JPS62157479A - Driving method for solid-state image pickup device - Google Patents

Abstract

PURPOSE:To expand a dynamic range by changing the width of a pulse to specify a charge reading period impressed to a charge transferring means in accordance with a light irradiating strength and making the barrier electric potential of an excessive charge discharging means lower than the barrier electric potential of a charge reading means. CONSTITUTION:The width of a pulse applied to a transferring electrode 6 is made longer in accordance with a light strength, and a constant low voltage is impressed to an overflowing control gate. The second electrode is kept to a low level, and then, the channel electric potential is the level shown by 31 and to an overflowing control gate 7, a constant voltage to give a level 32 a little higher than 31 is applied. Thus, when the shift channel is opened, the charging quantity to flow from a photodiode 2 to a transferring channel 3 is in proportion to the intensity of the light. Consequently, when the light is strong, to the whole of an N layer 2 of a photodiode and a transferring channel 3, the charge is accumulated, and when the electric potential of the transferring electrode 6 is lowered, the excessive charge can be thrown away to an overflowing drain side.

Description

[Detailed description of the invention] [Technical field of invention] The present invention relates to a method for driving a solid-state 1 lil image device.

[Technical background of the invention]

Solid lI! Imaging devices have outstanding features such as being small, lightweight, and suitable for Nagano fields, and are increasingly being used in video cameras and the like.

The configuration of a conventional solid ratio @ device will be explained with reference to the sectional view of FIG. A charge transfer channel 3 which is an n-type region, a photodiode 2 which is an n-type region, and an overflow drain 4 which is an n+ region are formed on the surface side of the p-type substrate 1.
A transfer electrode 6 is provided on the charge transfer channel 3 and the shift channel 5 between the charge transfer channel 3 and the photodiode 2, and an overflow control gate 7 is provided above the photodiode 2 and the overflow drain 4.

The operation of such a solid-state image transfer device will be explained with reference to FIGS. 9 to 13.

9th FJ t, t A predetermined pulse voltage is applied to the overflow control gate 7, and nothing is applied to the transfer electrode 6. In this case, the potential 12 of the shift channel 5 is almost 0, the shift channel is interdigitated, and the charge of the photodiode 2? The signal charges 13 accumulated in the S stack are not transferred. Note that the transfer channel potential is indicated by reference number 11.

Also, the potential under the overflow control gate 7 is pulsed ゛F
Level 1 changes from normal level 14a by applying IN pressure.
It rises to 4b. Therefore, when the charges accumulated by photoelectric conversion fall below the level 14a, the excess charges flow into the overflow drain 4 and form the drain potential 15.

FIG. 10 shows a state in which nothing is applied to the overflow control gate 7 and a pulsed voltage is applied to the shift channel 6, and the transfer channel potential 11'J3 and the shift channel 11 channel potential 12' are as shown in FIG. Since it is tilted upward compared to , the charges 13 accumulated by photoelectric conversion flow into between the transfer electrodes.

FIG. 11 shows the pulses P applied to the overflow control groups 1 to 7J5 and the transfer electrode 6 in driving the actual solid-state Il[1 imager. , and PTR,
For NT PTH, which is a short pulse generated every t, POF is a time t114 within t114T.
Only T1 is a high signal of level ■, and the remaining time tINT2 is a - signal.

The signal charge accumulated by the arrival of PTR is transferred to the charge transfer channel 3 and signal readout is performed. After that, the shift channel 5 is closed and the overflow control gate is opened to discharge excess charge.
During the period T2, the overflow potential is lowered to increase the maximum charge storage amount.

Here, the relationship between the amount of charge Q accumulated in the 0 layer 2 of the photodiode and the accumulation time t is shown in the graph of FIG. In this graph, the horizontal axis is the time t, the vertical axis is the accumulated charge mQ in the photodiode n layer, and the slope is the light intensity 1. represents.

Since the overflow potential is high during time tINTI, the maximum charge "P141" at this time is higher than tINTI.
If charge accumulation occurs in a shorter time than NT1, that is, the light intensity becomes the coordinate (t18□1.QPHI)
If the slope is steeper than the slope I when passing through H1, a buffer flow occurs. A straight line 16 in FIG. 12 shows such a case, and a straight line 17 shows an example where saturation does not occur.

When the time t has elapsed, the overflow control gate is closed, so the maximum storage capacity of 85 charges increases to Q PH2.

Therefore, the relationship between the charge ff1Q' read out by the accumulation operation over time t and the light intensity I is as shown in the graph of FIG.
1, and two straight lines A and B having a bending point C are combined. Such photoelectric conversion characteristics are called knee characteristics, and are effective in widening the dynamic range against strong light.

[Problems with background technology]

However, in order to achieve such a knee characteristic, a control pulse corresponding to a shift pulse that moves charges from the photodiode to the charge transfer channel must be applied to the overflow control gate, which makes control complicated.

In particular, it is difficult to realize knee characteristics in a vertical overflow drain whose cross-sectional structure is shown in FIG.

