JP3487575B2 - Photoelectric conversion device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoelectric conversion device having a large number of photoelectric conversion elements, and in particular, a transistor or a MOS switch circuit for reading charges of the photoelectric conversion elements can be used to expand the dynamic range and achieve a high S / N ratio. The present invention relates to a photoelectric conversion device.

[0002]

2. Description of the Related Art Conventionally, as a solid-state image pickup device, a CCD type photoelectric conversion element was often used, but recently, a movement to commercialize a MOS type photoelectric conversion element has appeared. Although it has been said that the MOS type photoelectric conversion device is inferior in image quality to the CCD type, it reduces noise, consumes less power than the CCD type, and is operated by a single power source, and has a light receiving section and peripheral circuits. Have been reviewed because of the merit that they can be manufactured by the same MOS process and are easily integrated. Here, in order to improve the image quality of the MOS type solid-state image pickup device, it is becoming possible to reduce random noise and fixed pattern noise.
In order to obtain N image signals, it is required to expand the dynamic range of the photoelectric conversion element.

FIG. 5 shows a block diagram of a conventional CMOS type area sensor. FIG. 5 is a configuration diagram of a 2 × 2 pixel two-dimensional sensor, but the number of pixels is not limited to this.

A pixel circuit of the CMOS type area sensor shown in FIG. 5 will be described. Photodiode 90 in the pixel
1, a transfer switch 911, a reset switch 902, a pixel amplifier 903, and a row selection switch 904 are provided.
The gate of the transfer switch 911 is connected to ΦTX (n, n + 1) from the vertical scanning circuit 910, and the gate of the reset switch 902 is ΦRES from the vertical scanning circuit 910.
(N, n + 1), and the gate of the row selection switch 904 is ΦSEL (n, n +) from the vertical scanning circuit 910.
Connected to 1).

Photoelectric conversion is performed by the photodiode 901, and a transfer switch 911 is provided during the accumulation period of the light quantity charge.
Is in the off state, and the electric charge photoelectrically converted by the photodiode 901 is not transferred to the gate of the source follower 903 that constitutes the pixel amplifier. The gate of the source follower 903 that constitutes the pixel amplifier is initialized to an appropriate voltage by turning on the reset switch 902 before starting the accumulation. That is, this is the dark level. Next, or at the same time, when the row selection switch 904 is turned on, the source follower circuit composed of the load current source 905 and the pixel amplifier 903 is activated, and the transfer switch 911 is turned on to turn on the photodiode. 90
The charge accumulated in 1 is transferred to the gate of the source follower 903 that constitutes the pixel amplifier.

Here, the output of the selected row is the vertical output line 906.
Occurs on. This output is transferred to transfer gates 909a and 909.
It is stored in the signal storage unit 907 via b. The output temporarily stored in the signal storage unit 907 is sequentially read by the horizontal scanning circuit 908 to the output unit V0.

FIG. 6 is an operation timing chart of the CMOS type area sensor of FIG. At the timing of the all-pixel reset period T1, ΦTX (n) and ΦTX (n + 1) become active, and the charges of the photodiodes 901 of all the pixels are transferred to the source follower 9 via the transfer switch 911.
03, the photodiode 901 is reset. In this state, the cathode charge of the photodiode 901 is transferred to the gate of the source follower 903 and averaged. By increasing the capacitor CFD913 component of the gate of the source follower 903,
This is the same as the level when the cathode of the photodiode 901 is reset.

At this time, the mechanical shutter 11 (not shown) that guides the light amount of the target image is open, and simultaneously with the end of the time T1, accumulation of all pixels starts simultaneously. The mechanical shutter 11 keeps the period T3 open, and the period is the accumulation period of the photodiode 901.

After the lapse of T3, the mechanical shutter is closed at the timing of T4, and the accumulation of the photocharges of the photodiode 901 is completed. In this state, the photodiode 9
The electric charge is accumulated in 01. Next, reading is started for each line. That is, the N-1th row is read and then the Nth row is read.

During a period of time T5, ΦSEL (n) is activated, the row selection switch 904 is turned on, and the source / follower circuits composed of the pixel amplifiers 903 of all the pixels connected to the nth row are in the operating state. become. Here, in the gate of the source follower formed by the pixel amplifier 903, ΦRES (n) becomes active in the period T2, the reset switch 902 is turned on, and the gate of the source follower 903 is initialized. That is, the dark level signal is output to the vertical output line 906.

