JPH11196331A - Photoelectric converted - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase dynamic range and to reduce dispersion due to manufacturing process by making each threshold of a transfer switching means and a reset switching means equal and making each threshold of the transfer switching means and the reset switching means different from the threshold of a field- effect transistor of a reading circuit. SOLUTION: The threshold voltage of each transistor can easily be made different by adjusting the concentration of a channel part 4 that is formed in a channel region of the transistor. A first channel doping forms the part 4 in channel regions of all transistors and a second channel doping further performs channel doping only to the channel region of a desired transistor and form a channel doped part 4' which has different concentration. The amounts and directions of dispersion of threshold production of a transfer switch and a reset switch become equal and a dynamic range is stabilized.

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 more particularly to a transistor and a MOS switch circuit for reading out charges of the photoelectric conversion elements, which can expand the dynamic range and enable high S / N. And a photoelectric conversion device.

[0002]

2. Description of the Related Art Conventionally, as a solid-state imaging device, there have been many examples using a CCD photoelectric conversion element, but recently, there has been a movement to commercialize a MOS photoelectric conversion element. It has been said that the MOS type photoelectric conversion device has inferior image quality as compared with the CCD type, but it reduces noise, operates with a single power supply with lower power consumption than the CCD type, and has a light receiving unit and peripheral circuits. Has been reviewed due to the merit that it can be manufactured in the same MOS process and is easy to integrate. Here, in order to improve the image quality of the MOS solid-state imaging device, it is 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 configuration diagram of a conventional CMOS 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.

[0004] A pixel circuit of the CMOS area sensor shown in FIG. 5 will be described. A photodiode 90 is provided 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 connected to ΦRES from the vertical scanning circuit 910.
(N, n + 1), and the gate of the row selection switch 904 is connected to the ΦSEL (n, n +) from the vertical scanning circuit 910.
1).

[0005] The photoelectric conversion is performed by the photodiode 901, and the transfer switch 911 during the accumulation period of the light quantity charge.
Is in an off state, and the electric charge photoelectrically converted by the photodiode 901 is not transferred to the gate of the source follower 903 constituting the pixel amplifier. Before the start of accumulation, the reset switch 902 of the gate of the source follower 903 constituting the pixel amplifier is turned on and initialized to an appropriate voltage. That is, this is the dark level. Next or simultaneously, 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 in an operating state, and the transfer switch 911 is turned on. 90
The charge accumulated in 1 is transferred to the gate of the source follower 903 constituting the pixel amplifier.

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

FIG. 6 is an operation timing chart of the CMOS 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, and the photodiode 901 is reset. This state is a state in which the cathode charge of the photodiode 901 is transferred to the gate of the source follower 903 and is averaged. By increasing the component of the capacitor CFD913 of the gate of the source follower 903,
The level is the same as the level at which the cathode of the photodiode 901 is reset.

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

After the elapse of the time T3, the mechanical shutter closes at the timing of T4, and the accumulation of the photoelectric charge of the photodiode 901 ends. In this state, the photodiode 9
01 stores electric charge. Next, reading starts for each line. In other words, the N-th row is read after the (N-1) -th row is read.

During the period of time T5, ΦSEL (n) is activated, the row selection switch 904 is turned on, and the source / follower circuit composed of the pixel amplifiers 903 of all the pixels connected to the n-th row operates. become. Here, in the gate of the source / follower constituted 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, this dark level signal is output to the vertical output line 906.

Next, ΦTN (n) becomes active, the transfer gate 909b turns on, and the signal is stored in the signal storage unit 907. This operation is performed simultaneously and in parallel for all the pixels connected to the N rows. When the transfer of the dark-level signal to the signal storage unit 907 is completed, the signal charge stored in the photodiode 901 is activated by ΦTX (n), so that the transfer switch 911 is turned on. Forward to configured source follower gate. At this time, the potential of the gate of the source / follower constituted by the pixel amplifier 903 fluctuates from the reset level by an amount corresponding to the transferred signal charge, and the signal level is determined.

