JP3246038B2 - Photosensor and photosensor driving method - Google Patents

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 of driving the photosensor, and more particularly, to a photosensor having a hysteresis characteristic by a charge trap and accurately detecting the illuminance of irradiation light, and a method of driving the photosensor. About.

[0002]

2. Description of the Related Art Conventionally, a photo sensor system is usually
A photodiode or a TFT (Thin Film Transistor) is used as a light receiving element (photo sensor), and a plurality of photo sensors are arranged in a matrix. Each photosensor generates an electric charge according to the amount of irradiated light, and detects the luminance by checking the amount of the electric charge. In the conventional photosensor system, a scanning voltage is applied to the photosensors arranged in a matrix from a horizontal scanning circuit and a vertical scanning circuit, and the charge amount of each photosensor is detected.

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

[0004]

However, in such a conventional photosensor, the charge accumulated in each photosensor is carried, amplified, and then A / D converted. Noise is superimposed during transportation, and peripheral circuits are complicated to eliminate the influence of the noise, which increases the size of the photosensor system and increases the S / N ratio.
There is a problem that the ratio is small, the illuminance of the irradiation light cannot be detected accurately, and the gradation display becomes inaccurate.

Accordingly, the present invention provides a photosensor formed of a transistor thin film having a hysteresis characteristic, which has an amplifying function, can be miniaturized with high accuracy, and has a high density of pixels. It is an object of the present invention to provide a photosensor that can be used and a method for driving the photosensor.

[0006]

According to the present invention, there is provided a photosensor, comprising: a silicon thin film having a channel region, a source region and a drain region;
Connected source and drain electrodes and transparent gate
An electrode; the gate electrode; and the channel of the silicon thin film.
Hysteresis provided by the charge trap
A layer having lysis characteristics, the source electrode and the
The voltage on the positive side of the voltage applied to the drain electrode is
Electrons are trapped by being applied to the gate electrode,
The voltage applied to the source electrode and the drain electrode
By applying a negative voltage to the gate electrode.
Irradiates the silicon thin film through the gate electrode.
The trapped electrons are placed in holes according to the illuminance of the trapped light.
The above object is achieved by providing a gate insulating film to be replaced .

In this case, the gate insulating film is formed, for example, by
As described in claim 2, it may be formed of silicon nitride whose composition ratio is higher than the stoichiometric ratio. The other photo sensor of the present invention, a silicon thin film having a channel region, a source region and a drain region, said Siri
The source electrode and the drain connected to the
An in-electrode, a transparent gate electrode, and the gate electrode
Provided between the silicon thin film and the channel region,
Voltage applied to the source electrode and the drain electrode
By applying a voltage on the more positive side to the gate electrode,
Trapping electrons, the source electrode and the drain
A voltage on the negative side of the voltage applied to the electrode is
Applied to the pole, through the gate electrode
Depending on the illuminance of the light applied to the silicon thin film, the trap
A gate insulating film in which the converted electrons are replaced with holes.
It is characterized by that.

[0008] A method of driving a photosensor according to the present invention includes a silicon thin film having a channel region, a source region, and a drain region, a gate insulating film including at least a layer formed on the channel region and having a hysteresis characteristic due to charge trapping, A method for driving a photo sensor, comprising: a transparent gate electrode formed on the gate insulating film, wherein a first step of applying a reset pulse to the gate electrode to trap electrons in the gate insulating film; A second step of irradiating the silicon thin film with light through the gate electrode in a state where a positive gate voltage is applied to replace electrons in the gate insulating film with holes in accordance with light intensity; The source-drain voltage was applied to replace
The above object is achieved by performing a third step of obtaining a drain current corresponding to holes .

[0009]

According to the present invention, a gate insulating film including a layer having a hysteresis characteristic due to charge trapping is formed on at least a channel region of a silicon thin film having a channel region, a source region, and a drain region. A transparent gate electrode is formed on the film.

