JP2000083198A - Optical sensor circuit and image sensor using the same - Google Patents

Abstract

(57) [Problem] To provide an optical sensor circuit capable of forming a pixel having a large aperture ratio and high sensitivity, and an image sensor using the circuit. SOLUTION: A photoconductive film L1 for converting an optical signal into a sensor current, a MOS transistor Q1 for converting the converted sensor current into a detection voltage having a logarithmic characteristic in a weak inversion state,
A capacitor C connected to the detection terminal of the MOS transistor; and a capacitor for reducing the drain-source impedance by applying a reset voltage to the gate voltage of the MOS transistor when detecting an optical signal. And a gate voltage VG for controlling the charging or discharging of the optical sensor circuit. This optical sensor circuit
If the sensor current converted by the photoconductive film L1 is a current equal to or less than a predetermined current, the photoconductive film L1 is provided with a linear response region for detecting a detection voltage proportional to a charging current or a discharging current. When the sensor current exceeds a predetermined current, a logarithmic response region for detecting a detection voltage having a logarithmic characteristic corresponding to the load characteristic of the MOS transistor Q1 is provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical sensor circuit for detecting a sensor output corresponding to illuminance and an image sensor using the same.
The present invention relates to an optical sensor circuit having high sensitivity and a large S / N ratio, and an image sensor using the same.

[0002]

2. Description of the Related Art A MOS or CCD image sensor in which photosensor circuits are combined in a matrix is already well known. In these image sensors, electric charges generated in the optical sensor circuit by irradiation light (incident light) are used as optical signals. For example, in a CCD image sensor, electric charges mainly generated by an optical signal are accumulated in each optical sensor circuit and used as an optical signal, and in a MOS image sensor, electric charges are previously stored in a junction capacitance of a photodiode constituting an optical sensor circuit. The light signal is detected by detecting the amount of charge discharged by the irradiation light at the time of recharging.

In order to expand the dynamic range when detecting an optical signal by such an optical sensor circuit, an FET (field effect transistor, for example, an enhancement type n-channel MOS transistor) or the like is used as a photodiode (light receiving element) for the purpose of expanding the dynamic range. Some have been developed that are connected in series and have the function of logarithmically compressing the output voltage. This utilizes the fact that when the current flowing through the FET is small, the resistance change shows a logarithmic characteristic.

FIG. 8 shows a configuration example of such an optical sensor circuit. The optical sensor circuit 100 has a gate connected to a photodiode PD, an enhancement type n-channel MOS transistor Q1 connected in series to the photodiode PD, and a connection point P (sensor detection terminal) between the photodiode PD and the enhancement type n-channel MOS transistor Q1. The enhancement type n-channel MOS transistor Q2 and the enhancement type n-channel MOS transistor Q2.
It comprises an enhancement type n-channel MOS transistor Q3 connected in series with the S transistor Q2. Further, at the connection point P, a photodiode PD, an enhancement type n-channel MOS transistor Q1,
A parasitic capacitance capacitor C composed of Q2 and a combined value of stray capacitances generated by wirings and the like interconnecting them is connected.

[0005] The photodiode PD detects the optical signal Ls and converts it into a sensor current Id proportional to the illuminance of the optical signal Ls. The enhancement type n-channel MOS transistor Q1 forms a load of the photodiode PD, converts a sensor current Id detected by the photodiode PD into a voltage, and generates a detection voltage Vd at a sensor detection terminal P.

Also, an enhancement type n-channel M
The OS transistor Q1 forms a MOS transistor load having a logarithmic characteristic in a weak inversion state where the sensor current Id is small, and converts the sensor current Id detected by the photodiode PD into a detection voltage Vd having a logarithmic characteristic. For this reason, even if the optical signal Ls greatly changes and the sensor current Id greatly changes (a large change such as a different number of digits),
The conversion having the logarithmic characteristic is performed as described above, and the detection voltage V
The change in d is suppressed and does not saturate, and the dynamic range of the output with respect to the input can be widened.

The n-channel MOS transistor Q2 forms an output transistor, and performs voltage-current conversion for extracting the detection voltage Vd to the outside of the optical sensor circuit 10 as a sensor current signal. The n-channel MOS transistor Q3 forms a switch for connecting or disconnecting the sensor current signal converted by the n-channel MOS transistor Q2 to an external circuit.

The operation of the conventional optical sensor circuit configured as described above will be described. Enhancement type n-channel M
The drain D and the gate G of the OS transistor Q1 are connected to a common power supply VD (for example, 5 V). In a state where the optical signal Ls is not detected (the photodiode PD is in a non-operating state), the enhancement type n-channel is supplied from the power supply VD. Charging current Ij via MOS transistor Q1
Flows into the capacitor C, and the capacitor C is charged. Therefore, the detection voltage Vd of the sensor detection terminal P rises to a value close to the voltage of the power supply VD, and this voltage value is a value indicating an initial state in which the photodiode PD has not detected the optical signal Ls.

