RELATED PATENT APPLICATION

This application is based on and claims the benefit of priority International Application No. PCT/JP2018/024892, filed on Jun. 29, 2018, which claims priority to Japanese Patent Application No. 2017-133466, filed on Jul. 7, 2017, the contents of which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a solid-state imaging device, a method for driving a solid-state imaging device, and an electronic apparatus.

BACKGROUND

Solid-state imaging devices (image sensors) including photoelectric conversion elements for detecting light and generating charges are embodied as CMOS (complementary metal oxide semiconductor) image sensors, which have been in practical use. The CMOS image sensors have been widely applied as parts of various types of electronic apparatuses such as digital cameras, video cameras, surveillance cameras, medical endoscopes, personal computers (PCs), mobile phones and other portable terminals (mobile devices).

A CMOS image sensor includes, for each pixel, a photodiode (a photoelectric conversion element) and a floating diffusion (FD) amplifier having a floating diffusion (FD). The mainstream design of the reading operation in CMOS image sensors is a column parallel output processing performed by selecting a row in a pixel array and reading the pixels simultaneously in the column direction.

To improve characteristics, various methods have been proposed for fabricating a CMOS image sensor that has a wide dynamic range and provides a high picture quality (see, for example, Patent Literature 1).

Patent Literature 1 discloses a dynamic range widening technology that involves dividing the exposure period into two or more exposure sessions of different durations, including short-exposure imaging for high illuminance and long-exposure imaging for low illuminance. Patent Literature 1 also discloses a dynamic range widening technology that involves employing a floating diffusion FD with a variable capacitance.

According to the dynamic range widening technology disclosed in Patent Literature 1, the imaging for low illuminance and the imaging for high illuminance are performed at different timings (during different periods), and this means that the signals obtained as a result of multiple exposure sessions are used. This disadvantageously creates defects in the images, causes moving object distortions and thus impair the quality of the moving image.

To address this issue, a solid-state imaging device has been proposed that has, in each pixel, two different photodiodes (PDs) with different responsivities so that two pieces of image data having different responsivities are obtained (see, for example, Non-Patent Literature 1).

FIG. 1shows a configuration of a pixel of a CMOS image sensor disclosed in Non-Patent Literature 1.FIG. 2is a timing chart including parts (A) to (E) to illustrate a reading operation for the pixel inFIG. 1.

The pixel shown inFIG. 1includes a small photodiode (photoelectric converting element) SPD having a small responsivity and a small well capacity and a large photodiode (LPD) having a large responsivity and a large well capacity. For the small photodiode SPD, a small transfer transistor TGS and a small floating diffusion FDS are provided. For the large photodiode LPD, a large transfer transistor TGL and a large floating diffusion FDL are provided. The small floating diffusion FDS and the large floating diffusion FDL are connected to each other via a connection switching transistor TDFD. Between the small floating diffusion FDS and a reset potential vrfd, a reset transistor TRST is connected. A source follower transistor TSF and a selection transistor TSEL are connected in series between a power supply line VDD and a vertical signal line vpix, and the gate of the source follower transistor TSF is connected to the large floating diffusion FDL.

In the pixel shown inFIG. 1, a low conversion gain (LCG) can be obtained by bringing the connection switching transistor TDFD into the conduction state. This increases the equivalent capacitance of the gate of the source follower transistor TSF. A high conversion gain (HCG) can be obtained by bringing the connection switching transistor TDFD into the non-conduction state. According to the pixel shown inFIG. 1, the large photodiode LPD can be used for both of the low conversion gain (LCG) reading and the high conversion gain (HCG) reading, and the small photodiode SPD can be used only for the low conversion gain (LCG) reading.

RELEVANT REFERENCES

List of Relevant Patent Literature

Patent Literature 1: Japanese Patent Application Publication No. 2000-165754.

List of Relevant Non-Patent Literature

Non-Patent Literature 1: T. Willassen, et al., “A 1280×1080 4.2Ξm Split-Diode Pixel HDR Sensor in 110 nm BSI CMOS Process” International Image Sensor Workshop (IISW), 8-11 Jun. 2015, Vaals, Netherlands.

SUMMARY

The dynamic range widening technology disclosed in Non-Patent Literature 1 has the following problems. When a dark signal (a signal obtained when the illuminance is low) is read out using the photodiode LPD having a large responsivity, the read-out noise deteriorates since the floating diffusion FD has a large capacitance. The floating diffusion FD, however, needs to have a large capacitance in order to read all the signals. The read-out noise can be reduced by reducing the capacitance of the floating diffusion FD for the large photodiode LPD. This, however, deteriorates the SNR gap for the reading of the other photodiode PD.

