JPS6086976A - Solid-state image pickup device - Google Patents

Abstract

PURPOSE:To suppress the low frequency noises and to improve the sensitivity with a solid-state image pickup device by subtracting the bias signal component which is transferred into a charge coupled element from the level of the output signal of said coupled element. CONSTITUTION:A photodiode group 8 is arrayed 2-dimensionally and connected to a charge coupled element 41 for horizontal transfer by a coupling part 5 and via plural vertical switches and vertical signal lines. The element 41 transfers the bias signal component even after it transferred the n-picture element signals within a horizontal scanning period tau1. Here a stage equivalent to (m) picture elements is set before a stage of (n) picture elements. Thus the m-picture element stage is not connected to the part 5. In such a way, it is possible to subtract the bias signal component from the output of the device 41.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a solid-state imaging device, and particularly to a solid-state imaging device suitable for increasing sensitivity.

[Background of the invention]

Solid-state imaging devices, in which photoelectric conversion elements are arranged in a play-like manner on a semiconductor substrate, are smaller in size, weight, and weight compared to conventional vacuum tube imaging devices.
It is attracting attention because of its advantages in terms of power consumption, burn-in, afterimage, lifespan, and stability. There are six types of solid-state imaging devices currently being commercialized: MOS type, OPD type, and CCD type. Of these, the OPD type is attracting attention as an imaging device that combines the advantages of the MOS type and the CCD type.

A conventional example of a CPD type imaging device will be described below with reference to FIGS. 1 to 3.

FIG. 1 shows a conventional example of an OPD type imaging device, in which 1 is a horizontal transfer 00D (charge coupled device), an input section 2, a transfer section 6,
It consists of an output section 4. Reference numeral 8 denotes an autodiode section, which includes a vertical signal line 9, a vertical MOS transistor 10, an autodiode 11, and a vertical gate line 12. Reference numeral 5 denotes a coupling section for coupling the CCD 1 and the autodiode section 8, and in this example, it consists of a transistor (MOS) transistor and a transfer gate line 7. The coupling section 5 usually has a more complicated circuit configuration, including a circuit for increasing the transfer efficiency from the vertical signal line 9 to the C0D1 and a circuit for sweeping out unnecessary signals such as vertical smear to the outside.・This example shows the simplest example.

13 is a vertical scanning circuit, and the output of the scanning circuit 13 is connected to the vertical gate line 12.

During the horizontal retrace period, the signal charge on the photodiode 11 of a designated line is read out to the vertical signal line 9 via the Taruyama transistor 10, and then read out to the OOD transfer unit 6 via the coupling unit 5.

In the next horizontal scanning period, C0D1 is scanned to obtain a video signal from the COD output section 4.

By adding appropriate signal processing to this video signal, NTS
O (or PAL, or SEOAM) standard signal is obtained.

It is effective to use bias charges to increase the transfer efficiency of 00D1 and the transfer efficiency from the vertical signal line 9 to 0OD1. This bias charge is injected from the COD input section.

Figure 2 is a diagram explaining the operation of the input section of 0OD1.
A two-phase buried channel COD is taken as an example. (α)
is a cross-sectional structure diagram, 21 is a square hole, 22 is a P-type well, 26 is a square diffusion layer, and 24 is a gate made of, for example, bosilicon. Φ) is a potential diagram, and the horizontal position is (α). This corresponds to the cross-sectional structure diagram of . 6 shown with diagonal lines
The 10 portion represents a negative charge. (C) is a timing chart of drive pulses. The potential diagram at the timing of 1it2 t3 is shown in (b).@As is clear from this diagram, the charge obtained by multiplying the potential difference between the Is gate and the IG gate by the IS gate capacitance is injected into the COD as a bias charge. . This charge injection method is called the potential equilibrium method, and it is said to be a charge injection method that is stable in layers.Figure 3 is a diagram explaining the operation of the output section of 0CD1, also using a two-phase buried channel COD as an example. . (α) is a cross-sectional structure diagram, 25 is a Le-type diffusion j-126
, 27 are MOS transistors, and 28 is a resistor.

The transistor 26 is a transistor used to reset the potential of the output diffusion layer 25, and the transistor 27 and the resistor 2
8 forms a floating gate source Holo 7 type output amplifier) is the potential diagram, and the horizontal level ll1
lt corresponds to the cross-sectional structure diagram in (a). (C)
Drive! It is a timing chart of iυ pulse.

Now, in the conventional Op D type imaging device described above, low frequency noise is generated and the sensitivity is limited.This noise becomes sideways drawing or flicker-like noise on the monitor screen.

Figure 4 is an example of the random noise spectrum of the OJ) D-type camera. The spectrum is almost flat at low frequencies, while it is almost 1/f at low frequencies.
(power spectral density is inversely proportional to frequency).

The former mainly occurs at the coupling section 5, while the latter mainly occurs at the input section 2 and output section 4 of the COD.

Generally, 1/f noise in a MOS transistor is generated due to the effect of traps in the channel, and is expressed by a circuit in which a noise voltage source with a 1/r spectrum is isometrically inserted into the gate of the transistor. When this is applied to the input section of COD, the potential of the Is gate fluctuates with a spectrum of 1/f as shown in Figure 2, and the injected/(Iasumi charge fluctuates with a spectrum of 1/f). When applied to MOS), the gate of transistor 27 is
This fluctuation is output as is.

Such 1/f noise (low frequency noise) is visually noticeable and seriously impairs image quality. [Object of the Invention] The object of the present invention is to provide a solid-state image sensor that suppresses the above-mentioned low frequency noise and improves sensitivity. The goal is to provide equipment.

