GB2122419A - A thin film transistor and an active matrix liquid crystal display device - Google Patents

Abstract

A thin film transistor formed on an insulating substrate (8) comprises a source electrode (15) connected to a source region (10), a drain electrode (16) connected to a drain region (11) and a channel region 9 between the source region and the drain region. The channel region is substantially covered by an extension of the source electrode 15 or the drain electrode. Another thin film transistor disclosed has a transparent contact connected to the drain region and lying on an insulation layer. The transistors are used in a LCD matrix including, in each element thereof, a display electrode 210 connected to a polysilicon layer on the transistor at a connection 209. The transistor is also connected to a signal electrode 207 at a connection 208. <IMAGE>

Description

SPECIFICATION A thin film transistor and an active matrix liquid crystal display device This invention relates to thin film transistors and to active matrix liquid crystal display devices.

Recently thin film transistor technology has been studied more actively. There are many applications of thin film transistors, for example, in thin active matrix liquid crystal display devices employing relatively inexpensive insulating substrates, three-dimensional integrated circuits in which active elements, such as thin film transistors, are formed on semiconductor integrated circuits, relatively inexpensive image sensors with high performance, high-density memory devices etc.

Large scale active matrix liquid crystal display devices having a matrix of picture elements are attracting particular attention and are being applied to various apparatus such as small-size personal computers, hand-held television receivers etc. The use of thin film transistors as switching devices for the picture elements is now under serious consideration particularly for planar large scale active matrix liquid crystal display devices.

According to one aspect of the present invention there is provided a thin film transistor formed on an insulating substrate comprising a source electrode connected to a source region, a drain electrode connected to a drain region, a channel region between the source region and the drain region, said channel region being substantially covered by an extension of the source electrode or drain electrode.

The source region may be substantially covered by the source electrode. The drain region may be substantially covered by the drain electrode.

According to another aspect of the present invention there is provided a thin film transistor comprising a source region, a drain region and a gate electrode, the drain region or source region being in direct contact with a transparent electrode.

Preferably the transparent electrode is made of indium oxide, tin oxide or indium tin oxide.

A metallic layer may overlie the region of direct contact between the source region or drain region and the transparent electrode.

The source region and drain region may be formed in a polycrystalline thin film.

According to another aspect of the present invention there is provided an active matrix liquid crystal display device comprising a matrix of picture elements each including a thin film transistor according to the present invention.

According to a still further aspect of the present invention there is provided an active matrix liquid crystal display device having a matrix of picture elements each of which is composed of a driving electrode, a capacitor for storing a data signal, a MOS transistor having a drain electrode, a source electrode and a gate electrode which is covered by a gate insulating layer, the MOS transistor being arranged selectively to apply the data signal to the capacitor, a gate line connected to said gate electrode, a source line orthogonal to the gate line and connected to said source electrode, and a further insulating layer between the gate line and the source line, the breakdown voltage of the further insulating layer being higher than the breakdown voltage of the gate insulating layer.

In the preferred embodiment the thickness of the further insulating layer at least in the region of the section of each gate line with each source line is greater than in other regions.

In each picture element at least a portion of said gate line or said MOS transistor may be covered by the respective driving electrode, an insulating layer being disposed therebetween.

The picture elements are preferably formed on a glass substrate.

The invention is illustrated, merely by way of example, in the accompanying drawings, in which: Figure 1 consisting of Figs 1 (a) and 1(b), illustrates the general construction of an active matrix liquid crystal display device; Figure 2 is a cross-sectional view of a conventional N-channel thin film transistor; Figure 3 shows graphically the voltage-current characteristics of the conventional thin film transistor of Fig. 2; Figure 4 is a cross-section of one embodiment of a thin film transistor according to the present invention; Figure 5 shows graphically the voltage-current characteristics of the thin film transistor of Fig. 4; Figure 6 is a cross-sectional view of another conventional thin film transistor; Figure 7 is an enlarged plan view of a picture element of the active matrix liquid crystal display device of Fig. 1;; Figure 8 illustrates the connection of the conventional thin film transistor of Fig. 6 with a transparent electrode of an active matrix liquid crystal display device; Figure 9 is a cross-sectional view of another embodiment of a thin film transistor according to the present invention; Figure 10 is a cross-sectional view of a yet further embodiment of a thin film transistor according to the present invention; Figure 11 consists of Figs. 1 1 (a) which is an enlarged plan view of a picture element of a conventional active matrix liquid crystal display device, and Fig. 1 1 (b) which is a section of the picture element of Fig. 1 1 (a) taken on the line A-B;; Figure 12 consists of Fig. 1 2(a) which is a cross-section of a picture element of an active matrix liquid crystal display device according to the present invention and Fig. 1 2(b) which is a plan view thereof; and Figure 13 consists of Fig. 1 3(a) which is a plan view of a picture element of another active matrix liquid crystal display device according to the present invention and Fig.

