JPH06232441A - Photosensor and driving method for photosensor - Google Patents

Abstract

PURPOSE:To provide a photosensor capable of accurately detecting the illuminance of irradiation light and a driving method for photosensor. CONSTITUTION:A photosensor 1 is formed on a insulating substrate 2 and has a carrier moving layer comprising a channel region 4, N<+> diffusion layers 5 and 6. A carrier generating layer made of amorphous silicon and an electric charge trap layer 11 are sequentially formed on the channel region 4, and these are covered with a gate insulating film 12. A gate electrode (G) 13 is formed on the electric charge trap layer 11, and a control electrode 14 is formed as an united body at both sides of the gate electrode (G) 13. Potential of each electrode is adjusted and held to a reset state and electrons are trapped in the electric charge trap layer 11, and a set state is created and light is applied; then the trapped electrons are replaced with holes, and threshold voltage of the photosensor 1 is varied and a drain current corresponding to total luminous energy can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photosensor and a method for driving the photosensor, and more particularly, a photosensor having a hysteresis characteristic due to a charge trap for accurately detecting the illuminance of irradiation light and a method for driving the photosensor. Regarding

[0002]

2. Description of the Related Art Conventionally, photosensor systems are usually
Photodiode and TFT (Thin Film Transistor)
Is used as the light receiving element (photo sensor), and a plurality of photo sensors are arranged in a matrix. Each photo sensor generates an electric charge according to the amount of light emitted,
The brightness is detected by looking at this charge amount. Then, in the conventional photo sensor system, a scanning voltage is applied from the horizontal scanning circuit and the vertical scanning circuit to the photo sensors arranged in a matrix, and the charge amount of each photo sensor is detected.

However, in such a conventional photosensor, when a closed circuit is formed, the generated charge is discharged as a current, so that conventionally, a selection transistor is formed separately from the photosensor for each photosensor. Then, the selection transistor is driven by the horizontal scanning circuit and the vertical scanning circuit to detect the charge amount of each photosensor.

[0004]

However, in such a conventional photosensor, the charge accumulated in each photosensor is conveyed, amplified, and then A / D converted. Noise is superposed during transportation, the peripheral circuit becomes complicated to remove the influence of this noise, the photosensor system itself becomes large, and the S / N ratio increases.
There is a problem that the ratio is small, the illuminance of the irradiation light cannot be accurately detected, and the gradation display becomes inaccurate.

Therefore, according to the present invention, by forming the photosensor with a transistor thin film having a hysteresis characteristic, the photosensor has an amplification function, can be miniaturized with high accuracy, and can increase the density of pixels. It is an object of the present invention to provide a photo sensor that can be used and a method for driving the photo sensor.

[0006]

A photosensor according to the present invention comprises a silicon layer having a channel region, a source region and a drain region, an electrification trap layer formed on the channel region and having a hysteresis characteristic due to charge trapping, The above object is achieved by including a transparent gate electrode formed on a substantially central portion of the channel region, and a pair of control electrodes formed on both sides of the gate electrode with the gate electrode interposed therebetween. .

In this case, the pair of control electrodes may be integrally formed of a transparent material as described in claim 2, for example.

The silicon layer may be made of polycrystalline silicon, for example.

Further, in this photosensor, for example, as described in claim 4, an amorphous silicon layer may be further laminated on the silicon layer, and the charge trap layer may be formed on the amorphous silicon layer. .

The charge trap layer may be, for example, as described in claim 5.
As described in (4) above, it may have a layer formed of silicon nitride in which the composition ratio of silicon is higher than the stoichiometric ratio.

