JP2545469B2 - Photoelectric conversion device - Google Patents

Description

The present invention relates to a photoelectric conversion device for reducing sensor noise.

[Problems to be Solved by the Related Art and Invention] An amplification type sensor is preferably used for a sensor used in a solid-state imaging device or the like in order to increase an output signal level.

The amplification type sensor is composed of transistors of MOS type, SIT type, FET type, bipolar type, etc., and amplifies the electric charge accumulated in those control electrodes and outputs it from the main electrode. . For example, Japanese Patent Publication Sho 55-
28456 discloses an example of an amplification type sensor. One of the problems of such an amplification type sensor is that the sensor noise is large.

There are two types of sensor noise: fixed pattern noise that generally appears fixedly (hereinafter referred to as FPN), and random noise (noise whose amplitude changes with each reset) that is taken into the control electrode when the control electrode is reset.

Of the sensor noise, the FPN appears statically, so it can be completely removed by subtracting the dark output of the sensor from the optical signal output of the sensor. The dark output can be obtained by reading the accumulation time to almost zero, that is, immediately after the sensor is reset.

On the other hand, in order to remove the random noise introduced into the control electrode, the sensor output (optical signal) after the accumulation may be subtracted from the sensor output (sensor noise) immediately after the accumulation starts. As a photoelectric conversion device capable of such subtraction processing, Japanese Patent Application No. Sho 63-4749 filed previously by the present applicant has been proposed.
There is one disclosed in No. 2. This photoelectric conversion device is provided with a means for storing an optical signal and a storage means for storing sensor noise, and takes the difference between the optical signal stored in both storage means and the sensor noise.

Such a photoelectric conversion device is very practical when used for a line sensor, but when used for an area sensor, the chip area of the storage means for storing the sensor noise becomes large. Even when the storage means is provided outside the image sensor, a field memory or frame memory is required, which is a cost problem.
Practical application is difficult.

Further, there is one in which a photoelectric conversion region is laminated on an amplification transistor in order to increase the sensitivity of the sensor and relatively reduce the influence of sensor noise. As such a solid-state imaging device, there is one disclosed in Japanese Patent Application No. 1-9089 filed by the present applicant earlier.

In this solid-state imaging device, when the photoconductive film is laminated, the aperture ratio of the photoelectric conversion region becomes very large, so that the sensor output (S) also becomes large. However, in order to achieve a higher S / N ratio, it is necessary to reduce the sensor noise (N).

In view of the above problems, an object of the present invention is to provide a photoelectric conversion device suitable for removing sensor noise.

[Means for Solving the Problems] A photoelectric conversion device of the present invention includes a storage unit that stores photoexcited charges, an amplification unit that amplifies charges of a control electrode,
A reset element for resetting the control electrode before transferring the charge of the storage means to the amplification means; and a reset element for resetting the output of the amplification means after resetting. A first read-out means for reading out as a first signal, a transfer means for electrically connecting the transfer element to transfer the charge of the storage means to the control electrode, and an output of the amplifying means after transfer of the charge for a second signal. And a subtraction processing unit that performs a subtraction process between the first signal and the second signal.

[Operation] The present inventor has already disclosed in Japanese Patent Application No. 1-136125 that the storage means for storing the photoexcited charge, the amplification means for amplifying the charge of the control electrode, and the storage means and the control electrode are connected. And a first read-out means for reading the output of the amplifying means of the switch means when the switch means is not conducting as a first signal, and the switch means for conducting the electric charge of the accumulating means to the control electrode. Transferring means, second reading means for reading the output of the amplifying means as a second signal after transfer of electric charge, the first signal and the second signal
And a subtraction processing unit that performs a subtraction processing with the signal of 1.

The photoelectric conversion device of Japanese Patent Application No. 1-136125 is related to the operation of the accumulating means by providing the switch means between the accumulating means for accumulating the photoexcited electric charge and the amplifying means for amplifying the electric charge of the control electrode. However, the sensor noise can be read out independently, and the dark current component and random noise of the amplifying element as well as the FPN can be removed.

The present invention is to improve the characteristics of the photoelectric conversion device of the above-mentioned Japanese Patent Application No. 1-136125, and is provided with reset means for resetting the control electrode of the amplifying means before transferring the charge of the accumulating means to the amplifying means. Later, the output of the amplification means
Read out as a signal of
After the charge of the storage means is transferred to the control electrode, the output of the amplifying means is read out as a second signal, whereby the dark current component of the amplifying means is removed before the transfer of the photocharge,
By setting the potential of the control electrode of the amplification means lower than the storage potential of the storage means, it is possible to completely transfer the photocharges from the storage means to the amplification means.