1 3(b) which is a cross-section thereof taken on the line C-D.

Generally an active matrix liquid crystal display device consists of an upper transparent substrate, a lower transparent substrate on which thin film transistors are formed, and a liquid crystal material which is sealed between the substrates. Liquid crystal driving elements associated with liquid crystal driving electrodes are arranged in a matrix on the thin film transistors. An external selecting circuit selects the liquid crystal driving elements, and the associated liquid crystal driving electrodes are excited to display desired letters, figures or pictures.

Fig. 1 illustrates the general construction of an active matrix liquid crystal display device having a matrix of liquid crystal driving elements 2 on a lower substrate (not shown) within a display area 1. Data signals and timing signals are applied to the liquid crystal driving elements 2 through orthogonal input lines 3, 4, respectively. Fig. 1 (b) is a circuit diagram of one of the liquid crystal driving elements 2. A thin film transistor 5 is provided for switching the data signal on the line 3 in response to the timing signal on the line 4. A capacitor 6, which is not necessarily requred if the capacitance of the liquid crystal material is sufficiently large, is provided for storing data.A liquid crystal cell 7 includes a liquid crystal driving electrode 7-1 on the lower substrate and a transparent electrode 7-2 on an upper transparent substrate (not shown).

The active matrix liquid crystal display device shown in Fig. 1 (a) suffers from faults due to bad patterning of the liquid crystal driving elements, poor electrical insulation at the intersection of the input lines 3, 4 and poor electrical insulation of the liquid crystal picture elements 2. Bad patterning can be overcome by improving the manufacturing techniques and making the manufacturing environment thoroughly clean so as to increase manufacturing yield. Improvement of quality and increase of thickness of an insulating layer of the thin film transistor of each picture element leads also to an increase in manufacturing yield by overcoming the problem of poor electrical insulation resulting at an early stage of manufacture.After the matrix of picture elements has been completely formed, however, poor electrical insulation between the input lines is apt to occur due to the effects of static electricity and other factors. When the input lines are subject to static electricity from outside the display area 1, breakdown of electrical insulation is caused at the intersections between the input lines. As a result of data signal from one of the input lines 3 may leak into one of the input lines 4 or a timing signal from one of the input lines 4 may leak into one of the input lines 3 at a region where the input lines 3, 4 intersect. This causes a so-called "line fault". As a result, the display of each picture element in the line where there is a breakdown of electrical insulation is reduced and the display produced by the active matrix liquid crystal display device as a whole deteriorates.When there is a breakdown of electrical insulation, this is usually compensated for by disconnecting the input line around the breakdown. However, this inevitably generates a line fault because the picture elements connected to the disconnected input line cannot be energised. In the case where the matrix of picture elements is formed on a single crystalline silicone substrate, protection from damage caused by static electricity can be achieved by providing a diode or resistor on the silicon substrate. On the other hand, in the case where the matrix of picture elements is formed on a glass substrate electrical insulation breakdown is likely to occur and it is very difficult to provide elements for protecting against damage due to static electricity. As a result, manufacturing yield is reduced.

Fig. 2 is a cross-sectional view of a conventional N-channel thin film transistor formed on an insulating substrate 8 of, for example, glass or quartz. A semiconductor thin film 9 of, for example, polycrystalline silicon, is doped with impurity, e.g. phosphorus or arsenic, to form a source region 10 and a drain region 11. A gate film 1 2 is formed on the semiconductor thin film 9 and is covered by a gate electrode 1 3. An insulating film 14 with windows is formed and a source electrode 1 5 and a drain electrode 1 6 connect with the source and drain regions respectively.

