JP2005136028A - Electrostatic protection circuit and electronic circuit comprising it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrostatic protection circuit being connected with an electronic circuit applied with a thin-film transistor, in which the voltage range of OFF voltage region (operating voltage region of electronic circuit) can be set relatively wide and electrostatic protection effect can be enhanced, by increasing the absolute value of on current in the ON voltage region abruptly, and to provide an electronic circuit equipped with it. <P>SOLUTION: The electrostatic protection circuit has the so-called "double gate thin-film transistor structure", comprising a source electrode 12 and a drain electrode 13 provided at the opposite ends of a semiconductor layer 11, and a top gate electrode TGx and a bottom gate electrode BGx provided on the upper and lower layers of the semiconductor layer 11. The top gate electrode TGx of a thin film transistor Tr 10 is connected to the source electrode 12 and one connection terminal N11, and the bottom gate electrode BGx is connected to the drain electrode 13 and the other connection terminal N12. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an electrostatic protection circuit and an electronic circuit including the electrostatic protection circuit, and more particularly to an electrostatic protection circuit that is suitable for prevention of electrostatic breakdown in an electronic circuit using a field effect thin film transistor and an electronic circuit including the electrostatic protection circuit. .

  Conventionally, in an electronic circuit, in order to prevent an overvoltage or an overcurrent due to static electricity or the like entering from the outside to be applied, the electronic circuit is prevented from being broken down. Some have a configuration provided with an electrostatic protection circuit. In particular, when the electronic circuit is configured by applying a field effect thin film transistor (TFT) (for example, a TFT liquid crystal display panel or an imaging device), the electronic circuit can be driven at a low voltage. On the other hand, since the breakdown voltage of the thin film transistor is relatively low, breakdown withstand voltage due to static electricity or the like is likely to occur, and the role of the electrostatic protection circuit (or electrostatic protection element) becomes extremely important. Therefore, conventionally, an electrostatic protection circuit using a thin film transistor is known by applying a manufacturing process for forming a thin film transistor inside an electronic circuit.

Hereinafter, a conventional electrostatic protection circuit will be briefly described.
FIG. 10 is a circuit configuration diagram showing an example of an electrostatic protection circuit using a thin film transistor in the prior art, and FIG. 11 is a characteristic diagram showing voltage-current (V-I) characteristics of the electrostatic protection circuit in the prior art. It is.
An example of the configuration of the electrostatic protection element in the prior art is, for example, as shown in FIG. 10A, a current path (between terminals Np11 and Np12 respectively connected to the signal input contact and ground potential of the electronic circuit. N-channel thin film transistors TN11 and TN12 connected in series between the source and drain terminals), and control terminals (gate terminals) of the thin film transistors TN11 and TN12 are connected to the terminals Np11 and Np12, respectively, The drain terminals of the TN 12 are connected to each other, and have an anti-series circuit configuration in which the TN 12 are connected in series in opposite directions.

  In addition, as shown in FIG. 10B, for example, another configuration example of the electrostatic protection circuit includes a current path (source) between the signal input contact of the electronic circuit and terminals Np21 and Np22 connected to the ground potential. N-channel thin film transistors TN21 and TN22 connected in parallel between the drain terminals), and control terminals (gate terminals) of the thin film transistors TN21 and TN22 are connected to the terminals Np21 and Np22, respectively, and the thin film transistors TN21 and TN22 The source electrode and the drain electrode are connected in parallel in opposite directions and have an anti-parallel circuit configuration.

  The voltage-current (V-I) characteristic in such an electrostatic protection circuit is 0 V as shown in FIG. 11 in both of the anti-series type and anti-parallel type electrostatic protection circuits shown in FIG. A very small current (approximately 0A) flows in the OFF voltage region where the positive / negative voltage range is uniform as the center, and the current flowing between the terminals in the positive voltage region other than the OFF voltage region (ON voltage region) (ON current) Current value increases according to the voltage, and in the negative voltage region, the current value tends to decrease according to the voltage. That is, the absolute value of the on-current tends to increase in the ON voltage region.

  In particular, in the anti-series electrostatic protection circuit shown in FIG. 10A, as shown in FIG. 11A, the OFF voltage region where the current value is approximately 0 A is a relatively wide voltage range (for example, In the ON voltage region, the thin film transistors TN11 and TN12 are connected in series, so that the ON resistance during conduction is relatively large (for example, ˜100 MΩ). Therefore, the absolute value of the on-current does not increase abruptly (has a gentle increase).

On the other hand, in the antiparallel electrostatic protection circuit shown in FIG. 10B, as shown in FIG. 11B, the OFF voltage region is a relatively narrow voltage range (for example, ± 5 centering on 0V). In the ON voltage region, since the thin film transistors TN21 and TN22 are connected in parallel, the ON resistance is relatively low (for example, a resistance value of about 1 MΩ). The absolute value of has a characteristic of rapidly increasing.
Note that an electrostatic protection circuit having an anti-series or anti-parallel circuit configuration as described above is described in, for example, Patent Document 1.

JP 2003-69028 A (page 5, page 7, FIG. 1, FIG. 2, FIG. 5)

However, the electrostatic protection circuit as described above has the following problems.
That is, in order to prevent a desired electronic circuit from breakdown due to static electricity, generally, the OFF voltage region corresponding to the operating voltage range of the electronic circuit connected to the electrostatic protection circuit (external terminal) is, for example, approximately ± In the ON voltage region that has a relatively wide voltage range of 20 V or more and defines a substantial electrostatic protection effect, the voltage-current characteristic in which the absolute value of the on-current increases rapidly (that is, a large value in the ON voltage region). It is desirable to have a voltage-current characteristic that can prevent an on-current from rapidly flowing to a ground potential or the like and an overcurrent from flowing into an electronic circuit connected to the electrostatic protection circuit.