Here, .phi.TS becomes active, the transfer gate 909a is turned on, and the signal level becomes
07. This operation is performed simultaneously and in parallel for all the pixels connected to the N rows. Here, the signal accumulation unit 907 holds the dark level and the signal level of all the pixels connected to the N rows, and obtains the difference between the dark level and the signal level between each pixel to obtain 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 from which a noise component having a high S / N is removed can be obtained.

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

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

[0015]

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

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 value of the reset switch), and the gate potential ΦRES of the reset switch RES.
From (n), the difference between the threshold of the gate and the source is obtained. 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 threshold value of the gate and source, and the dynamic range Dy for image reading is Dy (gate) = Vmax-Vmin = VG (RES) -VG (TX) + Vth (TX) -Vth (RES) (3) In equation (3) of this dynamic range Dy, Vt
h (TX) -Vth (RES) has a variation in manufacturing, and makes the stability of the dynamic range Dy extremely fuzzy.

Next, considering the threshold value of the MOS transistor in the pixel portion, when these MOS transistors are constituted by a single threshold value, in particular, in order to secure the stability of the switching characteristics of the selection switch. In addition, it is necessary to set the threshold value to some extent. But,
Setting the threshold value Vth (RES) of the reset switch low is advantageous in that the dynamic range is ensured. This is because the reset potential Vma set by the equation (1)
This is because x increases, and the linear region of the source follower circuit can be used to the maximum. Therefore,
With a single threshold setting, the dynamic range is restricted.

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

An object of the present invention is to realize both the expansion of the dynamic range and the reduction of variations due to the manufacturing process in view of the problem of abnormalities. Still another object is 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 signal charges generated in the photoelectric conversion element, and a gate for receiving the transferred signal charges at a gate. And a reset switch for resetting a gate of the read circuit, wherein the transfer switch and the reset switch have equal thresholds, and the transfer switch and the reset are reset. It is another object of the present invention to provide a photoelectric conversion device, wherein each threshold value of the switch means is different from the threshold value of the field effect transistor of the readout circuit.

According to the present invention, there is provided a photoelectric conversion element, a transfer switch for transferring a signal charge generated in the photoelectric conversion element, a gate region receiving the signal charge from the transfer switch, and a signal charge stored in the gate. A field effect transistor including a source and a drain for reading a corresponding signal, power supply means for supplying power to the field effect transistor, and selection switch means connected between the field effect transistor and the power supply means. In a photoelectric conversion device in which a plurality of pixel cells each including a reset unit for resetting the gate region are arranged, the selection switch unit and the reset switch unit include a field effect transistor, and respective threshold values are different from each other. It is characterized by.

Further, the present invention provides a photoelectric conversion element, a transfer switch for transferring a signal charge generated in the photoelectric conversion element, a gate region receiving the signal charge from the transfer switch, and a signal charge stored in the gate. A plurality of pixel cells each including a field effect transistor including a source / drain path for reading a corresponding signal, power supply means for supplying power to the field effect transistor, and a reset switch for resetting the gate region are arranged. In the photoelectric conversion device, the field effect transistor, the reset switch, and the transfer switch have different threshold values, and the threshold value of the field effect transistor is large.

[0023]

Embodiments of the present invention will be described in detail with reference to the drawings. The overall configuration of the photoelectric conversion device described below has a configuration as illustrated in FIG. 5, and each of the photoelectric conversion devices will be described in detail.

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

Here, the threshold voltage of each transistor can be easily varied by adjusting the concentration of the channel dope layer 4 formed in the channel region of the transistor. 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 is further performed only in the channel regions of the desired transistors by the second channel doping, so that the channel doping having different concentrations is performed. Layer 4 'can be formed. For example, in the example of FIG. 1, if the channel dope layer 4 is doped with the second N-type ion species for the second time, the threshold voltage can be reduced as compared with the case where the N-type ion species is not doped. The threshold voltage can be increased. The amount of the change can be accurately determined by controlling the concentration of the second channel doping layer 4 '.