The value of the detection voltage Vd in the initial state (initial value) increases as the detection voltage Vd at the sensor detection terminal P rises and approaches the voltage of the power supply VD as the capacitor C is charged. The voltage V (GS) between the gate G and the source S of the transistor Q1 (same as the voltage V (SD) between the drain D and the source S) decreases,
Since the impedance between the drain D and the source S sharply increases, the charging current Ij decreases, and is set to a value smaller than the power supply VD (for example, 4.5 V).

When the photodiode PD detects the optical signal Ls from the initial state of the optical sensor circuit 100, a sensor current Id flows through the photodiode PD, and the sensor detection terminal P
The detection voltage Vd decreases in accordance with the increase of the optical signal Ls in a logarithmic characteristic corresponding to the impedance between the drain D and the source S of the enhancement type n-channel MOS transistor Q1, and falls below the initial value. This detection voltage Vd
The optical signal Ls can be detected by detecting the absolute value of. The sensor current Id of the photodiode PD is proportional to the optical signal Ls, while the sensor detection terminal P
Is a value obtained by multiplying the sensor current Id by the impedance between the drain D and the source S having a logarithmic characteristic, so that the optical signal Ls can be detected with a logarithmic characteristic.

FIG. 9 shows a characteristic diagram of the sensor current Id and the detection voltage Vd in the optical sensor circuit 100. As can be seen from this figure, the detection voltage V when the optical sensor circuit 100 is close to the initial state (sensor current Id = 10 ⁻¹² A).
The value of d (initial value) is, for example, 4.5 V. When the sensor current increases by 5 digits (when the sensor current Id = 10 ⁻⁷ A), the detection voltage Vd becomes 4.2 V. As described above, when the optical sensor circuit 100 is used, a change of the optical signal at a 5-digit level (100,000 times) can be detected by the detection voltage Vd of 0.3V.
Therefore, an optical sensor circuit having a wide dynamic range with respect to the input of the optical signal Ls can be configured.

However, in the case of the optical sensor circuit 100 having the above-described configuration, since the sensor current Id is converted into the detection voltage Vd by a logarithmic characteristic in the entire range of the optical signal, the optical signal Ls is small and the sensor voltage is small. Current is in a very small range (Id
= 10 ⁻¹² to 10 ⁻¹¹ A), there is a problem that the change in the detection voltage Vd is too small and the sensor sensitivity is not very good.

In the optical sensor circuit 100, when the photodiode PD no longer detects the optical signal Ls, the photodiode PD is shut off, the charging current Ij flows through the capacitor C, and the detection voltage of the sensor detection terminal P is detected. V
Although d increases, as described above, the impedance between the drain D and the source S of the enhancement type n-channel MOS transistor Q1 rapidly increases and does not increase beyond a predetermined value (4.5 V). The time lapse characteristic when the detection voltage Vd increases in this way is shown by a dashed line L (100) in FIG. 10, but as can be seen from the characteristic shown in FIG. 10, the detection voltage Vd is cut off by the photodiode PD. Since the rate of increase decreases as the value approaches the predetermined value, it takes time to reach the predetermined value (4.5 V).

Therefore, when the photosensor circuits 100 are arranged in a matrix and applied to an image sensor, the response time until reaching the initial value (4.5 V) when resetting the detection voltage Vd is slow, The image sensor has a problem that the image is displayed as a long afterimage.

In the optical sensor circuit 100, the enhancement type n-channel MOS transistor Q1 and the capacitor C form a peak hold circuit for noise, and erroneously detect a noise level having a large amplitude as an optical signal Ls. There is also a problem that the N ratio decreases and the lower limit of detectable illuminance increases, resulting in a decrease in sensitivity.

In view of the above, the present applicant has devised an optical sensor circuit which can detect a minute optical signal with high accuracy, hardly causes an afterimage phenomenon, and has a high S / N ratio ( This has already been filed separately). This optical sensor circuit 200 is shown in FIG. 11, and the above-mentioned optical sensor circuit 100 is an enhancement type n-channel MO.
The difference is that a constant voltage power supply (for example, 5 V) VD is connected to the drain D of the S transistor Q1, and a gate voltage power supply VG capable of applying two kinds of high and low gate voltages is connected to the gate G.

In the case of the optical sensor circuit 200 having such a configuration, at a timing as shown in FIG. 12, a high voltage VH sufficiently higher than the drain voltage VD (= 5 V) and a high voltage VH equal to or lower than the drain voltage VD. The low voltage VL is applied to the gate voltage VG. First, when the high voltage VH is set as the gate voltage VG, the enhancement type n
The impedance between the drain D and the source S of the channel MOS transistor Q1 is in a low resistance state, and the capacitor C is rapidly charged as shown by a solid line L (200) in FIG.
Rises to a value substantially equal to the drain voltage VD (= 5V) (for example, 4.95V). Therefore, when the optical sensor circuits 200 are arranged in a matrix and applied to an image sensor, the response until the detection voltage Vd reaches the initial value (4.95 V) is improved when the detection voltage Vd is reset. The problem of afterimage can be prevented.

