JPH0752927B2 - Driving method for solid-state imaging device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving method of a solid-state image pickup device.

Configuration of Conventional Example and Its Problems In recent years, development of solid-state imaging devices has progressed, and performance has been improved. In a signal processing circuit of a solid-state image sensor, a clamp circuit is generally used to perform DC reproduction and dark current compensation. In the clamp circuit, the dark signal output from the image sensor is clamped to a constant voltage by a clamp pulse. However, when clamping, high frequency noise is included in the dark signal output period, and the high frequency noise is modulated in the cycle of the clamp pulse and converted to the low frequency range. Therefore, low frequency noise is increased in the video signal and the image quality is significantly deteriorated.

OBJECT OF THE INVENTION In order to overcome the above drawbacks, the present invention provides a dark signal output period
A method for driving a solid-state imaging device capable of improving the S / N ratio and eliminating deterioration of image quality due to clamping.

In order to achieve this object, a method for driving a solid-state imaging device according to the present invention is to apply a pulse that becomes a cutoff in a dark signal output period to an output gate electrode of a charge transfer unit, and after adding a dark signal charge. The configuration is such that the output gate is turned on and the added dark signal is read out. With this configuration, the dark signal is increased by the number of additions, and noise is generated in the output circuit section. Since it is dominant, the noise is constant and the S / N ratio is S / N (dB) = 20log {(Sn) / N}.

Here, S is a dark signal, n is the number of additions, and N is noise. Therefore, according to the present invention, the S / N ratio is improved by 20 log n (dB).

Description of Embodiments An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic diagram of an embodiment of a solid-state imaging device to which the present invention is applied.

In FIG. 1, 1 is a photodiode, 2 is a light-shielding aluminum, 3 is a signal reading MOS transistor, 4 is a signal reading gate, 5 is a charge transfer section, 6 is an output gate,
Reference numeral 7 is a reset transistor, 8 is a reset gate, 9 is a reset drain, 10 is a floating diffusion procedure, and 11 is a signal output terminal.

FIG. 2 is a cross-sectional view of a charge transfer portion in one embodiment to which the present invention is applied, and the same portions as those in FIG.
12 is an n ⁻ region, 13 is an n region, 14 is a p-type substrate, 15 is an oxide film,
Reference numeral 16 is a first transfer electrode, and 17 is a second transfer electrode.

FIG. 3 is a drive pulse timing chart in one embodiment of the present invention, where a is a pulse applied to the second transfer electrode 17, b is a pulse applied to the first transfer electrode 16, and c is an output gate 6. A pulse, d is an output signal waveform extracted from the output signal terminal 11, and t _{1 to} t ₂ are time.

FIG. 4 is a potential diagram of the charge transfer portion 5 in one embodiment of the present invention, where Q _{1 to} Q ₃ are dark signal charges and Q ₄ is an optical signal charge.

The operation of the driving method of the solid-state imaging device configured as described above will be described below with reference to FIGS. 1 to 4.
First, the signal charge accumulated for one cycle is read out to the charge transfer section 5 by the signal reading gate 4. Hereinafter, the movement of the charge at time t ₁ to t ₈ will be described. To the signal output terminal 11, the pulse of b in FIG. 3 is at the negative edge (direction indicated by the arrow) and is applied to the output gate 6. It is assumed that the output pulse (FIG. 3c) is output at the HI level. First, at time t = t ₁ , the potential under the gate electrode is such that the dark signal charges Q ₁ , Q ₂ , Q ₃ and the photo signal charge Q ₄ are accumulated under the respective gates, as shown in FIG. Since the HI level potential is applied to the output gate 6 at the time t = t _2, the photo signal charge Q ₄ is output to the signal output terminal 11 beyond the output gate 6 and at the same time, the dark signal charges Q ₁ and Q ₂ , Q ₃ is shifted one step. At the time t = t _3, the output gate 6 is closed (the pulse of c in FIG. 3 is at the LO level), and at the same time the output signal is reset. At time t = t _4, the signal charge Q ₃ is held below the final gate because the output gate 6 is closed, and the dark signal charges Q ₁ and Q ₂ are shifted by one stage. At time t = t _5, the dark signal charges Q ₁ and Q ₂ are further shifted by one stage, and Q ₂ is added to Q ₁ under the final transfer gate. At time t = t ₆ , the dark signal charge Q ₂ + Q ₃ is held under the final transfer gate,
Q ₁ is shifted one step. Dark signal charge Q _{1 at} time t = t ₇
+ Q ₂ and Q ₃ are added under the final transfer gate. Time t = t ₈
At this time, the output gate 6 is opened and the dark signal charge of Q ₁ + Q ₂ + Q ₃ is output.

As described above, according to the present embodiment, by applying a pulse that is cut off (LO level) only to the dark signal output period to the output gate 6, the dark signal charges Q ₁ , Q ₂ and Q ₃ are added and 3 The signal is amplified twice. In addition, since noise is generated in the output circuit, it is constant and the S / N ratio can be greatly improved. In addition,
In this embodiment, the one-dimensional image sensor is taken as an example, but the same applies to the two-dimensional image sensor.

EFFECTS OF THE INVENTION As described above, according to the present invention, a dark signal can be amplified by applying a predetermined pulse to the output gate side of the charge transfer section, and its practical effect is great.

[Brief description of drawings]

FIG. 1 is a schematic view of a solid-state image pickup device in one embodiment to which the present invention is applied, FIG. 2 is a cross-sectional view of a charge transfer section in one embodiment to which the present invention is applied, and FIG. 3 is used for carrying out the present invention. Driving pulse timing and signal output waveform, FIG. 4 is a charge potential diagram when the present invention is carried out. 1 ... photodiode, 2 ... light-shielding aluminum,
3 ... MOS transistor for signal reading, 4 ... signal reading gate, 5 ... charge transfer section, 6 ... output gate,
7: reset transistor, 8: reset gate,
9: reset drain, 10: floating diffusion layer, 11: signal output terminal, 12: n ^- region, 13: n region, 14: p-type substrate, 15: oxide film, 16: transfer Pulse b application terminal,
17 ... Transfer pulse a application terminals, a, b ... Charge transfer pulse, c ... Output gate pulse, d ... Output signal waveform, t ₁
〜T ₈ …… Time, Q ₁ 〜 Q ₃ …… Dark signal charge, Q ₄ …… Optical signal charge.

Claims (2)

[Claims] 1. A photoelectric conversion unit in which photoelectric conversion elements are arranged one-dimensionally or two-dimensionally, a dark signal output unit which shields a part of the photoelectric conversion unit from light, the photoelectric conversion unit and the dark signal output unit. For driving a solid-state imaging device having a charge transfer section for transferring the signal charge generated in 1. and an output gate provided between the charge transfer section and a signal output terminal, wherein the dark signal output section A method for driving a solid-state imaging device, comprising adding the generated signal charges at the output gate and outputting the added signal charges to the signal output terminal. 2. A pulse which cuts off a channel below the output gate electrode is applied to the output gate electrode during the time when the signal charge generated in the dark signal portion is transferred to the bottom of the final transfer gate of the charge transfer unit. The method for driving a solid-state imaging device according to claim 1, wherein