In the case of an active matrix liquid crystal display device, the thin film transistor controls the application of a data signal to the liquid crystal material associated with each liquid crystal driving element. In order to obtain a high quality display the thin film transistor should have the following characteristics: (1) When in the ON state, the thin film transistor should be capable of supplying sufficient current to charge the capacitor.

(2) When in the OFF state, there should be very little current leakage.

The first requirement is necessary because of writing data into the capacitor. Since the superiority of the display produced by the active matrix liquid crystal display device depends upon the potential of the capacitor, charge has to be stored in the capacitor in a relatively short period of time. In other words, the thin film transistor must be able to supply sufficient current for writing data thoroughly into the capacitor in a very short time. The amount of current for writing data (hereinafter referred to as "ON current") is determined by the capacitance of the capacitor and the time necessary for writing data. Accordingly, the thin film transistor should be formed so as to supply sufficient ON current as determined by the capacitor.The amount of ON current flowing through a thin film transistor depends on many factors such as the structure and manufacturing process of the thin film transistor, the size of the thin film transistor (channel length and/or channel width), the voltage applied to the gate region or drain region, etc.

The second requirement is necessary because written data must be held in the capacitor for a relatively long time. In general, data, once written in the capacitor, should be maintained for a time longer than the time taken to write the data into the capacitor. The capacitance of the capacitor is usually small, for example, approximately 1 pF. Therefore, the amount of charge stored in the capacitor is so small as to be easily affected by a small leakage current flowing between source and drain regions of the thin film transistor. In other words, if a leakage current flows when the thin film transistor is in the OFF state (hereinafter referred to as OFF current) the potential at the drain region to which the capacitor is coupled becomes close to that of the source region. As a result, the data written in the capacitor is not retained.Thus it is highly desirable to reduce OFF current between source and drain regions.

When irradiating a thin film transistor with light, the carrier density in the accumulation region is increased due to the light and the depletion layer at the PN junction is narrowed by the increased carrier density. As a result, the ON current and OFF current are increased, the increase in OFF current being particularly marked. In fact, as the increase in leakage current caused by irradiating the thin film transistor with light is proportional to the intensity of the light, the brighter the environment of the thin film transistor, the more the OFF current increases. Therefore, it is disadvantageous to use a conventional thin film transistor in an active matrix liquid crystal display device as a switching element.While an active matrix liquid crystal display device generally makes a profit from the brightness of light incident thereon for producing a superior display with improved contrast, the use of thin film transistors worsens the display due to the increased OFF current due to incident light.

Fig. 3 is a graph illustrating the characteristics of the conventional thin film transistor of Fig. 2, the graph being obtained by experiment. The abscissa represents gate voltage VGS relative to the source voltage and the ordinate represents drain current 1DS Drain voltage VDS relative to source voltage was kept constant at 4V. Solid line A shows drain current when the thin film transistor was not irradiated with light (dark current) and broken line B shows drain current when the thin film transistor was irradiated with light at an intensity of 10,000 lux. As seen from Fig. 3 while the ON current barely increases when the thin film transistor is irradiated with light, OFF current increased markedly. As a result, the difference, between ON current and OFF current is too small to give excellent characteristics to the thin film transistor.

Fig. 4 is a cross-sectional view of one embodiment of a thin film transistor according to the present invention. Like parts in Figs. 2 and 4 have been designated by the same reference numerals. A channel region of the thin film transistor is completely shielded from light because it is covered by an extended source electrode 1 5. As the source electrode and a drain electrode 1 6 are made of aluminium, a space 1 7 therebetween through which light may pass should be reduced as much as possible. The width of the space 1 7 is determined by the skill of the patterning techniques used to form the source and drain electrodes.Nevertheless, light passing through the space 1 7 generates carriers mainly in the drain region and these have little or no influence upon the generation of photo-induced current, because the density of impurity is so high that the generated carriers cannot exist for long and mobility of the carriers is extremely small. Accordingly, the thin film transistor as shown in Fig. 4 is substantially free from photo-induced current.

