JPH02280563A - Matrix drive type image sensor - Google Patents

Abstract

PURPOSE:To obtain an excellent gradation characteristic by selecting a block number (n) and a capacitance ratio (x) between a photodidode and a blocking diode so as to limit a current from a block other than that for reading. CONSTITUTION:A coefficient A representing the gradation characteristic is a function of a block number (n) and a capacitance ratio (x) of capacitors. Thus, an image sensor with a desired characteristic is realized by deciding the n, x to satisfy the A gradation. For example, in the case of block number n=128, Cp=2pF, CB=0.2pF, the relation of A=9.8 is obtained. Thus, crosstalk between blocks is less and 8-gradation number is sufficiently satisfied, where CB is a capacitance of a blocking diode, CP is a capacitance of a photodiode and A=(n-1+x)/(nx+1-x).

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a matrix-driven image sensor for reading images.

(Background of the Invention) Image sensors made of photodiode arrays are used in image reading devices such as facsimiles and image scanners.

Furthermore, amorphous silicon as a photoelectric conversion material has excellent properties and is used as a material for photodiodes.

Incidentally, as a method for driving an image sensor, matrix driving is often used, in which the number of driving ICs is small relative to the number of pixels. In this matrix drive, the photodiode array is divided into a plurality of blocks, and during reading, each block is sequentially switched and signals corresponding to the amount of light hitting the photodiode are sequentially read out for each block.

FIG. 2 is an explanatory diagram showing a schematic configuration when a matrix-driven image sensor is actually arranged on a substrate. In the figure, 1 is a substrate made of glass or ceramic, 2 is a diode part where a photodiode PD and a blocking diode BD are arranged, L is a lower electrode, and LLl
˜Lnw are lower electrodes for guiding the output of each photodiode PDIJ of the diode section 2, respectively. U is the upper electrode, U, ~U■ (shown from U+ to U6 in Figure 2) is the lower electrode, and I is used to read the output from ~Log.
This is the upper electrode leading to C4. The lower electrode and the upper electrode U are made of metal such as Al, Cr, TL, Mo, etc. 3 is an insulating film coated with SiO, SiN, polyimide, etc. for insulation between the lower electrode layer and the upper electrode U. Note that the connection point between the lower electrode and the upper electrode U is through a contact hole.

In this way, the upper electrode υ and the lower electrode are insulated by the insulating film 3, and necessary connection points are connected through contact holes.

(Problems to be Solved by the Invention) The electrical configuration of a conventional matrix drive type reading circuit is shown in FIG.

In this figure, n blocks each having m photoelectric conversion elements (shown as an equivalent circuit) are connected. B is a common connection terminal for each block, and is connected to the bias voltage -■ (when the block is selected) or to the ground (when the block is not selected) via the switch SB. In addition, B1 is l (1<l<
n) is the common electrode of the block, and SBI is!
This is the switch of the (1<I<n)th block. Each photoelectric conversion element includes a blocking diode BD, a photodiode PD, a storage capacitor CP considered to be connected in parallel with the photodiode PD, and a capacitor CB considered to be connected in parallel with the blocking diode BD. The equivalent circuit is shown below. In addition, BD
iJ, PDlj is l (1<l<n) j of block
This indicates that it is the (1<j<m)th element. A portion M surrounded by a broken line is a matrix wiring portion.

CR is a reading capacitor connected to each photoelectric conversion element, and its capacitance value is sufficiently thicker than the storage capacitor CP (
selected.

SR is a read switch. Furthermore, CRj, Sj, A
j and sRJ each represent the J (1<j<m)-th element.

FIG. 4 shows the common electrode Bl when driving the above reading circuit.
FIG. 3 is a timing diagram showing the timing of voltages supplied to the 1. The common electrode Bl (1<1<n) is sequentially -■
A voltage of Then, one cycle is completed in one line reading cycle.

FIG. 5 shows the switches S and SR when reading within a block when a voltage of -V is applied to the common electrode B.
FIG. 3 is a timing diagram showing on/off timing of the .

As shown in this figure, 5RI-SR2→...→SR- are turned on in the order of 5RI-SR2->...->SR- during the period when 12 is applied to the common electrode B. That is, turning on SR is for reading out each block (see FIG. 5) and resetting the capacitor CR.

The signal is then read by an amplifier connected to the end of SRi.

If such a reading circuit is used, the capacitor CR is charged with a current corresponding to the amount of light incident on the photodiode PD when a block is selected, so the optical signal is transferred to the capacitor CR.
can be taken out sequentially as the voltage across both ends.

However, in such a driving method, the following problems occur. That is, in the equivalent circuit of FIG. 3, the charge ΔQ flowing at the time of reset is expressed by the following formula: where T; - period time n; number of blocks x; CB/Cp CB; capacitance of blocking diode CP; capacitance of photodiode τ ; Time i, 1 from block selection to closing of reading switch SR; Photocurrent ikl flowing to the pixel of interest; Other photodiode PDII.

(Photocurrent flowing in k = 1 to n, h ≠ 9, 11 *1.