[Summary of the Invention] The main point of the present invention is that a portion (hereinafter referred to as 0
The low frequency noise is suppressed by providing a 0D overflow signal (called a 0D overflow signal) and taking the difference between the 0OD1 output signal and the COD overflow signal.
Schematically shown in the figure. Figure 5 (α) is an example of the output signal of 0OD1, where there is an OOD overflow signal between the video signals,
It shows off how these are modulated with large low-frequency noise.

Figure (b) is an example of a signal waveform when the COD overflow signal is subtracted from the output signal of 00D1 using the method of the present invention, and it can be seen that low frequency noise is significantly suppressed using the 0 line method. is very similar to the DC regeneration method using optical black (a photodiode covered with a black filter), but since optical black includes random noise generated at the coupling part, it is less likely to be caused by clamp error or subtraction than the COD feed signal. Large increase in random noise.

[Embodiments of the invention]

The present invention will be explained in detail using FIGS. 6 to 8.

FIG. 6 is a diagram schematically showing a COD output signal of a solid-state imaging device according to an embodiment of the present invention.

The shaded area is the COD feed signal and is located immediately after the video signal of the second pixel. Horizontal scanning period (T1)
By continuing to transfer the CO'D even after the signals of the pixels have been transferred, a COD overflow signal is obtained as shown in the figure. During the horizontal retrace period, during the period (? 3) other than the period (72) for obtaining the COD feed signal, operate the vertical MO8) rank 2/10 and the coupling unit 5 to transfer the video signal of the specified line to the COD. do.

FIG. 7 shows another embodiment of the present invention, in which (α) is 00D
FIG. 3 is a diagram schematically showing an output signal.

The shaded portion is the COD feed signal and is located immediately before the video signal of the second pixel. In order to obtain this COD flow signal, the transfer section of the OOD 41 is designed to be long as shown in (b). In the case of a COD that transfers one pixel signal in one stage, in order to obtain OOD overflow signals for m pixels, the embodiment shown in FIG. is superior to the embodiment shown in FIG. 6 because the response of the video signal does not affect the COD overflow signal when it passes through a low-pass filter.
＜The signal processing becomes easier.

FIG. 8 shows another embodiment of the present invention, in which (α) is 00D
This is a diagram schematically showing an output signal. The shaded part is a COD overflow signal, which is output alternately with a video signal. In order to obtain such a COD output signal, the COD 42 is connected to the coupling section at every other stage as shown in ([)].

The advantage of this embodiment over the embodiments shown in FIGS. 6 and 7 is that the distribution of the COD overflow signal is dense.
When distributed pixel by pixel in this way, horizontal noise can be almost completely suppressed.

A circuit that takes the difference between the 00D output video signal and the COD feed signal is well known, and an example is shown in FIGS. 9 and 10.

Figure 9 shows an example of a clamp circuit, where 51 is a signal input terminal;
52 is a signal output terminal, 56 is a clamp [pulse input terminal, 54 to 56 are resistors, 5758 is a capacitor, 59.
60 is a transistor, 61 is a diode, and 6265 is a voltage source. As a clamp pulse, a pulse that causes a sudden drop (during the period of the COD flow signal) is used. C.O.
During the period of the D-driving signal, the transistor 60 is conductive;
The base of the transistor 59 is reset to a predetermined voltage (the voltage of the voltage source 62) every period of the COD flow signal, as shown in FIG. - Low frequency noise can be suppressed.

FIG. 10 shows an example of a subtraction circuit, and 71 is a signal input terminal 72.
is a signal output terminal, 7374 is a sampling pulse input terminal, 75 is a differential amplifier, and 7677 is a sample hold circuit. Sample and hold the video signal at 76,
7 samples and holds the COD feed signal, and a differential amplifier 75 calculates the difference between the two. In the embodiments shown in FIGS. 816 and 7, the sample and hold circuit 76 for video signals is not required. Effects of the Invention An example of the effects of the present invention is shown in FIG. 11. This is an example of the power spectrum density of random noise when a signal is provided and clamped with this signal. The broken line shows the power spectral density when no clamping is performed.Low frequency noise can be significantly suppressed by the present invention.

[Brief explanation of drawings]

Figures 1 to 3 are explanatory diagrams of conventional solid-state imaging devices;
The figure is a diagram for explaining low frequency noise, Figure 5 is a detailed explanatory diagram of the present invention, Figures 6 to 10 are explanatory diagrams of an embodiment of the present invention, and Figure 11 is an explanatory diagram of an embodiment of the present invention. It is a line diagram showing an effect. 5...Coupling section 10...Vertical MOS transistor 41...
...C0D 51...Signal input terminal 52...Signal output terminal 63...Power source; t1 Figure 2 (2) CG) L, te21-3 → B'' mound 1 t3 figure (b) CG) t, t2- + last leap, ?4 figure circumferential liquid number (HX) year 5 figure (α) O6 figure off figure (α) i←I water thousand horizontal two →El! period (1)) 8th figure (0L) (b) f7 figure; f lo figure 11th figure liquid (missing (Hz)

Claims (1)

[Claims] 1. A group of photoelectric conversion elements arranged two-dimensionally, a group of vertical switches for switching the photoelectric conversion elements, and a plurality of vertical signals connected to the plurality of photoelectric conversion elements via the plurality of vertical switches, respectively. a charge-coupled device that transfers signal charges in the horizontal direction, and a coupling portion that couples the plurality of vertical signal lines and the charge-coupled device; A solid-state imaging device characterized in that low frequency noise is suppressed by providing means for periodically detecting the charge level transferred and output, and means for subtracting the charge level from the output charge level of the charge-coupled device. .