It will be appreciated that in an alternative embodiment the channel region may be covered by an extended drain electrode. Moreover the source region 10 and/or drain regipn 11 may be covered by the respective electrode in similar manner to the channel region, to block light incident on the thin film transistor. These measures further reduce the photoinduced current compared to the case where only the channel region is covered. Moreover, the thin film transistor of Fig. 4 does not require any special manufacturing process. In other words the thin film transistor can be manufactured conventionally changing only the patterning of the sourc electrode or drain electrode.

Fig. 5 is a graph showing the voltagecurrent characteristics of the thin film transistor of Fig. 4, the graph being obtained by experiment. The various parameters are the same as those of Fig. 3. A solid line C shows drain current when the thin film transistor was not irradiated with light (i.e. dark current) and a broken line D shows drain current when the thin film transistor was irradiated with light with an intensity of 10,000 lux. The line C corresponds to the line A of Fig. 3. As seen from Fig. 5 the photo-induced current is relatively small and the amount of OFF current increases by about 1 pA even if the light intensity is 10,000 lux. This small increase in OFF current is inevitable and caused by light passing through the space 1 7 between the source electrode and the drain electrode.

In an active matrix liquid crystal display device a transparent electrode made of, for example, indium oxide, tin oxide or indium tin oxide is conventionally used for the display driving electrodes provided on one substrate.

A transparent electrode is formed on a second substrate and liquid crystal material sealed between the substrates.

Fig. 6 shows a cross-section of another conventional thin film transistor which is employed as a switching element in an active matrix liquid crystal display device. An insulating layer 102, made of, for example, polycrystalline silicon, amorphous silicon or cadmium selenide, is formed on an insulating substrate 101. An insulating layer 103 covers the insulating layer 102 and a gate electrode 104 is provided. Source and drain regions are indicated by reference numeral 105. An insulating layer 106 is provided between conductive layers, and a metallic layer 107 is provided for electrical interconnection to the source and drain regions.

Fig. 7 is an enlarged plan view of the liquid crystal display element 2 shown in Fig. 1. The liquid crystal display element consists of a polycrystalline silicon layer 202, a timing signal line or scanning gate electrode 204 for the liquid crystal display element 2, and a data signal line or signal electrode 207 for the liquid crystal driving element. The signal electrode 207 is brought into contact with a polycrystalline silicon layer of the thin film transistor at a contact portion 208. A display electrode 210 is connected to the polycrystalline silicon layer at a contact portion 209. In the case of a transparent active matrix liquid crystal display device, a transparent electrode is utilised at the display electrode 210. Thus the transparent electrode of the display electrode 210 should be connected to the drain electrode.The connection of the conventional thin film transistor of Fig. 6 with a transparent electrode of an active matrix liquid crystal display device will now be described with reference to Fig. 8.

The metallic layer 107 is connected to the drain electrode 105 and to a transparent electrode 110 made of, for example, indium tin oxide. However, this construction is disadvantageous from the point of view of electrical contact characteristics. For example, if the metallic layer 107 is of alumium, electrical conductivity is reduced due to the fact that an aluminium oxide layer is formed between the aluminium of the metallic layer and the transparent electrode. As a result, there is relatively poor display contrast because the aluminium oxide acts as a insulating layer.

Fig. 9 illustrates another embodiment of a thin film transistor according to the present invention where a polycrystalline silicon layer 402 is formed on a transparent insulating substrate 401 and patterned to a predetermined configuration. Then, a gate insulating layer 403 is formed on the polycrystalline layer 402 and a gate electrode 404 is formed thereon. N-type impurity is doped into the polycrystalline silicon layer 402 by ion implantation utilising the gate electrode 404 as a mask to form diffusion regions which serve as source and drain regions 405 of the thin film transistor. Next, an insulating layer 406 is formed and contact holes 408, 409 in the region of the source and drain region 405 are produced by a photo-etching technique. Then a transparent electrode 410 made of, for example, indium oxide, tin oxide or indium tin oxide is formed and suitably patterned.

The transparent electrode 410 is in direct contact with the drain region 405. A metallic layer 407 made of, for example, aluminium is formed in direct contact with the source region. The direct contact between the drain region and the transparent electrode invites stable and favourable electrical contact characteristics. Moreover, there is the advantage that large contact holes can be provided and the driving electrode can be made large because there is no opaque material such as aluminium at the contact hole for the drain region.