Next, ΦTN (n) becomes active, the transfer gate 909b is turned on, and the signal is stored in the signal storage section 907. This operation is simultaneously executed in parallel for all pixels connected to N rows. At the time when the transfer of the dark-level signal storage unit 907 is completed, the signal charge stored in the photodiode 901 is activated by ΦTX (n), thereby turning on the transfer switch 911 and causing the pixel amplifier 903 to operate. Transfer to the configured source-follower gate. At this time, the potential of the gate of the source follower formed by the pixel amplifier 903 changes from the reset level by an amount corresponding to the transferred signal charges, and the signal level is determined.

At this time, ΦTS becomes active, the transfer gate 909a is turned on, and the signal level becomes the signal storage unit 9
It is held at 07. This operation is simultaneously executed in parallel for all pixels connected to N rows. Here, the signal accumulating unit 907 holds the dark level and the signal level of all the pixels connected to the N rows, and by obtaining the difference between the dark level and the signal level between the pixels, the source follower Of the fixed pattern noise (FPN) due to the variation of the threshold voltage Vth and the KTC noise generated when the reset switch 902 is reset, and a signal in which a noise component having a high S / N is removed is obtained.

This signal is horizontally scanned by the horizontal scanning circuit 908 for the difference signal between the dark level and the signal level accumulated in the signal accumulating section 907, and is output in time series at the timing of T7. This completes the output of the Nth row. Similarly, ΦSEL (n + 1) and ΦRES (n +
By driving 1), ΦTX (n + 1), ΦTN, and ΦTS in the same manner as in the Nth row as shown in FIG. 5, the signal in the N + 1th row can be read.

In the above conventional example, since the difference between the dark level and the signal level is output, high S / N can be realized and a high quality image signal can be obtained. Further, since the solid-state image sensor of the present embodiment can be realized by a CMOS compatible process, it can be integrated with a peripheral circuit on a single chip, so that cost reduction and high functionality can be realized.

[0015]

However, even if the noise component is reduced, if the dynamic range of image reading is small, a substantially high S / N cannot be obtained.

Here, considering the dynamic range of image reading, from the graph shown in FIG. 7, Vmax = VG (RES) -Vth (RES) (1) (where VG (RES) is the gate of the reset switch) potential,
Vth (RES) is the threshold of the reset switch), and the gate potential ΦRES of the reset switch RES
The difference between (n) and the gate-source threshold is: Vmin = VG (TX) −Vth (TX) (2) (where VG (TX) is the gate potential of the transfer switch, Vth
(TX) is the threshold value of the transfer switch), which is the difference between the gate potential ΦTX (n) of the transfer switch TX and the gate-source threshold, and the dynamic range Dy of image reading is Dy (gate). = Vmax-Vmin = VG (RES) -VG (TX) + Vth (TX) -Vth (RES) (3). In the equation (3) of this dynamic range Dy, Vt
Since h (TX) -Vth (RES) has a manufacturing variation, the stability of the dynamic range Dy becomes extremely unfavorable.

Next, considering the threshold values of the MOS transistors in the pixel portion, when these MOS transistors are configured with a single threshold value, especially in order to ensure the stability of the switching characteristics of the selection switch. In addition, it is necessary to set the threshold value to some extent high. But,
It is advantageous to set the threshold Vth (RES) of the reset switch low so that the dynamic range is secured. Because the reset potential Vma set by the equation (1)
This is because x becomes high and it becomes possible to make maximum use of the linear region of the source follower circuit. Therefore,
A single threshold setting limits the dynamic range.

As a method of improving this point, a method of setting the threshold value of the reset switch lower than the threshold values of other MOS transistors has been proposed. However, in this case, the threshold values of the transfer switch and the reset switch are different, and the fluctuation of the threshold value due to the manufacturing process varies independently of each other. As a result, there is a problem that the dynamic range Dy expressed by the equation (3) varies. Had occurred.

In view of the above problems, it is an object of the present invention to increase the dynamic range and reduce variations due to the manufacturing process. Furthermore, it is a further object to completely reset the photoelectric conversion element.