  For this reason, the anti-series electrostatic protection circuit shown in FIG. 10A has a problem that a large ON current does not flow and a sufficient electrostatic protection effect cannot be obtained because the ON resistance in the ON voltage region is large. In the anti-parallel type electrostatic protection circuit shown in FIG. 10B, the operating voltage range of the electronic circuit is narrowed, the circuit design is restricted, and the normal operation of the electronic circuit is performed. There has been a problem that leakage current in the voltage range becomes large, which may cause malfunction of the electronic circuit.

  As a circuit configuration for solving such a problem, for example, as shown in FIG. 12, thin film transistor circuits TN31, TN32 and TN33, TN34 connected in reverse parallel as shown in FIG. 10B are connected between terminals Np31 and Np32. An electrostatic protection circuit having a configuration in which two sets are connected in series can be considered. However, although the voltage-current characteristics in such a circuit configuration are substantially larger in the OFF voltage region than the voltage range shown in FIG. 11B (the operating voltage range of the electronic circuit can be widened), Since the thin film transistors are connected in series to each other, the ON resistance increases, and the absolute value of the ON current in the ON voltage region does not increase rapidly, and still a sufficient electrostatic protection effect can be obtained. Have the problem of not being able to.

  Therefore, in view of the above-described problems, the present invention can set a relatively wide voltage range in the OFF voltage region (operating voltage region of an electronic circuit) in an electrostatic protection circuit connected to an electronic circuit to which a thin film transistor is applied. In addition to providing an electrostatic protection circuit that can increase the absolute value of the on-current in the ON voltage region abruptly and improve the electrostatic protection effect, it offers a high degree of design freedom and excellent electrostatic withstand voltage characteristics. An object of the present invention is to provide an electronic circuit.

  According to a first aspect of the present invention, there is provided an electrostatic protection circuit that discharges an overcurrent caused by static electricity to a predetermined low-potential power source to protect the electronic circuit from electrostatic breakdown, and is connected to a signal input contact of the electronic circuit. And a second contact connected to the low-potential power source, at least one source and drain electrodes formed with a channel region made of a semiconductor layer sandwiched therebetween, above the channel region and A first gate electrode and a second gate electrode formed below each via an insulating film, wherein the first gate electrode is connected to the source electrode, and the second gate electrode is connected to the drain The circuit structure includes a transistor having a double gate structure connected to an electrode.

According to a second aspect of the present invention, in the electrostatic protection circuit according to the first aspect, the electrostatic protection circuit includes one transistor having the double gate structure between the first contact and the second contact. A source electrode and a drain electrode of the transistor are connected to the first contact or the second contact, respectively.
The invention according to claim 3 is the electrostatic protection circuit according to claim 1, wherein the electrostatic protection circuit includes two transistors having the double gate structure between the first contact and the second contact. A drain electrode of each transistor is connected to each other, and a source electrode of each transistor is connected to the first contact or the second contact.

  According to a fourth aspect of the present invention, there is provided an electronic circuit including an electrostatic protection circuit that discharges an overcurrent due to static electricity to a predetermined low potential power source to protect it from electrostatic breakdown, and the electronic circuit includes at least a channel region made of a semiconductor layer. A circuit configuration comprising a field effect transistor comprising a source electrode and a drain electrode formed with a gate electrode and a gate electrode formed via an insulating film at least above or below the channel region. And the electrostatic protection circuit includes at least one semiconductor layer between a first contact connected to a signal input contact of the electronic circuit and a second contact connected to the low potential power source. A source electrode and a drain electrode formed with a channel region made of the first and second gate electrodes formed above and below the channel region with an insulating film interposed therebetween, respectively. And a second gate electrode, wherein the first gate electrode is connected to the source electrode, and the second gate electrode is connected to the drain electrode. It is characterized by having.

According to a fifth aspect of the present invention, in the electronic circuit according to the fourth aspect, the field effect transistor in the electronic circuit includes a first gate formed above and below the channel region via an insulating film, respectively. It has a double gate structure comprising an electrode and a second gate electrode.
The invention according to claim 6 is the electronic circuit according to claim 4, wherein the electrostatic protection circuit includes one transistor having the double gate structure between the first contact and the second contact. A source electrode and a drain electrode of the transistor are connected to the first contact or the second contact, respectively.

According to a seventh aspect of the present invention, in the electronic circuit according to the fourth aspect, the electrostatic protection circuit includes two transistors having the double gate structure between the first contact and the second contact. The drain electrodes of the transistors are connected to each other, and the source electrodes of the transistors are connected to the first contact or the second contact, respectively.
According to an eighth aspect of the present invention, in the electronic circuit according to the fourth aspect, at least one of the transistor having the double gate structure constituting the electrostatic protection circuit and the field effect transistor constituting the electronic circuit. The structure of the part is formed simultaneously by the same manufacturing process.