Next, the gate voltage VG is set as a detectable period.
Is set to the low voltage VL, the enhancement type n-channel MOS transistor Q1 enters a weak inversion state.
When the photodiode PD is irradiated with light, the electric charge stored in the capacitor C is discharged. Here, when the light incident on the photodiode PD is weak, the sensor current Id hardly flows, so that the enhancement type n-channel MOS transistor Q1 is in a high impedance state, and mainly the charge charged in the capacitor C is used. . Therefore, the change of the output voltage becomes linear. On the other hand, when the light incident on the photodiode PD becomes stronger, the characteristic of the detection voltage Vd changes as shown by an arrow in FIG. 12, the electric charge stored in the capacitor C is rapidly consumed, and the sensor current flowing through the photodiode PD changes. Id becomes a current passing through the enhancement type n-channel MOS transistor Q1, and the change of the output voltage Vd becomes logarithmic.

FIG. 13 shows this relationship. When the light is weak and the sensor current Id is 10 ^-12 to 10 ^-11 , the charge of the capacitor C is discharged and the detection voltage Vd changes linearly. In a region where the light is strong and the sensor current exceeds 10 ^-11 , the detection voltage Vd changes logarithmically. That is, in the case of the light sensor circuit 200, when the light is weak (when the sensor current Id is small), a linear output characteristic equivalent to that of a normal MOS element is obtained, and when the light becomes strong (the sensor current The output characteristics equivalent to those of a logarithmic element can be obtained. Accordingly, when the sensor current is small, high sensitivity can be realized by utilizing the accumulation effect, and S / S when the incident light which is a problem in the logarithmic element is small is small.
The problem of N ratio can also be improved.

Each of the photosensor circuits 200 forms one pixel, and the photosensor circuits 200 are arranged in a matrix to form an image sensor. In this optical sensor circuit 200, the voltage V0 of the drain D of the enhancement type n-channel MOS transistor Q3 becomes the output voltage of each pixel. The relationship between this voltage V0 and the detection current of the photodiode PD is as shown in FIG. become.

The light sensor circuit 200 constituting each pixel is:
Photodiode PD, photodiode parasitic capacitance C
It is necessary to incorporate a MOS transistor Q1 for charging / discharging the electric charge, a MOS transistor Q2 for amplifying the detected voltage and converting the current, and a MOS transistor Q3 for extracting the converted current to the outside in one pixel area. is there. When configuring an image sensor, there is a demand to increase the number of pixels to improve the resolution of the image sensor. For this purpose, it is necessary to reduce the pixel area as much as possible.

[0022]

Here, when trying to reduce the size of a pixel, there is a certain limit in reducing the size of each transistor, and the size of the photodiode must be reduced to reduce the size of the pixel. Often. As described above, when the size of the photodiode is reduced, there is a problem that the aperture ratio of the pixel is reduced.

The present invention has been made in view of the above problems, and has as its object to provide an optical sensor circuit capable of forming a pixel having a large aperture ratio and high sensitivity, and an image sensor using the circuit.

[0024]

According to the present invention, there is provided an optical-electrical converting means comprising a photoconductive film for converting a received optical signal into a sensor current.
Conversion MOS transistors for converting the sensor current converted by the photoelectric conversion means into a detection voltage having a logarithmic characteristic in a weak inversion state (for example, the enhancement type n-channel MOS transistor Q1 and the depression type n-channel MOS transistor QD1 in the embodiment) ), A capacitor connected to the detection terminal of the conversion MOS transistor, and a reset voltage applied to the gate voltage of the conversion MOS transistor when a light signal is detected. An optical sensor circuit is configured to include an initial setting unit (for example, a gate voltage VG in the embodiment) that controls charging or discharging of the capacitor by lowering the impedance. The optical sensor circuit has a linear response region for detecting a detection voltage proportional to a charging current or a discharging current of a capacitor when a sensor current converted by the photoelectric conversion means is a current equal to or less than a predetermined current, A logarithmic response region for detecting a detection voltage having a logarithmic characteristic corresponding to the load characteristic of the conversion MOS transistor when the sensor current converted by the electric conversion means exceeds a predetermined current;

An amplification MOS transistor for amplifying and detecting the voltage signal at the detection terminal of the conversion MOS transistor to which the capacitor is connected (for example, the enhancement type n-channel MOS transistor in the embodiment)
It is preferable to provide a transistor Q2), and a selection MOS transistor (for example, an enhancement type n-channel MOS in the embodiment) for setting a timing for detecting a voltage signal of a detection terminal of the conversion MOS transistor to which the capacitor is connected. Transistor Q
Preferably, 3) is provided.

