JPH0473834B2 - - Google Patents

Description

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

[Technical background of the invention]

Solid-state imaging devices have excellent features such as small size, light weight, and long life, and are increasingly being used in video cameras and the like.

The configuration of a conventional solid-state imaging device will be explained with reference to the cross-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, and the charge transfer channel 3
A transfer electrode 6 is provided on the shift channel 5 between the photodiode 2 and the photodiode 2, and an overflow control gate 7 is provided on the shift channel 5 between the photodiode 2 and the overflow drain 4.

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

FIG. 9 shows a state in which 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 approximately 0, the shift channel is closed, and the signal charges 13 accumulated in the charge storage layer of the photodiode 2 are not transferred. Note that the transfer channel potential is indicated by reference number 11. Further, the potential under the overflow control gate 7 is brought to the normal level 14a by applying a pulse voltage.
It rises to level 14b. Therefore, when the charge accumulated by photoelectric conversion falls below the level 14a, the excess charge goes to the overflow drain 4.
and forms a 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' and shift channel potential 12' are different from those in FIG. Since the current is rising, the charges 13 accumulated by photoelectric conversion flow into between the transfer electrodes.

FIG. 11 shows the pulses P _OF and P _TR applied to the overflow control gate 7 and _the transfer electrode 6, respectively, in driving an actual solid _- state imaging device. On the other hand, P _OF is a high signal at level V _B for time t _INT1 within t _INT and is a low signal for the remaining time t _INT2 .

The signal charge accumulated by the arrival of P _TR is transferred to the charge transfer channel 3 and signal readout is performed, after which the excess charge is discharged by closing the shift channel 5 and opening the overflow control gate. , _tINT2 , the overflow potential is lowered to increase the maximum charge storage amount.

The graph of FIG. 12 shows the relationship between the amount of charge Q _P accumulated in the n-layer 2 of the photodiode and the accumulation time t. In this graph, the horizontal axis is time t, and the vertical axis is the amount of charge accumulated in the photodiode n layer.
Q _P , and the slope represents the light intensity I _P.

Since the overflow potential is high during time t _INT1 , the maximum charge amount Q _PM1 at this time is higher than t _INT1.
An overflow occurs when the charge is accumulated in a time shorter than , that is, when the slope of the light intensity is steeper than the slope I _PM1 when passing through the coordinates (t _INT1 , Q _PM1 ). A straight line 16 in FIG. 12 shows such a case, and a straight line 17 shows an example where saturation does not occur.

After the time t _INT1 has elapsed, the overflow control gate closes, so the maximum accumulated charge amount increases to Q _PM2 .

Therefore, the relationship between the amount of charge Q _P ′ read out by the accumulation operation at time t _INT and the light intensity I _P is as follows.
As shown in the graph of FIG. 3, two straight lines A and B having a bending point C at I _PM1 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, this structure has a photodiode 1 formed in a p-well 22 formed in an n-type substrate 21, a charge transfer channel 25 in which charges are shifted by a gate 26, and a p-well 22 directly below the photodiode 1. The well is a shallow well 23 with a low impurity concentration. In this structure, a reverse bias voltage is applied to the n-type substrate 21, and excess charge in the photodiode n layer 24 is removed from the shallow well 2.
However, due to the two-dimensional spread of the electric field, the potential change in this shallow well 23 is about 1/5 of the change in the reverse bias voltage of the n-type substrate. Quite small. 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 an overflow potential and can easily realize knee characteristics.

[Summary of the invention]

The present invention includes a plurality of photoelectric conversion sections that photoelectrically convert incident light to generate signal charges and have a charge storage layer that accumulates the signal charges, and a method that reads out and accumulates the signal charges accumulated in the photoelectric conversion sections. A method for driving a solid-state imaging device comprising an accumulation region and a charge reading means including a transfer means for transferring signal charges accumulated in the accumulation region, and an excess charge discharge means for discharging excess charge generated in the photoelectric conversion section. In this step, the width of the pulse that defines the charge readout period applied to the charge transfer means is varied according to the light irradiation intensity, and the barrier potential at the gate between the photoelectric conversion section and the excess charge discharge means is varied as described above. The present invention is characterized in that the barrier potential is lower than the barrier potential in the channel between the photoelectric conversion section and the transfer means when pulses are applied. 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 imaging device to which the present invention is applied. FIG. 3 is a plan view of the solid-state imaging device, and FIG. 4 is a cross-sectional view taken along X ₁ -X ₂ thereof. The same parts as shown in FIG. 8 are given the same reference numerals, and detailed explanation thereof will be omitted. According to this, in FIG. 4, the next electrode 8 of the charge transfer channel,
The only difference is that 9 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.
Figure 2 is a waveform diagram showing pulses applied to the transfer electrodes, where Figure 2a shows a pulse with a length t _FS1 that is approximately the same as the conventional control pulse _PTR , Figure 2b shows this pulse, and Figure 2c shows this pulse. 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 transfer electrode 8 is kept at a low level, the channel potential below it will be at the level indicated by 31, and a constant voltage that gives a level 32 slightly higher than 31 is applied to the overflow control gate 7. It is assumed that there is Also, the potential under the shift channel 5 is 3 due to the voltage applied to the transfer electrode 6.
3, the potential of 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, the n-layer 2 of the photodiode
When the potential of the transfer electrode 6 decreases, the barrier potential of the overflow control gate becomes lower than the barrier potential of the transfer electrode, so the excess charge is transferred to the overflow drain side. You can throw it away.