That is, the electrostatic protection circuit according to the present invention includes a signal input contact (first contact) of an electronic circuit to be prevented from electrostatic breakdown, and a low potential power source (second contact) such as a ground potential. A top gate electrode (first gate electrode) and a source electrode of a thin film transistor having a so-called double gate structure are connected, a bottom gate electrode (second gate electrode) and a drain electrode are connected, and the source electrode is connected to a signal input contact. Alternatively, it has a circuit configuration in which the drain electrode is connected to one side of the low potential power source and the drain electrode is connected to the signal input contact or the other side of the low potential power source.
Here, a thin film transistor having a single double gate structure may be provided between the signal input contact and the low potential power source, or a thin film transistor having two double gate structures is provided, and the source of each thin film transistor is provided. The electrodes may be connected to each other in series.

  In the electrostatic protection circuit having such a configuration, since the ON resistance in the ON voltage region can be suppressed to a low level, it is possible to quickly discharge the overcurrent to the ground potential and realize a good electrostatic withstand voltage characteristic. . In addition, since a relatively wide OFF voltage region can be obtained, the operating voltage region of the electric circuit can be set wide, and the electronic circuit can be operated satisfactorily by suppressing the leakage current in the operating voltage region. .

  In particular, in the former configuration (a configuration in which a thin film transistor having a single double gate structure is provided between a signal input contact and a low potential power source), two individual thin film transistors (field effect transistors) are connected. Since the electrostatic protection characteristic equivalent to that of the electrostatic protection circuit can be realized with a simple circuit configuration (element structure), the circuit area of the electrostatic protection circuit can be reduced.

  In the latter configuration (a configuration in which two thin film transistors having a double-gate structure are connected in series between the signal input contact and the low-potential power supply), the voltage is symmetric in the positive voltage region and the negative voltage region, and a wider voltage range. Since the OFF voltage region having the above can be obtained, the operating voltage region of the electronic circuit can be set wider, the degree of freedom in circuit design can be further improved, and the leakage current can be further suppressed. The electronic circuit can be operated well.

  Furthermore, an electronic circuit including an electrostatic protection circuit according to the present invention includes a circuit configuration including a field effect thin film transistor and a thin film transistor having a double gate structure similar to the electrostatic protection circuit, such as a TFT liquid crystal display panel and a photo It is desirable to have a sensor array or the like. According to this, it is the same as when a TFT liquid crystal display panel or a photo sensor array (that is, a field effect thin film transistor or a thin film transistor having a double gate structure) is formed on a single insulating substrate on which an electronic circuit is formed. Since the above-described electrostatic protection circuit (thin film transistor having a double gate structure) can be simultaneously formed together with the TFT liquid crystal display panel and the photosensor array by using a manufacturing process, the manufacturing process is almost increased. In addition, an electrostatic protection circuit having a high electrostatic protection effect can be formed with a relatively small circuit area, and an electronic circuit excellent in electrostatic withstand voltage characteristics can be realized at low cost.

Hereinafter, an electrostatic protection circuit according to the present invention will be described with reference to the drawings.
<First Embodiment>
FIG. 1 is an equivalent circuit and a schematic cross-sectional view showing a first embodiment of an electrostatic protection circuit according to the present invention, and FIG. 2 is a characteristic showing voltage-current characteristics of the electrostatic protection circuit according to this embodiment. FIG.
As shown in FIGS. 1A and 1B, the electrostatic protection circuit (electrostatic protection element) according to this embodiment is roughly composed of a source electrode 12 and a drain electrode 13 provided at both ends of the semiconductor layer 11. And a so-called double gate type thin film transistor structure comprising a top gate electrode (upper gate electrode) TGx and a bottom gate electrode (lower gate electrode) BGx provided in the upper and lower layers of the semiconductor layer 11, A top gate electrode TGx (top gate terminal TG) of a thin film transistor (hereinafter referred to as “double gate transistor”) Tr10 is connected to a source electrode 12 (source terminal S) and one connection terminal N11, and a bottom gate electrode BGx (bottom gate) The gate terminal BG) is connected to the drain electrode 13 (drain terminal D) and the other connection terminal N12.

Here, the double-gate transistor Tr10 applied to the present invention specifically includes a semiconductor layer (channel region) 11 such as amorphous silicon and both ends of the semiconductor layer 11 as shown in FIG. The source electrode 12 and the drain electrode 13 formed through impurity layers (ohmic contact layers) 17 and 18 each made of n ⁺ silicon, and a block insulating film (stopper film) 14 above the semiconductor layer 11 (upper side in the drawing). And a top gate electrode (first gate electrode) TGx formed via the upper gate insulating film 15 and a bottom gate electrode (lower gate insulating film 16) formed below the semiconductor layer 11 (downward in the drawing). Second gate electrode) BGx. Note that the double-gate transistor Tr10 as described above is formed on an insulating substrate SUB such as a glass substrate, for example. A protective insulating film (passivation film) 19 is formed on the entire surface of the insulating substrate SUB including the double gate transistor Tr10.

  The top gate electrode TGx of the double-gate transistor Tr10 having such a configuration is connected to the lower source electrode 12 through the contact hole H11 provided in the upper gate insulating film 15, and the drain electrode 13 is connected to the lower gate insulation. By connecting to the lower bottom gate electrode BGx through the contact hole H12 provided in the film 16, an electrostatic protection circuit having the equivalent circuit shown in FIG. 1A is obtained.