Consequently, the stable and excellent contact characteristics can be achieved as a result of direct contact between the polycrystalline silicon of the drain region and the transparent electrode.

Fig. 10 shows a yet further embodiment of a thin film transistor according to the present invention. The thin film transistor of Fig. 10 is manufactured in the same manner as that of Fig. 9. In the last step of manufacture, a metallic layer 411 for example of aluminium overlies the region of contact between the drain electrode 405 and the transparent electrode 410. Thus even if direct contact between the drain electrode and transparent electrode is inferior, the conductivity with a driving electrode is never cut off because of the metallic layer on the contact portion which serves as a conductor. Thus in an an active matrix liquid crystal display device utilising the thin film transistors illustrated in Figs. 9 and 10, display contrast can be free from variation caused by poor electrical contact characteristics.

In Figs. 9 and 10 an N-type polycrystalline silicon layer 402 on the substrate 401 has been used. However, the polycrystalline silicon layer may be a P-type polycrystalline silicon layer. Additionally the drain region is in direct contact with the transparent electrode, but it is contemplated that the source region can also be in direct contact with the transpar ent electrode.

Fig. 11 (a) is an enlarged plan view of a conventional active matrix liquid crystal display device and Fig. 11 (b) is a cross-section thereof. A polycrystalline silicon layer 508 is formed on a glass substrate 515 and a gate insulating layer 513 is formed by thermally oxidising the surface of the polycrystalline layer 508. A second layer of polycrystalline silicon is formed and is patterned by photoetching thereby simultaneously forming a gate electrode and a gate line 509 of a thin film transistor, and one electrode 51 2 of a capacitor (such as the capacitor 6 of Fig. 1(b)).

Then, impurity is diffused into the second polycrystalline silicon layer 509 and into the first polycrystalline silicon layer 508 except in the region masked by the gate electrode 509 in order to form a source region and a drain region. Subsequently an insulating layer 514 is formed all over the surface and contact holes are formed for the source region and the drain region. Lastly, a source line 510 and a picture element driving electrode 511 are formed.

The insulating layer 514 which insulates the source line 510 from the gate line 509 is disposed between the electrode 512 and an electrode 511 of the capacitor. As the capacitance of the capacitor is in inverse proportion to the thickness of the insulating layer, the insulating layer 514 is required to be thin so that the capacitor has a relatively large capacitance. For example, in the case where each picture element is 1 mm square, the size of the capacitor is limited to 200,um2 so as not to effect the brightness of the display. If the insulating layer 514 is formed of silicon oxide and has a thickness of 5,000Â, the capacitance of the capacitor is only about 2.5 pF.

On the other hand, the capacitance of the liquid crystal material of the picture element is about 9 pF when the thickness of the liquid crystal material is 10 microns. The capacitance of the capacitor should be at least equal to, but preferably twice or three times as great as that of the liquid crystal material. In order to satisfy this condition, the thickness of the insulating layer 514 should be reduced to one-fifth or one-tenth or the area of the capacitor should be increased by five to ten times.

However, as the area of the capacitor must be limited in order not to affect the brightness of the display, the reduction of the thickness of the insulating layer is the only way to make the capacitance of a capacitor larger than the capacitance of the of the liquid crystal material. Consequently, the insulating layer should be 1000 A in thickness. Even in the case of utilising silicon nitride for the insulating layer, silicon nitride having a relatively large dielectric constant, the thickness of the insulating layer should be 1 oooA to 2000A.

On the other hand, the gate insulating layer 51 3 of a thin film transistor usually is at least 1000 to 2000A in thickness. When the breakdown voltage of a transistor is to be relatively high, the gate insulating layer is required to have a thickness greater than 5000 . The breakdown voltage of the gate insulating layer 51 3 is twice that of the insulating layer 514 when both layers are of the same thickness.Because the gate insulating layer 513 is of silicon oxide and is thermally formed and the insulating layer 514 is of silicon oxide and is deposited by a chemical vapour deposition technique, and the thickness of the gate insulating layer 513 and the insulating layer 514 is 1000 to 2000A, the breakdown voltage of the insulating layer 514 is necessarily lower than that of the gate insulating layer 513. As a result, static electricity inevitably damages the gate line or the source line where they intersect. Increasing the thickness of the insulating layer 514 in order to protect against damage due to static electricity decreases the capacitance of the capacitor to less than the capacitance of the liquid crystal material.