According to the method of driving a photosensor of the present invention, a silicon layer having a channel region, a source region and a drain region, a charge trap layer formed on the channel region, and a substantially central portion of the channel region are formed. A transparent gate electrode and a pair of control electrodes formed on both sides of the gate electrode with the transparent gate electrode sandwiched between the source electrode and the drain region. The gate electrode and the control electrode to a positive potential to trap electrons in the charge trapping layer, and the control electrode to a positive potential and the gate electrode to a negative potential with respect to the source region and the drain region. In this state, the silicon layer is irradiated with light through the gate electrode so that the electrons in the charge trap layer are converted into holes depending on the light intensity. By performing a second step of substituting, and a third step of obtaining a drain current by setting the gate electrode to a negative potential with the control electrode having a positive potential with respect to the source region and the drain region. , Achieves the above purpose.

[0012]

According to the present invention, in the photosensor, a charge trap layer having hysteresis characteristics due to charge traps is formed on a silicon layer having a channel region, a source region and a drain region, and the channel region is substantially A transparent gate electrode is formed on the central portion, and a pair of control electrodes are formed on both sides of the gate electrode with the gate electrode sandwiched therebetween.

Therefore, after the gate electrode and the control electrode are set to a positive potential with respect to the source region and the drain region to trap electrons in the charge trap layer, the control electrode is set to a positive potential with respect to the source region and the drain region. When the silicon layer is irradiated with light through the gate electrode with a negative potential, electrons in the charge trap layer can be replaced with holes according to the light intensity, and then the source region and the drain region can be replaced. On the other hand, if the gate electrode is set to the negative potential while the control electrode is set to the positive potential, the drain current can be taken out. As a result, the photosensor can have a high amplification factor, high accuracy, and a small size.

[0014]

EXAMPLES The present invention will be described below based on examples.

1 to 5 are views showing an embodiment of a photosensor and a photosensor driving method according to the present invention.

FIG. 1 is a side sectional view of the photosensor 1, which has a silicon layer 3 formed on an insulating substrate 2 made of glass or the like.

The silicon layer 3 is composed of a carrier moving layer 10a and a carrier generating layer 10b, and the carrier moving layer 10a has a channel region 4 formed at the center thereof.
And N ⁺ diffusion regions 5 and 6 formed on both sides sandwiching the channel region 4. This carrier moving layer 10a
Is made of polycrystalline silicon, and has an N ⁺ diffusion layer 5,
6 is formed by diffusing a dopant such as phosphorus.

The carrier generation layer 10b is arranged substantially above the center of the channel region 4 and is made of amorphous silicon having a high light absorption coefficient.

A source electrode (S) 7 and a drain electrode (D) 8 are formed on the carrier moving layer 10a so as to be opposed to each other at a predetermined distance with the carrier moving layer 10a interposed therebetween. The electrode (S) 7 and the drain electrode (D) 8 are connected to the N ⁺ diffusion regions 5 and 6 of the carrier moving layer 10a. Then, the source electrode (S) 7 and the drain electrode (D) 8 and the insulating substrate 2
The insulating film 9 is formed between the insulating film 9 and
For example, it is formed of silicon nitride.

A charge trap layer 11 is formed on the carrier generation layer 10b, and the charge trap layer 11 is formed.
Is a light-transmissive silicon nitride, especially Si /
The N composition ratio is larger than the stoichiometric ratio (Si / N = 0.75), for example, the Si / N composition ratio is 0.85 or more, so-called Si-rich silicon nitride.

Si-rich silicon nitride has a charge trapping function and changes the threshold voltage during writing and erasing. This voltage-current characteristic draws a hysteresis loop, and this state is maintained even if the applied voltage is removed, so that it is possible to configure a non-volatile memory transistor. The details are disclosed in Japanese Patent Laid-Open No. 2-119183.

Then, the gate insulating film 12 having a light transmitting property is formed so as to cover the silicon layer 3 and the charge trapping layer 11.
And the gate insulating film 12 is formed of silicon nitride having a normal stoichiometric ratio.

A gate electrode 13 is formed on the charge trap layer 11 in the gate insulating film 12, and the gate electrode (G) 13 is formed of, for example, a transparent conductive film (ITO). Has been done.