  That is, the electrostatic protection circuit (electrostatic protection element) according to the present embodiment is configured by the source electrode 12, the drain electrode 13, and the top gate electrode TGx provided at both ends of the semiconductor layer 11, and diode connected (source electrode). 12 and a top gate electrode TGx are connected) and is composed of an n-channel field effect transistor (first field effect transistor), a source electrode 12, a drain electrode 13, and a bottom gate electrode BGx. The n-channel field effect transistor (second field effect transistor) to which the electrode 13 and the bottom gate electrode BGx are connected) is connected between the contacts N11 and N12 (first and second contacts). 12 and drain electrode 13 are common and connected in parallel in opposite directions. 2 has an element structure of a terminal element. Here, since the first and second field effect transistors each have a predetermined diode characteristic, the electrostatic protection circuit (double gate transistor Tr10) functions as a two-terminal element having a varistor characteristic.

  Specifically, the electrostatic protection circuit described above has a gate voltage applied to the top gate electrode TGx and the bottom gate electrode BGx constituting the double gate type thin film transistor Tr10 (first and second field effect transistors). The ON / OFF state is controlled based on (that is, the voltage applied to the contacts N11 and N12). For example, when 0 V (ground potential) is applied to the source electrode 12 (that is, one contact N11) of the double gate transistor Tr10 described above, and a positive voltage is applied to the drain electrode 13 (that is, the other contact N12), Since the bottom gate electrode BGx is connected to the drain electrode 13, a bottom gate voltage equivalent to that of the drain electrode 13 is applied. Therefore, when the applied voltage (bottom gate voltage) to the drain electrode 13 is equal to or lower than the threshold voltage of the second field effect transistor, the second field effect transistor is turned off, while the application When the voltage (bottom gate voltage) is equal to or higher than the threshold voltage, the second field effect transistor is turned on.

  On the other hand, when 0 V (ground potential) is applied to the drain electrode 13 (ie, the other contact N12) and a positive voltage is applied to the source electrode 12 (ie, the one contact N11), the top gate electrode TGx is Since it is connected to the source electrode 12, a top gate voltage equivalent to that of the source electrode 12 is applied. Therefore, the voltage applied to the source electrode 12 (top gate voltage) is, for example, about 3 to 5 times the threshold voltage of the first field effect transistor (hereinafter referred to as “switching voltage” for convenience). 1), the first field effect transistor is turned off. When the applied voltage (top gate voltage) is equal to or higher than the switching voltage, the first field effect transistor is turned on.

  Here, the threshold voltage and the switching voltage for the ON / OFF operation in the first and second field effect transistors are different in the double gate transistor because of the element structure, the bottom gate voltage is higher than the top gate voltage. Since the force (dominant power) that is substantially involved in the ON / OFF operation of the thin film transistor (field effect transistor) is larger, the bottom gate voltage is set to 0V (when 0V is applied to the drain electrode 13). Since the field effect transistor is to be held in the OFF state, the field effect transistor cannot be turned on unless the top gate voltage is set to about several times the original threshold voltage.

  Therefore, in the electrostatic protection circuit (double gate type transistor Tr10) according to the present embodiment, as shown in FIG. 2, in the positive voltage region, the original threshold voltage of the field effect transistor is approximately +5 to 8V. Is the OFF state (OFF voltage region) in which no current flows in the voltage range of 0 V to the threshold voltage, and the ON state in the voltage range equal to or higher than the threshold voltage. Current characteristics are shown. Here, by setting the conduction resistance of the double-gate thin film transistor to be small (˜1 MΩ), the on-current in the ON state (ON voltage region) can be rapidly increased.

  In addition, as shown in FIG. 2, in the negative voltage region, the voltage is several times (for example, three times) the original threshold voltage of the field-effect transistor. In the voltage range of the switching voltage, an OFF state (OFF voltage region) in which no current flows, and an ON state is generated in the voltage range equal to or lower than the switching voltage, indicating a voltage-current characteristic in which an ON current flows. Even in this case, since the ON resistance of the double gate type thin film transistor is set to be small (˜1 MΩ), the ON current in the ON state (ON voltage region) is drastically decreased (that is, the absolute value of the ON current is reduced). Increase).

  Therefore, in the electrostatic protection circuit according to the present embodiment, as shown in FIG. 2, the asymmetric voltage-current characteristics in which the threshold voltage and the switching voltage of the ON / OFF operation are different between the positive voltage region and the negative voltage region. However, as in the case of the antiparallel electrostatic protection circuit (see FIG. 10B) as shown in the prior art, the ON resistance is kept low and the antiseries type as shown in the prior art is used. Similarly to the electrostatic protection circuit (see FIG. 10A), it is possible to obtain an electrostatic protection circuit capable of realizing a relatively wide OFF voltage region (about 20 to 30 V) and reducing the leakage current. .

  In particular, as shown in this embodiment, by applying a double gate transistor as an electrostatic protection circuit, the electrostatic protection formed by connecting two individual thin film transistors (field effect transistors) as shown in the prior art. Effective circuit characteristics (that is, both a wide OFF voltage region and a low ON resistance as shown in FIGS. 11A and 11B) can be realized, so that the circuit configuration (element structure) is greatly increased. It is possible to simplify and realize an electrostatic protection circuit having a small circuit area (element area) and having an excellent electrostatic protection effect.

<Second Embodiment>
Next, a second embodiment of the electrostatic protection circuit according to the present invention will be described.
FIG. 3 is an equivalent circuit showing a second embodiment of the electrostatic protection circuit according to the present invention, and FIG. 4 is a schematic sectional view showing the second embodiment of the electrostatic protection circuit according to the present invention. . FIG. 5 is a characteristic diagram showing voltage-current characteristics of the electrostatic protection circuit according to the present embodiment. Here, about the structure equivalent to 1st Embodiment mentioned above, the same or equivalent code | symbol is attached | subjected and the description is simplified or abbreviate | omitted.