[0020]

In order to achieve the above object, the present invention provides a photoelectric conversion element, transfer switch means for transferring the signal charge generated in the photoelectric conversion element, and a gate for receiving the transferred signal charge. In a photoelectric conversion device having a read circuit for reading out by read and reset switch means for resetting the gate of the read circuit, the transfer switch means and the reset switch means have equal threshold values, and the transfer switch means and the reset There is provided a photoelectric conversion device characterized in that each threshold value of the switch means is lower than the threshold value of the field effect transistor of the read circuit.

The present invention also relates to a photoelectric conversion element and a transfer switch for transferring the signal charge generated in the photoelectric conversion element.
Means , a field effect transistor including a gate region for receiving signal charge from the transfer switch means and a source / drain for reading out a signal corresponding to the signal charge accumulated in the gate, and power is supplied to the field effect transistor. A photoelectric conversion device comprising a plurality of pixel cells, each of which includes a power supply unit, a selection switch unit connected between the field effect transistor and the power supply unit, and a reset switch unit for resetting the gate region, The selection switch means, the reset switch means , and the transfer switch means are field effect transistors, and the transfer switch means and the reset
The threshold values of the switch means are equal, and the transfer switch
Means and the threshold value of the reset switch means.
It is characterized in that it is lower than the threshold value of the selection switch means .

[0022]

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail with reference to the drawings. The overall structure of the photoelectric conversion device described below is as shown in FIG. 5, and each of the photoelectric conversion devices will be described in detail.

[First Embodiment] FIG. 1 shows a sectional view of a photoelectric conversion device according to an embodiment of the present invention. In the figure, 1 is a semiconductor substrate, and in the figure, an example of a P-type semiconductor is shown. Reference numeral 2 denotes a gate electrode formed on the semiconductor substrate 1 via a gate oxide film, which is made of, for example, polysilicon or polycide. Reference numerals 3 and 3 ′ are a source electrode and a drain electrode having a conductivity type opposite to that of the semiconductor substrate 1, which are formed in the semiconductor substrate 1 by ion implantation or the like, and a field effect transistor is constituted by the above. In addition, 4,
4'is a channel region between the drain and source of the MOS field effect transistor, and 5'is an insulating oxide film layer between the gate electrode and the semiconductor substrate.

Here, by adjusting the concentration of the channel dope layer 4 formed in the channel region of the transistor, the threshold voltage of each transistor can be easily made different. That is, the channel doping layer 4 is formed in the channel regions of all the transistors by the first channel doping, and the channel doping of only the desired transistor is further performed by the second channel doping. The layer 4'can be formed. For example, in the example of FIG. 1, if the channel dope layer 4 is doped with N-type ion species for the second time, the threshold voltage can be lowered as compared with the case where it is not doped, and conversely if it is doped with P-type ion species. The threshold voltage can be raised. The amount of change can be accurately determined by controlling the concentration of the channel dope layer 4 ′ for the second time.

Therefore, the transfer switch and the reset switch are channel-doped only for the first time, and the read system of the source follower and the selection switch has a structure in which the channel-doping is executed for the first time and the second time. Since the steps of determining the threshold voltage of the switch are the same, the amount and direction of manufacturing variation of the thresholds Vth (TX) and Vth (RES) of the transfer switch and the reset switch are equal to each other. (3) Medium Vth (TX) -Vth
The value of (RES) stabilizes and the dynamic range itself stabilizes.

Since the channel dope of the transfer switch and the reset switch is small, the reset switch
Since the threshold value becomes low and the reset voltage in the equation (1) becomes high, the dynamic range can be expanded in this respect as well.

In the above-mentioned embodiment, the number of times of channel dope injection is increased in the case of the source follower and the selection switch with respect to the transfer switch and the reset switch, but the gate width and the gate length of the MOS transistor of each switch are shown. May be provided separately. That is, due to the narrow channel effect and / or the short channel effect, if the gate width of the MOS transistor is made small, its threshold value becomes large, and if the channel length is made small, its threshold value becomes small . Utilizing this point, it is desired to expand the dynamic range by setting the number of times of channel dope implantation in the same manner as in the case of providing a difference.