It is to be noted that when a storage area serving as storage means and an amplification area serving as amplification means are formed on a substrate, a photoelectric conversion area is stacked on the storage area, and the photoelectric conversion area and the amplification area are connected to each other, the aperture ratio of the socket Can be increased and the S / N ratio can be increased.

EXAMPLES Examples of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a partial circuit diagram of a first embodiment of a photoelectric conversion device of the present invention.

FIG. 2 is a schematic plan view of a pixel of the photoelectric conversion device.

FIG. 3 is a sectional view taken along line XX ′ of the photoelectric conversion device of FIG.

FIG. 4 is a cross-sectional view of the photoelectric conversion device of FIG. 2 taken along the line YY ′.

As shown in FIG. 1, one pixel includes a PMOS transistor Tr _A for resetting the pixel, a capacitance Co _X for controlling transient reset of the pixel and signal accumulation / reading,
A photo diode D which is a storage means for accumulating the photoexcited electric charge, an amplifying transistor Tr _C for amplifying a signal from the photo diode D, and a photo diode D.
Transistor Tr _C for amplifying charges generated in the photoelectric conversion portion
Element for controlling transfer to the base, which is the control electrode of the
Composed of Tr _B. φ _VR and φ _TB are pulses for controlling the PMOS transistor Tr _A and the transfer element Tr _B , respectively.

Amplifying transistor Tr _C Emitter is a MOS transistor
It is connected to the storage capacitors C ₁ and C ₂ via T ₁ and T ₂ . φ _T1 , φ
_T2 is a pulse for controlling the MOS transistors T ₁ and T ₂ , respectively. T _VC is a MOS transistor that resets the vertical signal line, and is set to the potential V _VC by controlling the pulse φ _VC .

As shown in FIGS. 2 and 3, each pixel is formed on the P substrate, but may be on the N substrate. Each pixel of this embodiment is separated by a PMOS transistor Tr _A (shown by a broken line in FIG. 3), and a part of the base B of the amplifying transistor Tr _C is a poly layer L ₁ provided on SiO _2. Thereby forming a capacitance Co _X. The emitter E is connected by the Al layer L ₂ as a vertical output line above the pixel separation. As shown in FIGS. 2 and 4, the photodiode D, which is a photoelectric conversion unit, and the base B of the amplification transistor Tr _C are the transfer element T.
They are separated by r _B (shown by the broken line in FIG. 4). n '
Indicates a channel dope region. It should be noted that, except for the photodiode D, it is normally shielded from light, but is omitted in FIGS. 2 to 4.

The operation of the photoelectric conversion device will be described below with reference to FIGS. 1 to 4.

FIG. 5 is a timing chart for explaining the operation of the photoelectric conversion device.

In FIG. 5, first, the period T ₁ is a complete reset period of the horizontal amplification transistor Tr _C , and is also a period of outputting signals and sensor noise from the storage capacitors C ₁ and C ₂ described later. For complete reset, pulse φ _{VR is set} to a negative potential (illustration
V _L ), and the PMOS transistor Tr _A is turned on.

This complete reset clamps the base potential to GND.

In the period T _2, following the complete reset period T _1, the transient reset is performed. Positive potential of pulse φ _VR (V _H shown)
When set to, the PMOS transistor Tr _A is turned off and the capacitance
The base potential of the amplifying transistor Tr _C rises via C _OX . At the same time, the pulse φ _{VC is set} to the high level to turn on the MOS transistor T _VC, and the emitter of the amplifying transistor Tr _C is set to the predetermined potential V _VC . At this time, the base charge of the amplifying transistor Tr _C sharply drops due to recrystallization with the emitter current. Further, the pulse φ _{T1 is set} to the high level, the MOS transistor T ₁ is turned on, and the storage capacitor C ₁ is set to the predetermined potential V _VC .

The period T ₃ is a period for reading out the sensor noise. The pulse φ _VR is kept at the positive potential V _H , the pulse φ _{VC is set} to the low level, the MOS transistor T _VC is turned off, and the emitter of the amplification transistor Tr _C is floated. In this state, the voltage at the emitter terminal becomes a potential that reflects the base voltage.

The base potential at this time is the output potential in the dark, and the output voltage that depends on the characteristics of the amplifying transistor Tr _C appears at the emitter. This is commonly called the offset voltage,
Since a plurality of amplifying transistors have slightly different parameters, the emitter output voltage, that is, the offset voltage also differs. Here, the difference between these offset potentials is called sensor noise. This sensor noise is stored in the storage capacitor C ₁ .

