JPH07114469B2 - Driving method for solid-state imaging device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving method for a MOS type solid-state imaging device, and more particularly to a driving method for a solid-state imaging device that can reduce unread signals.

[Conventional technology]

Currently, there are MOS type and CCD type solid-state image pickup devices that have been put to practical use (Japanese Patent Laid-Open No. 54-43615). FIG. 2 shows a schematic diagram of a MOS type element. The MOS type image sensor is horizontal on the semiconductor substrate,
A large number of light-receiving elements 1 (for example, photodiodes) are formed in the vertical (X, Y) direction, and a horizontal shift register and a horizontal MOS transistor form a vertical shift register and a vertical shift register.
The signal is read by designating the coordinate position in the Y direction by the MOS transistor. The signal is from Photodiode 1 to the above vertical
It is led to the external preamplifier 8 via the MOS transistor 2, the vertical signal line 6, the horizontal MOS transistor 3, and the output signal line 7.

[Problems to be solved by the invention]

An equivalent circuit of this signal reading circuit is shown in FIG. The solid-state image sensor has various capacitors inside. In the figure, C ₀ is the parasitic capacitance of the photodiode, C ₁ is the capacitance of the vertical signal line 6, and C ₂
Represents the sum of the capacitance of the output signal line 7 and the input capacitance of the preamplifier 8. Further, the vertical and horizontal MOS transistors 2 and 3 have a resistance in a conductive state, and these on-resistances are represented as R ₀ and R ₁ , respectively.

Signal current flows in accordance with the light intensity in photodiode 1 which is incident light, the signal charges Q ₀ is charged to C _0. The vertical shift register 4 outputs one pulse in one horizontal scanning period,
The vertical MOS transistor 2 is turned on. At this time, the signal charge Q ₀ accumulated in the capacitor C ₀ is transferred to the capacitor C ₁ via the on-resistance R ₁ . This signal readout circuit has a capacitance C ₀ and an on-resistance R ₁
It is a discharge circuit consisting of. The capacitance C ₁ (about 3 pF) is the capacitance C
On-resistance R ₀ is 5KΩ, which is sufficiently larger than ₀ (about 0.1pF)
Therefore, the discharge time constant τ is R ₀ × C ₀ ≈0.5n sec. The conduction time of the vertical MOS transistor 2 (about 2 μsec) is sufficiently longer than this time constant τ. Therefore, most of Q ₀ moves to the capacitance C ₁ in the form of capacitance division. Next, the horizontal MOS transistor 3 is rendered conductive by the scanning pulse of the horizontal shift register 5, and the signal charge in the capacitor C ₁ is transferred to the output signal line 7.
The temporal change of the signal charge of the capacitance C _{1 at} this time is given by the following equation, assuming that the input impedance Z _in of the preamplifier 8 is purely resistive. The elapsed time from the start of reading is t However A = C ₁ · C ₂ · R ₁ · Z _in … (4) B = C ₁ · R ₁ + C ₂ · Z _in + C ₁ · Z _in … (5) D = C ₂ R ₁ · Z _in … (6 ) E = R ₁ + Z _in (7) When C ₁ = 3pF, C ₂ = 20pF, R ₁ = 5KΩ, Z _in = 500Ω, the discharge characteristic of this discharge circuit is as shown in Fig. 4, and the time constant τ ₁ is about 15nsec. On the other hand, the conduction time of the horizontal MOS transistor 3 in the conventional device driving method is almost half of the value obtained by dividing one horizontal scanning period (63,55 μsec in NTSC) by the number of pixels in the horizontal direction. When the number of pixels is increased, the conduction time of the horizontal MOS transistor 3 becomes shorter. For example, if the number of horizontal pixels is 1000, the horizontal MO
The conduction time of the S transistor 3 is about 30 nsec, which cannot be said to be long compared to τ ₁ . Capacity for short conduction times
The electric charge of C ₁ is not completely discharged and is left unread on the vertical signal line 6. This signal charge mixes with the signal of the pixel on the next scanning line, causing an afterimage, color mixture, and deterioration of vertical resolution. The same problem occurs when the drive frequency of the horizontal shift register 5 is increased for reasons such as increasing the frame frequency.