That is, in this structure, the p
Photodiode 1 and gate 2 formed in well 22
The p-well directly under the photodiode 1 is a shallow well 23 with a low impurity concentration. In this structure, by applying a reverse bias voltage to the n-type substrate 21, the excess charge of the photodiode nt124 can be discharged to the n-type substrate 21 through the shallow well 23.
The change in the potential of this shallow well 23 is approximately 1 compared to the change in the reverse bias voltage of the n-type substrate due to the two-dimensional spread of the electric field.
It's quite small, about 15. Therefore, the knee characteristic cannot be obtained unless the amplitude of the voltage pulse for overflow control is set to a high voltage of several tens of volts, which is difficult to achieve.

[Purpose of the invention]

The present invention has been made to solve such problems, and an object of the present invention is to provide a method for driving a solid-state imaging device that does not require pulse modulation of the O-puff O-potential and can easily realize knee characteristics.

[Summary of the invention]

The present invention includes a plurality of photoelectric conversion units that photoelectrically convert incident light to generate signal charges, a charge storage layer that generates the signal charges, and stores the signal charges, and a signal that is accumulated in the photoelectric conversion units. In a method for driving a solid-state image device, which includes a charge reading means with an accumulation region for reading charges, and an excess charge discharging means for discharging excess charges generated by the photoelectric conversion unit C, the charge transfer means is applied with a charge. The present invention is characterized in that the width of the pulse that defines the charge readout period is varied in accordance with the intensity of light irradiation, and the barrier potential of the excess charge extraction means is lower than the barrier potential of the charge readout means. Therefore, the dynamic range can be expanded without changing the overflow control potential.

[Embodiments of the invention]

Embodiments of the present invention will be described in detail below with reference to the drawings.

3 and 4 illustrate the configuration of a solid-state III imaging device to which the present invention is applied. FIG. 3 is a plan view of the solid-state silver (g! Figure 8
The same parts as those shown in the figures are given the same reference numerals, and detailed explanation thereof will be omitted. According to this, the only difference in FIG. 4 is that the next electrode 8.9 of the charge transfer channel 1F channel is drawn, and the structure is exactly the same.

FIG. 1 is a potential distribution diagram showing the driving method according to the present invention, and is drawn in correspondence with FIG. 4.

In the present invention, the width of the pulse applied to the transfer electrode 6 is increased in accordance with the light intensity, and a constant low voltage is applied to the overflow control gate.

FIG. 2 is a waveform diagram showing pulses applied to the transfer electrodes, in which FIG. 2(a) is a pulse with approximately the same length t as the conventional control pulse P, and FIG. 2(b) is a pulse of TRFSI. (C) shows a longer pulse. These pulse widths can be selected from a wide range of, for example, 1 μs to 1 ms.

If the second electrode is kept at a low level, the channel potential below it will be at the level 31, and the overflow control gate 7 will be at a level 32, which is slightly higher than 31.
Assume that a constant voltage is applied that gives . Further, due to the voltage applied to the transfer electrode 6, the potential under the shift channel 5 becomes 33, and the potential on the transfer channel 3 becomes 34.

In this way, when the shift channel is open, the amount of charge flowing from the photodiode 2 toward the transfer channel 3 is proportional to the intensity of light. Therefore, when the light is strong, by increasing the width of the pulse applied to the transfer electrode 6, charge can be accumulated in the nFN2 of the photodiode and the entire transfer channel 3, and when the potential of the transfer electrode 6 decreases, excess charge can be accumulated. can be discarded to the overflow drain side.

In Fig. 1, 36 indicates the charge stored in the photodiode and read out within the time tFS, 37 indicates the charge photoelectrically converted within the period tFS and is used as a signal component, and 38
is the excess charge generated during the period t and 8 during excessive light irradiation, and tF
Each of the figures shows what is discarded into the overflow drain 4 after the S period has passed.

Here, charge f is read out to the buried channel 3 under the transfer electrode 6 by applying a shift pulse voltage for a period tFS.
ftQ3 is expressed by the following formula.

Q8=ηI p (i + t r6)−'−”
−”'(T p≦IP, >NT = ηI t +Q (IPH
) P FS PH −QIH (lSH × IP ) where η : Photoelectric conversion coefficient, ■, : Irradiation light intensity, I : Irradiation light intensity for which Q = 01H, 3M
S t : Storage gate of the photo diode to which no shift pulse is applied, Q : Final saturation charge amount.

H Therefore, in the period when IP is IPH x IP x IsH, Q
is proportional to the product of I and t, and if the time of tFS is
By selecting P FS as appropriate, the slope of the photoelectric conversion curve Q3/ηt
FS can be set.

Figure 5 shows this situation, and Figures 2 (a) and (b)
, (C), the read charge amount straight line 41°42.43
It can be seen that is determined. Further, it can be seen that this photoelectric conversion curve has a knee characteristic with a bending point at IPH.