The present invention is designed to provide a solid-state imaging device, a method for driving a solid-state imaging device, and an electronic apparatus that are capable of realizing a widened dynamic range, preventing the read-out noise from affecting the performance, and achieving improved image quality.

A first aspect of the present invention provides a solid-state imaging device including a pixel part having a pixel arranged therein. The pixel has at least one first photoelectric converting part for storing therein, in a storing period, charges generated by photoelectric conversion, at least one second photoelectric converting part for storing therein, in a storing period, charges generated by photoelectric conversion, at least one first transfer element for transferring, in a designated transfer period, the charges stored in the first photoelectric converting part, at least one second transfer element for transferring, in a designated transfer period, the charges stored in the second photoelectric converting part, a floating diffusion to which the charges stored in at least one selected from the group consisting of the first photoelectric converting part and the second photoelectric converting part are transferred through at least one selected from the group consisting of the first transfer element and the second transfer element, a source follower element for converting the charges in the floating diffusion into a voltage signal with a gain determined by the quantity of the charges, and a capacitance changing part for changing a capacitance of the floating diffusion depending on a capacitance changing signal. Here, the first photoelectric converting part has a first well capacity and a first responsivity, the second photoelectric converting part has a second well capacity and a second responsivity, and the second well capacity and the second responsivity are respectively different from the first well capacity and the first responsivity.

A second aspect of the present invention provides a method for driving a solid-state imaging device including a pixel part having a pixel arranged therein. The pixel has at least one first photoelectric converting part for storing therein, in a storing period, charges generated by photoelectric conversion, at least one second photoelectric converting part for storing therein, in a storing period, charges generated by photoelectric conversion, at least one first transfer element for transferring, in a designated transfer period, the charges stored in the first photoelectric converting part, at least one second transfer element for transferring, in a designated transfer period, the charges stored in the second photoelectric converting part, a floating diffusion to which the charges stored in at least one selected from the group consisting of the first photoelectric converting part and the second photoelectric converting part are transferred through at least one selected from the group consisting of the first transfer element and the second transfer element, a source follower element for converting the charges in the floating diffusion into a voltage signal with a gain determined by the quantity of the charges, a reset element for discharging, in a reset period, the charges from the floating diffusion, and a capacitance changing part for changing the capacitance of the floating diffusion depending on a capacitance changing signal. Here, the first photoelectric converting part has a first well capacity and a first responsivity, the second photoelectric converting part has a second well capacity and a second responsivity, and the second well capacity and the second responsivity are respectively different from the first well capacity and the first responsivity. Here, in a read-out scanning period including reading, in a reading period following a reset period in which the floating diffusion is reset through the reset element, a signal in a reset state, and reading, in a reading period following a transfer period in which the charges stored in the first photoelectric converting part or the second photoelectric converting part are transferred to the floating diffusion through the first transfer element or the second transfer element after the reading period following the reset period, a signal determined by the stored charges, where the first photoelectric converting part has a first well capacity and a first responsivity and the second photoelectric converting part has a second well capacity and a second responsivity, in a single reading period, at least one selected from the group consisting of first conversion gain mode reading of reading the pixel signal corresponding to the charges stored in the first photoelectric converting part, with a first conversion gain corresponding to a first capacitance set by the capacitance changing part and second conversion gain mode reading of reading the pixel signal corresponding to the charges stored in the second photoelectric converting part, with a second conversion gain corresponding to a second capacitance set by the capacitance changing part is performed.

A third aspect of the present invention provides an electronic apparatus including a solid-state imaging device, and an optical system for forming a subject image on the solid-state imaging device. The solid-state imaging device includes a pixel part having a pixel arranged therein, and the pixel has at least one first photoelectric converting part for storing therein, in a storing period, charges generated by photoelectric conversion, at least one second photoelectric converting part for storing therein, in a storing period, charges generated by photoelectric conversion, at least one first transfer element for transferring, in a designated transfer period, the charges stored in the first photoelectric converting part, at least one second transfer element for transferring, in a designated transfer period, the charges stored in the second photoelectric converting part, a floating diffusion to which the charges stored in at least one selected from the group consisting of the first photoelectric converting part and the second photoelectric converting part are transferred through at least one selected from the group consisting of the first transfer element and the second transfer element, a source follower element for converting the charges in the floating diffusion into a voltage signal with a gain determined by the quantity of the charges, and a capacitance changing part for changing the capacitance of the floating diffusion depending on a capacitance changing signal. Here, the first photoelectric converting part has a first well capacity and a first responsivity, the second photoelectric converting part has a second well capacity and a second responsivity, and the second well capacity and the second responsivity are respectively different from the first well capacity and the first responsivity.

Advantages

The present invention is capable of realizing a widened dynamic range, preventing the read-out noise from affecting the performance, and achieving improved image quality.