An object of the present invention is to reduce the signal charge left on the vertical signal line when driving a MOS solid-state image pickup device at high speed to reduce the afterimage,
It is an object of the present invention to provide a driving method of a solid-state image sensor capable of preventing color mixture and reduction in vertical resolution.

[Means for solving problems]

In order to achieve the above-mentioned object, in the driving method of the solid-state image pickup device of the present invention, the pulse width of the horizontal shift register 5 is set to be a multiple of the driving clock of the shift register so that it can be completely discharged, and it is sufficiently longer than the time constant of the discharge circuit. The horizontal MOS transistor 3 has a sufficient conduction time. On this occasion,
The scanning pulses are allowed to overlap with each other, and all the horizontal MOS transistors are made conductive once in one horizontal scanning period. Since the signals are obtained by overlapping a plurality of pixels as in the case of the pulses, the high frequency signal is compensated by the frequency characteristic correction circuit.

[Action]

The solid-state imaging device of the present invention can lengthen the time for reading out a signal from the vertical signal line. As a result, it is possible to reduce the unread portion of the signal at the time of reading the signal charge, and prevent the afterimage mixture and the vertical resolution from decreasing.

〔Example〕

Examples of the present invention will be described below. FIG. 1 shows the drive pulse timing of the solid-state imaging device according to the present invention. Reference numeral 10 is a vertical scanning pulse from the vertical shift register 4, 11 is a driving clock of the horizontal shift register 5, and 12 to 17 are horizontal MOS transistors 2 generated by the horizontal shift register 5.
Represents a pulse for conducting. Hereinafter, a method of reading a signal using these pulses will be described. First, the signal charge in the photodiode 1 is transferred to the vertical signal line 6 by the scanning pulse 10 of the vertical shift register 4. Horizontal scan pulse (12 to 17) generated by the horizontal shift register 5
Thus, the horizontal MOS transistors 3 are sequentially turned on, and the signal charges on the vertical signal line 6 are sequentially read out to the output signal line 7. In the present embodiment, in order to explain the case of driving a solid-state image pickup device in which 384 pixels are arranged in the horizontal direction, the horizontal shift register has 384 stages, which outputs 384 pulses. This scanning pulse is set to be sufficiently longer than the time constant τ _{1 of the} discharge circuit composed of the on-resistance R ₁ capacitance C _{1 in} FIG. The first horizontal MOS in the horizontal scanning pulse of FIG.
The pulse 12 that turns on the transistor 3 rises at the leading edge of the first clock and remains conductive until the third clock rises. Second horizontal MO
The on-pulse 13 of the S-transistor 3 remains conductive from the second clock to the fourth clock. Hereinafter, horizontal scanning is performed by the same operation up to the third to 384th pixels.

In order to read out signals from all the pixels in one horizontal scanning period, a period in which both the horizontal MOS transistors 3 adjacent to each other are brought into a conductive state is required as in the relationship between the pulses 12 and 13, and this is also the point of the present invention. In this embodiment, the width of the pulse for turning on the horizontal MOS transistor 3 is set to be two cycles of the clock. However, if it is two cycles or more, any multiple can satisfy the gist of the present invention. However, the discharge characteristics are shown in FIG. 4, and even if the discharge time is lengthened, the effect gradually decreases. Therefore, the horizontal scanning pulse width is preferably set in the range of twice to several times the clock period.
According to the driving method of the present invention, signals between pixels in the vertical direction are not mixed, but signals of pixels in the horizontal direction are mixed. When signals are mixed in the vertical direction and the image quality deteriorates, a delay circuit that is several times the horizontal scanning period is required to improve the image quality, and the scale of the correction circuit becomes large. On the other hand, when mixing in the horizontal direction, a small-scale circuit that compensates for the high frequency range is sufficient.