Furthermore, since the amount of charge of QpH varies depending on the height of the overflow control gate, the bending point (knee point) of the knee curve can be changed by changing the potential of the overflow control gate 7.

FIG. 6 is a graph showing this situation, and shows the overflow potential v1. It can be seen that the knee point changes to 51.52.53 corresponding to V and V (V,>V2>V3).

Note that the knee point can be varied by lowering the shift pulse voltage.

FIG. 6 shows a solid-state decoy system that enables the driving method of the present invention, which includes a solid-state imaging device 61, a driving pulse generation circuit 62 for driving the solid-state imaging device 61, and a signal processing circuit 63 for processing read signals. In addition to the normal configuration in which a light amount detector 65 is connected to detect the amount of light, and a read pulse width setting circuit 64 that inputs the output and sets the read pulse width.
is provided to exchange signals with the drive pulse generation circuit 62. Here, the light amount detector is, for example, a phototransistor, which detects the average light amount of the subject, and the readout pulse width setting circuit is equipped with, for example, a counter, and sends an appropriate count value to the drive pulse generation circuit according to the obtained light amount. This generates a read pulse having a width.

Note that it is also possible to set the readout pulse width by the output of a specific pixel in the solid-state imaging device, as shown by the dotted line in FIG. 6, without using a special light amount detector.

As described above, arbitrary knee characteristics can be obtained by appropriately selecting the time of the read pulse tFS, the overflow potential, and the read potential, making it possible to read out a solid-state 1I11@ device having a desired dynamic range.

In addition, by reading the maximum transfer charge amount QTH of the charge transfer device and designing it to be larger than the maximum charge ff101N so that the overflow drain functions effectively, overflow of excess charge (blooming) can be reliably prevented. The present invention can also be applied to solid-state imaging devices with photodiodes having anti-blooming drains.

Further, the present invention, which can fix the overflow potential, is most suitable for solid-state imaging devices having a vertical overflow drain structure, which have poor overflow potential control efficiency.

(Effects of the Invention) As described above, according to the driving method of the solid state 11a (! Since the pulse width that defines the pulse width is changed in accordance with the light irradiation intensity, it is possible to read out a solid-state image transfer device with a wide dynamic range without performing complicated control of the excess charge discharging means.

Also, is there a means of draining excess charge? 12 Since no complicated control is required, the present invention can be easily applied to a solid-state imaging device having a vertical overflow drain.

[Brief explanation of drawings]

FIG. 1 is a potential distribution diagram explaining the present invention in detail, FIG. 2 is a waveform diagram showing pulses applied to the transfer electrode 6, and FIG. 3 is a solid state! The plan view of li@sou, Figure 4 is its sectional view, and Figure 5 is
The figure is a graph showing the relationship between light intensity and readout charge amount in J3 of the present invention, Figure 6 is a graph showing how the knee point is changed, and Figure 7 is a block diagram showing the configuration of a device realizing the present invention. , FIG. 8 is a sectional view of a solid-state image transfer device, FIGS. 9 and 10 are potential distribution diagrams showing the operation of a conventional solid-state image transfer device, and FIG. 11 is a waveform diagram showing conventionally used control pulses. , Figure 12 shows time and savings in ft? FIG. 13 is a graph showing the relationship with the + charge group, FIG. 13 is a graph explaining knee characteristics, and FIG. 14 is a cross-sectional view showing an example of a solid-state imaging device having a vertical overflow drain. DESCRIPTION OF SYMBOLS 1...Substrate, 2...Photodiode, 3...Transfer channel, 4...Overflow drain, 5...
Shift channel, 6... Transfer gate, 7... Overflow control gate, 51.52.53... Knee point. Applicant's Representative Sato - Rooster 1 Figure 2 Figure 3 Tail Figure 6 Figure 7'- Figure 7

Claims (1)

[Claims] 1. A plurality of photoelectric conversion sections each having a charge storage layer that photoelectrically converts incident light to generate signal charges and accumulates the signal charges; In a method for driving a solid-state imaging device, the solid-state imaging device includes a charge readout means with an accumulation region to be read out, and an excess charge discharge means for discharging excess charge generated in the photoelectric conversion section, comprising: a charge readout period applied to the charge transfer means; Driving a solid-state imaging device, characterized in that the width of the pulse that defines the pulse is varied in accordance with the intensity of light irradiation, and the barrier potential of the excess charge discharging means is lower than the barrier potential of the charge reading means when the pulse is applied. Method. 2. The method for driving a solid-state imaging device according to claim 1, wherein the barrier potential of the excess charge discharge means is given at a constant potential. 3. The method for driving a solid-state imaging device according to claim 1 or 2, wherein the maximum amount of charge stored in the storage region for charge readout is also smaller than the maximum amount of charge transferred by the charge readout means. 4. The device according to any one of claims 1 to 3, wherein the charge discharge means has a vertical overflow drain structure in which the discharge drain is provided deep in the photoelectric conversion section. A method for driving a solid-state imaging device.