JPS6353968A - image sensor - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention (Industrial Field of Application) This invention is a one-dimensional or two-dimensional battery that expands the dynamic range by varying the amount of accumulated B and variably controlling charge accumulation according to the amount of incident light. Regarding dimensional image sensors.

(Prior Art) As a conventional MO3 type image sensor of this type, the one shown in FIGS. 2(a) and 2(b), for example, is known.

In FIG. 2(a), numeral 9 is a P-type substrate, and an N diffusion layer 7 is formed on this P-type substrate 9 to form a photodiode, and a switching MOS is formed by similarly forming an N diffusion layer.
It constitutes the source 8 of the transistor, and the surface of the P-type substrate 9 has S! Further, the source electrode 3 for signal extraction formed on the oxide film 5 is connected to the source 8, and the diffusion layer 7 and the source 8 forming the photodiode on the oxide film 5 are formed. A gate electrode 2 of a switching MOS transistor is formed between the two. 2
0 is a gate terminal, and 30 is a source terminal. Also, the second
In Figure (a), 4 is a light shielding film 4 formed so as to cover the thinning film 5, the gate electrode 2, and the source electrode 3, and this light shielding film 4 has an opening 4A formed therein. Therefore, the incident light is incident only on the photodiode through the aperture 4A.

FIG. 2(b) shows a similar circuit having the structure shown in FIG. 2(a), and the diode shown in FIG. 2(a) is a parallel circuit of a junction diode 11 and a junction capacitor 12. It is indicated by
The switching MOS transistor 13 is configured as a whole.

The conventional example configured in this manner operates as follows.

First, the output terminal 30 is held at a predetermined precharge voltage vp, and the switching MOS transistor 13 is turned on to precharge the junction portion 12 of the photodiode to the precharge voltage Vp. Turn off.

If left in this state, the junction capacitance of the photodiode will decrease by 1
Since the charge accumulated in 2 is discharged according to the amount of incident light,
The charging voltage of the photodiode junction portion 12 decreases from the voltage Vp.

After discharging due to the incident light for a certain period of time, the switching MOS transistor] 3 is turned on, and the amount of discharge of the accumulated charge in the junction portion 12 of the photodiode and the charging voltage V are read out from the output terminal 3.

By repeating the above operations for each pixel, it functions as an image sensor.

(Problem to be solved by this invention) However,
In such conventional image sensors, the amount of incident light is converted into an electrical signal and output to the outside by sequentially reading out the amount of discharge or charge due to incident light of the charge accumulated in the junction capacitor 12 of the photodiode. Therefore, if the accumulated charge is completely discharged by strong incident light, the output will reach a saturated state and a normal output signal will no longer be obtained.

Therefore, in order to obtain an output signal that does not become saturated even with strong incident light, the junction ff1 of the photodiode must be
It is necessary to increase the amount of stored charge by increasing the capacitance (a) of 12.

In order to increase the capacitance of the junction capacitance 12 of the photodiode, there are methods such as increasing the diffusion area of the diffusion layer 7 constituting the photodiode or increasing the impurity concentration of the silicon substrate 9. However, if such a method is used to increase the junction capacitance 12, the dark current flowing through the junction increases and the operating speed of the device decreases. Further, in the method of reading out the charging voltage of the junction capacitor 12, the change in the charging voltage V becomes small with respect to weak incident light, and the sensitivity decreases.

In order to cope with such strong incident light, the junction capacitance is 12
Increasing the dark current increases the operating speed,
In addition, since the sensitivity decreases in the charging voltage readout method,
There was a problem in that the characteristics against weak incident light were sacrificed.

(Means for Solving the Problems) The present invention has been made in view of the above problems, and it is an object of the present invention to provide an image sensor whose dynamic range is expanded by making the storage capacity variable. It is an object.