Therefore, the transfer switch and the reset switch have the first channel doping, and the readout system of the source follower and the selection switch has a structure in which the first and second channel dopings are performed. Since the process of determining the threshold voltage of the switch is the same, the amount and direction of the manufacturing variation between the threshold values Vth (TX) and Vth (RES) of the transfer switch and the reset switch are equal, and (3) Medium Vth (TX) -Vth
The value of (RES) is stabilized, and the dynamic range itself is stabilized.

Further, since the channel doping of the transfer switch and the reset switch is small, the thresholds of the source follower and the selection switch become larger than the thresholds of the transfer switch and the reset switch, and the reset voltage of the equation (1) becomes high. Therefore, this point can also achieve the expansion of the dynamic range.

In the above embodiment, the number of times of channel doping is increased in the case of the source follower and the selection switch with respect to the transfer switch and the reset switch. However, the gate width and the gate length of the MOS transistor of each switch are described. May be provided separately. That is, if the gate width of a MOS transistor is reduced due to the narrow channel effect and / or the short channel effect, the threshold value is increased, and if the channel length is reduced, the threshold value is reduced. ) And Expression (6), the channel width of the selection switch is made smaller than the channel width of the reset switch and smaller than the channel width of the source follower. Alternatively, by setting the channel length of the selection switch larger than the channel length of the reset switch and larger than the channel length of the source follower, the linear operation range can be increased. It is desired that the dynamic range be expanded by setting this point together with the case where a difference is provided in the number of times of channel doping described above.

In addition to the above two examples, a method of increasing the thickness of the gate oxide film 5 'of the source follower and the selection switch (readout circuit) with respect to the gate oxide film 5 of the transfer switch and the reset switch. You may take it. Furthermore, by simultaneously employing a plurality of these methods, it is possible to further enhance the effect of expanding and stabilizing the dynamic range.

[Second Embodiment] In a second embodiment of the present invention, a case where the selection switch shown in FIG. 5 is not provided will be described. FIG. 2 shows a circuit configuration diagram of the photoelectric conversion element. FIG. 2 shows that the photocharge accumulated in the photodiode 901 is supplied to the gate of the source follower 903 by the transfer switch 911, the source follower 903 is activated together with the selection pulse ΦSEL being high, and the vertical output line 906 is supplied by the load of the current source 905. The signal is read out, and a photocharge corresponding to the photocharge of the photodiode 901 is stored in the signal storage unit 907 when ΦTS is high. On the other hand, source follower 9
In resetting the gate of 03, by turning ΦRES high before turning on the transfer switch 911, the gate is reset to a high level. Immediately after that, the source follower 903 is activated together with the high level of the selection pulse ΦSEL, and the vertical line Then, the transfer gate 909b is turned on to accumulate a noise component in the signal accumulation unit 907.

In the configuration of the photoelectric conversion element having such a configuration, since there is no selection switch, the upper limit of the range in which the source follower 903 operates linearly is eliminated. Therefore, by setting the reset potential high, the range in which the source follower 903 operates linearly can be naturally increased. 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 (RST) −Vth (R
Since ST) = VG (SF), the reset potential VG (SF) can be increased.

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

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

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 the photoelectric charge, 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. And a power supply line VDD and a vertical output line 90.
6 are formed.

At the time of manufacturing such a configuration, channel doping is performed entirely during the first channel doping, and then the photodiode 901 is performed during the second channel doping.
And the transfer switch 911 and the reset switch 902,
As shown by the resist at the time of implantation of CD2 in the dotted frame in the figure, the resist is covered and channel-doped ion implantation is performed.

Here, in plan view, the gate length is made shorter by the transfer switch 911 and the reset switch 902, and made longer by the source follower 903 and the selection switch 904. Thus, by setting 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 increased.

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

As a result, the dynamic range of the source follower can be expanded, and the fluctuation of the dynamic range due to the variation in the manufacturing stage can be reduced.

Although the photoelectric conversion device according to the above-described embodiment has a photoelectric conversion element having a photodiode structure, it can be formed together with a shift register of a scanning circuit by a CMOS process in manufacturing. Can be used for general purposes.

[0040]

As described above, according to the present invention,
The dynamic range of the photoelectric conversion device can be expanded, and variations in the manufacturing stage can be suppressed as much as possible. In addition, the linear operation range of the source follower can be widened, and the variation in its manufacture can be reduced.