When the reading of the sensor noise is completed, the pulse φ _VR changes from the high voltage V _H to the intermediate voltage V _M. Due to this potential change, the base of the amplifying transistor Tr _C is reverse-biased through the capacitor C _OX .

The period T ₄ is a period for transferring the photocharge of the photodiode D to the base of the amplifying transistor Tr _C , and when the pulse φ _TB is lowered to a negative potential (V _L ), the transfer element Tr _B Is turned on, and the photocharges accumulated in the photodiode D are transferred to the base of the amplifying transistor Tr _C.

Since the base potential of the amplifying transistor Tr _C is set lower than the potential of the photo diode D, the transfer of photocharges from the photo diode D to the amplifying transistor Tr _C is completed.

After the period T ₄ , the pulse φ _{VC is set to the} high level to turn on the MOS transistor T _VC , the pulse φ T ₂ is set to the high level to _turn on the MOS transistor T ₂ , and the storage capacitor C ₂ is set to the predetermined potential V _VC . .

The period T ₅ is a read period of the optical signal, and the pulse φ _VR
The When a positive potential (V _H), the potential of the base of the amplifying transistor Tr _C rises through the capacitor Co _X, the optical signal output is read from the amplifying transistor Tr _C. This optical signal contains the sensor noise described above and is stored in the storage capacitor C ₂ .

The column of the storage capacitor C ₁ that stores the sensor noise and the column of the storage capacitor C ₂ that stores the optical signal are driven by the horizontal shift register (not shown), and the horizontal output line from the horizontal transfer switch (not shown) is selected. Transferred to. Since the output terminal of the horizontal output line is connected to the differential amplifier, as a result, the sensor noise is subtracted from the optical signal, and only the optical signal can be obtained.

The sensor noise subtraction process will be described in detail below.

According to our experimental results, the FPN after transient reset is
Depending on the difference in parameters such as h _FE of the amplifying transistor Tr _C, the base potential V _B ΔV varies after the transient reset. This variation ΔV is due to the read operation and Is done. Here, C _B is a combined capacitance of the base-collector capacitance C _bC , the base-emitter capacitance C _be , and the capacitance C _OX .

Further, the random noise components are kTC noise at the time of resetting which depends on the combined capacitance C _B in the period T ₂ , and random noise at the time of the sensor transistor reading operation. The kTC noise at reset is a little smaller due to the transient reset, However, it is removed by the above subtraction processing.

Further, random noise in a read operation base capacitance C _B, the load capacitance C _V and the storage capacitor C _T as viewed from the emitter, although depends on the current amplification factor h _FE, non-destructive to increase the current amplification factor h _FE It was found that if the degree is increased, it can be made much smaller than the random noise at the time of reset. This means that the sensor's FPN and the residual kTC noise at reset are
This means that the S / N ratio can be greatly improved by the subtraction process of the optical signal and the sensor noise. In CCD, by charge / voltage conversion output amplifier post-correlation double sampling method,
Although the kTC noise caused by the reset transistor is removed, the amplifier noise is dominant because of the high speed driving.

The one in which each pixel is composed of the amplifying element as in the present invention is a low speed scan for every 1H, and therefore the amplifier noise, that is, the read noise is extremely small.

Further, since the dark current component of the amplifying transistor resets the base before the transfer of the photocharge, there is no problem.

Although the transient reset is performed in the period T ₂ at the timing of the embodiment shown in FIG. 5, the transient reset is not an essential condition. For example, when the amplifying transistor is a bipolar transistor, transient reset can be performed, but when the amplifying transistor is a MOS transistor, it is impossible.

FIG. 6 is a partial circuit diagram of a second embodiment of the photoelectric conversion device of the present invention.

The characteristic part of this embodiment is that the amplifying element is a MOS transistor.

In the figure, the MOS transistor Tr _A ′ is a reset MOS transistor for clearing the residual charge in the gate region which is the control electrode of the MOS transistor Tr _C ′.
In the first embodiment, since the amplifying element is composed of the bipolar transistor, the transient reset is performed. However, in this embodiment, since the amplifying element is the MOS transistor, the transient reset operation is unnecessary. Random noise when transferring the photocharge accumulated in the photo diode D to the gate and reading the optical signal from the MOS transistor of the amplifying element is the control electrode at the time of reading if MOS or FET is used as the amplifying element of the sensor MOS. Since the gate charge is not destroyed, the correlation between the sensor noise and the sensor noise included in the optical signal output becomes very high, and as a result, a high S / N sensor can be obtained.

FIG. 7 is a sectional view of a sensor according to the third embodiment of the present invention.