Further, in the optical sensor circuit of the present invention,
Preferably, a MOS transistor, initial setting means, etc. are formed on a silicon substrate, and a photoconductive film serving as a photoelectric conversion means is formed on the surface of the silicon substrate, covering the MOS transistor, the initial setting means, etc. Aperture ratio 10
A 0% light sensor circuit can be obtained.

Further, it is preferable that the above-mentioned capacitor is constituted mainly by using the capacity of the photoconductive film, that is, it is preferable that the stray capacitance other than this is made extremely small.

In the optical sensor circuit according to the present invention configured as described above, since the photoelectric conversion means is formed of a photoconductive film, the photoconductive film can be provided on a MOS transistor, wiring, or the like. It is possible to increase the aperture ratio by increasing the light receiving area of the photoelectric conversion means, for example,
The aperture ratio can be made 100% by covering the entire top surface of the circuit. When the light receiving area of the photoelectric conversion device is increased by using the photoconductive film as described above, the detection current for the optical signal increases, the S / N ratio increases, and as a result, the sensitivity improves. In addition, the parasitic capacitance of the photoconductive film itself increases proportionally with the size, but other transistors,
Since the stray capacitance of the wiring and the like does not change, the stray capacitance is relatively smaller than the parasitic capacitance of the photoconductive film. That is, the capacitor is formed mainly using the capacity of the photoconductive film, and the influence of the parasitic capacitance as a whole on the detection current becomes relatively small, and as a result, the sensitivity is improved.

Further, in the conventional optical sensor circuit, transistors and the like, including the photodiode, are arranged side by side on the substrate surface. However, in the optical sensor circuit of the present invention, the transistor, the wiring and the like are formed on the substrate. Since the surface is covered with the photoconductive film, the degree of freedom in arranging transistors, wirings, and the like on the substrate is high. In other words, it is possible to reduce the arrangement area of the transistor, the wiring, and the like, thereby making the photosensor circuit, that is, the pixel smaller.

The image sensor according to the present invention is configured by arranging the optical sensor circuits having the above-described configuration in a one-dimensional or two-dimensional array. , The S / N ratio is high, and the sensitivity is high, so that an image sensor with high sensitivity can be obtained. Further, since the photosensor circuit forming each pixel can be reduced in size, an image sensor in which pixels are arranged at high density can be manufactured, and an image sensor with high resolution can be obtained.

[0031]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a first configuration example of the optical sensor circuit 10 according to the present invention. The optical sensor circuit 10 includes a photoconductive film (optical-electrical conversion means) L
1. An enhancement type n-channel MOS transistor Q1 connected in series with this, an enhancement type n-channel MOS transistor having a gate connected to a connection point P (sensor detection terminal) between the photoconductive film L1 and the enhancement type n-channel MOS transistor Q1 Q2 and an enhancement-type n-channel MOS transistor Q3 connected in series with the enhancement-type n-channel MOS transistor Q2. Also,
At the connection point P, a photoconductive film L1, an enhancement-type n-channel MOS transistor Q1, an enhancement-type n-channel MOS transistor Q2, and a parasitic capacitor C which is a combined value of stray capacitances generated by wiring and the like connecting these are connected. Connected.

The photoconductive film L1 detects the optical signal Ls and converts it into a sensor current Id proportional to the illuminance of the optical signal Ls. Enhancement type n-channel MOS transistor Q1
Forms a load on the photoconductive film L1, converts the sensor current Id detected and converted by the photoconductive film L1 into a voltage, and generates a detection voltage Vd at the sensor detection terminal P.

The enhancement type n-channel M
The OS transistor Q1 forms a MOS transistor load having a logarithmic characteristic in a weak inversion state in a range where the sensor current Id is small, and converts the sensor current Id detected by the photoconductive film L1 into a detection voltage Vd having a logarithmic characteristic. For this reason,
Even if the optical signal Ls largely changes and the sensor current Id changes greatly (a large change such as a different number of digits), the conversion having the logarithmic characteristic is performed as described above, and the change in the detection voltage Vd is suppressed. Without saturation, the dynamic range of the output with respect to the input can be widened.

The enhancement type n-channel MOS transistor Q2 forms an output transistor, and functions to amplify and convert the detection voltage Vd into a current signal and to take out the amplified voltage signal V0 outside the optical sensor circuit 10. Further, the enhancement type n-channel MOS transistor Q3 is
A switch for connecting or disconnecting the voltage signal amplified by the S transistor Q2 to an external circuit is formed.