In particular, CMOS type photoelectric conversion devices in recent years are expanding to applications requiring higher definition (multiple pixels) and lower power consumption (low voltage), such as digital still cameras and video camcorders. In such a case, if it is desired to expand the drive dynamic range by increasing the definition (multiple pixels), expand the linear operation range, and reduce the power consumption (lower voltage), the dynamic range is increased as a whole. S /
N image signals can be obtained.

[Brief description of the 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 according to 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 the 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 a diagram illustrating the operation of the solid-state imaging device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor substrate 2 Gate electrode 3 Source / drain electrode 4 Channel doping part 901 Photodiode 902 Reset switch 903 Source follower 905 Current source 906 Vertical output line 907 Signal storage part 908 Horizontal scanning circuit 910 Vertical scanning circuit 911 Transfer switch 913 Capacitor

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Tohru Koizumi 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Katsuhisa 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon (72) Inventor Katsuhito Sakurai 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Narutoshi Sugawa 3-30-2, Shimomaruko, Ota-ku, Tokyo Canon Inc.

Claims (11)

[Claims] 1. A photoelectric conversion element, transfer switch means for transferring a signal charge generated in the photoelectric conversion element, a read circuit for receiving and reading the transferred signal charge at a gate, and resetting a gate of the read circuit. A reset switch means, wherein each threshold value of the transfer switch means and the reset switch means is equal, and each threshold value of the transfer switch means and the reset switch means is the electric field effect of the read circuit. A photoelectric conversion device, which is different from a threshold value of a transistor. 2. The read-out circuit according to claim 1, wherein the read-out circuit includes a source follower that amplifies in a source follower type via the transfer switch, and a selection switch that is connected to the source follower in series and activated. Photoelectric conversion device. 3. The photoelectric conversion device according to claim 1, wherein said readout circuit and said reset switch means are comprised of field effect transistors, and the respective transistor regions have different impurity concentrations in channel regions. 4. The transfer switch means, the reset switch means, and the readout circuit are subjected to a first channel doping, and the readout 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 said readout circuit and said reset switch means comprise field effect transistors, and the well regions of each transistor have different impurity concentrations. 6. The photoelectric conversion device according to claim 1, wherein said readout circuit and said reset switch means are composed of field effect transistors, and the gate lengths of the respective transistors are different from each other. 7. The photoelectric conversion device according to claim 1, wherein said readout circuit and said reset switch means comprise field effect transistors, and the gate widths of the respective transistors are different from each other. 8. The photoelectric conversion device according to claim 1, wherein said readout circuit and said reset switch means comprise field effect transistors, and the gate lengths and channel widths of the respective transistors are different from each other. 9. The photoelectric conversion device according to claim 1, wherein said readout circuit and said reset switch means comprise field effect transistors, and the thicknesses of oxide films of gates of respective transistors are different from each other. 10. A photoelectric conversion element, a transfer switch for transferring a signal charge generated in the photoelectric conversion element, a gate region receiving the signal charge from the transfer switch, and a signal corresponding to the signal charge stored in the gate. A field effect transistor including a source and a drain for reading, and power supply means for supplying power to the field effect transistor;
A photoelectric conversion device comprising a plurality of pixel cells arranged in a plurality of pixel cells each including a selection switch unit connected between the field effect transistor and the power supply unit and a reset unit for resetting the gate region, wherein the selection switch unit; A photoelectric conversion device, wherein the reset switch means comprises a field effect transistor, and respective threshold values are different from each other. 11. A photoelectric conversion element, a transfer switch for transferring a signal charge generated in the photoelectric conversion element, a gate region receiving the signal charge from the transfer switch, and a signal corresponding to the signal charge stored in the gate. A photoelectric conversion device in which a plurality of pixel cells including a field effect transistor including a source / drain path for reading, power supply means for supplying power to the field effect transistor, and a reset switch for resetting the gate region are arranged. 2. The photoelectric conversion device according to claim 1, wherein the field effect transistor, the reset switch, and the transfer switch have different threshold values, and the threshold value of the field effect transistor is large.