Therefore, a reset pulse is applied to the gate electrode to trap electrons in the gate insulating film, and in a state where a positive gate voltage is applied, the silicon thin film is irradiated with light through the gate electrode to form a gate insulating film. When the electrons inside are replaced with holes according to the light intensity, holes corresponding to the total amount of light irradiation are trapped in the gate insulating film, and a positive source-drain voltage is applied to the trapped holes. As a result, the photosensor can have high amplification factor, high accuracy, and small size.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to embodiments.

FIGS. 1 to 5 show an embodiment of a photosensor and a method of driving the photosensor according to the present invention.

FIG. 1 is a side sectional view of the photosensor 1. The photosensor 1 basically has a configuration of a coplanar thin film transistor.

That is, in the photosensor 1, a semiconductor layer 3 is formed on an insulating substrate 2 made of glass or the like, and the semiconductor layer 3 is made of i-type amorphous silicon (i-type amorphous silicon).
a-Si).

A source electrode (S) 4 and a drain electrode (D) 5 are formed on the semiconductor layer 3 at positions opposing each other with a predetermined interval, with the semiconductor layer 3 interposed therebetween.
These source electrode (S) 4 and drain electrode (D) 5
Is formed through n ⁺ silicon layers 6 and 7 made of amorphous silicon in which a dopant such as phosphorus is diffused.
Is connected to

An optically transparent first gate insulating film 8 made of silicon nitride (SiN) is formed so as to cover the semiconductor layer 3, the source electrode (S) 4 and the drain electrode (D) 5. The first gate insulating film 8 is made of the Si
It is formed of a so-called Si-rich silicon nitride having a / N composition ratio larger than the stoichiometric ratio (Si / N = 0.75), for example, a Si / N composition ratio of 0.85 or more. In FIG. 5, the first gate insulating film 8 is formed so as to cover the whole of the semiconductor layer 3, the source electrode (S) 4, and the drain electrode (D) 5, but is not limited thereto. It is sufficient that at least the semiconductor layer 3 sandwiched between the electrode (S) 4 and the drain electrode (D) 5 is covered.

A light-transmitting second gate insulating film 9 made of silicon nitride is formed on the first gate insulating film 8, and the second gate insulating film 9 has a Si / N composition ratio that is generally equal to that of the first gate insulating film. Of stoichiometric silicon nitride.

On the second gate insulating film 9, a gate electrode (G) 10 is formed at a position facing the semiconductor layer 3, and the gate electrode (G) 10 is formed of a transparent conductive film (ITO). Have been. The gate electrode (G) 10 includes not only a channel region of the semiconductor layer 3 for generating an electron-hole pair described later, but also an n ⁺ silicon layer 4 as shown in FIG.
5 It is desirable to be formed in a size that also covers the upper surface. Although not shown, the gate electrode (G) 10 and the second
A transparent overcoat film made of silicon nitride is formed so as to cover the gate insulating film 9, and the overcoat film protects the gate electrode (G) 10 and the second gate insulating film 9.

As described above, this photosensor 1 has a semiconductor layer 3, n ⁺ silicon layers 6, 7, a source electrode (S) 4, a drain electrode (D) 5, and a first gate insulating film 8 on a substrate 2. , The second gate insulating film 9 and the gate electrode are sequentially laminated to form a so-called coplanar thin film transistor.

The photosensor 1 can be formed by, for example, a vacuum evaporation method, a sputtering method, a plasma CVD method, or the like. The thickness of each film is appropriately set. Is, for example, 20
It is formed in 00 angstrom.

Next, the operation will be described.