In addition to the above two examples, a method of thickening the source follower and the gate oxide film 5'of the selection switch (readout circuit) with respect to the gate oxide film 5 of the transfer switch and the reset switch is adopted. Very good. Furthermore, by adopting a plurality of these methods at the same time, it is possible to further enhance the effects of expanding and stabilizing the dynamic range.

[Second Embodiment] In the second embodiment of the present invention, the case where the selection switch shown in FIG. 5 is not provided will be described. A circuit configuration diagram of the photoelectric conversion element is shown in FIG. In FIG. 2, the photocharges accumulated in the photodiode 901 are supplied to the gate of the source follower 903 by the transfer switch 911, the source follower 903 is activated along with the high level of the selection pulse ΦSEL, and the load of the current source 905 causes the vertical output line 906 to flow. When the signal is read and ΦTS is high, the photocharge corresponding to the photocharge of the photodiode 901 is accumulated in the signal accumulating unit 907. On the other hand, Source Follower 9
For resetting the gate of 03, before the transfer switch 911 is turned on, it is reset to a high level by making ΦRES high. Immediately thereafter, the source follower 903 is activated along with the high level of the selection pulse ΦSEL to activate the source output through the vertical output line 906. The transfer gate 909b is turned on to store the noise component in the signal storage unit 907.

In the photoelectric conversion element having such a configuration, since there is no selection switch, there is no upper limit of the range in which the source follower 903 operates linearly. Therefore, by setting the reset potential high, the range of linear operation of the source follower 903 can be naturally expanded. Further, the reset potential VG (SF) is reduced by lowering the threshold value of the reset switch 902, so that the reset gate potential VG ( RES ) -Vth ( R
Since ES ) = VG (SF), the reset potential VG (SF) can be raised.

On the other hand, the threshold value Vth (SF) of the MOS transistor of the source follower 903 is stabilized without variation and cannot be lowered so much. Therefore, the condition is Vth (SF)> Vth (RES) (4).

As described in the first embodiment with reference to FIG. 1, this condition is that the threshold of the source follower is larger than the threshold of the reset switch because the channel doping of the reset switch is small. In addition, since the expression (4) is satisfied, it is possible to increase the dynamic range and reduce manufacturing variations.

[Third Embodiment] A third embodiment of the present invention will be described with reference to FIG. FIG. 3 is a plan view of one pixel of the photoelectric conversion device shown in FIG. The photoelectric conversion device includes a photodiode 901 having a wide opening, a transfer switch 911 for transferring photocharges thereof, a source follower 903 having a floating diffusion portion, a reset switch 902 for resetting the floating diffusion portion, and a drain of the source follower 903. The selection switch 904 is connected to the power supply line VDD and the vertical output line 90.
6 and 6 are formed.

When manufacturing such a structure, channel doping is performed over the entire surface during the first channel doping, and then the photodiode 901 is formed during the second channel doping.
The transfer switch 911 and the reset switch 902,
As shown by the resist at the time of CD2 implantation in the dotted line frame, the resist is covered and channel-doped ion implantation is performed.

Here, in plan view, the gate length is shortened by the transfer switch 911 and the reset switch 902, and long by the source follower 903 and the selection switch 904. Therefore, by making the channel length of the selection switch 904 larger than the channel length of the reset switch 902 and larger than the channel length of the source follower 903, the linear operation range can be enlarged.

FIG. 4 is a plan view showing FIG. 3 more specifically. According to the figure, the channel length of the transfer switch 911 is 0.6 μm, and the channel length of the reset switch 902 for resetting the floating diffusion portion is 0.
6 μm and the source follower 903 has a channel length of 1.0
and the channel length of the selection switch 904 is 1.0 μm.
m.

As a result, it is possible to widen the dynamic range of the source follower and reduce fluctuations in the dynamic range due to variations in the manufacturing stage.

In the photoelectric conversion device according to the above embodiment, the photoelectric conversion element has the structure of a photodiode, but it can be formed together with the shift register of the scanning circuit or the like by the CMOS process in manufacturing, so that the above characteristics are improved as a so-called CMOS sensor. Can be used universally.

[0040]

As described above, according to the present invention,
It is possible to widen the dynamic range of the photoelectric conversion device and suppress variations in the manufacturing stage as much as possible. Further, it is possible to widen the linear operation range of the source follower and reduce the manufacturing variations.