A control electrode (GC) 14 is formed on both sides of the gate electrode (G) 13 and above the channel layer 4 with the gate insulating film 12 interposed therebetween. Like the gate electrode (G) 13, it is made of, for example, a transparent conductive film (ITO). The control electrodes (GC) 14 on both sides of the gate electrode (G) 13 are continuously and integrally formed so as to cover the upper part of the gate electrode (G) 13, and thus the control electrode (GC) is formed. 14
Is formed so as to cover the gate electrode (G) 13, the formation area of the photosensor 1 can be reduced, and the pixel density can be improved.

Although not shown, a transparent overcoat film made of silicon nitride is formed so as to cover these control electrode (GC) 14, source electrode (S) 7, drain electrode (D) 8 and other parts. Has been done.

This photo sensor 1 is manufactured by using a normal film forming technique,
For example, vacuum deposition method, sputtering method or plasma CV
It can be formed by the D method or the like. In this case, if the insulating film 9 and the gate insulating film 12 are formed of the same material, the manufacture of the photosensor 1 can be simplified. Each film thickness is set appropriately, but the charge trap layer 11 is formed to have a thickness of 2000 angstrom, for example.

Next, the operation will be described.

The photosensor 1 can be used as a photosensor having an amplification function and a selection transistor function by driving in the following procedure.

That is, first, as shown in FIG. 2, the photosensor 1 is provided with a source electrode (S) 7 and a drain electrode (D) 8, a gate electrode (G) 13 and a control electrode (GC) 14. Is biased to a positive potential. For example, 0 [V] is applied between the source electrode (S) 7 and the drain electrode (D) 8 and +20 [V] is applied to the gate electrode (G) 13.
And a voltage of +20 [V] is applied to the control electrode (GC) 14. By doing so, electrons are injected into the channel region 4, and the channel region 4 becomes a strong enhancement type. The electrons injected into the channel region 4 are supplied from the N ⁺ diffusion regions 5 and 6 of the carrier moving layer 10a through the control electrode 14 as shown in FIG. The electrons injected into the channel region 4 are trapped in the charge trap layer 11 through the carrier generation layer 10 because the charge trap layer 11 is Si-rich as described above. Therefore, an n channel is formed in the channel region 4 of the carrier moving layer 10a and the reset state is established. When the film thickness of the charge trap layer 11 is 2000 angstrom, the gate voltage V _G and the control voltage V _GC are +20 [V].
Then, the electrons are trapped in the charge trap layer 11 in a short time of about 100 microseconds.

From this reset state, as shown in FIG. 3, the gate electrode (G) 13 is set to a negative potential with respect to the source electrode (S) 7 and the drain electrode (D) 8 and the control electrode (GC) 14 is set. Bias to positive potential. For example, +20 [V] is applied to the source electrode (S) 7 and the drain electrode (D) 8.
When +20 [V] is applied to the control electrode (GC) 14 and 0 [V] is applied to the gate electrode (G) 13, the photosensor 1 enters the sensing state, and in this state, the gate electrode (G) 13 Light from the side (indicated by an arrow in FIG. 4)
When the irradiation is performed, the irradiation light is transmitted through the control electrode (GC) 14, the gate electrode (G) 13, the gate insulating film 12 and the charge trap layer 11, and is applied to the carrier generation layer 10b. When the carrier generation layer 10b is irradiated with light, as shown in FIG. 3, an amount of electron-hole pairs corresponding to the illuminance of the irradiation light is generated, and the electron-holes generated in the carrier generation layer 10b are generated. Of the pair, the electrons are N of the carrier transfer layer 10a.
⁺ Move to the diffusion regions 5 and 6, and holes are trapped in the charge trap layer 1.
It is taken in by 1 and is replaced with the electron trapped at the time of reset. That is, the holes in the electron-hole pairs generated by the light irradiation from the time of the light irradiation are sequentially charged.
Are trapped in the charge trap layer 11 and are replaced with the electrons trapped in the charge trap layer 11, and the threshold value changes in the depletion type direction. In this case, the charge trap layer 1
Since No. 1 has a hysteresis characteristic, the charges taken into the charge trap layer 11 are retained unless reset. As a result, the threshold voltage V _{th of the} photosensor 1 changes according to the light irradiation amount from the light irradiation time to the end of the sensing time, that is, the total exposure amount, and even a slight change in the light irradiation amount is sufficient. Can be detected.