As described above, since the photo sensor 1 uses the Si-rich SiN film for the first gate insulating film 8, the drain voltage V _D of the photo sensor 1 is set to 1 in FIG.
As 0V, the drain current I _D with respect to the gate voltage V _G
(Current flowing between the source electrode (S) 4 and the drain electrode (D) 5) has a hysteresis characteristic. That is, the gate voltage V _G, the drain current I _D gradually to a positive value from a negative value is gradually increased until saturation, then, lowering the gate voltage V _G, the drain current I _D, the gate voltage V _For the value of _G , the gate voltage V _G
And when increasing the, after the drain current I _D is saturated, in the case of lowering the gate voltage V _G is lowered by taking different values. Then, change the gate voltage V _G drain current I _D is where the 1nA in when the magnitude [Delta] V _th (the gate voltage V _G of the hysteresis in this case is changed from the negative side, the gate voltage V _G from the positive side the drain current I _D at the time obtained by the difference) between the gate voltage V _G at which the 1 nA, as described below, depends on the amount of exposure light applied to the photo sensor 1,
It depends on the thickness of the first gate insulating film 8.

Therefore, as shown in FIG. 3, for example, 0 [V] is applied between the source electrode (S) 4 and the drain electrode (D) 5 and +2 is applied to the gate electrode (G) 10 as shown in FIG.
When a pulse voltage of 0 [V] is applied, electrons are supplied from the n ⁺ layer on the side of the source electrode (S) 4 and the drain electrode (D) 5 to the channel region of the semiconductor layer 3, and the electrons in this channel region are The first gate insulating film 8 is made of Si as described above.
Since the first gate insulating film 8 is rich,
It is taken in. Since electrons are trapped in the first gate insulating film 8, holes are induced in the channel region of the semiconductor layer 3, and the photosensor 1 becomes a strong enhancement type as a transistor. This state is a reset state. For example, when the film thickness of the first gate electrode 8 is 2000 angstroms, the gate voltage V _G and +20 [V], electrons are trapped in the first gate insulating film 8 in a short time of about 100 microseconds.

[0024] From the reset state, then, as shown in FIG. 4, the bias state of the gate electrode (G) 10 is a negative potential, for example, the gate voltage V _G and 0 [V], the source electrode (S) +20 [V] to the drain electrode 4 and the drain electrode (D) 5
Is applied, the photosensor 1 enters the sensing state.
In this state, light from the gate electrode (G) 10 side (in FIG. 4,
It is indicated by an arrow. ), The light irradiation generates electron-hole pairs in the semiconductor layer 3 in an amount corresponding to the illuminance of the irradiation light. Of the electron-hole pairs generated in the semiconductor layer 3, the electrons move to the n ⁺ layer, and the holes are taken into the first gate insulating film 8 and replaced with the electrons trapped at the time of reset. . In this case, since the first gate insulating film 8 has a hysteresis characteristic, the holes of the electron-hole pairs generated during the period from the start of the light irradiation to the end of the light irradiation are sequentially converted to the first gate insulating film. The electrons are taken into the film 8 and replaced by the electrons trapped in the first gate insulating film 8. As a result, the amount of light irradiation from the time of light irradiation,
That is, the threshold voltage _Vth of the photosensor 1 changes according to the total exposure amount, and the photosensor 1 changes in a depletion type direction as a transistor.

In this sense state, when there is no light irradiation, the threshold voltage V _th hardly changes.

At the time of reading, as shown in FIG. 5, by setting the drain electrode (D) 5 to a positive potential with respect to the source electrode (S) 4, for example, 0 [V] is applied to the source electrode (S) 4. ,
By applying +5 [V] to the drain electrode (D) 5 and 0 [V] to the gate electrode (G), a drain current I _DS corresponding to the light irradiation amount, that is, a channel current can be read. it can.

As described above, since the photosensor 1 is made of Si-rich silicon nitride as the first gate insulating film 8, the photosensor 1 can have a hysteresis characteristic (memory function). By resetting, holes are trapped in the first gate insulating film 8, and thereafter, when light is irradiated in a sense state, electrons corresponding to the total amount of light irradiation are trapped in the first gate insulating film 8. And the output ratio with respect to non-irradiation can be increased, and the amount of light irradiation can be detected with high sensitivity. As a result, the photosensor 1 can be made small with high accuracy, and the density of pixels can be increased.

