JPH066514A - Switching element - Google Patents

Abstract

(57) [Summary] [Purpose] The switching speed is improved by reducing the reverse current flowing immediately after the change from the forward bias to the reverse bias. [Structure] On a glass substrate 12, a lower electrode 14 and a semiconductor layer 16 of a pin structure made of amorphous silicon.
In the switching element 10 including the upper electrode 18 and the upper electrode 18, the lower electrode 14 and the upper electrode 18 are formed of ITO. As a result, light leakage from the periphery is made incident on the semiconductor layer 16 and the recombination speed is increased to improve the switching speed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching element, and more particularly to a switching element such as a diode formed on a substrate such as glass.

[0002]

2. Description of the Related Art For example, in an image reading unit such as a facsimile or an image scanner, a C which requires a reduction optical system.
In place of the CD type document reading device, a document reading device generally called a contact image sensor is widely adopted. For example, as shown in FIG. 5, the document reading apparatus 1 reads out a photodiode 3 which is a photoelectric conversion element, a blocking diode 4 which is a switching element, and an electric signal from the photodiode 3 on a glass substrate 2. Channel wirings C _1, C _{2, ...} C _n are formed.

The photodiode 3 and the blocking diode 4 are both opaque lower electrodes 3a and 4a made of metal and a pin made of amorphous silicon.
Structured semiconductor layers 3b and 4b and ITO (Indium Tin Oxi
de) and transparent upper electrodes 3c and 4c are sequentially deposited. The photodiode 3 and the blocking diode 4 are covered with a transparent interlayer insulating film 5 made of SiO _x, and are connected in series in reverse polarity by a connecting wire 7 through a contact hole 6 formed in the interlayer insulating film 5. Has been done. On the other hand, the lower electrode 3a forming the photodiode 3 has the channel wirings C _1, C via the contact hole 8 formed in the interlayer insulating film 5.
_{2, ...} C _n . Further, these are entirely covered with the protective film 9. Here, the reason that the upper electrode 4c of the blocking diode 4 is transparent is that the manufacturing process is simplified by depositing the upper electrode 4c of the photodiode 3 at the same time as that of the upper electrode 3c of the photodiode 3, so that light is incident. It's not. Therefore, the entire blocking diode 4 is covered with the connection wiring 7 so that light does not enter.

Further, the photodiode 3 and the blocking diode 4 are m-dimensionally arranged as shown in FIG.
× n blocks are arranged, and m blocks B _{1 and} B are arranged for every n blocks.
_{2, ...} have been divided into B _m, anode electrode blocks B _1, B 2 blocking diode _{4, ...} are connected in common in a B _m, the anode electrode of the photo diode 3 channel wiring C _{1 ,} C _{2, ...} C _n by blocks B _1, B
_{2, 2, ...,} B _m , which are relatively in the same position, are commonly connected. These blocking diodes 4
Are necessary for sequentially selecting the photodiodes 3 for each block B _1, B _{2, ...} B _m .

This document reading apparatus 1 operates by a charge storage method, and as shown in the time chart of FIG.
Drive pulse _{Vp 1, Vp 2, ... Vp} m blocks B _1, B _{2, ...} B
It is applied in cycle T for each _m . Drive pulse Vp _1, Vp
_2, when ... Vp _m is applied, the block B _1,
The blocking diodes 4 in B _{2, ...} B _m are forward biased, and the blocks B _1, B _{2, ...} B _m are in a read state. On the other hand, the drive pulse Vp _1, Vp _2, ... when the Vp _m is not applied, the block B _1, B _{2, ...} blocking diode 4 in B _m becomes reverse biased, the block B _{1 ,} B _{2, ...} B _m are in a storage state. That is, each block B _1, B _{2, ...} B _m has the read state at time t and the read state at time T.
The accumulation state of −t is repeated. From the blocks B _1, B _{2, ...} B _{m in the} read state, output currents Iout _1, Iout corresponding to the amount of light incident during the storage state up to that point.
_{2, ...} Iout _n flow out via the channel wirings C _1, C _{2, ...} C _n , and these output currents Iout _1, Iout _{2, ...} Iout _n are amplified and integrated by an external signal processing circuit. After that, it will be output in time series. For example, the first block B
_{When 1} becomes the read state, the output current from the first block B ₁
When Iout _1, Iout _{2, ...} Iout _n flow out, then the first block B ₁ is in the accumulation state and the second block B ₂ is in the reading state, the output currents Iout _1, Iout from the second block B _{2
2, ...} Iout _n will flow out.

