charge transfer device having a floating diffusion for temporarily storing signal charges from a charge transfer section, and a reset transistor consisting of a reset gate and a reset drain, in the vicinity of the reset transistor and correlation with an electrostatic potential under the reset gate. Charge transfer device characterized in that a potential detection transistor for generating a voltage having a characteristic is provided, and an output terminal of the potential detection transistor is connected to a reset drain of the reset transistor. 2. The charge transfer device according to claim 1, wherein the reset drain of the reset transistor is used as the source of the potential detection transistor. 3. A charge transfer device according to claim 1, wherein the gate of the potential detection transistor is grounded.