In the optical sensor circuit 10 having such a configuration, at the timing shown in FIG.
(= 5V) High voltage VH and drain voltage VD sufficiently higher than
Is applied as the gate voltage VG. The high voltage VH acts only for a short time t1 as a signal voltage for circuit reset, and when the high voltage VH is set as the gate voltage VG,
Enhancement type n-channel MOS transistor Q
1 is in a low resistance state between the drain D and the source S, and the capacitor C is indicated by a solid line L in FIG.
As shown by (200), the battery is rapidly charged, and the detection voltage Vd of the sensor detection terminal P rapidly rises to a value substantially equal to the drain voltage VD (= 5V) (for example, 4.95V). For this reason, as described later (FIG. 7), when applied to an image sensor in which the optical sensor circuits 10 are arranged in a matrix, the detection voltage Vd is reset until the detection voltage Vd reaches an initial value (4.95 V). Responsiveness is improved, and occurrence of an afterimage of the image sensor can be prevented.

Next, the gate voltage VG is set to the low voltage VL only during the detectable period t2. In this state, enhancement type n-channel MOS transistor Q1 is in a weakly inverted state. When the photoconductive film L1 is irradiated with light, the charge stored in the capacitor C is discharged first. Here, when the light incident on the photoconductive film L1 is weak, the sensor current Id hardly flows, so that the enhancement type n-channel MOS transistor Q1 is in a high impedance state, and mainly the electric charge charged in the capacitor C is used. You. Therefore, the output voltage Vd is
The change is linear (linear)
Become The detection of the output voltage Vd at this time is for detecting the accumulated discharge amount within the detectable time t2, and is not affected by peak noise, and S / S
Detection with a high N ratio is performed.

On the other hand, when the light incident on the photoconductive film L1 becomes stronger, the characteristic of the detection voltage Vd changes as shown by an arrow in FIG. 12, and the charge stored in the capacitor C is rapidly consumed, and The sensor current Id flowing through L1 is a current passing through the enhancement type n-channel MOS transistor Q1, and the change in the output voltage Vd is logarithmic.

This relationship is as shown in FIG.
The same characteristics as those of the optical sensor circuit 200 shown in FIG.
That is, according to the optical sensor circuit 10, when the light is weak and the sensor current Id is 10 ^-12 to 10 ^-11 , the capacitor C
Is discharged and the detection voltage Vd changes linearly, but in a region where the light is strong and the sensor current exceeds 10 ^-11 ,
The detection voltage Vd changes logarithmically. That is, in the case of the optical sensor circuit 10, when the light is weak (the sensor current Id
Is small), a linear output characteristic equivalent to that of a normal MOS element is obtained, and an output characteristic equivalent to that of a logarithmic element is obtained when the light intensity is high (when the sensor current is increased to some extent). Thus, when the sensor current is small, high sensitivity can be realized by utilizing the accumulation effect, and the problem of a decrease in the S / N ratio when the incident light, which is a problem in the logarithmic element, is small can be improved.

The output voltage Vd thus obtained is
Enhancement type n-channel MOS transistor Q
2 and is extracted from the line 11 to the outside as a detection voltage V0 at a switch timing set by the enhancement type n-channel MOS transistor Q3. The relationship between the detection voltage V0 and the sensor current is as shown in FIG.

FIG. 2 shows a specific configuration of the photodetector circuit 10 having such a configuration. In manufacturing the photodetector circuit 10, a p-type well 22 is first formed on a silicon substrate 21, and polysilicon layers 24a and 24b serving as gates of MOS transistors are formed. Then, a plurality of n-type silicon layers 23a to 23d are formed on the upper surface of the p-type well 22 using the polysilicon layers 24a and 24b as a mask.
Then, a first interlayer film 25 made of an insulating material is formed on the surface, and the insulating material is removed from a part of the upper surface of the n-type silicon layer 23b and a part of the upper surface of the polysilicon layer 24b to expose this part. Let it. An aluminum wiring layer 26 is provided on the first interlayer film 25 so as to connect a part of the exposed upper surface of the n-type silicon layer 23b and a part of the upper surface of the polysilicon layer 24b. Next, a second interlayer film 27 made of an insulating material is formed on the upper surface of the wiring layer 26, and a part of the insulating material on the upper surface of the wiring layer 26 is removed. Further, a pixel electrode 28 made of molybdenum (Mo) or tungsten (W) covering almost the entire surface is formed thereon. A photoconductive layer 29 made of hydrogenated amorphous silicon (a-Si: H) is formed on the entire upper surface including the pixel electrode 28.
A transparent electrode 30 of ITO or the like is formed thereon.