FIG. 5 shows a processing circuit of a signal obtained from the solid-state imaging device driven by the present invention. Reference numeral 24 is a solid-state image pickup device, 25 is a frequency characteristic compensation circuit, and 26 is a signal processing circuit of a television camera. The signal from the solid-state image pickup device is obtained by overlapping from the two vertical signal lines at the time when the scanning pulses overlap, as shown in FIG. Since this signal is a signal in which the high frequency component is deteriorated by a wide read pulse, the frequency characteristic correction circuit 25 emphasizes the high frequency component of the signal. Then, it is processed by the signal processing circuit and output as a video signal.
Get from It goes without saying that a video signal can be obtained without performing high frequency compensation.

FIG. 7 shows a schematic diagram of another solid-state imaging device in which the present invention can be implemented. In this device, horizontal MOS transistors 3 are alternately connected to two output signal lines 7-1 and 7-2. Therefore, the signals of adjacent pixels can be taken out from different output signal lines and obtained without overlapping. FIG. 8 shows the relationship between the scanning pulse and the signal. Due to the scanning pulses 11 and 12, the signal 28 is obtained from the output signal line 7-1 and the signal 29 is obtained from the signal line 7-2 without being mixed. If these signals are integrated and sampled, the signals can be obtained without performing high frequency compensation of the signals, and a video signal with good S / N can be obtained. The present invention can be implemented by using a plurality of output signal lines as described above.

〔The invention's effect〕

As described above, according to the driving method of the solid-state imaging device of the present invention, it is possible to lengthen the time for reading a signal from the vertical signal line, reduce the unread portion of the signal, and prevent afterimages, color mixture, and deterioration of vertical resolution. Therefore, when an image pickup device having an increased number of pixels is used, it is possible to take an image with good image quality when the solid-state image pickup device is operated at high speed by increasing the frame frequency and the like, which is effective.

[Brief description of drawings]

FIG. 1 is a waveform diagram showing a driving method of an embodiment of the present invention, FIG. 2 is a schematic diagram of a MOS type solid-state image pickup device suitable for applying the present invention, and FIG. 3 is a read-out portion of FIG. Equivalent circuit diagram,
FIG. 4 is a diagram showing the time response of the signal readout circuit, FIG. 5 is a block diagram of the signal processing circuit of the present invention, FIG. 6 is a waveform diagram of the signal obtained by the driving method of FIG. 1, and FIG. FIG. 8 is a diagram showing another MOS type solid-state image pickup device for carrying out the driving method of the present invention, and FIG. 8 is a waveform diagram of horizontal scanning pulses and signals of the embodiment of FIG. 1 ... Light receiving element (photodiode), 2 ... Vertical MOS
Transistor, 3 ... Horizontal MOS transistor, 4 ... Vertical shift register, 5 ... Horizontal shift register, 6 ...
Vertical signal line, 7 ... Output signal line, 8 ... Preamplifier, 9
...... Signal output terminal, 10 ...... Vertical scan pulse, 11 ...... Horizontal shift register drive clock, 12 to 17 ...... Horizontal MOS transistor ON pulse, 18 to 23 ...... Horizontal MOS transistor, 24 ...... Solid-state imaging device, 25 ...... Frequency characteristic compensation circuit,
26 …… Signal processing circuit, 27 …… Video signal output terminal, 28,2
9 …… Obtained signal.

Claims (1)

[Claims] 1. A plurality of photoelectric conversion elements arranged in a matrix, a group of switching transistors for reading out signals accumulated in the photoelectric conversion elements, and a shift for opening and closing the switching transistor groups in order. In a solid-state imaging device including a register, the output pulse width of a horizontal shift register for turning on the horizontal switching transistor is set to a multiple of the clock pulse width for driving the horizontal shift register, and a plurality of horizontal switching transistors are simultaneously operated. A method for driving a solid-state imaging device, which is set so as to be in a conductive state.