[0028]

According to the photosensor and the method for driving the photosensor of the present invention, a layer having hysteresis characteristics due to charge trapping is formed on at least the channel region of the silicon thin film having the channel region, the source region and the drain region. Is formed, and a transparent gate electrode is formed on the gate insulating film, so that a reset pulse is applied to the gate electrode to trap electrons in the gate insulating film, and a positive gate voltage is applied. In this state, the silicon thin film is irradiated with light through the gate electrode, and electrons in the gate insulating film are replaced with holes according to the light intensity. The trapped holes can be taken out as a drain current by applying a positive source-drain voltage. High photosensor amplification factor, with high precision, can be of small size.

[Brief description of the drawings]

FIG. 1 is a side sectional view of one embodiment of a photosensor according to the present invention.

FIG. 2 is a diagram showing a hysteresis characteristic of the photosensor of FIG.

FIG. 3 is an explanatory diagram showing a bias relationship and carrier movement at the time of resetting the photosensor of FIG. 1;

FIG. 4 is an explanatory diagram showing a bias relationship and a carrier movement at the time of sensing of the photosensor of FIG. 1;

FIG. 5 is a diagram showing a bias relationship at the time of reading of the photosensor of FIG. 1;

[Explanation of symbols]

 Reference Signs List 1 photosensor 2 insulating substrate 3 semiconductor layer 4 source electrode (S) 5 drain electrode (D) 6, 7 ohmic contact layer 8 first gate insulating film 9 second gate insulating film 10 gate electrode (G)

──────────────────────────────────────────────────の Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01L 31/10-31/119 H01L 27/14-27/148

Claims (4)

(57) [Claims] 1. A silicon thin film having a channel region, a source region and a drain region, and a source electrode respectively connected to the silicon thin film.
And a drain electrode; a transparent gate electrode; and the channel region of the gate electrode and the silicon thin film.
And the hysteresis characteristic by the charge trap.
The source electrode and the drain
A voltage more positive than the voltage applied to the electrode is
Trapping electrons by applying to the pole
More negative than the voltage applied to the electrode and the drain electrode
The gate voltage is applied to the gate electrode,
Of the light applied to the silicon thin film through the gate electrode
Depending on the degree, the trapped electrons are replaced with holes.
A photo- insulating film . 2. The photosensor according to claim 1, wherein said gate insulating film is formed of silicon nitride whose composition ratio is larger than a stoichiometric ratio. 3. A silicon thin film having a channel region, a source region and a drain region, and a source electrode respectively connected on the silicon thin film.
And a drain electrode; a transparent gate electrode; and the channel region of the gate electrode and the silicon thin film.
Between the source electrode and the drain electrode.
A voltage more positive than the voltage applied to the pole is applied to the gate electrode.
Trapping electrons by applying voltage to the
On the negative side of the voltage applied to the electrode and the drain electrode.
By applying a voltage to the gate electrode, the gate
Illuminance of light applied to the silicon thin film through the gate electrode
In which trapped electrons are replaced by holes
Photo sensor comprising: the gate insulating film. 4. A silicon thin film having a channel region, a source region and a drain region, a gate insulating film including at least a layer formed on the channel region and having a hysteresis characteristic due to charge trapping, and formed on the gate insulating film. A transparent gate electrode, comprising: a first step of applying a reset pulse to the gate electrode to trap electrons in the gate insulating film; and a voltage of the source region and A second method of irradiating the silicon thin film with light through the gate electrode in a state where a negative gate voltage is applied to the voltage of the drain region to replace electrons in the gate insulating film with holes according to light intensity; Step and applying positive source-drain voltage to displace holes
And a third step of obtaining a drain current according to the following .