In Figure 1, 36 indicates the charge stored in the photodiode and read out within time t _FS , and 37 indicates
Charges photoelectrically converted within the period t _FS and used as signal components, and 38 indicate excess charges generated during the t _FS period during excessive light irradiation and are discarded to the overflow drain 4 after the t _FS period has elapsed.

Here, the amount of charge Q _S read out to the buried channel 3 under the transfer electrode 6 by applying the shift pulse voltage during the period t _FS is expressed by the following equation.

_{Here _ _ _ _ _ _ _ _ _ _ _} η: Photoelectric conversion coefficient, I _P : Irradiation light intensity, I _SM : Irradiation light intensity where Q _S = Q _IM , t _IMT : Photodiode accumulation period when no shift pulse is applied, Q _IM : Final saturated charge amount .

Therefore, in the period when I _P is I _PM < I _P < I _SM , Q _S is equal to I _P
The slope Q _S /ηt _FS of the photoelectric conversion curve can be set by appropriately selecting the time of t _FS , which is proportional to the product of t _FS .

Figure 5 shows this situation, and Figure 2a,
Corresponding to b, c, read charge amount straight lines 41, 42,
It can be seen that 43 is determined. Furthermore, it can be seen that this photoelectric conversion curve has a knee characteristic with a bending point at I _PM .

Furthermore, since the amount of charge of Q _PM 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 that the knee points change to 51, 52, and 53 in response to overflow potentials V ₁ , V ₂ , and V ₃ (V ₁ >V ₂ >V ₃ ). I understand.

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

FIG. 6 shows a solid-state imaging system that enables the driving method of the present invention.
A drive pulse generation circuit 62 for driving this
In addition to the normal configuration in which a signal processing circuit 63 for processing read signals is connected, a light amount detector 65 for detecting the amount of light and a read pulse width setting circuit 64 for inputting the output and setting the read pulse width are provided. Signals are exchanged with the drive pulse generation circuit 62 provided therein. 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 certain time 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 mentioned above, any knee characteristic is determined by the readout pulse
This can be obtained by appropriately selecting the t _FS time, overflow potential, and readout potential, and enables readout driving of a solid-state imaging device having a desired dynamic range.

Also, read out the maximum transfer charge amount Q _TM of the charge transfer device and design it to be larger than the maximum charge amount Q _IM .
By allowing the overflow drain to function effectively, overflow of excess charge (blooming) can be reliably prevented, and the present invention is also applied to a solid-state imaging device equipped with a photodiode having a blooming prevention drain. be able to.

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.

〔Effect of the invention〕

As described above, according to the solid-state imaging device driving method according to the present invention, the barrier potential at the gate between the photoelectric conversion section and the excess charge discharging means is changed between the photoelectric conversion section and the transfer means when the pulse is applied. Since the barrier potential is set to be lower than the barrier potential in the channel between the two, it is possible to read out the solid-state imaging device with a wide dynamic range without performing complicated control of the excess charge discharging means.

Further, since complicated control of the excess charge discharging means is not necessary, 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 illustrating the method of the present invention;
FIG. 2 is a waveform diagram showing pulses applied to the transfer electrode 6, FIG. 3 is a plan view of the solid-state imaging device, FIG. 4 is a cross-sectional view thereof, and FIG. 5 is a diagram showing the light intensity and readout charge amount in the present invention. FIG. 6 is a graph showing how the knee point is changed; FIG. 7 is a block diagram showing the configuration of an apparatus for realizing the present invention;
FIG. 8 is a sectional view of a solid-state imaging device, FIGS. 9 and 10 are potential distribution diagrams showing the operation of a conventional solid-state imaging device, FIG. 11 is a waveform diagram showing conventionally used control pulses, and FIG. FIG. 13 is a graph showing the relationship between time and accumulated charge amount, 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.

Claims (1)

[Claims] 1. A plurality of photoelectric conversion sections that photoelectrically convert incident light to generate signal charges and have charge storage layers that accumulate the signal charges, and read out the signal charges accumulated in the photoelectric conversion sections. A solid-state imaging device comprising: a charge reading means including an accumulation region for accumulating signal charges; and a transfer means for transferring signal charges accumulated in the accumulation region; and an excess charge discharging means for discharging excess charges generated in the photoelectric conversion section. In the driving method, the width of the pulse that defines the charge readout period applied to the charge transfer means is varied according to the light irradiation intensity, and a barrier is provided at the gate between the photoelectric conversion section and the excess charge discharge means. A method for driving a solid-state imaging device, characterized in that the potential is lower than a barrier potential in a channel between the photoelectric conversion section and the transfer means when the pulse is applied. 2. The method for driving a solid-state imaging device according to claim 1, wherein a barrier potential at a gate between the photoelectric conversion section and the excess charge discharging means is given as a constant potential. 3. The method of driving a solid-state imaging device according to claim 1 or 2, wherein the maximum amount of charge accumulated in the accumulation region is smaller than the maximum amount of charge transferred by the transfer means. 4. According to any one of claims 1 to 3, the excess 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.