In the photodetector circuit 10 having this configuration, the n-type silicon layer 23a serves as the drain D and is connected to the drain voltage VD via the wiring (not shown), and the polysilicon layer 24a serves as the gate G and connects the wiring (not shown). The n-type silicon layer 23b serves as a source S via the gate voltage VG, and these components constitute an enhancement type n-channel MOS transistor Q1. Also, n
The type silicon layer 23c serves as a drain D, and is connected to the amplified voltage VDD via a wiring (not shown) and an enhancement type n channel MOS transistor Q3 which does not appear in this figure. The polysilicon layer 24b serves as a gate G, and the n type silicon layer 23d serves as a gate. The source S is connected to the ground via a wiring (not shown), and these components constitute an enhancement type n-channel MOS transistor Q2. Although the position is different, not shown here, the enhancement type n-channel MOS transistor Q3 is formed similarly.

N-type silicon layer 23b serving as source S of enhancement type n-channel MOS transistor Q1
Is connected via a wiring layer 26 to a polysilicon layer 24b that will be the gate G of the enhancement type n-channel MOS transistor Q2. Further, the wiring layer 26 is connected to the photoconductive layer 29 via the pixel electrode 28, and the photoconductive layer 29 is grounded via the transparent electrode 30 on the surface.

In this circuit, in addition to the parasitic capacitance of the photoconductive film layer 29, the capacitance of the n-type silicon layer 23b serving as the source S of the enhancement-type n-channel MOS transistor Q1, the gate G of the enhancement-type n-channel MOS transistor Q2, The total capacitance such as the capacitance of the polysilicon layer 24b and the capacitance of the wiring layer 26 becomes the capacitance of the capacitor C. However, since the parasitic capacitance of the photoconductive film layer 29 is large in this circuit, the capacitor C is mainly constituted by the parasitic capacitance.

Since the photoconductive film layer 29 is formed so as to cover the entire surface, the aperture ratio is almost 100%, a large detection current can be obtained for an optical signal, and S / S of the detection signal can be obtained. The N ratio increases. The parasitic capacitance of the photoconductive layer 29 itself increases in proportion to its size, but the detection current of the photoconductive layer 29 also increases in proportion to its size.
Considering only the photoconductive layer 29, the sensitivity does not change much. However, since the size of the other floating capacitance with respect to the parasitic capacitance of the photoconductive film layer 29 does not increase, the photoconductive film 2
The ratio of the parasitic capacitance of No. 9 is larger than that of a conventional photosensor using a photodiode. For this reason, the detection sensitivity as a whole of the photodetector circuit is improved as compared with the related art.

In the case of the structure shown in FIG. 2, a MOS transistor and a related wiring layer may be formed on the surface of the silicon substrate 21 and a photoconductive layer 29 may be formed thereon. The degree of freedom of layer design is high, and further miniaturization is possible.

FIG. 3 shows a second configuration example of the optical sensor circuit according to the present invention. This optical sensor circuit 10 'is configured as shown in FIG.
1 has a configuration in which an enhancement-type n-channel MOS transistor Q3 constituting a switch is removed from the optical sensor circuit 10. Although this circuit does not provide a switch function, detection of an optical signal having characteristics similar to those in FIG. It can be performed.

FIG. 4 shows a third configuration example of the optical sensor circuit according to the present invention. This optical sensor circuit 10 ″ is configured as shown in FIG.
Has a configuration in which an enhancement type n-channel MOS transistor Q2 serving as a signal amplifying means is removed from the optical sensor circuit 10 described above.
Optical signals having the same characteristics as those in the case of FIG. 1 can be detected.

Further, instead of the enhancement type n-channel MOS transistor Q1 in the above configuration, a light sensor circuit may be configured using a depression type n-channel MOS transistor. In FIG. 5, the characteristics of the depletion type n-channel MOS transistor QD1 are indicated by solid lines, and the characteristics of the enhancement type n-channel MOS transistors Q1, Q2, Q3 are indicated by chain lines. As can be seen from this characteristic, the enhancement type n
When the gate voltage VG = 0, the channel MOS transistors Q1, Q2, and Q3 do not output the detection current Id.
Although it is always in the OFF state, in the case of the depletion-type n-channel MOS transistor QD1, the weak inversion state can be achieved with the gate voltage VG = 0.

Specifically, when the gate voltage VG of the depletion type n-channel MOS transistor QD1 is VG = 0, the low voltage VL is applied to the gate of the enhancement type n-channel MOS transistor Q1 in the optical sensor circuit 10 shown in FIG. It becomes the same state as the state that was done. Further, if the gate voltage VG of the depletion type n-channel MOS transistor QD1 is VG = VS, the drain D
A low resistance state between the sources S can be achieved. Therefore, the gate voltage VG of the depletion-type n-channel MOS transistor QD1 at the timing shown in FIG.
12 in the optical sensor circuit 10 of FIG.
The same detection as in the case where the high voltage VH and the low voltage VL are applied at the timing described above can be performed. Therefore, the number of power supplies can be reduced.

