JP2799015B2 - Driving method of solid-state imaging device - Google Patents

Description

The present invention relates to a solid-state imaging device used in, for example, an electronic still camera or a video camera, and particularly to an improvement in a driving method thereof.

(Prior Art) As is well known, for example, in an electronic still camera or a video camera, in order to convert an optical image of a subject into an electric television signal, for example, a CCD (Charge Coupled Device) Solid-state imaging devices such as those described above have been used. The solid-state imaging device is generally provided with a plurality of photodiodes arranged in a plane in the horizontal and vertical directions, and installed corresponding to each of the plurality of photodiodes. A plurality of vertical transfer stages capable of transferring charges in the directions, a horizontal transfer stage to which the signal charges transferred vertically by the plurality of vertical transfer stages are supplied, and a signal charge transferred to the horizontal transfer stages. And an output amplifying unit for amplifying and outputting.

Here, a case is considered in which the charge transfer from the photodiode to the vertical transfer stage is an incomplete transfer type solid-state imaging device and the electronic shutter operation is performed. The electronic shutter operation is a means for improving the resolution for a moving subject by setting the signal charge accumulation time for the photodiode to a time shorter than the field rate (1 / 59.94 sec in the case of the NTSC system) (for example, 1/1000 sec). This is realized by sweeping out unnecessary charges in the photodiode just before the signal reading by the exposure time. This signal reading and unnecessary charge sweeping are performed by applying a field shift pulse for signal reading and charge sweeping to the transfer gate electrode.

By the way, in a solid-state imaging device in which charge transfer from the photodiode to the vertical transfer stage is incomplete, when the electronic shutter operation is performed, as shown in FIG. 10, when the signal amount is small, the incident light amount and the output signal The problem that the linearity becomes worse. The cause is that the existence probability of the energy level of the electrons stored in the photodiode follows the Fermi-Dirac distribution which is in a thermal equilibrium state, so that the number of electrons existing in the pixel and the number of read signal electrons are different. This is because a linear relationship is not established.

That is, the Fermi-Dirac distribution P _FD (V, V _FL ) according to the probability of existence of electrons in the photodiode is P _FD (V, V _FL ) = 1 / [1 + exp {e (V-V _FL ) / kT}]. Given by Here, V _FL is the Fermi level of the electrons in the photodiode, V is the energy level that the electrons can have, e is the electron charge (1.602 × 10 ⁻¹⁹ C), and k is the Boltzmann constant ( 1.3804 × 10 ^-23 J / K)
T is the absolute temperature. Then, charge transfer from the photodiode to the vertical transfer stage (so-called field shift)
Is at an energy level higher than the potential of the transfer gate TG by lowering the potential of the transfer gate TG provided between the photodiode PD and the vertical transfer stage V-CCD, as shown in FIG. Electrons (indicated by oblique lines in FIG. 11) are transferred to the vertical transfer stage V-CCD and performed.

Therefore, the vertical transfer stage V-CC
Charge is transferred to the D Q _R is the potential of the transfer gate TG and V _TG, twice the level density of the photodiode PD g
given that, Given by Doing this _{integration, Q R = (egkT / e} ) · ln [1 + exp {(V FL -V TG) / V T}] become. However, V _T is a thermal voltage, a kT / e. And
The Fermi level (V _FL −V
_TG ) / V _T Is shown in FIG. Photodiode PD
The charge transfer in the vertical transfer stages V-CCD from the Fermi level in the photodiode PD decreases by Q _R / eg. For this reason, the relationship between the Fermi levels before and after the field shift is shown in Fig. 12 by drawing the vertical line corresponding to the Fermi level before the field shift, drawing the line lowered at an angle of 45 degrees from the intersection with the curve, and the horizontal axis. Is the Fermi level after the field shift.

After the unnecessary charges are swept out by the unnecessary charge sweeping field shift pulse, the charges in the photodiode PD increase due to the incident light within the electronic shutter exposure time. Thereafter, the signal charge is read out from the photodiode PD, and the signal charge amount E read out at this time is larger than the charge amount D generated during the exposure time of the electronic shutter as shown in FIG. Has become. Then, the relationship between the amount of incident light and the amount of signal charge obtained in this way is as shown in FIG. 10, and the linearity is impaired at the time of a small signal.

Further, as shown in FIG. 13 (a), a large amount of residual charges corresponding to the signal charge amount are present in the photodiode PD after the unnecessary charges have been swept out, and are indicated by oblique lines in FIG. 13 (b). Thus, there is also a disadvantage that an afterimage occurs.