In this sense state, when there is no light irradiation, the potential of the control electrode (GC) 14 is the same as that of the source electrode (S) 7 and the drain electrode (D) 8. Even if the gate electrode (G) 13 has a negative potential, holes are hardly supplied from the N ⁺ diffusion layers 5 and 6 when there is no light irradiation. As a result, the threshold voltage V _th hardly changes and the light irradiation amount can be detected with high sensitivity.

At the time of reading, as shown in FIG. 4, the control electrode (GC) 14 is set to a positive potential with respect to the source electrode (S) 7 and the drain electrode (D) 8 and the gate electrode (G).
13 is set to a negative potential. For example, 0 is set to the source electrode (S) 7.
[V], +5 [V] for the drain electrode (D) 8, 0 [V] for the gate electrode (G) 13, and the control electrode (GC).
By applying +10 [V] to 14, the drain current I _DS having a magnitude corresponding to the light irradiation amount, that is, the channel current can be amplified and read.

The drain current I _DS is shown in FIG. 5 in relation to the voltage (control electrode voltage) V _GC of the control electrode (GC) 14 using the light irradiation amount, that is, the exposure amount as a parameter. That is, the drain current I _DS, during a reset is about 1pA, the sensing operation, light in a non-irradiation state, as described above, since there is no hole in the supply, is the same 1pA about when a reset. However, when light is irradiated during sensing, as described above, electron-hole pairs corresponding to the amount of exposure are generated, and of these holes, holes are included in the charge trap layer 1.
The threshold voltage V _th of the photo sensor 1 changes according to the exposure amount because it moves to 1 and is replaced with the electrons trapped in the charge trap layer 11 at the time of reset. as a result,
As shown in FIG. 5, as the exposure amount increases,
The drain current I _DS exponentially increases, and the exposure amount can be detected with high sensitivity by the drain current I _DS .

As described above, in the photosensor 1, the charge trap layer 11 formed on the channel region 4 of the carrier transfer layer 10a and having the hysteresis characteristic by the charge trap.
Since the transparent gate electrode 13 is formed on the substantially central portion of the channel region 4 and the control electrodes 14 are formed on both sides of the gate electrode 13 with the gate electrode 13 interposed therebetween, the photosensor 1 can have a hysteresis characteristic (memory function), and by resetting, the charge trap layer 11 can trap electrons,
After that, when light is irradiated in the sense state, holes generated according to the total amount of light irradiation can be replaced with the electrons trapped in the charge trap layer 11, and
It is possible to increase the output ratio with that without light irradiation, and it is possible to detect the light irradiation amount with high sensitivity. As a result, the photo sensor 1 can be made highly precise and small, and the pixels can be made highly dense.

Further, since the carrier moving layer 10a is formed of polysilicon, the charge moving speed is high and the driving performance of the photosensor 1 can be improved.

Further, since the carrier generation layer 10b is made of amorphous silicon, it is possible to efficiently generate electron-hole pairs by irradiation light, and the photosensor 1 can have good detection sensitivity.

Since the control electrode (GC) 14 is formed so as to sandwich the source electrode (S) 7 side and the drain electrode (D) 8 side of the gate electrode (G) 13, the control electrode ( By controlling the voltage of the GC) 14, holes can be prevented from flowing into the channel region 4 when there is no light irradiation, and the sensitivity of the photosensor 1 can be further improved.