Next, an image sensor 50 in which the optical sensor circuits 10 according to the present invention having the above-described configuration are arranged in a matrix will be described with reference to FIG. The image sensor 50 is formed in a rectangular or square shape by arranging photosensor circuits (pixels) 10 in an array on a plane, and here, the constant voltage power supplies VD and VDD are omitted.

In order to perform image detection by the image sensor 50, the detection voltage V0 of each pixel 10 located in a column at a time is taken out from the detection line 55, and the column detection is performed by sequentially scanning each column. . Note that the detection line 55 corresponds to the output line 11 shown in FIG.
The extraction of the detection voltage V0 is performed by applying the switch power supply VC at a predetermined timing. Each time the detection is completed, the gate voltage VG = VS is applied to the gate of the enhancement type n-channel MOS transistor Q1 for each column. And reset this.

The application terminal of the switch power supply VC for extracting the detection voltage V0 is indicated by SEL in each pixel 10, and the gate voltage application terminal for reset is indicated by RST. As shown in FIG. 5, a takeout signal line 5 is connected to each takeout application terminal SEL of a column of pixels.
2 are connected, and a reset signal line 53 is connected to each reset application terminal RST. Further, each extraction signal line 52 is connected to a reset signal line 53 which is connected to a terminal RST of a pixel in the column on the left in the figure.

When the image sensor 50 having the above configuration is used, a take-out signal (switch power supply voltage VC) is applied (scanned) to the take-out signal line 52 sequentially from the vertical pixel row on the left end side to the right. And the detection voltage V0
Take out. Thus, image detection is performed by scanning each vertical pixel row from left to right. Here, the extraction of the detection voltage V0 of the leftmost vertical pixel column is completed, and then the extraction signal is applied to the extraction signal line 52 of the second vertical pixel column from the left to extract the detection signal from this vertical pixel column. When performing, the take-out signal line 52 is connected to the reset signal line 53 of the leftmost vertical pixel column, so a reset signal is applied to this reset signal line 53, all the leftmost vertical pixel columns are reset, and the next light detection Is performed. Hereinafter, when the detection signal is extracted in the same manner, the vertical pixel row on the left is reset at the same time.

With this configuration, the detection signal can be taken out and reset by one signal, and the control is simplified. The extraction and reset of the detection signal are performed at the end of time t2 in FIG.

[0055]

As described above, according to the present invention,
An optical-electrical conversion unit configured of a photoconductive film and converting a received optical signal into a sensor current, and a conversion unit for converting the sensor current converted by the optical-electrical conversion unit into a detection voltage having a logarithmic characteristic in a weak inversion state. A MOS transistor, a capacitor connected to the detection terminal of the conversion MOS transistor, and a drain-source impedance by applying a reset voltage to the gate voltage of the MOS transistor when detecting an optical signal. And an initial setting means for controlling the charging or discharging of the capacitor by reducing the capacitance. The optical-to-electrical conversion means is constituted by a photoconductive film. It is possible to provide a photoconductive film on
The aperture ratio can be increased by enlarging the light receiving area of the photoelectric conversion means. For example, the aperture ratio can be set to 100% by covering the entire upper surface. When the light receiving area of the photoelectric conversion unit is increased by using the photoconductive film in this manner, the detection current for the optical signal is increased, the S / N ratio is increased, and as a result, the sensitivity can be improved. In addition, the parasitic capacitance of the photoconductive film itself increases proportionally with an increase in size, but the stray capacitance of other transistors and wiring does not change. Relatively small. That is, the capacitor is formed mainly using the capacitance of the photoconductive film, and the influence of the parasitic capacitance as a whole on the detection current becomes relatively small, so that the sensitivity can be improved as a result. .

Further, in the conventional optical sensor circuit, MOS transistors and the like, including the photodiode, are arranged in a plane on the substrate surface. However, in the optical sensor circuit of the present invention, the MOS transistor, the wiring, and the like are provided. After being formed on the substrate, this surface is covered with the photoconductive film, so that the degree of freedom in the arrangement of the MOS transistors, wirings and the like on the substrate is high. Conversely, it is possible to reduce the arrangement area of the MOS transistor, the wiring, and the like, thereby making the photosensor circuit, that is, the pixel smaller.

The optical sensor circuit of the present invention detects a detection voltage proportional to a charging current or a discharging current of a capacitor when the sensor current converted by the photoelectric conversion means is a current equal to or less than a predetermined current. With a linear response region,
When the sensor current converted by the electric conversion means is a current exceeding a predetermined current, a logarithmic response region for detecting a detection voltage having a logarithmic characteristic corresponding to the load characteristic of the n-channel MOS transistor is provided, so that a strong optical signal In addition, light detection with a logarithmic characteristic can be performed to detect a large dynamic range. For a weak optical signal, the output signal becomes linear and weak light can be detected with high sensitivity.