In the first embodiment described above, the case where a single double gate type transistor is applied as it is as an electrostatic protection circuit is shown. However, in this embodiment, two double gate type transistors are placed in opposite directions. It has an anti-series circuit configuration connected in series.
That is, as shown in FIG. 3, the electrostatic protection circuit 20 according to the present embodiment includes double gate transistors Tr20a and Tr20b having the same element configuration as that of the first embodiment described above, and the double gate transistor Tr20a. Source terminal S and top gate terminal TG are commonly connected to one contact N21, and the source terminal S and top gate terminal TG of the double gate transistor Tr20b are commonly connected to the other contact N22, and the double gate type Each drain terminal D and bottom gate terminal BG of the transistors Tr20a and Tr20b are connected to each other.

  Here, as shown in FIG. 4, the double-gate transistors Tr20a and Tr20b applied to the present embodiment are roughly formed on a single insulating substrate SUB and have an element structure equivalent to that of the first embodiment described above. This is realized by simultaneously manufacturing each electrode layer and insulating layer of two double-gate transistors Tr20a and Tr20b having the same structure (see FIG. 1B) in the same manufacturing process.

Specifically, a bottom gate electrode BGa made of a conductive material having a predetermined planar shape on one surface side of an insulating substrate SUB such as a glass substrate and selected from chromium, chromium alloy, aluminum, aluminum alloy, and the like. After BGb is formed at the same time, a bottom gate insulating film 16 made of, for example, silicon nitride or silicon oxide is formed on the entire surface of one surface of the insulating substrate SUB.
Next, after simultaneously forming semiconductor layers (channel regions) 11a and 11b made of amorphous silicon or the like having a predetermined planar shape on the upper surface of the bottom gate insulating film 16 and above the bottom gate electrodes BGa and BGb Block insulating films (stopper films) 14a and 14b are simultaneously formed on the semiconductor layers 11a and 11b.

Next, impurity layers (ohmic contact layers) 17a and 18a, 17b and 18b made of n ⁺ silicon are simultaneously formed so as to be in contact with both ends of the respective semiconductor layers 11a and 11b, and then the impurity layers 17a and 18a, 17b and It is made of a conductive material selected from chromium, chromium alloy, aluminum, aluminum alloy or the like having a predetermined planar shape (wiring shape) so as to be electrically connected to each semiconductor layer 11a, 11b via 18b. The source electrode 12a, the drain electrode 13a, the source electrode 12b, and the drain electrode 13b are formed simultaneously. Here, the drain electrodes 13a and 13b are integrally formed so as to be connected to each other, and are respectively connected to the bottom gate electrode through the contact holes HBa and HBb opened in a predetermined region of the bottom gate insulating film 16. It is formed so as to be connected to the upper surfaces of BGa and BGb.

Next, a top gate insulating film 15 made of, for example, silicon nitride or silicon oxide is formed on the entire surface of one surface of the insulating substrate SUB, and then the top surface of the top gate insulating film 15 includes the semiconductor layers 11a, Top gate electrodes TGa and TGb having a predetermined planar shape and made of a transparent electrode layer such as a tin oxide film or an ITO film (indium-tin oxide film) are simultaneously formed in a region above 11b. Here, the top gate electrodes TGa and TGb are formed so as to be connected to the upper surfaces of the source electrodes 12a and 12b through contact holes HTa and HTb opened in predetermined regions of the top gate insulating film 15, respectively. .
Then, by forming the protective insulating film 19 made of, for example, silicon nitride, silicon oxide, or the like over the entire surface of the insulating substrate SUB, the electrostatic protection circuit having the equivalent circuit shown in FIG. 3 can be obtained.

In the electrostatic protection circuit having such a configuration, the gate voltage applied to the top gate electrode TGa or TGb constituting the double gate transistor Tr20a or Tr20b (that is, the voltage applied to the contacts N21 and N22) is set. Based on this, the ON / OFF state is controlled. That is, when 0 V (ground potential) is applied to one of the contacts (for example, contact N21) and a positive voltage is applied to the other contact (for example, contact N22), for example, the source electrode of the double gate transistor Tr20a 12V and the top gate electrode TGa are applied with 0V, and a positive voltage is applied to the source electrode 12b and the top gate electrode TGb of the double gate type transistor Tr20b. Therefore, the voltage applied to the contact N22 (the top of the double gate type transistor Tr20b). As in the case of the first embodiment described above, the gate voltage) is the original threshold voltage of the bottom gate side field effect transistor (that is, the second field effect transistor described above). For example, a double gate transistor T is used until a switching voltage of about 3 to 5 times is reached. 20b (the field effect transistor on the top gate side) is in an OFF state, and almost no current flows between the contacts N21 and N22, and when the applied voltage at the contact N22 is equal to or higher than the switching voltage, a double gate transistor The Tr 20b is turned on, and a predetermined on-current flows between the contacts N21 and N22.
Here, in the electrostatic protection circuit according to the present embodiment, as shown in FIGS. 3 and 4, double-gate transistors Tr20a and Tr20b having exactly the same element structure are connected in series in opposite directions ( Since the above-described configuration is reversed, the above-described ON / OFF operation is similarly controlled in any direction between the contacts N21 and N22.