(Problems to be Solved by the Invention) As described above, in the driving method of the conventional solid-state imaging device,
When an electronic shutter operation is performed by a non-perfect transfer type solid-state imaging device, the amount of read signal charge is larger than the amount of charge generated during the exposure time due to the relationship between the Fermi level before and after the field shift at the time of a small signal. As a result, the relationship between the output signal and the amount of incident light is not linear, and many afterimages are generated.

Therefore, the present invention has been made in view of the above circumstances, and when an electronic shutter operation is performed by a non-perfect transfer type solid-state imaging device, the relationship between an incident light amount and an output signal at the time of a small signal becomes non-linear. It is an object of the present invention to provide a very good driving method of a solid-state imaging device that can prevent the occurrence of afterimages and reduce afterimages.

[Structure of the Invention] (Means for Solving the Problems) In a method for driving a solid-state imaging device according to the present invention, a charge is discharged to a photoelectric conversion element, and after a predetermined exposure time to the photoelectric conversion element has elapsed. And a device for reading out electric charges from a photoelectric conversion element. Then, a predetermined amount of charge is injected into the photoelectric conversion element before the charge is discharged, and the potential of the transfer gate for taking out the charge from the photoelectric conversion element is set at the time of reading the charge from the photoelectric conversion element. Is also set low.

(Operation) According to the above-described method, a predetermined amount of electric charge is injected into the photoelectric conversion element before the electric charge is discharged, the electric charge in the photoelectric conversion element after the electric charge is discharged is made constant, and the potential of the transfer gate is reduced. Is set to be lower than that at the time of sweeping out the charges, and by reading out the charges from the photoelectric conversion element, the charge in the photoelectric conversion element after reading out the charges can be kept constant. Can be prevented from becoming non-linear and the afterimage can be reduced.

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. That is, as shown in FIG. 1, before applying the field shift pulses FS1 and FS2 for sweeping out unnecessary charges and reading out signal charges to the transfer gate TG, a pulse DT1 for charge injection is generated and the photodiode PD is generated. In addition to injecting a predetermined amount of charge, the level of the field shift pulse FS2 for reading out signal charges is increased by ΔV _FS with respect to the level of the field shift pulse FS1 for sweeping out unnecessary charges. The difference from the related art is that the potential of the gate TG is set lower than that at the time of sweeping out unnecessary charges so that charges in the photodiode PD are read out. FIG. 2 shows the relationship between the Fermi level of the charge in the photodiode PD and the amount of charge transferred from the photodiode PD when sweeping out unnecessary charges and when reading out signal charges.

Here, assuming that the Fermi level of the charge in the photodiode is increased by ΔVinj by performing charge injection prior to the sweeping of the unnecessary charge, the Fermi level V _S of the charge in the photodiode after the charge sweeping is the amount of the unnecessary charge. Δ
It is a function of V _Q and if ΔV _FS is large enough (the Fermi level does not change after signal reading), Becomes Here, if (ΔVinj−ΔV _FS + ΔV _Q )> 2V _T, it is possible to approximate ln (1 + X) = X with an error within 6.3%, so that V _S (ΔV _Q ) = − V _T × exp ｛− ( ΔVinj−ΔV _FS + ΔV _Q ) / V _T ＝= − V _T × exp ｛− (ΔVinj−ΔV _FS ) / V _T ｝ × exp ｛− (ΔV _Q / V _T )｝ The Fermi level of the charge in the photodiode PD can be approximated. Thereafter, signal charges are accumulated, and the Fermi level of the charges in the photodiode PD increases.

Next, at the time of reading signal charges from the photodiode PD, the potential of the transfer gate TG is set to be lower by ΔV _FS to read charges. The read signal charge amount is converted into the capacitance C _PD (= e
The value V _RO divided by g) is ΔV _FS > 2V _T, and ΔVsig is the increment of the Fermi level of the charge accumulated in the photodiode PD. Becomes In the above equation, ΔVsig is a signal amount, and ΔV _FS
Since is a constant, the term including those [Delta] Vsig, [Delta] V _FS is not a non-linear. Therefore, what degrades the linearity is the remaining term including exp (−ΔVsig / V _T ) exp (−ΔV _Q / V _T ). Since these terms are multiplied by exp (−ΔV _FS / V _T ) exp ｛− (ΔVinj−ΔV _FS ) / V _T非, by increasing ΔV _FS or ΔVinj−ΔV _FS , Ingredients can be reduced.