Incidentally, in the optical sensor circuit of the present invention,
Preferably, a MOS transistor, initial setting means, etc. are formed on a silicon substrate, and a photoconductive film serving as a photoelectric conversion means is formed on the surface of the silicon substrate, covering the MOS transistor, the initial setting means, etc. Aperture ratio 10
A 0% light sensor circuit can be obtained.

The image sensor according to the present invention is configured by arranging the optical sensor circuits having the above-described configuration in a one-dimensional or two-dimensional array. , The S / N ratio is high, and the sensitivity is high, so that an image sensor with high sensitivity can be obtained. Further, since the photosensor circuit forming each pixel can be reduced in size, an image sensor in which pixels are arranged at high density can be manufactured, and an image sensor with high resolution can be obtained.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a first configuration example of an optical sensor circuit according to the present invention.

FIG. 2 is a sectional view showing a specific configuration of the optical sensor circuit.

FIG. 3 is a circuit diagram showing a second configuration example of the optical sensor circuit according to the present invention.

FIG. 4 is a circuit diagram showing a third configuration example of the optical sensor circuit according to the present invention.

FIG. 5 shows a depletion type and an enhancement type n
4 is a graph showing characteristics of a channel MOS transistor.

FIG. 6 is a graph showing a temporal change of a gate voltage VG (a takeout signal) and a detection voltage Vd in an optical sensor circuit using a depletion type n-channel MOS transistor.

FIG. 7 is a schematic diagram showing a configuration of an image sensor according to the present invention.

FIG. 8 is a circuit diagram showing a conventional optical sensor circuit.

FIG. 9 is a graph showing characteristics of a sensor current Id and a detection voltage Vd of a conventional optical sensor circuit.

FIG. 10 is a graph showing a change over time of a detection voltage Vd of the optical sensor circuits of the related art and the present invention.

FIG. 11 is a circuit diagram showing a configuration of a conventional optical sensor circuit.

FIG. 12 is a graph showing a temporal change of a gate voltage VG (a take-out signal) and a detection voltage Vd in the optical sensor circuits of the related art and the present invention.

FIG. 13 is a graph showing characteristics of a sensor current Id and a detection voltage Vd of the optical sensor circuits of the related art and the present invention.

FIG. 14 is a graph showing characteristics of a sensor current Id and an output voltage V0 of the optical sensor circuits of the related art and the present invention.

[Explanation of symbols]

10, 10 ', 10 "photosensor circuit 50 image sensor C capacitor L1 photoconductive film (photo-electric conversion means) Q1 enhancement type n-channel MOS transistor

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Katsuhiko Takebe 1-4-1 Chuo, Wako-shi, Saitama F-term in Honda R & D Co., Ltd. (Reference) 4M118 AA02 AA05 AB01 BA07 BA14 CA02 CA14 CB06 CB14 DD09 DD12 FA06 FA08 5C024 AA01 CA12 CA15 FA01 FA11 GA04 GA31 5F049 MA01 MA20 RA06 UA20

Claims (6)

[Claims] 1. An optical-electrical converting means which is formed of a photoconductive film and converts a received optical signal into a sensor current, and has a logarithmic characteristic in a weakly inverted state of the sensor current converted by the optical-electrical converting means. Conversion MOS to convert to detection voltage
A transistor connected to a detection terminal of the conversion MOS transistor, and a capacitor connected to a detection terminal of the conversion MOS transistor. Initializing means for lowering the impedance of the capacitor and controlling the charging or discharging of the capacitor, and when the sensor current converted by the photoelectric conversion means is a current equal to or less than a predetermined current, the charging current of the capacitor Or a linear response area for detecting a detection voltage proportional to the discharge current, and when the sensor current converted by the photoelectric conversion means exceeds the predetermined current, the load characteristic of the conversion MOS transistor Having a logarithmic response region for detecting a detection voltage having a logarithmic characteristic corresponding to the optical sensor circuit. 2. The optical sensor circuit according to claim 1, further comprising an amplification MOS transistor for amplifying and detecting a voltage signal at a detection terminal of the conversion MOS transistor to which the capacitor is connected. . 3. The conversion MOS transistor according to claim 1, further comprising a selection MOS transistor for setting a timing for detecting a voltage signal of a detection terminal of the conversion MOS transistor to which the capacitor is connected. Optical sensor circuit. 4. The MOS transistor, the initial setting means, wiring connecting them, etc. are formed on a silicon substrate, and the photoconductive film constituting the photoelectric conversion means is the MOS transistor, the initial setting means. 4. The optical sensor circuit according to claim 1, wherein the optical sensor circuit is formed on a surface of the silicon substrate so as to cover the wiring and the like. 5. The capacitor according to claim 1, wherein the capacitor is mainly configured by using a capacity of the photoconductive film.
5. The optical sensor circuit according to any one of items 1 to 4. 6. An image sensor comprising a plurality of photosensor circuits according to claim 1 arranged in a one-dimensional or two-dimensional array.