In order to achieve this object, the present invention provides an image sensor in which a plurality of photodiodes are arranged on a semiconductor substrate, and the amount of discharge or charge of the accumulated charge of the junction capacitor of each photodiode is sequentially read out by incident light. , an electrode for 11 is provided which is connected in parallel to the junction capacitance of the photodiode and interposed between the auxiliary storage capacitance formed by the diffusion layer and both capacitances;
By arbitrarily setting the potential of the control electrode, the storage capacitance is changed to variably control the amount of charge storage with respect to the incident light.

(Function) In the present invention having such a configuration, by providing an auxiliary storage capacitor adjacent to the photodiode and changing the potential of the control electrode, only the junction capacitance of the photodiode is reduced in response to weak incident light. Capacitance can be used to cope with strong incident light, and two large capacitances, a junction capacitance and an auxiliary storage capacitance, can be used to cope with strong incident light. That is,
The storage capacity can be increased or decreased, and the dynamic range can be expanded.

(Example) Embodiments of the present invention will be described below based on the drawings.

1(a) and 1(b) are diagrams showing an embodiment of the present invention.The same components as those of the conventional example are given the same numbers and the explanation thereof will be omitted.

First, to explain the structure, in FIG. 1(a), 6 is a diffusion layer formed by N2 diffusion on a P-type substrate 9, and this diffusion layer 6 is provided adjacent to the diffusion layer 7 of the photodiode. . Further, 1 is a control electrode formed on the oxide film 5, and this control electrode 1 is provided between the diffusion layer 6 and the diffusion layer 7 of the photodiode, and the diffusion layer 6 and the control electrode 1 are different from each other. Switching MOS transistor 1 as a whole
4. Therefore, by arbitrarily setting the potential of the control electrode 1, the storage capacitance can be changed by making both the diffusion layers 6 and 7 freely disconnectable.

Although a photodiode is constituted by the diffusion layer 6 and the P-type substrate 9, only the junction capacitance functions by blocking light with the light-shielding film 4. 10 is a control terminal connected to the control electrode 1.

FIG. 1(b) shows an equal circuit having the structure shown in FIG. 1(a), in which the diffusion layer 6 is shown as an auxiliary storage capacitor 15,
This auxiliary storage capacitor 15 is connected in parallel to a parallel circuit consisting of a junction diode 11 and a junction capacitor 12.

Next, the operation will be explained.

First, a first case in which the potential of the control electrode 1 is set to O will be explained.

In this case, since the switching MOS transistor 14 is turned off, the auxiliary storage capacitor 15 is not always connected to the photodiode. In other words, the capacitance for storing charge is the junction capacitance 12 of the photodiode.
The structure is the same as the conventional one. Expose this ff
To explain based on FIG. 3 showing the relationship between 1E and charging voltage V, if the potential of control electrode 1 is O in the figure, a
The discharge characteristics are shown by the broken line. Precharge voltage Vp
As the exposure amount E increases, the charging voltage decreases, and when the exposure amount E reaches Ea, the charging voltage becomes V +,t O and the output signal becomes saturated. In this way, when only the junction capacitance of the photodiode is used, the storage capacity (2) becomes small, so the sensitivity becomes high, but the saturation exposure amount EC becomes small.

Next, a second case in which the potential of the control electrode 1 is set to Vp will be described. The MOS transistor 14 is always on, and the auxiliary storage capacitor 15 is always connected to the photodiode. Therefore, the capacitance for storing charges is the sum of the junction capacitance 12 of the photodiode and the auxiliary storage capacitor 15. The discharge characteristics in this state are as shown by the broken line C in FIG. By increasing the storage capacity in this way, the saturation exposure ff1EC becomes higher, but the sensitivity becomes lower.

Furthermore, a third case will be explained in which the potential of the control electrode 1 is set to Vx between ○ and Vp.
4 is turned on and the auxiliary storage capacitor 15 is connected to the photodiode, so that the junction capacitance 12 between the auxiliary storage capacitor 15 and the photodiode is charged to Vx. However, when the charging voltage exceeds Vx, the MOS transistor 14 is turned off, the auxiliary storage capacitor 15 is not connected to the photodiode, and only the junction capacitance 12 of the photodiode is charged to vp.