The first item in the above equation is the current of the pixel of interest in the g block of interest, the second item is the current of the corresponding pixel in the previous N-1 block, and the third item is the current of the corresponding pixel in the other blocks. This is the pixel current.

Therefore, ΔQ, corresponds to the corresponding) odd diodes PD J (b-1 to n, h≠g) of other blocks.

h#-9-1) is affected by the photocurrent.

Generally, a practical optical sensor is required to have gradation characteristics of about 8 gradations, but due to the above-mentioned problems, it has been difficult to satisfy the required gradation characteristics.

The present invention has been made in view of these problems, and an object of the present invention is to provide an image sensor that can obtain good gradation characteristics.

(Means for Solving the Problems) The present invention for solving the above-mentioned problems includes a photoelectric conversion means in which n blocks each consisting of m photoelectric conversion elements are arranged, and the output of the photoelectric conversion means is divided for each block. In a matrix drive type image sensor which is read out externally by matrix drive, each photoelectric conversion element is composed of a photodiode with a capacitance of C and a blocking diode with a capacitance of CB, and when CB/CP-X is used, The structure is characterized in that the number of blocks n and the capacitance ratio X between the photodiode and the blocking diode are selected to satisfy the requirements.

(Function) In the image sensor of the present invention, the number of blocks n and the capacitance ratio X between the photodiode and the blocking diode
By selecting and configuring these, desired gradation characteristics can be obtained.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a circuit diagram showing an equivalent circuit of a schematic configuration of an image sensor of the present invention. In the figure, the same parts as those in FIG. 3 are designated by the same numbers, and the explanation will be omitted. Note that A is a buffer amplifier for receiving the read signal.

In the circuit shown in Figure 1, the charge Δ flowing at the time of reset is
Q can be expressed by the following formula.

ΔQ# Therefore, if the first term in the above equation is A times greater than the sum of the second and third terms, the characteristics of the A gradation will be cleared.

That is, here, T; - period time n; number of blocks Photocurrent i flowing through the pixel, B and other photodiodes PDh.

(--1 to n, h≠l b≠g -1).

The first item in the above equation is the current of the pixel of interest in the g block of interest, the second item is the current of the corresponding pixel in the previous N-1 block, and the third item is the current of the corresponding pixel in the other blocks. This is the pixel current.

Therefore, ΔQ is the corresponding photodiode PD jl (b=1~n, b-1-1
It is affected by a photocurrent of ゜h≠1-1). here,
When τ(T) is approximated as τ/T−0, the above equation becomes as follows.

Therefore, the coefficient A indicating the gradation characteristic becomes a function of the number of blocks n and the capacitance ratio X of the capacitor. Therefore, by determining n and X so as to satisfy the A gradation, an image sensor with desired characteristics can be realized.

Expanding the above formula, we get nx+1-x becomes.

For example, the number of blocks n = 128. CI+ -2
1) F.

If CB = 0.2pF, then A-9゜2 from the above formula.
It becomes 8. Therefore, there is little crosstalk between blocks, and 8 gradations are sufficiently cleared.

In addition, when reading an A4 size (216 ms) width, if 1792 pixels are divided into 128 blocks (14 pixels in each block) at 8 pixels/ms, when the capacitance ratio X of the capacitor is 0.1, A-9, 8 at 28
Satisfy the gradation.

Note that when the desired gradation characteristics have been determined, it is also possible to first determine the number of blocks n and then determine the capacitance ratio X of the capacitor. You may also calculate n.

(Effect of the invention) As explained in detail above, in the present invention, the number of blocks n
By selecting the capacitance ratio X between the photodiode and the blocking diode, the current from the blocks outside the reading range is limited. Therefore, it is possible to realize a matrix-driven image sensor that provides good gradation characteristics.

[Brief Description of the Drawings] Fig. 1 is a configuration diagram showing the configuration of an embodiment of the present invention, Fig. 2 is a pattern diagram showing an example of a circuit pattern on a substrate of an image sensor, and Fig. 3 is a circuit diagram of an image sensor. A circuit diagram showing an example, FIG. 4 is a timing diagram showing the timing of the block selection signal, and FIG. 5 is a timing diagram showing the driving waveform when driving by a conventional driving method. SB...Block selection switch BD...Blocking diode PD...Photodiode CP...Photodiode capacitance CB...Blocking diode capacitance CR...Reading capacitor SR...Reading switch

Claims (1)

[Scope of Claims] A matrix drive type device comprising photoelectric conversion means in which n blocks each consisting of m photoelectric conversion elements are arranged, and the output of the photoelectric conversion means is read out to the outside for each block by matrix drive. In an image sensor, each photoelectric conversion element is composed of a photodiode with a capacitance C_P and a blocking diode with a capacitance C_B, and when C_B/C_P=x, (1/n)+
(1/[1+x])(1-[2/n])>8([1/n
]+{x/[1+x]}×{[n-2]/n}) The number of blocks n and the capacitance ratio x between the photodiode and the blocking diode are selected to satisfy Matrix-driven image sensor.