FIG. 3 shows the effect of improving the nonlinearity according to the above embodiment. The relationship between the amount of incident light and the amount of output signal when an electronic shutter operation is performed at a field frequency of 59.94 Hz and 1/1000 sec is performed. , ΔV _FS = ΔVinj−ΔV _FS = n × V _T (n = 0, 1, 2, 3, 4,...). As is apparent from Figure _{3, ΔV FS = ΔVinj-ΔV FS} > nonlinearity in 3V _T It can be seen that hardly. At room temperature (27 ° C.), V _T = 25.6 mV, so that if ΔV _FS = ΔVinj−ΔV _FS > 80 mV, it can be said that the linearity hardly deteriorates.

Therefore, according to the above-described embodiment, the bias charge is injected into the photodiode PD before the unnecessary charge is swept out, and the signal charge at the level deeper than that at the time of the sweeping is read out when reading out the signal charge. When the operation is performed, it is possible to prevent the relationship between the amount of incident light and the output signal from becoming non-linear at the time of a small signal.

Here, the lowest level of the charges transferred from the photodiode PD can be realized because the potential under the transfer gate TG electrode can be controlled by changing the pulse voltage applied to the transfer gate TG as shown in FIG. . The method of injecting the bias charge into the photodiode PD includes an electric method and an optical method. Therefore, an example of the electric injection method will be described. The solid-state imaging device is provided with an overflow drain (OFD) as shown in FIG. 5 so that an excessive current does not flow from the photodiode PD to the signal transmission line even when strong light enters.
Normally, a low potential V _OFD is _applied to the overflow drain to allow excess current to flow. When a high-potential pulse is applied to this drain, charges can be injected as shown in FIG.
In the optical injection method, as shown in FIG. 7, a bias current can be applied by applying a pulse current to the light emitting diode LED and irradiating the photodiode PD with a light pulse. In the case of a photoconductive film stacked type image sensor as shown in FIG. 8, it can be realized by applying a pulse to the transparent electrode (ITO film).

Further, according to the method of the above embodiment, the signal amount dependency of the residual charge amount in the photodiode PD after the unnecessary charges have been swept out is reduced, so that an effect of reducing the afterimage is also produced. In FIG. 9, the residual image amount when the bias charge of 2C _PD V _T is injected and the field shift pulses FS1 and FS2 are offset by V _T is shown by oblique lines. Thus, the thirteenth first
It can be seen that the afterimage is significantly reduced as compared with the one shown in the figure. Further, when the bias charge amount and the offset amount of the field shift pulse are increased, the afterimage can be reduced exponentially.

It should be noted that the present invention is not limited to the above-described embodiment, and can be implemented with various modifications without departing from the scope of the invention.

[Effects of the Invention] As described in detail above, according to the present invention, when an electronic shutter operation is performed by a non-perfect transfer type solid-state imaging device,
It is possible to provide a very good method of driving a solid-state imaging device that can prevent the relationship between the amount of incident light and the output signal from becoming non-linear at the time of a small signal and can reduce afterimages.

[Brief description of the drawings]

FIG. 1 is a timing chart showing an embodiment of a method for driving a solid-state imaging device according to the present invention, FIG. 2 is a diagram for explaining the charge transfer amount of the embodiment, and FIG. FIG. 4 is a diagram for explaining the charge transfer of the embodiment, FIGS. 5 to 8 are diagrams for explaining a bias charge injection method, and FIGS. FIG. 10 is a diagram for explaining reduction of an afterimage, FIG. 10 is a diagram for explaining non-linearity at the time of an electronic shutter operation according to a conventional driving method,
FIG. 11 is a diagram for explaining charge transfer from a photodiode to a vertical transfer stage, FIG. 12 is a diagram for explaining generation of nonlinearity in a conventional driving method, and FIG. 13 is a diagram for explaining a conventional driving method. FIG. 6 is a diagram for explaining the occurrence of an afterimage in FIG.

Claims (1)

(57) [Claims] 1. A method for driving a solid-state imaging device, comprising: discharging electric charges from a photoelectric conversion element; and reading charges from the photoelectric conversion element after a predetermined exposure time has elapsed. A predetermined amount of charge is injected into the photoelectric conversion element before, and at the time of reading out the charge from the photoelectric conversion element, the potential of the transfer gate for taking out the charge from the photoelectric conversion element is set to:
A method for driving a solid-state imaging device, wherein the driving speed is set lower than at the time of discharging electric charges.