  As a result, in the electrostatic protection circuit according to the present embodiment, as shown in FIG. 5, a relatively wide voltage range of about ± 15 to 25 V, for example, is symmetrical about the positive voltage region and the negative voltage region around 0V. In FIG. 4, an OFF state (OFF voltage region) in which no current flows is shown, and in a voltage range equal to or higher than the switching voltage, an ON state (ON voltage region) is obtained, and voltage-current characteristics in which an on-current flows rapidly can be obtained.

  Here, the electrostatic protection circuit according to the present embodiment has a circuit configuration in which double-gate transistors Tr20a and Tr20b are connected in series, so that the on-resistance in the ON voltage region is the first embodiment. As compared with the circuit configuration using a single double gate type transistor as shown in FIG. 1, the electrostatic protection circuit as shown in the prior art (FIG. 10A, FIG. 12), a sufficiently low resistance value (˜1 MΩ) can be set.

  Therefore, in the electrostatic protection circuit according to the present embodiment, an OFF voltage region having a relatively wide voltage range is obtained in a symmetric manner in the positive voltage region and the negative voltage region, so that the operating voltage region of the electronic circuit is set wide. Thus, the degree of freedom in circuit design can be improved, and the leakage current in the operating voltage region can be suppressed, and the electronic circuit can be operated favorably. In addition, since the ON resistance in the ON voltage region can be suppressed to a low level, an electrostatic protection circuit having an excellent electrostatic protection effect can be realized.

Next, an electronic circuit to which the electrostatic protection circuit as described above can be applied will be briefly described with reference to the drawings.
Since the electrostatic protection circuit according to the present embodiment has a circuit configuration (element structure) using a double gate type transistor as described above, an electronic circuit including a double gate type transistor as an internal functional element, For example, the present invention can be satisfactorily applied to an image reading apparatus (photosensor system) provided with a photosensor including a double gate transistor.

<Image reading device>
FIG. 6 is a schematic configuration diagram showing an example of an image reading apparatus to which the electrostatic protection circuit according to the present invention can be applied. FIG. 7 is a schematic cross-sectional view showing the element structure of a photosensor applicable to the image reading apparatus according to this configuration example.
As shown in FIG. 6, the image reading apparatus (photosensor system) to which the electrostatic protection circuit according to the present invention is applicable can be roughly divided into a large number of photosensors PS, for example, n rows × m columns (n, m Is an arbitrary natural number) of photosensor arrays 300, a top gate line 301 extending by connecting the top gate terminals TG of the photosensors PS in the row direction, and a bottom gate terminal BG of the photosensors PS. A bottom gate line 302 extending in the row direction, a drain line (data line) 303 extending in the column direction by connecting the drain terminal D of each photosensor PS, and the source terminal S to a predetermined low potential voltage A source line (common line) 304 commonly connected to (for example, ground potential) Vss and a top gate connected to each top gate line 301 A driver 310, a bottom gate driver 320 connected to each bottom gate line 302, a drain driver 330 connected to each drain line 303 and including a column switch 331, a precharge switch 332, an output amplifier 333, and the like; It is comprised.

  In FIG. 6, φtg is a control signal for generating signals φT1, φT2,... ΦTi,... ΦTn selectively output as either a reset voltage (reset pulse) or a carrier storage voltage, and φbg Is a control signal for generating signals φB1, φB2,... ΦBi,... ΦBn that are selectively output as either a read voltage or a non-read voltage, and φpg controls the timing of applying the precharge voltage Vpg. This is a precharge signal.

Here, the photosensor PS applicable to this configuration example is configured to have a double-gate thin film transistor structure, similarly to the above-described electrostatic protection circuit. That is, as shown in FIG. 7, the photosensor PS generally includes a semiconductor layer (channel region) 31 such as amorphous silicon in which electron-hole pairs are generated by the incidence of excitation light (here, visible light), and Visible at both ends of the semiconductor layer 31 through impurity layers (ohmic contact layers) 37 and 38 each made of n ⁺ silicon and made of a conductive material selected from chromium, chromium alloy, aluminum, aluminum alloy, etc. A source electrode 32 (source terminal S) and a drain electrode 33 (drain terminal D) that are opaque to light, and a block insulating film (stopper film) 34 and an upper gate insulating film 35 above the semiconductor layer 31 (upward in the drawing). A top gate electrode that is formed through a transparent electrode layer such as a tin oxide film or an ITO film (indium-tin oxide film) and is transparent to visible light TGy (first gate electrode; top gate terminal TG) is formed below the semiconductor layer 31 (downward in the drawing) via the lower gate insulating film 36, and is selected from chromium, chromium alloy, aluminum, aluminum alloy, etc. A bottom gate electrode BGy (second gate electrode; bottom gate terminal BG) made of a conductive material and opaque to visible light is configured. The double-gate photosensor PS having such a configuration is formed on an insulating substrate SUB such as a glass substrate as shown in FIG. A protective insulating film (passivation film) 39 is formed on the entire surface of the insulating substrate SUB including the photosensor PS.

In FIG. 7, the top gate insulating film 35, the block insulating film 34, the insulating film constituting the bottom gate insulating film 36, and the protective insulating film 39 provided over the top gate electrode TGy are all formed of the semiconductor layer 31. Because it is made of a material having a high transmittance with respect to visible light to be excited, for example, silicon nitride, silicon oxide, or the like, the irradiation light from a light source (not shown) provided below the drawing The light is transmitted and reflected by a subject placed on a detection surface (subject detection surface) DTC provided on the upper surface of the protective insulating film 20, and is incident on the photosensor PS (specifically, the semiconductor layer 31) from above the drawing. It has a structure that detects only light.
The image reading apparatus described above constitutes the photosensor array 300 by two-dimensionally arranging such photosensors PS on a transparent insulating substrate SUB.