[0006]

[Problems to be Solved by the Invention] However, in reality,
Immediately after the blocks B _1, B _{2, ...} B _m are switched from the read state to the storage state, the output currents Iout _1, Iout _{2, ...} Iout _n
Current in the opposite direction (hereinafter referred to as "reverse current") Ir _1, Ir
_{2, ...} Ir _n flows. This reverse current Ir _1, Ir _{2, ...} Ir _n
The normal output current is Iout _1, Iout _{2, ...} Iout _n 1
It reaches 0 to 20%, and its convergence time Tr reaches the order of 10 ⁻³ seconds. Therefore, for example, the first block B ₁
When white is read by the second block B ₂ and black is read by the third block B ₃ , output currents Iout _1, Iout flowing when the second block B ₂ is in the read state
_{2, ...} Iout _n becomes smaller than usual, and the output currents Iout _1, Iout _{2, ...} Iout _n that should not flow when the third block B ₃ is in the read state are the reverse currents Ir _{1 ,} Ir
_{2, ...} Ir _n will flow in the opposite direction.

The cause of this reverse current is that even if the voltage applied to the blocking diode 4 is changed from forward bias to reverse bias, the carriers injected during forward bias cannot be instantly lost, so this carrier is generated. It is considered that this is because the flow flows in the opposite direction for a fixed time. That is,
It is considered that the switching speed of the blocking diode 4 is limited by this reverse current.

Therefore, the inventors of the present invention have conducted intensive studies in order to improve the switching speed by reducing the reverse current generally associated with a switching element such as a blocking diode.

[0009]

A gist of a switching element according to the present invention is that a lower electrode and a lower electrode are provided on a substrate.
In a switching element including a semiconductor layer deposited on the lower electrode and an upper electrode deposited on the semiconductor layer, at least one of the lower electrode and the upper electrode is transparent. There is something.

Further, in the switching element, the transparent electrode is made of ITO.

Further, in such a switching element,
The semiconductor layer has a pin structure.

Further, in the switching element, the semiconductor layer is made of amorphous silicon.

[0013]

According to such a switching element, since the lower electrode, the upper electrode, or both of them are made of ITO or the like and are transparent, the light leaked from the periphery passes through the lower electrode and the upper electrode and is incident on the semiconductor layer. To do. As a result, the recombination level in the semiconductor layer is increased and the recombination rate is increased. Therefore, even when the voltage applied to the switching element is changed from the forward bias to the reverse bias, the carriers injected during the forward bias are considered to disappear promptly due to recombination. As a result, the reverse current is rapidly converged, the peak value thereof is reduced, and the switching speed is improved.

[0014]

Embodiments of the switching element according to the present invention will be described in detail with reference to the drawings.

As shown in FIG. 1, a switching element 10 according to the present invention comprises a transparent substrate 12 made of glass or the like.
A transparent lower electrode 14 made of ITO (Indium Tin Oxide) or the like, a semiconductor layer 16 made of amorphous silicon or the like, and a transparent upper electrode 18 made of ITO or the like.
And are formed. Here, the semiconductor layer 16
Is p in which holes become majority carriers in order from the substrate 12 side.
The type amorphous silicon layer 16a, the i-type amorphous silicon layer 16b serving as an intrinsic semiconductor, and the n-type amorphous silicon layer 16c serving as majority carriers of electrons are stacked to form a pin structure.

In order to manufacture this switching element 10, first, a transparent conductive film such as ITO is deposited on the substrate 12 by a vacuum vapor deposition method using an electron beam or resistance heating, or a sputtering method using DC or RF. Further, a p-type amorphous silicon film, an i-type amorphous silicon film, and an n-type amorphous silicon film are successively deposited on this by a plasma CVD method or the like. Then, a transparent conductive film such as ITO is deposited again on this by a vacuum vapor deposition method, a sputtering method, or the like. The thickness of each of these transparent conductive films is preferably several hundreds to several thousand liters, but is appropriately determined in consideration of the performance of the deposited amorphous silicon film and the characteristics of the transparent conductive film.

Next, these films are sequentially patterned to form the lower electrode 14, the semiconductor layer 16 and the upper electrode 18. For example, in the case of patterning by photolithography, first apply a resist solution on the uppermost transparent conductive film, perform pre-baking, and then perform exposure using a mask in which a predetermined pattern is engraved. Bake. Then, when ITO is used as the transparent conductive film, the transparent conductive film is etched with a mixed solution of hydrochloric acid and nitric acid to form the upper electrode 18. Next, the amorphous silicon film is etched using a parallel plate type etching device. That is,
After the chamber was evacuated to 10 ^-3 Torr or less, CF ₄ gas and O ₂ gas were introduced, and while maintaining the pressure at 5.0 Pa, a high frequency power source of 13.56 MHz was used to apply 0.1 to 0.1% to the electrodes. A power of 0.7 W / cm ² is supplied. In this way, the amorphous silicon film is etched to form the semiconductor layer 16. Then, after the resist used for patterning is once removed, the lowermost transparent conductive film is patterned by photolithography or the like to form the lower electrode 14 like the above-mentioned uppermost transparent conductive film. 14, semiconductor layer 16, and upper electrode 18
The switching element 10 composed of is manufactured.

Here, the method of forming the lower electrode 14, the semiconductor layer 16 and the upper electrode 18 by the photolithography method is illustrated, but the film is not deposited on the unnecessary portion from the beginning by the mask method or the like. The manufacturing method is not limited at all, such that it may be formed.

According to this switching element 10, since the lower electrode 14 and the upper electrode 18 are transparent, and the leaked light from the periphery passes through them and is incident on the semiconductor layer 16, the reverse current is 10 ⁻⁵ to 10 ^−. It can be converged in the order of ⁶ seconds, and its peak value can be reduced to improve the switching speed. The main reason for this is that a large number of trap levels exist in the semiconductor layer 16 made of amorphous silicon, and when light is incident on the semiconductor layer 16, these trap levels change and recombination levels change. It is expected to increase. Therefore, it is considered that the recombination speed increases and even when the voltage applied to the switching element 10 is changed from the forward bias to the reverse bias, the carriers injected during the forward bias are promptly lost by the recombination.
Further, it is considered that the provision of the interface between the semiconductor layer 16 and ITO or the like has a considerable influence. Therefore,
If this switching element 10 is used in a document reading device, more accurate signal output can be obtained and the signal reading speed can be increased.

Although one embodiment of the switching element according to the present invention has been described in detail above, the present invention is not limited to the above-mentioned embodiment and can be implemented in other modes.

For example, as shown in FIG. 2, a transparent lower electrode 14 made of ITO or the like is formed on a substrate 12 such as glass.
Alternatively, the switching element 24 may be configured by forming the semiconductor layer 20 composed only of the i-type amorphous silicon layer and the opaque upper electrode 22 composed of metal or the like. In this switching element 24, a Schottky barrier is formed at the interface between the i-type amorphous silicon layer (20) and the lower electrode 14, and the leakage of light from the periphery is lower electrode 1.
4 is transmitted and enters the semiconductor layer 20.

Further, as shown in FIG. 3, a transparent lower electrode 14 made of ITO or the like is provided on a substrate 12 such as glass.
a semiconductor layer 26 formed by stacking a p-type amorphous silicon layer 26a and an i-type amorphous silicon layer 26b,
The switching element 28 may be formed by forming the opaque upper electrode 22 made of metal or the like. As is clear from this example, the semiconductor layer may have a pi structure.

Further, as shown in FIG. 4, a transparent lower electrode 14 made of ITO or the like is formed on a substrate 12 such as glass.
An insulating layer 30 and an i-type amorphous silicon layer 32a
A switching element 34 may be formed by forming a semiconductor layer 32 formed by laminating a p-type amorphous silicon layer 32b and an opaque upper electrode 22 formed of a metal or the like. The switching element 34 is of the MIS type, and the semiconductor layer 32 is deposited on the lower electrode 14 via the insulating layer 30. As is clear from this example, the semiconductor layer may be deposited not directly on the lower electrode but indirectly, and the same applies to the other layers.

Further, the semiconductor layer 16 illustrated first is laminated in the order of pin from the substrate 12 side, but conversely, it may be laminated in the order of nip to have a pin structure. Also, the Schottky barrier type described above, p
Besides i-type and MIS type, it may be a single-layer or multi-layer deposited on ni-type, pn-type, hetero-junction type, homo-junction type or a combination thereof. Furthermore, as the amorphous silicon constituting the semiconductor layer, hydrogenated amorphous silicon a-Si: H, hydrogenated amorphous silicon carbide a-SiC: H, amorphous silicon nitride, and the like, simple amorphous silicon a-Si and the like are preferable. However, an amorphous or fine crystal of an amorphous silicon semiconductor made of an alloy of silicon and another element such as carbon, germanium or tin may be deposited.
That is, the switching element according to the present invention may have any structure as long as it has at least one surface barrier such as a potential barrier or a Schottky barrier and has a switching function.

Further, the transparent electrode may be a lower electrode or an upper electrode, and both may be transparent. That is, at least one of the lower electrode and the upper electrode may be transparent. In the conventional document reading apparatus 1, the upper electrode 4c forming the blocking diode 4 is transparent for the purpose of simplifying the manufacturing process and is shielded by the connection wiring 7 and the like. The present invention does not have any particular effect according to the present invention.

Other than ITO, SnO ₂ or TiO ₂ may be used as the material of the lower electrode and the upper electrode.
That is, the material is not limited as long as it is a transparent and electrically conductive material, and the present invention includes various improvements and modifications based on the knowledge of those skilled in the art without departing from the spirit of the invention.
It can be implemented in a modified form.

[0027]

In the switching element according to the present invention, since at least one of the lower electrode and the upper electrode is transparent, the light leaked from the periphery passes through the lower electrode and the upper electrode and is incident on the semiconductor layer. However, the reverse current is rapidly converged, and the peak value becomes small. Therefore, the switching speed can be significantly improved. Further, when the switching element according to the present invention is used in a document reading apparatus, more accurate signal output can be obtained and the signal reading speed can be increased. The present invention has various excellent effects. Play.

[Brief description of drawings]

FIG. 1 is a schematic sectional view showing an embodiment of a switching element according to the present invention.

FIG. 2 is a schematic sectional view showing another embodiment of the switching element according to the present invention.

FIG. 3 is a schematic sectional view showing still another embodiment of the switching element according to the present invention.

FIG. 4 is a schematic sectional view showing still another embodiment of the switching element according to the present invention.

FIG. 5 is a schematic cross-sectional view showing an example of a document reading apparatus using a conventional switching element.

FIG. 6 is a circuit diagram of the document reading apparatus shown in FIG.

FIG. 7 is a time chart for explaining the operation of the document reading apparatus shown in FIGS. 5 and 6.

[Explanation of reference signs] 10, 24, 28, 34; switching element 12; substrate 14; lower electrode 16, 20, 26, 32; semiconductor layer 18, 22; upper electrode

Claims (4)

[Claims] 1. A switching element configured by forming a lower electrode, a semiconductor layer deposited on the lower electrode, and an upper electrode deposited on the semiconductor layer on a substrate. And at least one of the upper electrodes is transparent. 2. The switching element according to claim 1, wherein the transparent electrode is made of ITO. 3. The switching element according to claim 1, wherein the semiconductor layer has a pin structure. 4. The switching element according to claim 1, wherein the semiconductor layer is made of amorphous silicon.