In this embodiment, a photoconductive film formed by stacking layers in the photoelectric conversion region is used.

As shown in the figure, a pixel electrode made of a photoconductive film is connected to the photodiode portion of FIG. 4 which is the first embodiment of the photoelectric conversion device.

In the figure, 1 is a photoelectric film, 2 is a transparent electrode, 3 is a pixel electrode,
4 is an insulating layer, the photoconductive film 1 is through the pixel electrode 3,
It is connected to the p-type semiconductor region that constitutes the photodiode portion.

According to the present embodiment, the photoconductive film can be provided also on the amplification element portion, so that the aperture ratio of the receptacle and the S / N ratio can be increased.

FIG. 8 is a schematic configuration diagram of a solid-state imaging device to which the present invention is applied.

In the figure, the vertical scanning unit 202 and the horizontal scanning unit 203 perform television scanning on the image sensor 201 in which optical sensors are arranged in an area.

The signal output from the horizontal scanning unit 203 is output as a standard television signal through the processing circuit 204.

Driving pulse φ _{HS for} the vertical and horizontal scanning units 202 and 203,
φ _H1 , φ _H2 , φ _VS , φ _V1 , φ _V2, etc. are supplied by the driver 205. The driver 205 is also limited by the controller 206.

[Effects of the Invention] As described in detail above, according to the photoelectric conversion device of the present invention, the switch means is provided between the storage means for accumulating the photoexcited charge and the amplification means for amplifying the charge of the control electrode. As a result, the sensor noise of the amplification element can be independently read regardless of the operation of the storage unit, and not only the FPN but also the dark current component and random noise of the amplification element can be removed, and the amplification element can be driven at low speed. It is possible to realize an image pickup device with a high S / N ratio, with very small noise during reading.

Further, according to the present invention, reset means for resetting the control electrode of the amplifying means is provided before the charge of the accumulating means is transferred to the amplifying means, and after the reset, the output of the amplifying means is read as the first signal and the transfer element And the charge of the accumulating means is transferred to the control electrode and then the output of the amplifying means is read out as a second signal, whereby the dark current component of the amplifying means can be removed before the transfer of the photocharges. It is possible to set the potential of the control electrode of the amplifying means lower than the accumulated potential, and to completely transfer the photocharges from the accumulating means to the amplifying means.

It is to be noted that when a storage area serving as storage means and an amplification area serving as amplification means are formed on a substrate, a photoelectric conversion area is stacked on the storage area, and the photoelectric conversion area and the amplification area are connected to each other, the aperture ratio of the socket And the S / N ratio can be increased.

[Brief description of drawings]

FIG. 1 is a partial circuit diagram of a first embodiment of a photoelectric conversion device of the present invention. FIG. 2 is a schematic plan view of a pixel of the photoelectric conversion device. FIG. 3 is a sectional view taken along line XX ′ of the photoelectric conversion device of FIG. FIG. 4 is a cross-sectional view of the photoelectric conversion device of FIG. 2 taken along the line YY ′. FIG. 5 is a timing chart of the photoelectric conversion device. 3 is a partial circuit diagram of FIG. FIG. 6 is a partial circuit diagram of a second embodiment of the photoelectric conversion device of the present invention. FIG. 7 is a sectional view of a sensor according to the third embodiment of the present invention. FIG. 8 is a schematic configuration diagram of a solid-state imaging device to which the present invention has been applied. Tr _A : PMOS transistor, Tr _B : transfer element, Co _X : capacitance, D: photo diode D, Tr _C : amplification transistor, φ _VR ,
φ _TB , φ _T1 , φ _T2 , φ _VC : pulse, T _VC , T ₁ , T ₂ : MOS transistors, C ₁ , C ₂ : capacitors.

Claims (2)

(57) [Claims] 1. Storage means for storing photoexcited charges,
A reset means for resetting the control electrode before transferring the charge of the storage means to the amplification means, and an amplification means for amplifying the charge of the control electrode, and a transfer element for connecting the storage means and the control electrode. And, after resetting, first reading means for reading the output of the amplifying means as a first signal, transfer means for electrically connecting the transfer element, and transferring the charge of the accumulating means to the control electrode, and transfer of the charge. Second read means for reading the output of the amplifying means as a second signal later, and subtraction processing means for performing subtraction processing of the first signal and the second signal. Photoelectric conversion device. 2. A storage region serving as the storage unit and an amplification region serving as the amplification unit are formed on a substrate, and a photoelectric conversion region is stacked on the storage region, and the photoelectric conversion region and the amplification region are electrically connected to each other. The photoelectric conversion device according to claim 1, wherein the photoelectric conversion device is connected.