  Here, the photosensor PS according to this configuration example and the electrostatic protection circuit (double gate type transistor) described in each of the above-described embodiments are each formed of a conductive layer or an insulating layer formed by the same manufacturing process. Layers can be applied. That is, the photosensor PS and the double gate type transistors Tr10 or Tr20a, Tr20b are made of the same conductive material (transparent electrode layer such as tin oxide film or ITO film) using the same insulating substrate SUB such as glass substrate. TGy, TGx, TGa, and TGb made of), and bottom gate electrodes BGy, BGx, and BGa made of the same conductive material (chrome, chrome alloy, aluminum, aluminum alloy, etc.). , BGb, source electrode S, drain electrode D, and top gate insulating film 35 made of the same insulating material (for example, a transparent insulating material such as silicon nitride or silicon oxide). 15, block insulating films 34 and 14, bottom gate insulating films 36 and 16, and protective insulating films 39 and 19 are applied. It is possible.

Next, a drive control method for the above-described image reading apparatus will be briefly described with reference to the drawings.
FIG. 8 is a timing chart showing a basic drive control method in the image reading apparatus described above. FIG. 9 is a cross-sectional view of a main part when the above-described image reading apparatus is applied to a fingerprint reading apparatus. Here, a case where a fingerprint is read as a subject will be described as a drive control method of the photosensor system. In FIG. 9, for convenience of illustration, a part of hatching that represents a cross-sectional portion of the photosensor system is omitted.

In the above-described drive control method of the image reading apparatus, as shown in FIG. 8, a reset period Trst, a charge accumulation period Ta, a precharge period Tprch, and a readout period Tread are set in a predetermined processing operation period (processing cycle). It is realized by.
As shown in FIG. 8, first, in the reset period Trst, the i-th row (i is an arbitrary natural number; i = 1, 2,... N) is passed through the top gate line 301 by the top gate driver 310. Applying a reset pulse (for example, a high level of Vtg = + 15V) φTi to the top gate terminal TG of the photosensor PS to release carriers (here, holes) accumulated in the semiconductor layer 31 (initial stage) ).

Next, in the charge accumulation period Ta, the top gate driver 310 applies a low level (for example, Vtg = −15 V) bias voltage φTi to the top gate terminal TG, thereby ending the reset operation, and charge accumulation operation ( Carrier accumulation operation) is started.
Here, in the charge accumulation period Ta, as shown in FIG. 9, detection is performed from the backlight (light source) BL provided below the transparent insulating substrate SUB on which the photosensor PS shown in FIG. 7 is formed. Irradiation light La is irradiated onto a subject (for example, a finger) FG placed on the surface DTC, and the reflected light Lb passes through the top gate electrode TGy made of a transparent electrode layer and enters the semiconductor layer 31. Thereby, electron-hole pairs are generated in the incident effective region (carrier generation region) of the semiconductor layer 31 according to the amount of light incident on the semiconductor layer 31 during the charge accumulation period Ta, and the semiconductor layer 31 and the block insulating film 34 are generated. Holes are accumulated in the vicinity of the interface (around the channel region).

In the precharge period Tprch, in parallel with the charge accumulation period Ta, the drain driver 330 applies a precharge pulse (precharge voltage) Vpg to the drain terminal D via the drain line 303 based on the precharge signal φpg. Is applied, and a precharge operation for holding the charge in the drain electrode 12 is executed.
Next, in the read period Tread, after the precharge period Tprch has elapsed, the bottom gate driver 320 applies a read pulse φBi of high level (for example, Vbg = + 10 V) to the bottom gate terminal BG via the bottom gate line 302. By applying the voltage, a read operation is performed in which the drain driver 330 (column switch 331) reads the drain voltage VD corresponding to the carriers (holes) accumulated in the channel region during the charge accumulation period Ta.

  Here, the change tendency of the drain voltage VD shows a tendency that the drain voltage VD sharply decreases when there are many carriers accumulated in the charge accumulation period Ta (bright state), while there are few accumulated carriers. In this case (dark state), the light tends to decrease gradually. For example, the light incident on the photosensor PS is detected by detecting the drain voltage VD (= Vrd) after the elapse of a predetermined time from the start of the read period Tread. , That is, brightness data (brightness information) corresponding to the brightness pattern of the subject can be detected.

  Then, a series of brightness data detection operations for such a specific row (i-th row) is regarded as one cycle, and each row (i, i + 1,...) Of the photosensor array 300 described above is equivalent. By repeating the processing procedure, the photosensor system using the photosensor PS can be operated as a monochrome type image reading apparatus that reads a two-dimensional image (for example, a fingerprint pattern) of a subject as lightness data.

  Therefore, in the image reading apparatus (photosensor system) according to this configuration example, for example, any of the top gate line 301, the bottom gate line 302, the drain line 303, and the source line 304 disposed in the photosensor array 300, or By providing the electrostatic protection circuit composed of the double gate transistor shown in each of the above-described embodiments in all the lines, it is possible to realize an image reading apparatus that has excellent electrostatic withstand voltage characteristics and good operating characteristics. it can.

  In particular, in the image reading apparatus according to this configuration example, the static manufacturing method described in each of the above-described embodiments is performed by using the same manufacturing process as that for forming the photosensor PS constituting the photosensor array 300 on the insulating substrate. Since the double gate type transistors constituting the electric protection circuit can be simultaneously formed together with the photosensor, the electrostatic protection circuit can be formed with a relatively small circuit area with almost no increase in the manufacturing process. Can be formed. As a result, an image reading apparatus having excellent electrostatic withstand voltage characteristics can be realized without increasing the scale of the apparatus and at a low manufacturing cost.

  In the application example described above, the case where the electrostatic protection circuit according to the present invention is applied to an image reading apparatus including a photosensor array including a double gate type photosensor has been described. However, the present invention is not limited thereto. However, the present invention can be favorably applied to a liquid crystal display device or the like provided with a thin film transistor (field effect transistor) in a pixel transistor as shown in the prior art. In short, if a functional element inside an electronic circuit to which an electrostatic protection circuit is connected and at least a part of the element structure (conductive layer, insulating layer, etc.) are formed simultaneously by the same manufacturing process, significant manufacturing is required. An electronic circuit including an electrostatic protection circuit having a high electrostatic protection effect can be realized at a relatively low cost without any change or increase in process.

It is the equivalent circuit and schematic sectional drawing which show 1st Embodiment of the electrostatic protection circuit which concerns on this invention. It is a characteristic view which shows the voltage-current characteristic of the electrostatic protection circuit which concerns on this embodiment. It is an equivalent circuit which shows 2nd Embodiment of the electrostatic protection circuit which concerns on this invention. It is a schematic sectional drawing which shows 2nd Embodiment of the electrostatic protection circuit which concerns on this invention. It is a characteristic view which shows the voltage-current characteristic of the electrostatic protection circuit which concerns on this embodiment. 1 is a schematic configuration diagram illustrating an example of an image reading apparatus to which an electrostatic protection circuit according to the present invention is applicable. It is a schematic sectional drawing which shows the element structure of the photosensor applicable to the image reading apparatus which concerns on this structural example. 6 is a timing chart illustrating a basic drive control method in the image reading apparatus according to the present configuration example. It is principal part sectional drawing at the time of applying the image reading apparatus which concerns on this structural example to a fingerprint reading apparatus. It is a circuit block diagram which shows the example of the electrostatic protection circuit using the thin-film transistor in a prior art. It is a characteristic view which shows the voltage-current (V-I) characteristic of the electrostatic protection circuit in a prior art. It is a circuit block diagram of the electrostatic protection circuit used as the examination object for solving the problem in a prior art.

Explanation of symbols

Tr10 Double gate type transistor 11 Semiconductor layer 12 Source electrode 13 Drain electrode TGx Top gate electrode BGx Bottom gate electrode H11, H12 Contact hole

Claims (8)

In an electrostatic protection circuit that discharges an overcurrent due to static electricity to a predetermined low-potential power supply to protect the electronic circuit from electrostatic breakdown,
Between a first contact connected to the signal input contact of the electronic circuit and a second contact connected to the low potential power supply,
At least one source and drain electrodes formed across a channel region made of a semiconductor layer, and a first gate electrode and a second electrode formed above and below the channel region via insulating films, respectively. And a circuit configuration comprising a transistor having a double gate structure in which the first gate electrode is connected to the source electrode and the second gate electrode is connected to the drain electrode. An electrostatic protection circuit. The electrostatic protection circuit includes one transistor having the double gate structure between the first contact and the second contact, and a source electrode and a drain electrode of the transistor are respectively connected to the first contact or the drain. The electrostatic protection circuit according to claim 1, wherein the electrostatic protection circuit is connected to the second contact. The electrostatic protection circuit includes two transistors having the double gate structure between the first contact and the second contact, the drain electrodes of the transistors are connected to each other, and the source electrodes of the transistors are connected to each other. The electrostatic protection circuit according to claim 1, wherein each of the electrostatic protection circuits is connected to the first contact or the second contact. In an electronic circuit equipped with an electrostatic protection circuit that discharges an overcurrent caused by static electricity to a predetermined low-potential power supply and protects against electrostatic breakdown
The electronic circuit includes at least a source electrode and a drain electrode formed across a channel region made of a semiconductor layer, and a gate electrode formed via an insulating film at least above or below the channel region. A circuit configuration including a field-effect transistor provided;
The electrostatic protection circuit includes at least one channel formed of a semiconductor layer between a first contact connected to a signal input contact of the electronic circuit and a second contact connected to the low potential power source. A source electrode and a drain electrode formed on both sides of the region; and a first gate electrode and a second gate electrode formed above and below the channel region via an insulating film, respectively. An electronic circuit having a circuit configuration including a transistor having a double gate structure in which a gate electrode is connected to the source electrode and the second gate electrode is connected to the drain electrode. The field effect transistor in the electronic circuit has a double gate structure including a first gate electrode and a second gate electrode formed above and below the channel region via insulating films, respectively. The electronic circuit according to claim 4. The electrostatic protection circuit includes one transistor having the double gate structure between the first contact and the second contact, and a source electrode and a drain electrode of the transistor are respectively connected to the first contact or the drain. The electronic circuit according to claim 4, wherein the electronic circuit is connected to the second contact. The electrostatic protection circuit includes two transistors having the double gate structure between the first contact and the second contact, the drain electrodes of the transistors are connected to each other, and the source electrodes of the transistors are connected to each other. The electronic circuit according to claim 4, wherein each of the electronic circuits is connected to the first contact or the second contact. At least a part of the structure of the transistor having the double gate structure constituting the electrostatic protection circuit and the field effect transistor constituting the electronic circuit is formed simultaneously by the same manufacturing process. The electronic circuit according to claim 4.

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