In the above-mentioned embodiment, the carrier moving layer 10a and the carrier generating layer 10b are made of the same semiconductor material, for example, amorphous silicon or polycrystalline silicon, and have a carrier moving function and a carrier generating function. It is also possible to

The control electrode (GC) 14 has a shape covering the entire upper surface of the gate electrode (G) 13,
A pair may be formed on the left and right sides of the gate electrode (G) 13.

[0040]

According to the photosensor and the method of driving the photosensor of the present invention, the photosensor has a charge trap layer formed on the channel region of a silicon layer having a channel region, a source region and a drain region. A transparent gate electrode is formed on the substantially central portion of the channel region, and a pair of control electrodes are formed on both sides of the gate electrode with the gate electrode sandwiched between them, so that the gate is formed on the source region and the drain region. After the electrodes and the control electrode are set to a positive potential to trap electrons in the charge trap layer, the control electrode is set to a positive potential and the gate electrode is set to a negative potential with respect to the source region and the drain region. When the layer is irradiated with light, the electrons in the charge trap layer can be replaced with holes according to the light intensity, and then the source region and the In a state where the control electrode is at a positive potential with respect to the drain region and the gate electrode to the negative potential, it can be taken out drain current. As a result, the photosensor can have a high amplification factor, high accuracy, and a small size.

[Brief description of drawings]

FIG. 1 is a side sectional view of an embodiment of a photo sensor according to the present invention.

FIG. 2 is an explanatory diagram showing a bias relationship and carrier movement at the time of resetting the photosensor of FIG.

FIG. 3 is an explanatory diagram showing a bias relationship and carrier movement during sensing of the photo sensor of FIG.

FIG. 4 is a diagram showing a bias relationship at the time of reading from the photo sensor of FIG.

FIG. 5 is a characteristic curve diagram showing the drain current in relation to the gate voltage with the exposure amount as a parameter.

[Explanation of symbols]

1 Photosensor 2 Insulating Substrate 3 Silicon Layer 4 Channel Region 5, 6 N ⁺ Diffusion Region 7 Source Electrode (S) 8 Drain Electrode (D) 9 Insulating Film 10a Carrier Transfer Layer 10b Carrier Generation Layer 11 Charge Trap Layer 12 Gate Insulation Membrane 13 Gate electrode (G) 14 Control electrode (GC)

Claims (6)

[Claims] 1. A silicon layer having a channel region, a source region and a drain region, a charge trap layer formed on the channel region and having hysteresis characteristics due to charge traps, and formed on a substantially central portion of the channel region. A photosensor comprising: a transparent gate electrode, and a pair of control electrodes formed on both sides of the gate electrode with the gate electrode interposed therebetween. 2. The photosensor according to claim 1, wherein the pair of control electrodes are integrally formed of a transparent material. 3. The photosensor according to claim 1, wherein the silicon layer is made of polycrystalline silicon. 4. An amorphous silicon layer is further laminated on the silicon layer, and the charge trap layer is formed on the amorphous silicon layer.
4. The photosensor according to claim 3. 5. The charge trap layer according to claim 1, wherein the charge trap layer includes a layer formed of silicon nitride having a composition ratio of silicon larger than a stoichiometric ratio. Photo sensor. 6. A silicon layer having a channel region, a source region and a drain region, a charge trap layer formed on the channel region, and a transparent gate electrode formed on a substantially central portion of the channel region. A method of driving a photosensor, comprising: a pair of control electrodes formed on both sides of the gate electrode with the gate electrode sandwiched between the gate electrode and the control electrode with respect to the source region and the drain region. A positive potential for trapping electrons in the charge trap layer, the gate electrode having a positive potential with respect to the source region and the drain region and a negative potential with respect to the gate electrode. A second step of irradiating the silicon layer with light through an electrode to replace the electrons in the charge trap layer with holes according to the light intensity; A third step of obtaining a drain current by setting the gate electrode to a negative potential while the control electrode is set to a positive potential with respect to the source region and the drain region; .