In particular, recent CMOS type photoelectric conversion devices are expanding to applications such as digital still cameras and video camcorders, which require higher definition (multiple pixels) and lower power consumption (low voltage). At this time, if it is desired to increase the driving dynamic range by increasing the definition (increasing the number of pixels), the linear operating range, and lower power consumption (lower voltage), the total dynamic range is increased to S /
It is possible to obtain N image signals.

[Brief description of drawings]

FIG. 1 is a schematic explanatory view of a photoelectric conversion device of the present invention.

FIG. 2 is a schematic circuit diagram of a solid-state imaging device of the present invention.

FIG. 3 is a plan view of a photoelectric conversion device according to the present invention.

FIG. 4 is a specific plan view of a photoelectric conversion device according to the present invention.

FIG. 5 is a schematic circuit diagram of a conventional solid-state imaging device.

FIG. 6 is an operation timing chart of a conventional solid-state imaging device.

FIG. 7 is an operation explanatory diagram of the solid-state imaging device.

[Explanation of symbols] 1 Semiconductor substrate 2 Gate electrode 3 Source / drain electrodes 4 channel dope 901 photodiode 902 reset switch 903 Source Follower 905 current source 906 Vertical output line 907 Signal storage unit 908 Horizontal scanning circuit 910 Vertical scanning circuit 911 transfer switch 913 capacitor

─────────────────────────────────────────────────── ─── Continuation of front page (72) Toru Koizumi 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Katsuhisa Ogawa 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. Co., Ltd. (72) Inventor Katsuhito Sakurai 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Shigetoshi Sugawa 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (56) References JP 10-256520 (JP, A) JP 11-103043 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H04N 5/335 H01L 27 / 146

Claims (9)

(57) [Claims] 1. A photoelectric conversion element, transfer switch means for transferring signal charges generated in the photoelectric conversion element, a read circuit for receiving and transferring the transferred signal charge at a gate, and resetting the gate of the read circuit. In the photoelectric conversion device having a reset switch means, the transfer switch means and the reset switch means have the same threshold value, and the transfer switch means and the reset switch means have the same threshold value for the field effect of the readout circuit. A photoelectric conversion device characterized by being lower than a threshold value of a transistor. 2. The read circuit comprises a source follower that amplifies in a source follower type via the transfer switch, and a selection switch connected to the source follower in series and activated. Photoelectric conversion device. Wherein said readout circuit and said reset switch means comprises a field effect transistor, photoelectric conversion device according to claim 1 in which the impurity concentration of the channel region of each transistor is different from each other. 4. The transfer switch means, the reset switch means, and the read circuit are subjected to a first channel doping, and the read circuit is further subjected to a second channel doping. The photoelectric conversion device according to claim 3. 5. The photoelectric conversion device according to claim 1, wherein the readout circuit and the reset switch means are field effect transistors, and the channel lengths of the gates of the respective transistors are different from each other. 6. The photoelectric conversion device according to claim 1, wherein the readout circuit and the reset switch means are field effect transistors, and the channel widths of the gates of the respective transistors are different from each other. 7. The photoelectric conversion device according to claim 1, wherein the readout circuit and the reset switch means are field effect transistors, and gate lengths and channel widths of gates of the respective transistors are different from each other. 8. The photoelectric conversion device according to claim 1, wherein the readout circuit and the reset switch means are field effect transistors, and the thickness of the oxide film of the gate of each transistor is different from each other. 9. A photoelectric conversion element, transfer switch means for transferring the signal charge generated in the photoelectric conversion element, a gate region for receiving the signal charge from the transfer switch means , and a signal charge accumulated in the gate. A field effect transistor including a source / drain for reading a signal;
A plurality of pixel cells each including a power supply unit for supplying power to the field effect transistor, a selection switch unit connected between the field effect transistor and the power supply unit, and a reset switch unit for resetting the gate region are arranged. In the photoelectric conversion device, the selection switch means, the reset switch means , and the transfer switch means are field effect transistors, and the transfer switch means and the reset switch means are provided.
Switch thresholds are equal, and the transfer switch
Means and the threshold value of the reset switch means.
A photoelectric conversion device characterized by being lower than the threshold value of the selection switch means .