In this way, the charging voltage is Vx for the auxiliary storage capacitor 15 and Vp for the junction capacitance 12 of the diode.

After the precharging is completed, a discharge due to the incident light is started, and the discharge characteristic of the junction capacitance 12 of the photodiode becomes the characteristic shown by b in FIG. 3.

That is, if the exposure amount at which the charging voltage becomes VX is EX, the MOS transistor 14 is off and the auxiliary storage capacitor 15 is not connected until the exposure amount reaches EX. The discharge characteristics are the same as in the case of . However, when the exposure amount exceeds EX and the charging voltage of the photodiode junction capacitor 12 becomes below VX, the MOS transistor 14 is turned on and the auxiliary storage capacitor 15 is connected, so that the slope of the discharge characteristic becomes second. It will be the same as in the case of Therefore, storage capacity is automatically switched according to the amount of exposure.

As described above, by providing the auxiliary storage capacitor 15 and its single control electrode 1 and changing the potential of the control electrode 1, the storage capacitance can be decreased or increased, and as a result, the dynamic range can be expanded. Become.

Although the embodiments have been explained using an MO3 type image sensor as an example, the present invention is not limited to this, and for example, COD
This method can also be applied to image sensors, in which case the amount of charge of the junction capacitance of each photodiode due to incident light is sequentially read out.

The present invention can be applied not only to one-dimensional image sensors but also to two-dimensional image sensors.

In the case of a one-dimensional image sensor, if an auxiliary storage capacitor is provided in a direction perpendicular to the direction of pixel parallelism/υ,
It can be configured without reducing pixel density.

(Effects of the Invention) As explained above, according to the present invention, the storage capacitance can be arbitrarily selected according to the amount of incident light, so the storage capacitance can be made small for weak incident light and the dark current can be reduced. By increasing the storage capacity and increasing the saturation exposure amount for strong incident 0=1 light, sacrifice the 1"jt property for weak incident q4 light. Therefore, the dynamic range can be expanded.

Furthermore, if applied to an image sensor that reads the charging voltage of a junction capacitance, it can provide excellent characteristics such as high sensitivity to weak incident light and high saturation exposure amount to strong incident light.

[Brief explanation of drawings]

FIG. 1 (a> is a cross-sectional structural diagram showing an embodiment of the present invention,
Fig. 1 (b> is an equivalent circuit diagram thereof, Fig. 2 (a> is a sectional structural diagram showing a conventional example, Fig. 2 (b) is an equivalent circuit diagram thereof, and Fig. 3 t is an embodiment of the present invention. It is an explanatory diagram for explaining the operation. 1... Control electrode, 2... Gate electrode of switching MOS transistor 3... Source electrode, 4... Light shielding film, 5... Silicon oxide film , 6... Diffusion layer of auxiliary storage capacitor, 7... Diffusion layer of photodiode, 8... Source of switching MO3 transistor, 9
... Silicon substrate, 10 ... Control terminal, 11 ... Junction diode, 12 ... Junction capacitance of photodiode, 13 ... Switching MO3) - transistor, 14 ... MOS transistor, 15: Auxiliary Storage capacity.

Claims (1)

[Claims] In an image sensor in which a plurality of photodiodes are arranged on a semiconductor substrate and the amount of charge accumulated in the junction capacitance of each photodiode is sequentially read out due to incident light, the amount of charge accumulated in the junction capacitance of each photodiode is sequentially read out. A layer-based auxiliary storage capacitor is provided, a switching element is provided between the two capacitors, and the switching element enables the auxiliary storage capacitor to be connected in parallel to the junction capacitance of the photodiode, thereby variably controlling the amount of charge storage with respect to the amount of incident light. An image sensor characterized by: