LIGHT DETECTION DEVICE AND METHOD FOR DRIVING PHOTOSENSOR

In a light detection device, a control unit performs a first charge transfer process for transferring charge generated in a charge generation region to a charge storage region by applying an electric potential to a transfer gate electrode so that a potential energy of a region immediately below the transfer gate electrode is lower than a potential energy of the charge generation region and a first read process for reading an amount of charge stored in the charge storage region. In the first charge transfer process, the control unit applies an electric potential to an overflow gate electrode so that a potential energy of a region immediately below the overflow gate electrode is lower than the potential energy of the charge generation region.

TECHNICAL FIELD

An aspect of the present disclosure relates to a light detection device including a photosensor and a method for driving a photosensor.

BACKGROUND ART

Patent Literature 1 describes a photosensor including a photodiode for generating charge in response to incident light, a floating region for storing the charge from the photodiode, and a storage capacity element for storing the charge overflowing from the floating region.

CITATION LIST

Patent Literature

Patent Literature 1: International Publication WO 2005/083790

SUMMARY OF INVENTION

Technical Problem

In the photosensor described in Patent Literature 1, the charge overflowing from the floating region can be stored in the storage capacity element. However, when the charge is stored in the floating region to the extent that the charge overflows to the storage capacity element, a part of the charge remains in the photodiode. In this case, the light detection accuracy may be reduced due to the charge remaining in the photodiode. In particular, if the above situation occurs when the photosensor has a gating function of detecting only the light that arrives at a predetermined timing instead of detecting light over the entire period, the charge remaining in the photodiode in one period is read as charge generated in another period, and accordingly, it may not be possible to accurately detect the amount of charge generated at a predetermined timing.

It is an object of an aspect of the present disclosure to provide a light detection device and a method for driving a photosensor that can improve the detection accuracy.

Solution to Problem

A light detection device according to an aspect of the present disclosure includes: a photosensor; and a control unit that controls the photosensor. The photosensor includes a charge generation region that generates charge in response to incident light, a charge storage region, an overflow region, a transfer gate electrode arranged on a region between the charge generation region and the charge storage region, and an overflow gate electrode arranged on a region between the charge storage region and the overflow region. The control unit performs a first charge transfer process, which is for transferring charge generated in the charge generation region to the charge storage region by applying an electric potential to the transfer gate electrode so that a potential energy of a region immediately below the transfer gate electrode is lower than a potential energy of the charge generation region, and a first read process, which is for reading an amount of charge stored in the charge storage region after the first charge transfer process. In the first charge transfer process, an electric potential is applied to the overflow gate electrode so that a potential energy of a region immediately below the overflow gate electrode is lower than the potential energy of the charge generation region.

In the light detection device, the photosensor includes the overflow region and the overflow gate electrode disposed on the region between the charge storage region and the overflow region. Therefore, since the charge overflowing from the charge storage region can be stored in the overflow region, it is possible to suppress the saturation of the storage capacity. In addition, during the execution of the first charge transfer process for transferring the charge generated in the charge generation region to the charge storage region, the potential energy of the region immediately below the overflow gate electrode is lower than the potential energy of the charge generation region. Therefore, even when the charge is stored in the charge storage region to the extent that the charge overflows into the overflow region, it is possible to suppress the charge from remaining in the charge generation region. Therefore, according to the light detection device, it is possible to improve the detection accuracy.

The control unit may perform the first read process after performing the first charge transfer process multiple times. In this case, it is possible to improve the S/N ratio.

The charge generation region may include an avalanche multiplication region. In this case, since the avalanche multiplication can be caused in the charge generation region, it is possible to increase the detection sensitivity of the photosensor. On the other hand, when the avalanche multiplication region is included in the charge generation region, the amount of charge generated is extremely large. In the light detection device, even in such a case, it is possible to sufficiently suppress the saturation of the storage capacity, and it is possible to sufficiently suppress the charge from remaining in the charge generation region.

The control unit may perform a second charge transfer process, which is for transferring the charge stored in the charge storage region to the overflow region by applying an electric potential to the overflow gate electrode so that the potential energy of the region immediately below the overflow gate electrode is reduced after the first read process, and a second read process, which is for reading a total amount of charge stored in the charge storage region and the overflow region after the second charge transfer process. In this case, not only is the amount of charge stored in the charge storage region read in the first read process, but also the total amount of charge stored in the charge storage region and the overflow region is read in the second read process. As a result, it is possible to improve the charge amount detection accuracy.

The photosensor may further include an unnecessary charge discharge region and an unnecessary charge transfer gate electrode arranged on a region between the charge generation region and the unnecessary charge discharge region. The control unit may perform an unnecessary charge transfer process for transferring the charge generated in the charge generation region to the unnecessary charge discharge region by applying an electric potential to the unnecessary charge transfer gate electrode so that a potential energy of a region immediately below the unnecessary charge transfer gate electrode is lower than the potential energy of the charge generation region in a period other than a period during which the first charge transfer process is performed. In this case, since the charge generated in the charge generation region can be transferred to the unnecessary charge discharge region in a period other than the period during which the first charge transfer process is performed, it is possible to further suppress the charge from remaining in the charge generation region.

The light detection device according to an aspect of the present disclosure may further include a light source that emits detection light, and the control unit may perform the first charge transfer process in a period during which reflected light of the detection light on an object is incident on the charge generation region. In this case, it is possible to accurately detect the amount of charge generated in the charge generation region in a period during which the reflected light of the detection light on the object is incident on the charge generation region.

The light detection device according to an aspect of the present disclosure may further include a photogate electrode arranged on the charge generation region. In the first charge transfer process, the control unit may apply an electric potential to the photogate electrode and the overflow gate electrode so that the potential energy of the region immediately below the transfer gate electrode is lower than the potential energy of the charge generation region and the potential energy of the region immediately below the overflow gate electrode is lower than the potential energy of the charge generation region. In this case, it is possible to accurately adjust the magnitude of the potential energy.

The overflow region may have a charge storage capacity larger than a charge storage capacity of the charge storage region. In this case, it is possible to effectively suppress the saturation of the storage capacity.

A method for driving a photosensor according to an aspect of the present disclosure is a method for driving a photosensor. The photosensor includes a charge generation region that generates charge in response to incident light, a charge storage region, an overflow region, a transfer gate electrode arranged on a region between the charge generation region and the charge storage region, and an overflow gate electrode arranged on a region between the charge storage region and the overflow region. The method for driving the photosensor includes: a charge transfer step for transferring charge generated in the charge generation region to the charge storage region by applying an electric potential to the transfer gate electrode so that a potential energy of a region immediately below the transfer gate electrode is lower than a potential energy of the charge generation region; and a read step for reading an amount of charge stored in the charge storage region after the charge transfer step. In the charge transfer step, an electric potential is applied to the overflow gate electrode so that a potential energy of a region immediately below the overflow gate electrode is lower than the potential energy of the charge generation region.

In the method for driving the photosensor, the photosensor includes the overflow region and the overflow gate electrode disposed on the region between the charge storage region and the overflow region. Therefore, since the charge overflowing from the charge storage region can be stored in the overflow region, it is possible to suppress the saturation of the storage capacity. In addition, during the execution of the charge transfer step for transferring the charge generated in the charge generation region to the charge storage region, the potential energy of the region immediately below the overflow gate electrode is lower than the potential energy of the charge generation region. Therefore, even when the charge is stored in the charge storage region to the extent that the charge overflows into the overflow region, it is possible to suppress the charge from remaining in the charge generation region. Therefore, according to the method for driving the photosensor, it is possible to improve the detection accuracy.

Advantageous Effects of Invention

According to an aspect of the present disclosure, it is possible to provide a light detection device and a method for driving a photosensor capable of improving the detection accuracy.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of the present disclosure will be described in detail with reference to the diagrams. In addition, in the following description, the same or equivalent elements are denoted by the same reference numerals, and repeated description thereof will be omitted.

First Embodiment

Configuration of Distance Measurement Device

As shown inFIG.1, a distance measurement device (light detection device)1includes a light source2, a distance measurement sensor (distance measurement image sensor, photosensor)10A, a signal processing unit3, a control unit4, and a display unit5. The distance measurement device1is a device that acquires a distance image of an object OJ (an image including information regarding a distance d to the object OJ) by using an indirect TOF method.

The light source2emits pulsed light (detection light) L. The light source2is formed by, for example, an infrared LED. The pulsed light L is, for example, near-infrared light, and the frequency of the pulsed light L is, for example, 10 kHz or higher. The distance measurement sensor10A detects the pulsed light L that is emitted from the light source2and reflected by the object OJ. The distance measurement sensor10A is configured by monolithically forming a pixel unit11and a CMOS read circuit unit12on a semiconductor substrate (for example, a silicon substrate). The distance measurement sensor10A is mounted on the signal processing unit3.

The signal processing unit3controls the pixel unit11and the CMOS read circuit unit12of the distance measurement sensor10A. The signal processing unit3performs predetermined processing on the signal output from the distance measurement sensor10A to generate a detection signal. The control unit4controls the light source2and the signal processing unit3. The control unit4generates a distance image of the object OJ based on the detection signal output from the signal processing unit3. The display unit5displays the distance image of the object OJ generated by the control unit4.

Configuration of Distance Measurement Sensor

As shown inFIGS.2and3, the distance measurement sensor10A includes a semiconductor layer20and an electrode layer40in the pixel unit11. The semiconductor layer20has a first surface20aand a second surface20b. The first surface20ais a surface on one side of the semiconductor layer20in the thickness direction. The second surface20bis a surface on the other side of the semiconductor layer20in the thickness direction. The electrode layer40is provided on the first surface20aof the semiconductor layer20. The semiconductor layer20and the electrode layer40form a plurality of pixels11aarranged along the first surface20a. In the distance measurement sensor10A, the plurality of pixels11aare arranged in a two-dimensional manner along the first surface20a. Hereinafter, the thickness direction of the semiconductor layer20is referred to as a Z direction, one direction perpendicular to the Z direction is referred to as an X direction, and a direction perpendicular to both the Z direction and the X direction is referred to as a Y direction. In addition, one side in the Z direction is referred to as a first side, and the other side in the Z direction (side opposite to the first side) is referred to as a second side. In addition, inFIG.2, the arrangement of charge storage regions P1to P4, overflow regions Q1to Q4, an unnecessary charge discharge region R, a photogate electrode PG, transfer gate electrodes TX1to TX4, overflow gate electrodes OV1to OV4, and an unnecessary charge transfer gate electrode RG, which will be described later, is schematically shown, and other elements are omitted as appropriate.

In the semiconductor layer20, each pixel11ahas a semiconductor region21, an avalanche multiplication region22, a charge distribution region23, a first charge storage region P1, a second charge storage region P2, a third charge storage region P3, a fourth charge storage region P4, a first overflow region Q1, a second overflow region Q2, a third overflow region Q3, a fourth overflow region Q4, two unnecessary charge discharge regions R, a well region31, and a barrier region32. Each of the regions21to23, P1to P4, Q1to Q4, R, and31and32is formed by performing various processes (for example, etching, film formation, impurity injection, and the like) on a semiconductor substrate (for example, a silicon substrate).

The semiconductor region21is a p-type (first conductive type) region, and is provided along the second surface20bin the semiconductor layer20. The semiconductor region21functions as a light absorption region (photoelectric conversion region). As an example, the semiconductor region21is a p-type region having a carrier concentration of 1 × 1015cm-3or less, and the thickness of the semiconductor region21is about 10 µm. In addition, the avalanche multiplication region22and the like also function as a light absorption region (photoelectric conversion region).

The avalanche multiplication region22includes a first multiplication region22aand a second multiplication region22b. The first multiplication region22ais a p-type region, and is formed on the first side of the semiconductor region21in the semiconductor layer20. As an example, the first multiplication region22ais a p-type region having a carrier concentration of 1 × 1016cm-3or more, and the thickness of the first multiplication region22ais about 1 µm. The second multiplication region22bis an n-type (second conductive type) region, and is formed on the first side of the first multiplication region22ain the semiconductor layer20. As an example, the second multiplication region22bis an n-type region having a carrier concentration of 1 × 1016cm-3or more, and the thickness of the second multiplication region22bis about 1 µm. The first multiplication region22aand the second multiplication region22bform a pn junction. The avalanche multiplication region22is a region that causes avalanche multiplication. The electric field strength generated in the avalanche multiplication region22when a reverse bias having a predetermined value is applied is, for example, 3 × 105to 4 × 105V/cm.

The charge distribution region23is an n-type region, and is formed on the first side of the second multiplication region22bin the semiconductor layer20. As an example, the charge distribution region23is an n-type region having a carrier concentration of 5 × 1015to 1 × 1016cm-3, and the thickness of the charge distribution region23is about 1 µm.

Each of the charge storage regions P1to P4is an n-type region, and is formed on the first side of the second multiplication region22bin the semiconductor layer20. Each of the charge storage regions P1to P4is connected to the charge distribution region23. As an example, each of the first charge storage regions P1to P4is an n-type region having a carrier concentration of 1 × 1018cm-3or more, and the thickness of each of the first charge storage regions P1to P4is about 0.2 µm.

Each of the overflow regions Q1to Q4is an n-type region, and is formed on the first side of the second multiplication region22bin the semiconductor layer20. The charge storage capacity of the first overflow region Q1is larger than the charge storage capacity of the first charge storage region P1. The charge storage capacity of the second overflow region Q2is larger than the charge storage capacity of the second charge storage region P2. The charge storage capacity of the third overflow region Q3is larger than the charge storage capacity of the third charge storage region P3. The charge storage capacity of the fourth overflow region Q4is larger than the charge storage capacity of the fourth charge storage region P4. For example, the charge storage capacities of the charge storage regions P1to P4are equal, and the charge storage capacities of the overflow regions Q1to Q4are equal. A PN junction capacitor is used in the charge storage regions P1to P4, while an additional capacitor is provided in the overflow regions Q1to Q4. Therefore, the storage capacities of the overflow regions Q1to Q4are larger than the storage capacities of the charge storage regions P1to P4. Examples of the capacitor to be added include an MIM (Metal Insulator Metal) capacitor, a MOS capacitor, a trench capacitor, a PIP capacitor, and the like.

Each unnecessary charge discharge region R is an n-type region, and is formed on the first side of the second multiplication region22bin the semiconductor layer20. Each unnecessary charge discharge region R is connected to the charge distribution region23. The unnecessary charge discharge region R has the same configuration as, for example, the charge storage regions P1to P4.

The well region31is a p-type region, and is formed on the first side of the second multiplication region22bin the semiconductor layer20. The well region31surrounds the charge distribution region23when viewed from the Z direction. The well region31forms a plurality of read circuits (for example, a source follower amplifier, a reset transistor, and the like). The plurality of read circuits are electrically connected to the charge storage regions P1to P4and the overflow regions Q1to Q4, respectively. As an example, the well region31is a p-type region having a carrier concentration of 1 × 1016to 5 × 1017cm-3, and the thickness of the well region31is about 1 µm.

The barrier region32is an n-type region, and is formed between the second multiplication region22band the well region31in the semiconductor layer20. The barrier region32includes the well region31when viewed from the Z direction. That is, the well region31is located within the barrier region32when viewed from the Z direction. The barrier region32surrounds the charge distribution region23. The n-type impurity concentration in the barrier region32is higher than the n-type impurity concentration in the second multiplication region22b. As an example, the barrier region32is an n-type region having a carrier concentration from the carrier concentration of the second multiplication region22bto about twice the carrier concentration of the second multiplication region22b, and the thickness of the barrier region32is about 1 µm. Since the barrier region32is formed between the second multiplication region22band the well region31, even if a depletion layer formed in the avalanche multiplication region22spreads toward the well region31due to the application of a high voltage to the avalanche multiplication region22, the depletion layer is prevented from reaching the well region31. That is, it is possible to prevent the current from flowing between the avalanche multiplication region22and the well region31due to the depletion layer reaching the well region31.

Here, the positional relationship of the respective regions will be described. The first charge storage region P1faces the second charge storage region P2in the X direction with the charge distribution region23interposed therebetween. The first overflow region Q1is arranged on a side opposite to the charge distribution region23with respect to the first charge storage region P1. The second overflow region Q2is arranged on a side opposite to the charge distribution region23with respect to the second charge storage region P2.

The third charge storage region P3faces the fourth charge storage region P4in the X direction with the charge distribution region23interposed therebetween. The third overflow region Q3is arranged on a side opposite to the charge distribution region23with respect to the third charge storage region P3. The fourth overflow region Q4is arranged on a side opposite to the charge distribution region23with respect to the fourth charge storage region P4. The first charge storage region P1and the fourth charge storage region P4are aligned in the Y direction. The second charge storage region P2and the third charge storage region P3are aligned in the Y direction. The first overflow region Q1and the fourth overflow region Q4are aligned in the Y direction. The second overflow region Q2and the third overflow region Q3are aligned in the Y direction. The two unnecessary charge discharge regions R face each other in the Y direction with the charge distribution region23interposed therebetween.

In the electrode layer40, each pixel11aincludes a photogate electrode PG, a first transfer gate electrode TX1, a second transfer gate electrode TX2, a third transfer gate electrode TX3, a fourth transfer gate electrode TX4, a first overflow gate electrode OV1, a second overflow gate electrode OV2, a third overflow gate electrode OV3, a fourth overflow gate electrode OV4, and two unnecessary charge transfer gate electrodes RG. Each of the gate electrodes PG, TX1to TX4, OV1to OV4, and RG is formed on the first surface20aof the semiconductor layer20with an insulating film41interposed therebetween. The insulating film41is, for example, a silicon nitride film or a silicon oxide film.

The photogate electrode PG is arranged on the charge distribution region23. The photogate electrode PG is formed of a material having conductivity and light transmission (for example, polysilicon). As an example, the photogate electrode PG has a rectangular shape having two sides facing each other in the X direction and two sides facing each other in the Y direction when viewed from the Z direction. Of the semiconductor region21, the avalanche multiplication region22, and the charge distribution region23, a region immediately below the photogate electrode PG functions as a charge generation region24that generates charge according to incident light. In other words, the photogate electrode PG is arranged on the charge generation region24. In the charge generation region24, the charge generated in the semiconductor region21is multiplied in the avalanche multiplication region22and distributed in the charge distribution region23. Unlike in the embodiment, when the pulsed light L is incident on the semiconductor layer20from the side of a counter electrode50(in the case of back surface incidence), the photogate electrode PG does not have to have light transmission. The region immediately below the photogate electrode PG is a region that overlaps the photogate electrode PG when viewed from the Z direction. This point is the same for the other gate electrodes TX1to TX4, OV1to OV4, and RG.

The first transfer gate electrode TX1is arranged on a region between the first charge storage region P1and the charge generation region24in the charge distribution region23. The second transfer gate electrode TX2is arranged on a region between the second charge storage region P2and the charge generation region24in the charge distribution region23. The third transfer gate electrode TX3is arranged on a region between the third charge storage region P3and the charge generation region24in the charge distribution region23. The fourth transfer gate electrode TX4is arranged on a region between the fourth charge storage region P4and the charge generation region24in the charge distribution region23.

Each of the transfer gate electrodes TX1to TX4is formed of a conductive material (for example, polysilicon). As an example, each of the transfer gate electrodes TX1to TX4has a rectangular shape having two sides facing each other in the X direction and two sides facing each other in the Y direction when viewed from the Z direction.

The first overflow gate electrode OV1is arranged on a region between the first charge storage region P1and the first overflow region Q1in the well region31. The second overflow gate electrode OV2is arranged on a region between the second charge storage region P2and the second overflow region Q2in the well region31. The third overflow gate electrode OV3is arranged on a region between the third charge storage region P3and the third overflow region Q3in the well region31. The fourth overflow gate electrode OV4is arranged on a region between the fourth charge storage region P4and the fourth overflow region Q4in the well region31.

Each of the overflow gate electrodes OV1to OV4is formed of a conductive material (for example, polysilicon). As an example, each of the overflow gate electrodes OV1to OV4has a rectangular shape having two sides facing each other in the X direction and two sides facing each other in the Y direction when viewed from the Z direction.

One of the unnecessary charge transfer gate electrodes RG is arranged on a region between one of the pair of unnecessary charge discharge regions R and the charge generation region24in the charge distribution region23. The other one of the unnecessary charge transfer gate electrodes RG is arranged on a region between the other one of the pair of unnecessary charge discharge regions R and the charge generation region24in the charge distribution region23. Each unnecessary charge transfer gate electrode RG is formed of a conductive material (for example, polysilicon). As an example, each unnecessary charge transfer gate electrode RG has a rectangular shape having two sides facing each other in the X direction and two sides facing each other in the Y direction when viewed from the Z direction.

The distance measurement sensor10A further includes a counter electrode50and a wiring layer60in the pixel unit11. The counter electrode50is provided on the second surface20bof the semiconductor layer20. The counter electrode50includes a plurality of pixels11awhen viewed from the Z direction. The counter electrode 50 faces the electrode layer40in the Z direction. The counter electrode50is formed of, for example, a metal material. The wiring layer60is provided on the first surface20aof the semiconductor layer20so as to cover the electrode layer40. The wiring layer60is electrically connected to each pixel11aand the CMOS read circuit unit12(seeFIG.1). A light incidence opening60ais formed in a portion of the wiring layer60facing the photogate electrode PG of each pixel11a.

FIG.4shows an example of the circuit configuration of each pixel11a. As shown inFIG.4, each pixel11ahas a plurality of (four in this example) reset transistors RST connected to the overflow regions Q1to Q4and a plurality of (four in this example) selection transistors SEL used for selecting the pixel11a.

Method for Driving Distance Measurement Sensor

An operation example of the distance measurement sensor10A will be described with reference toFIGS.5and6. The following operation is realized by the control unit4controlling the driving of the distance measurement sensor10A. In each pixel11aof the distance measurement sensor10A, a negative voltage (for example, -50 V) is applied to the counter electrode50with the electric potential of the photogate electrode PG as a reference (that is, a reverse bias is applied to the pn junction formed in the avalanche multiplication region22), so that an electric field strength of 3 × 105to 4 × 105V/cm is generated in the avalanche multiplication region22. In this state, when the pulsed light L is incident on the semiconductor layer20through the light incidence opening60aand the photogate electrode PG, electrons generated by the absorption of the pulsed light L are multiplied in the avalanche multiplication region22and move to the charge distribution region23at high speed.

When generating a distance image of the object OJ (seeFIG.1), first, a reset process (reset step) for applying a reset voltage to each reset transistor RST of each pixel11ais performed. The reset voltage is a positive voltage with the electric potential of the photogate electrode PG as a reference. Then, the charge stored in the charge storage regions P1to P4and the overflow regions Q1to Q4is discharged to the outside, so that no charge is stored in the charge storage regions P1to P4and the overflow regions Q1to Q4(time T1,FIG.6(a)). The charge is discharged to the outside through, for example, a read circuit configured by the well region31and the wiring layer60. Hereinafter, the operation will be described focusing on one selected pixel11a.

After the reset process, the charge is stored in the charge storage regions P1to P4and the overflow regions Q1to Q4in a storage period T2(FIG.6(b)). In the storage period T2, charge transfer signals having different phases are applied to the transfer gate electrodes TX1to TX4. As a result, a charge distribution process (charge distribution step) for distributing the charge generated in the charge generation region24between the charge storage regions P1to P4is performed.

As an example, the charge transfer signal applied to the first transfer gate electrode TX1is a voltage signal in which a positive voltage and a negative voltage are alternately repeated with the electric potential of the photogate electrode PG as a reference, and is a voltage signal having the same period, pulse width, and phase as the intensity signal of the pulsed light L emitted from the light source2(seeFIG.1). The charge transfer signals applied to the second transfer gate electrode TX2, the third transfer gate electrode TX3, and the fourth transfer gate electrode TX4are the same voltage signals as the pulse voltage signal applied to the first transfer gate electrode TX1except that the phases are 90°, 180°, and 270°, respectively.

In a first period during which a positive voltage is applied to the first transfer gate electrode TX1, the potential energy ϕTX1of a region immediately below the first transfer gate electrode TX1is lower than the potential energy ϕPGof a region (charge generation region24) immediately below the photogate electrode PG. In other words, in the first period, the electric potential is applied to the photogate electrode PG and the first transfer gate electrode TX1so that the potential energy ϕTX1is lower than the potential energy ϕPG. As a result, the charge generated in the charge generation region24is transferred to the first charge storage region P1(first charge transfer process, first charge transfer step). InFIG.6(b), the potential energy ϕTX1when a positive voltage is applied to the first transfer gate electrode TX1is shown by the broken line, and the potential energy ϕTX1when a negative voltage is applied to the first transfer gate electrode TX1is shown by the solid line. In addition, the charge stored in the first charge storage region P1and the first overflow region Q1is shown by hatching.

For adjusting the magnitude of the potential energy of a region immediately below the gate electrode, the magnitude of the electric potential applied to the gate electrode may be adjusted, or instead of or in addition to this, the carrier concentration in the region immediately below the gate electrode may be adjusted. When the potential energy ϕPGof the region (charge generation region24) immediately below the photogate electrode PG is set to a predetermined magnitude by adjusting the carrier concentration, the photogate electrode PG may not be provided. In this case, the negative voltage described above does not necessarily have to be applied.

In the first period, a negative voltage is applied to the second to fourth transfer gate electrodes TX2to TX4, and the potential energy ϕTX2of a region immediately below the second transfer gate electrode TX2, the potential energy ϕTX3of a region immediately below the third transfer gate electrode TX3, and the potential energy ϕTX4of a region immediately below the fourth transfer gate electrode TX4are higher than the potential energy ϕPG. As a result, a potential energy barrier is generated between the charge generation region24and the second to fourth charge storage regions P2to P4, so that the charge generated in the charge generation region24is not transferred to the second to fourth charge storage regions P2to P4. In other words, in the first period, the electric potential is applied to the photogate electrode PG and the second to fourth transfer gate electrodes TX2to TX4so that the potential energies ϕTX2, ϕTX3and ϕTX4are higher than the potential energy ϕPG.

In addition, in the first period, the electric potential is applied to the photogate electrode PG and the first overflow gate electrode OV1so that the potential energy ϕOV1of a region immediately below the first overflow gate electrode OV1is lower than the potential energy ϕPGof the region (charge generation region24) immediately below the photogate electrode PG. In other words, the electric potential applied to the first overflow gate electrode OV1in the first period is set with the electric potential of the photogate electrode PG as a reference so that the potential energy ϕOV1is lower than the potential energy ϕPG. As a result, as shown inFIG.6(b), even when the first charge storage region P1is saturated with charge, the charge overflowing from the first charge storage region P1flows into the first overflow region Q1and stored in the first overflow region Q1.

In a second period during which a positive voltage is applied to the second transfer gate electrode TX2, the potential energy ϕTX2of the region immediately below the second transfer gate electrode TX2is lower than the potential energy ϕPGof the region (charge generation region24) immediately below the photogate electrode PG. In other words, in the second period, the electric potential is applied to the photogate electrode PG and the second transfer gate electrode TX2so that the potential energy ϕTX2is lower than the potential energy ϕPG. As a result, the charge generated in the charge generation region24is transferred to the second charge storage region P2(first charge transfer process, first charge transfer step). In the second period, the electric potential is applied to the photogate electrode PG and the first, third, and fourth transfer gate electrodes TX1, TX3, and TX4so that the potential energies ϕTX1, ϕTX3, and ϕTX4are higher than the potential energy ϕPG.

In addition, in the second period, the electric potential is applied to the photogate electrode PG and the second overflow gate electrode OV2so that the potential energy ϕOV2of a region immediately below the second overflow gate electrode OV2is lower than the potential energy ϕPGof the region (charge generation region24) immediately below the photogate electrode PG. As a result, even when the second charge storage region P2is saturated with charge, the charge overflowing from the second charge storage region P2flows into the second overflow region Q2and stored in the second overflow region Q2.

In a third period during which a positive voltage is applied to the third transfer gate electrode TX3, the potential energy ϕTX3of the region immediately below the third transfer gate electrode TX3is lower than the potential energy ϕPGof the region (charge generation region24) immediately below the photogate electrode PG. In other words, in the third period, the electric potential is applied to the photogate electrode PG and the third transfer gate electrode TX3so that the potential energy ϕTX3is lower than the potential energy ϕPG. As a result, the charge generated in the charge generation region24is transferred to the third charge storage region P3(first charge transfer process, first charge transfer step). In the third period, the electric potential is applied to the photogate electrode PG and the first, second, and fourth transfer gate electrodes TX1, TX2, and TX4so that the potential energies ϕTX1, ϕTX2, and ϕTX4are higher than the potential energy ϕPG.

In addition, in the third period, the electric potential is applied to the photogate electrode PG and the third overflow gate electrode OV3so that the potential energy ϕOV3of a region immediately below the third overflow gate electrode OV3is lower than the potential energy ϕPGof the region (charge generation region24) immediately below the photogate electrode PG. As a result, even when the third charge storage region P3is saturated with charge, the charge overflowing from the third charge storage region P3flows into the third overflow region Q3and stored in the third overflow region Q3.

In a fourth period during which a positive voltage is applied to the fourth transfer gate electrode TX4, the potential energy ϕTX4of the region immediately below the fourth transfer gate electrode TX4is lower than the potential energy ϕPGof the region (charge generation region24) immediately below the photogate electrode PG. In other words, in the fourth period, the electric potential is applied to the photogate electrode PG and the fourth transfer gate electrode TX4so that the potential energy ϕTX4is lower than the potential energy ϕPG. As a result, the charge generated in the charge generation region24is transferred to the fourth charge storage region P4(first charge transfer process, first charge transfer step). In the fourth period, the electric potential is applied to the photogate electrode PG and the first to third transfer gate electrodes TX1to TX3so that the potential energies ϕTX1to ϕTX3are higher than the potential energy ϕPG.

In addition, in the fourth period, the electric potential is applied to the photogate electrode PG and the fourth overflow gate electrode OV4so that the potential energy ϕOV4of a region immediately below the fourth overflow gate electrode OV4is lower than the potential energy ϕPGof the region (charge generation region24) immediately below the photogate electrode PG. As a result, even when the fourth charge storage region P4is saturated with charge, the charge overflowing from the fourth charge storage region P4flows into the fourth overflow region Q4and stored in the fourth overflow region Q4.

After the charge distribution process in the storage period T2, a first read process (high-sensitivity read process) (first read step) for reading the amount of charge stored in each of the charge storage regions P1to P4is performed (time T3,FIG.6(c)). After each of the process in which the charge generated in the charge generation region24is transferred to the first charge storage region P1, the process in which the charge generated in the charge generation region24is transferred to the second charge storage region P2, the process in which the charge generated in the charge generation region24is transferred to the third charge storage region P3, and the process in which the charge generated in the charge generation region24is transferred to the fourth charge storage region P4is performed multiple times, the first read process is performed.

After the first read process, a voltage higher than the voltage applied in the first period is applied to the first overflow gate electrode OV1to reduce the potential energy ϕOV1of the region immediately below the first overflow gate electrode OV1, thereby performing a charge transfer process (charge transfer step) (second charge transfer process, second charge transfer step) for transferring the charge stored in the first charge storage region P1to the first overflow region Q1(FIG.6(d)). In other words, in the charge transfer process, the charge stored in the first charge storage region P1is transferred to the first overflow region Q1by applying the electric potential to the first overflow gate electrode OV1so that the potential energy ϕOV1is reduced.

Similarly, in the charge transfer process, the charge stored in the second charge storage region P2is transferred to the second overflow region Q2by applying the electric potential to the second overflow gate electrode OV2so that the potential energy ϕOV2of the region immediately below the second overflow gate electrode OV2is reduced. By applying the electric potential to the third overflow gate electrode OV3so that the potential energy ϕOV3of the region immediately below the third overflow gate electrode OV3is reduced, the charge stored in the third charge storage region P3is transferred to the third overflow region Q3. By applying the electric potential to the fourth overflow gate electrode OV4so that the potential energy ϕOV4of the region immediately below the fourth overflow gate electrode OV4is reduced, the charge stored in the fourth charge storage region P4is transferred to the fourth overflow region Q4.

After the charge transfer process, a second read process (low-sensitivity read process) (second read step) for reading the total amount of charge stored in the first charge storage region P1and the first overflow region Q1is performed (time T4,FIG.6(d)). Similarly, in the second read process, the total amount of charge stored in the second charge storage region P2and the second overflow region Q2is read. The total amount of charge stored in the third charge storage region P3and the third overflow region Q3is read. The total amount of charge stored in the fourth charge storage region P4and the fourth overflow region Q4is read. After the second read process, the reset process described above is performed again (time T1,FIG.6(a)), so that the series of processes described above are repeatedly performed.

In addition, in a period other than the first to fourth periods, an unnecessary charge transfer process (unnecessary charge transfer step) for transferring the charge generated in the charge generation region24to the unnecessary charge discharge region R is performed. In the unnecessary charge transfer process, by applying a positive voltage to the unnecessary charge transfer gate electrode RG, the potential energy ϕRGof a region immediately below the unnecessary charge transfer gate electrode RG is made lower than the potential energy ϕPGof the region (charge generation region24) immediately below the photogate electrode PG. In other words, the electric potential is applied to the photogate electrode PG and the unnecessary charge transfer gate electrode RG so that the potential energy ϕRGis lower than the potential energy ϕPG. As a result, the charge generated in the charge generation region24is transferred to the unnecessary charge discharge region R. The charge transferred to the unnecessary charge discharge region R is discharged to the outside. For example, the unnecessary charge discharge region R is connected to the fixed electric potential, so that the charge transferred to the unnecessary charge discharge region R is discharged to the outside without passing through the read circuit.

As shown inFIG.1, when the pulsed light L is emitted from the light source2and the pulsed light L reflected by the object OJ is detected by the distance measurement sensor10A, the phase of the intensity signal of the pulsed light L detected by the distance measurement sensor10A is shifted from the phase of the intensity signal of the pulsed light L emitted from the light source2in accordance with the distance d to the object OJ. Therefore, by acquiring a signal based on the amount of charge stored in the charge storage regions P1to P4and the overflow regions Q1to Q4(that is, the amount of charge read in the first read process and the second read process) for each pixel11a, it is possible to generate the distance image of the object OJ.

Functions and Effects of First Embodiment

In the distance measurement device1, the distance measurement sensor10A has the first overflow region Q1having a charge storage capacity larger than the charge storage capacity of the first charge storage region P1, the second overflow region Q2having a charge storage capacity larger than the charge storage capacity of the second charge storage region P2, the first overflow gate electrode OV1arranged on a region between the first charge storage region P1and the first overflow region Q1, and the second overflow gate electrode OV2arranged on a region between the second charge storage region P2and the second overflow region Q2. Therefore, the charge overflowing from the first charge storage region P1can be stored in the first overflow region Q1, and the charge overflowing from the second charge storage region P2can be stored in the second overflow region Q2. As a result, it is possible to suppress the saturation of the storage capacity. In addition, in the first period of the charge distribution process, the potential energy ϕOV1of the region immediately below the first overflow gate electrode OV1is lower than the potential energy ϕPGof the charge generation region24, and in the second period of the charge distribution process, the potential energy ϕOV2of the region immediately below the second overflow gate electrode OV2is lower than the potential energy ϕPGof the charge generation region24. As a result, even when the charge is stored in the first charge storage region P1to the extent that the charge overflows into the first overflow region Q1and when the charge is stored in the second charge storage region P2to the extent that the charge overflows into the second overflow region Q2, it is possible to suppress the charge from remaining in the charge generation region24. Therefore, according to the distance measurement device1, it is possible to improve the accuracy of distance measurement. In addition, it is possible to achieve high sensitivity and high dynamic range.

This point will be further described with reference to a comparative example shown inFIGS.7and8. In the image sensor of the comparative example, the potential energy ϕTXof a region immediately below the transfer gate electrode TX is higher than the potential energy ϕPGof a region immediately below the photogate electrode PG over the entire storage period T2(FIG.8(b)). In addition, the potential energy ϕOVof a region immediately below the overflow gate electrode OV is higher than the potential energy ϕPGof the region immediately below the photogate electrode PG over the entire storage period T2. After the storage period T2, the potential energy ϕTXof the region immediately below the transfer gate electrode TX is lower than the potential energy ϕPGof the region (charge generation region) immediately below the photogate electrode PG, so that the charge stored in the charge generation region is transferred to the charge storage region P. Thereafter, the amount of charge stored in the charge storage region P is read (time T3,FIG.8(c)).

In the image sensor of the comparative example, in the storage period T2, the potential energy ϕOVof the region immediately below the overflow gate electrode OV is higher than the potential energy ϕPGof the region immediately below the photogate electrode PG. Therefore, as shown inFIG.8(c), when the charge is stored in the charge storage region P to the extent that the charge overflows into the overflow region Q, a part of the charge remains in the region (charge generation region) immediately below the photogate electrode PG. In this case, the accuracy of distance measurement may decrease due to the charge remaining in the charge storage region.

In contrast, as described above, in the distance measurement device1, the potential energy ϕOV1of the region immediately below the first overflow gate electrode OV1and the potential energy ϕOV2of the region immediately below the second overflow gate electrode OV2are lower than the potential energy ϕPGof the charge generation region24during the execution of the charge distribution process. As a result, even when the charge is stored in the first charge storage region P1or the second charge storage region P2to the extent that the charge overflows into the first overflow region Q1or the second overflow region Q2, it is possible to suppress the charge from remaining in the charge generation region24.

After performing the first charge transfer process for transferring the charge generated in the charge generation region24to the first charge storage region P1multiple times, the control unit4performs a first read process for reading the amount of charge stored in the charge storage region P1. In this manner, it is possible to improve the S/N ratio.

The charge generation region24includes the avalanche multiplication region22. In this case, since the avalanche multiplication can be caused in the charge generation region24, it is possible to increase the detection sensitivity of the distance measurement sensor10A. On the other hand, when the avalanche multiplication region22is included in the charge generation region24, the amount of charge generated is extremely large. In the distance measurement device1, even in such a case, it is possible to sufficiently suppress the saturation of the storage capacity, and it is possible to sufficiently suppress the charge from remaining in the charge generation region24.

The control unit4performs a first read process for reading the amount of charge stored in the first charge storage region P1and the second charge storage region P2, a second charge transfer process for transferring the charge stored in the first charge storage region P1to the first overflow region Q1and transferring the charge stored in the second charge storage region P2to the second overflow region Q2, and a second read process for reading the total amount of charge stored in the first charge storage region P1and the first overflow region Q1and reading the total amount of charge stored in the second charge storage region P2and the second overflow region Q2. Therefore, not only is the amount of charge stored in the first and second charge storage regions P2read in the first read process, but also the total amount of charge stored in the first charge storage region P1and the first overflow region Q1and the total amount of charge stored in the second charge storage region P2and the second overflow region Q2are read in the second read process. As a result, it is possible to improve the charge amount detection accuracy.

The control unit4performs an unnecessary charge transfer process for transferring the charge generated in the charge generation region24to the unnecessary charge discharge region R by using the unnecessary charge transfer gate electrode RG in a period other than the first period and the second period (that is, a period other than the period during which the first charge transfer process is performed). Therefore, since the charge generated in the charge generation region24can be transferred to the unnecessary charge discharge region R in a period other than the first and second periods, it is possible to further suppress the charge from remaining in the charge generation region24. The unnecessary charge transfer process is particularly useful in an environment in which there is a lot of ambient light.

In the first period, the control unit4applies the electric potential to the photogate electrode PG and the first transfer gate electrode TX1so that the potential energy ϕTX1of the region immediately below the first transfer gate electrode TX1is lower than the potential energy ϕPGof the region (charge generation region24) immediately below the photogate electrode PG and the potential energy ϕOV1of the region immediately below the first overflow gate electrode OV1is lower than the potential energy ϕPGof the region immediately below the photogate electrode PG. In the second period, the control unit4applies the electric potential to the photogate electrode PG and the second transfer gate electrode TX2so that the potential energy ϕTX2of the region immediately below the second transfer gate electrode TX2is lower than the potential energy ϕPGof the region immediately below the photogate electrode PG and the potential energy ϕOV2of the region immediately below the second overflow gate electrode OV2is lower than the potential energy ϕPGof the region immediately below the photogate electrode PG. In the third period, the control unit4applies the electric potential to the photogate electrode PG and the third transfer gate electrode TX3so that the potential energy ϕTX3of the region immediately below the third transfer gate electrode TX3is lower than the potential energy ϕPGof the region immediately below the photogate electrode PG and the potential energy ϕOV3of the region immediately below the third overflow gate electrode OV3is lower than the potential energy ϕPGof the region immediately below the photogate electrode PG. In the fourth period, the control unit4applies the electric potential to the photogate electrode PG and the fourth transfer gate electrode TX4so that the potential energy ϕTX4of the region immediately below the fourth transfer gate electrode TX4is lower than the potential energy ϕPGof the region immediately below the photogate electrode PG and the potential energy ϕOV4of the region immediately below the fourth overflow gate electrode OV4is lower than the potential energy ϕPGof the region immediately below the photogate electrode PG. As a result, it is possible to accurately adjust the magnitude of each potential energy.

The distance measurement sensor10A has not only the first and second charge storage regions P1and P2, the first and second overflow regions Q1and Q2, the first and second transfer gate electrodes TX1and TX2, and the first and second overflow gate electrodes OV1and OV2but also the third and fourth charge storage regions P3and P4, the third and fourth overflow regions Q3and Q4, the third and fourth transfer gate electrodes TX3and TX4, and the third and fourth overflow gate electrodes OV3and OV4. Then, in the charge distribution process, the control unit4applies charge transfer signals having different phases to the transfer gate electrodes TX1to TX4, so that the charge generated in the charge generation region24is distributed between the charge storage regions P1to P4. Therefore, since charge distribution by the first to fourth transfer gate electrodes TX1to TX4can be realized, it is possible to improve the accuracy of distance measurement.

Modification Examples of First Embodiment

In a distance measurement sensor10B according to a first modification example shown inFIG.9, the unnecessary charge discharge region R and the unnecessary charge transfer gate electrode RG are not provided in each pixel unit11. The third charge storage region P3faces the fourth charge storage region P4in the Y direction with the charge generation region24(photogate electrode PG) interposed therebetween. The distance measurement sensor10B is driven, for example, as shown inFIG.10. In this driving method, the unnecessary charge transfer process for transferring the charge generated in the charge generation region24to the unnecessary charge discharge region R is not performed. Also in the first modification example, as in the embodiment described above, it is possible to improve the accuracy of distance measurement by suppressing the saturation of the storage capacity and suppressing the charge from remaining in the charge generation region24.

In a distance measurement sensor10C according to a second modification example shown inFIG.11, the third and fourth charge storage regions P3and P4, the third and fourth overflow regions Q3and Q4, the third and fourth transfer gate electrodes TX3and TX4, and the third and fourth overflow gate electrodes OV3and OV4are not provided in each pixel unit11. Each pixel unit11has four unnecessary charge discharge regions R1, R2, R3, and R4and four unnecessary charge transfer gate electrodes RG. The unnecessary charge discharge regions R1and R2face each other in the X direction with the charge generation region24(photogate electrode PG) interposed therebetween. The unnecessary charge discharge regions R3and R4face each other in the X direction with the charge generation region24interposed therebetween. The unnecessary charge discharge regions R1and R4face each other in the Y direction with the first charge storage region P1interposed therebetween. The unnecessary charge discharge regions R2and R3face each other in the Y direction with the second charge storage region P2interposed therebetween.

The distance measurement sensor10C is driven, for example, as shown inFIG.12. In this driving method, in the storage period T2, a first period during which a positive voltage is applied to the first transfer gate electrode TX1, a second period during which a positive voltage is applied to the second transfer gate electrode TX2, and a period during which an unnecessary charge transfer process for transferring the charge generated in the charge generation region24to the unnecessary charge discharge region R are repeated in this order. A distance image of the object OJ can also be generated by such a driving method. Also in the second modification example, as in the embodiment described above, it is possible to improve the accuracy of distance measurement by suppressing the saturation of the storage capacity and suppressing the charge from remaining in the charge generation region24.

As in a third modification example shown inFIG.13, the reset transistor RST may be arranged at a position different from that in the embodiment. InFIG.13, only the circuit configuration of a part of the pixel11ais shown. Also in the third modification example, as in the embodiment described above, it is possible to improve the accuracy of distance measurement by suppressing the saturation of the storage capacity and suppressing the charge from remaining in the charge generation region24.

Second Embodiment

As shown inFIG.14, a light detection device100includes a light source2, an image sensor10D, a control unit4, and an optical system6. The light detection device100is configured as a range gate camera having a gating function (shutter function) for detecting light arriving at a predetermined timing (in a predetermined period). The optical system6guides the pulsed light L, which is emitted from the light source2and reflected by the object OJ, to the pixel unit11of the image sensor10D.

As shown inFIG.15, the image sensor10D is different from the above-described distance measurement sensor10A in that the second to fourth charge storage regions P2to P4, the second to fourth overflow regions Q2to Q4, the second to fourth transfer gate electrodes TX2to TX4, and the second to fourth overflow gate electrodes OV2to OV4are not provided. In the image sensor10D, the first charge storage region P1and the unnecessary charge discharge region R are arranged on one side in the X direction with respect to the charge generation region24(photogate electrode PG). The first charge storage region P1and the unnecessary charge discharge region R are aligned in the Y direction. The first transfer gate electrode TX1and the unnecessary charge transfer gate electrode RG are aligned in the Y direction.

The light detection device100is driven, for example, as shown inFIG.16. In this driving method, in the storage period T2, instead of the charge distribution process, a first charge transfer process (first charge transfer step) for transferring the charge generated in the charge generation region24to the first charge storage region P1is repeatedly performed. As an example, the charge transfer signal applied to the first transfer gate electrode TX1is a voltage signal in which a positive voltage and a negative voltage are alternately repeated with the electric potential of the photogate electrode PG as a reference, and is a voltage signal having the same period and pulse width as the intensity signal of the pulsed light L emitted from the light source2except that the phases are shifted by a predetermined amount.

In a period during which a positive voltage is applied to the first transfer gate electrode TX1, the potential energy ϕTX1of the region immediately below the first transfer gate electrode TX1is lower than the potential energy ϕPGof the region (charge generation region24) immediately below the photogate electrode PG. In other words, in the period, the electric potential is applied to the photogate electrode PG and the first transfer gate electrode TX1so that the potential energy ϕTX1is lower than the potential energy ϕPG. As a result, the charge generated in the charge generation region24is transferred to the first charge storage region P1.

On the other hand, in a period during which a negative voltage is applied to the first transfer gate electrode TX1, the potential energy ϕTX1of the region immediately below the first transfer gate electrode TX1is higher than the potential energy ϕPGof the region (charge generation region24) immediately below the photogate electrode PG. In other words, in the period, the electric potential is applied to the photogate electrode PG and the first transfer gate electrode TX1so that the potential energy ϕTX1is higher than the potential energy ϕPG. As a result, a potential energy barrier is generated between the charge generation region24and the first charge storage region P1, so that the charge generated in the charge generation region24is not transferred to the first charge storage region P1.

In addition, in the storage period T2, the electric potential is applied to the photogate electrode PG and the first overflow gate electrode OV1so that the potential energy ϕOV1of the region immediately below the first overflow gate electrode OV1is lower than the potential energy ϕPGof the region (charge generation region24) immediately below the photogate electrode PG. As a result, even when the first charge storage region P1is saturated with charge, the charge overflowing from the first charge storage region P1flows into the first overflow region Q1and stored in the first overflow region Q1.

In addition, in a period other than the period during which the first charge transfer process is performed, an unnecessary charge transfer process (unnecessary charge transfer step) for transferring the charge generated in the charge generation region24to the unnecessary charge discharge region R is performed. In the unnecessary charge transfer process, the electric potential is applied to the photogate electrode PG and the unnecessary charge transfer gate electrode RG so that the potential energy ϕRGof the region immediately below the unnecessary charge transfer gate electrode RG is lower than the potential energy ϕPGof the region (charge generation region24) immediately below the photogate electrode PG. As a result, the charge generated in the charge generation region24is transferred to the unnecessary charge discharge region R.

After the charge transfer process is performed multiple times in the storage period T2, a first read process (high-sensitivity read process) (first read step) for reading the amount of charge stored in the first charge storage region P1is performed (time T3). After the first read process, a voltage higher than the voltage applied in the storage period T2is applied to the first overflow gate electrode OV1to reduce the potential energy ϕOV1of the region immediately below the first overflow gate electrode OV1, thereby performing a second charge transfer process (second charge transfer step) for transferring the charge stored in the first charge storage region P1to the first overflow region Q1. In other words, in the second charge transfer process, the charge stored in the first charge storage region P1is transferred to the first overflow region Q1by applying the electric potential to the first overflow gate electrode OV1so that the potential energy ϕOV1is reduced. After the second charge transfer process, a second read process (low-sensitivity read process) (second read step) for reading the total amount of charge stored in the first charge storage region P1and the first overflow region Q1is performed (time T4).

The gating function that can be realized by the above operation will be described with reference toFIG.17. As in the example of (1) shown inFIG.17, when the pulsed light L reflected in the vicinity of the object OJ1away from the image sensor10D by a distance d1 is detected, a voltage signal having a phase shifted by an amount corresponding to the distance d1 is applied to the first transfer gate electrode TX1. Therefore, in a period during which the pulsed light L reflected in the vicinity of the object OJ1(that is, a range away from the image sensor10D by a predetermined distance) is incident on the charge generation region24, the first charge transfer process for transferring the charge generated in the charge generation region24to the first charge storage region P1can be performed. As a result, it is possible to detect only the pulsed light L reflected in the vicinity of the object OJ1. Similarly, as in the examples of (2) to (4) shown inFIG.17, when the pulsed light L reflected in a range away from the image sensor10D by a predetermined distance is detected, a voltage signal having a phase shifted by an amount corresponding to the distance is applied to the first transfer gate electrode TX1. As described above, according to the light detection device100, it is possible to realize the gating function for detecting only the light arriving at a predetermined timing. The gating function can be suitably used, for example, for measuring the fluorescence lifetime.

Functions and Effects of Second Embodiment

In the light detection device100, the image sensor10D has the first overflow region Q1having a charge storage capacity larger than the charge storage capacity of the first charge storage region P1and the first overflow gate electrode OV1arranged on a region between the first charge storage region P1and the first overflow region Q1. Therefore, since the charge overflowing from the first charge storage region P1can be stored in the first overflow region Q1, it is possible to suppress the saturation of the storage capacity. In addition, during the execution of the first charge transfer process for transferring the charge generated in the charge generation region24to the first charge storage region P1, the potential energy ϕOV1of the region immediately below the first overflow gate electrode OV1is lower than the potential energy ϕPGof the charge generation region24. Therefore, even when the charge is stored in the first charge storage region P1to the extent that the charge overflows into the first overflow region Q1, it is possible to suppress the charge from remaining in the charge generation region24. Therefore, according to the light detection device100, it is possible to improve the detection accuracy.

The control unit4performs the first read process after performing the first charge transfer process multiple times. In this manner, it is possible to improve the S/N ratio.

The charge generation region24includes the avalanche multiplication region22. Therefore, since the avalanche multiplication can be caused in the charge generation region24, it is possible to increase the detection sensitivity of the image sensor10D. On the other hand, when the avalanche multiplication region22is included in the charge generation region24, the amount of charge generated is extremely large. However, even in this case, in the light detection device100, it is possible to sufficiently suppress the saturation of the storage capacity, and it is possible to sufficiently suppress the charge from remaining in the charge generation region24.

The control unit4performs the second charge transfer process for transferring the charge stored in the first charge storage region P1to the first overflow region Q1and the second read process for reading the total amount of charge stored in the first charge storage region P1and the first overflow region Q1. Therefore, not only is the amount of charge stored in the first charge storage region P1read in the first read process, but also the total amount of charge stored in the first charge storage region P1and the first overflow region Q1is read in the second read process. As a result, it is possible to improve the charge amount detection accuracy.

The control unit4performs an unnecessary charge transfer process for transferring the charge generated in the charge generation region24to the unnecessary charge discharge region R by using the unnecessary charge transfer gate electrode RG in a period other than the period during which the first charge transfer process is performed. Therefore, since the charge generated in the charge generation region24can be transferred to the unnecessary charge discharge region R in a period other than the period during which the first charge transfer process is performed, it is possible to further suppress the charge from remaining in the charge generation region24. The unnecessary charge transfer process is particularly useful in an environment in which there is a lot of ambient light.

The control unit4performs the first charge transfer process in a period during which the pulsed light L reflected by the object OJ is incident on the charge generation region24. Therefore, it is possible to accurately detect the amount of charge generated in the charge generation region24in the period during which the pulsed light L reflected by the object OJ is incident on the charge generation region24.

Modification Examples of Second Embodiment

As a modification example, in the image sensor10D, the unnecessary charge discharge region R and the unnecessary charge transfer gate electrode RG may not be provided in each pixel unit11. The image sensor of this modification example is driven, for example, as shown inFIG.18. In this driving method, the unnecessary charge transfer process for transferring the charge generated in the charge generation region24to the unnecessary charge discharge region R is not performed. Also in this modification example, as in the second embodiment described above, it is possible to improve the detection accuracy by suppressing the saturation of the storage capacity and suppressing the charge from remaining in the charge generation region24. The image sensor of this modification example can be suitably used, for example, when it is difficult for ambient light to be incident on the charge generation region24in a period other than the period during which the first charge transfer process is performed. As an example of such a case, for example, there is a case where light detection is performed in a dark room.

The present disclosure is not limited to the above-described embodiments and modification examples. For example, the material and shape of each component are not limited to the materials and shapes described above, and various materials and shapes can be adopted. In the distance measurement sensors10A and10C and the image sensor10D, the charge transferred to the unnecessary charge discharge regions R and R1to R4may be stored and read without being discharged to the outside. That is, the unnecessary charge discharge regions R and R1to R4may function as charge storage regions. In this case, light (light that does not include distance information) other than signal light can be read and used.

The avalanche multiplication region22may not be formed in the semiconductor layer20. That is, the charge generation region24may not include the avalanche multiplication region22. At least one of the well region31and the barrier region32may not be formed in the semiconductor layer20. The signal processing unit3may be omitted, and the control unit4may be directly connected to the distance measurement sensors10A to10C. The second charge transfer process and the second read process may not be performed. The first read process may be performed after the first charge transfer process is performed once.

In the distance measurement sensors10A to10C and the image sensor10D, it is possible to make light incident on the semiconductor layer20from either the first side or the second side. For example, when light is incident on the semiconductor layer20from the second side, the counter electrode50may be formed of a material having conductivity and light transmission (for example, polysilicon). In any of the distance measurement sensors10A to10C and the image sensor10D, the p-type and n-type conductive types may be the opposite of those described above. In any of the distance measurement sensors10A to10C and the image sensor10D, the plurality of pixels11amay be aligned in a one-dimensional manner along the first surface20aof the semiconductor layer20. Each of the distance measurement sensors10A to10C and the image sensor10D may have only a single pixel11a. The charge storage capacity of the first overflow region Q1may be equal to or less than the charge storage capacity of the first charge storage region P1. The charge storage capacity of the second overflow region Q2may be equal to or less than the charge storage capacity of the second charge storage region P2.

Reference Signs List