KR970010689B1 - Thin film transistor semiconductor device - Google Patents

Abstract

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Description

Thin film transistor for liquid crystal display device

1 is a cross-sectional structure of a conventional thin film transistor,

2 is a cross-sectional structure of a thin film transistor according to an embodiment of the present invention,

3 is a process flowchart showing a thin film transistor manufacturing method according to an embodiment of the present invention,

4 is a cross-sectional view of a thin film transistor according to another embodiment of the present invention.

* Explanation of symbols for main parts of the drawings

1 Insulation substrate 2 Gate electrode

3: gate insulating film 4: amorphous silicon active layer

5: channel protective layer 6: doped amorphous silicon layer

7 source electrode and drain electrode 8 insulating layer

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to thin film transistors for liquid crystal display devices, and more particularly, to a thin film transistor structure capable of reducing leakage current of thin film transistors during light irradiation.

In the conventional thin film transistor for liquid crystal display device, as shown in FIG. 1, the gate electrode 2 is formed on the glass substrate 1, and the gate insulating film 3 and the amorphous silicon (a-Si: H) are formed on the entire surface thereof. ) Active layer (4), channel protective layer (5) of amorphous nitride (a-SiN: H), phosphorus (P) doped amorphous silicon layer (6) for ohmic contact, source and drain electrode (7) ) Is formed in sequence.

In the above-described prior art, the leakage current at the time of turning off the thin film transistor serving as a switch in the liquid crystal display device changes the voltage of the pixel of the liquid crystal display device, which adversely affects the image quality.

In particular, when the backlight is irradiated with light, light is absorbed near the source and drain junctions (hatched portions of FIG. 1) of the active layer made of amorphous silicon to generate electrons and holes, thereby increasing leakage current.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object thereof is to provide a thin film transistor structure suitable for reducing leakage current of a thin film transistor during light irradiation.

The thin film transistor of the present invention for achieving the above object is a gate electrode (2) formed on the insulating substrate 1, the gate insulating film (3) formed on the entire surface of the insulating substrate (1) formed with the gate electrode (2), A channel protection layer 5 formed on the gate insulating film 3, an amorphous silicon active layer 4 defined only below the channel protection layer 5, and the gate insulating film 3 at both ends of the channel protection layer 5. And a doped semiconductor layer 6 for ohmic contact formed over the top, and a source electrode and a drain electrode 7 formed on the doped semiconductor layer 6.

Hereinafter, with reference to the accompanying drawings will be described in detail the present invention.

According to the present invention, unlike the conventional thin film transistor, the amorphous silicon active layer of the source and drain junction portions has different characteristics from those of the channel portion so as to reduce leakage current due to light during off.

When a negative voltage is applied to the gate electrode of the transistor, holes are accumulated in the amorphous silicon active layer, but are blocked by phosphorus (P) doped amorphous silicon in contact with the source and drain electrodes so that very little current flows. At this time, when light is irradiated to the source and drain junctions, electrons and holes are generated to increase the current. This current is proportional to the product of the number of electron-holes generated by light and their average survival time.

As described above, the leakage current during light irradiation of the transistor is closely related to the photoelectric conductivity near the source and drain junctions. The present invention uses the same to reduce the leakage current by including a new bonding layer having a low photoelectric conductivity.

2 shows a cross-sectional structure of a thin film transistor according to the present invention.

The thin film transistor of the present invention includes a gate electrode 2 formed on the insulating substrate 1, a gate insulating film 3 formed on the gate electrode 2, and an amorphous silicon active layer 4 formed on the gate insulating film 3. A channel protection layer 5 formed on the amorphous silicon layer 4, a doped semiconductor layer 6 for ohmic contact formed over the gate insulating layer 3 at both ends of the channel protection layer 5, and And a source electrode and a drain electrode 7 formed on the doped semiconductor layer 6.

In the thin film transistor of the present invention, it is also possible to reduce the photoelectric conductivity by forming the bonding layer 8 between the doped semiconductor layer 6 and the source electrode and the drain electrode 7.

Referring to Figure 3 described above the thin film transistor manufacturing method of the present invention as follows.

First, as shown in FIG. 3A, a conductive layer is formed on the glass substrate 1, and then patterned to form a gate electrode 2. Then, as shown in FIG. A gate insulating film 3, an amorphous silicon active layer 4, and a channel protective layer 5 are sequentially formed on the entire glass substrate 1 on which the gate electrode 2 is formed.

Subsequently, as shown in FIG. 3 (c), the channel protective layer 5 and the amorphous silicon active layer 4 are selectively etched to remove the amorphous silicon active layer 4 other than the channel region. As shown in Fig. 6), a junction layer 8 is formed on the entire surface of the resultant, and then an amorphous silicon layer 6 doped with phosphorus (P) for ohmic contact is formed thereon. The silicon layer 6 and the bonding layer 8 are selectively etched so as to remain only in an area over the gate insulating layer 3 at both ends of the upper portion of the channel protection layer 5.

The bonding layer 8 is preferably formed by depositing amorphous silicon having a large band gap in order to reduce the absorption coefficient of light.

Typically, the amorphous silicon has a bandgap of about 1.7 eV, which can be controlled by changing the deposition conditions (temperature, reactor pressure, hydrogen gas concentration, etc.).

By increasing the band gap of the bonding layer as described above, even if light is irradiated, the amount of absorption is reduced and the generation of battery-holes is reduced, thereby reducing leakage current.

The bonding layer 8 may be formed of an amorphous silicon alloy or an amorphous silicon-carbon alloy, and the band gap has a large value of ˜1.9 eV or more depending on the amount of carbon. At this time, in order to reduce the junction resistance in the transistor on state, the doping of phosphorus (P) into the amorphous silicon-carbon is performed using a uniform doping or delta doping technique of about 1 to 1000 ppm.

Next, as shown in FIG. 3E, a metal is deposited on the entire surface of the resultant, and then patterned to form a source electrode and a drain electrode 7 on the doped amorphous silicon layer 6. Complete the manufacture.

On the other hand, in the manufacture of the thin film transistor of the present invention, even when the bonding layer is not formed, the leakage current during light irradiation can be reduced.

That is, as shown in FIG. 4, the amorphous silicon active layer 4 is formed so as to be limited to the lower portion of the channel protective layer 5, and the doped amorphous silicon layer 6 for ohmic contact is directly formed with the gate insulating film 3. Form contact.

In this case, since there is no bonding layer made of amorphous silicon under the source and drain electrodes 7, generation of electron-holes is suppressed during light irradiation. Therefore, leakage current during light irradiation can be reduced as in the case of ion doping.

In the method of manufacturing a thin film transistor in which the bonding layer 8 does not exist, the process of forming the bonding layer 8 in the process of FIG. 3 is omitted, and the description thereof is omitted.

As described above, according to the present invention, by forming a bonding layer made of amorphous silicon having a large band gap and a small light absorption coefficient, it is possible to reduce the leakage current due to the photoelectric current when the thin film transistor is turned off.

Therefore, the leakage current of the transistor in the thin film transistor liquid crystal display device can be reduced to improve the image quality.

Claims (3)

A gate electrode 2 formed on the insulating substrate 1, a gate insulating film 3 formed on the entire surface of the insulating substrate 1 on which the gate electrode 2 is formed, and a channel protective layer formed on the gate insulating film 3; 5), an amorphous silicon active layer 4 formed only under the channel protection layer 5, and a doped semiconductor layer for ohmic contact formed over the gate insulating film 3 at both ends of the channel protection layer 5. (6), and a source electrode and a drain electrode (7) formed on the doped semiconductor layer (6). The thin film transistor according to claim 1, further comprising a bonding layer (8) formed between the doped semiconductor layer (6) and the source electrode and the drain electrode (7) to reduce photoelectric conductivity. . The liquid crystal display device according to claim 2, wherein the bonding layer (8) is made of any one selected from amorphous silicon, amorphous silicon alloy, phosphorus doped amorphous silicon-carbon alloy, and delta-doped amorphous silicon-carbon alloy. Thin film transistor.