Referring now to Fig. 12(a), there is shown a cross-section of a picture element of an active matrix liquid crystal display device according to the present invention, the crosssection being taken on the same line as Fig.

11(a). Fig. 12(b) is a plan view of the picture element of Fig. 12(a). Like parts in Figs. 11 and 1 2 have been designated by the same reference numerals. In addition to the process steps followed for forming the insulating layer 514 of Fig. 11 (indicated by reference numeral 514-1 in Fig. 12) a second silicon oxide insulating layer 51 4-2 is formed all over the surface and is removed by photo-etching except in the region where the gate line 509 and the source line 510 intersect. Subsequently, contact holes are provided in the source region and the drain region of the first insulating layer 514-1. Lastly the source line 510 is formed. The thickness of the first insulating layer 514-1 is 1000A to give sufficient capacitance to the capacitor.On the other hand, the thickness of the second insulating layer 514-2 is 5000 or more for raising the breakdown voltage at the intersection between the gate line and the source line.

This construction has the advantage that the breakdown voltage at the intersection between the gate line and the source line is higher that that of the gate insulating film and the capacitance of the capacitor is sufficiently large. The second insulating layer 514-2 may be formed and photo-etched before the first insulating layer 514-1 is formed. In the case where the first and second insulating layers are formed of the same material, such as silicon oxide, the thicker insulating layer 514-2 insulating the gate line 509 from the source line 510 is much more easily photo-etched than the thinner insulating layer 514-1.Further, when the first insulating layer 514-1 is formed first and the second insulating layer 514-2 is then formed thereon, it is better to form the two insulating layers of different materials, for example, the first insulating layer may be silicon nitride and the second insulating layer may be silicon oxide. This is because either of these layers is alternatively photo-etched at the time of patterning.

Fig. 13(a) is a plan view of another embodiment of an active matrix liquid crystal display device according to the present invention and Fig. 13(b) is a cross-section thereof. Again like parts in Figs. 11, 1 2 and 1 3 have been designated by the same reference numerals.

As in Fig. 12, the gate insulating layer 513 is formed on the surface of the polycrystalline silicon layer 508 by thermal oxidation and a second silicon oxide layer is formed and patterned to provide the gate line 509 and the electrode 512 of the capacitor. Impurity is diffused into the gate line 509 and the electrode 51 2 as well as the first silicon layer 508, except in the region covered by the gate electrode 509. After forming the first insulating layer 514-1 and the second insulating layer 514-2, subsequently the second insulating layer 514-2 in the region of the electrode 1 2 is removed by photo-etching, thereby the first insulating layer 514-1 is solely formed on the electrode 1 2. Then contact holes are formed at the source region and the drain region.The source line 510 and the picture element driving electrode 511 are then formed. In this embodiment the thickness of the first insulating layer 514-1 is 1000A and that of the second insulating layer 514-2 is 50008, or more. Thus the breakdown voltage at the intersection between the source line and the gate line is higher than that of the gate insulating layer of the thin film transistor.

Further, the capacitor has a sufficient capacitance. In addition the picture element is protected by the second insulating layer which is relatively thick and this permits improvement of reliability. As indicated by a broken line 51 6 in Fig. 13(a), the second insulating layer 514-2 may substantially agree with or partially cover the capacitor.The larger the area of the capacitor not covered by the second insulating layer 514-2, the greater its capacitance.

The insulating layers 514-1, 514-2 are usually formed of silicon oxide, but can be made of silicon nitride and/or aluminium oxide. In addition, the order of forming the two insulating layers can be reversed similar to that described in relation to Fig. 1 2.

Further, the display area of the driving electrode 511 can be increased because it covers the gate line 509 and a part of the first silicon layer 508. Therefore image brightness is much improved. To cover the gate line 509 and a part of the first silicon layer 508 with the driving electrode 511 can be done in the embodiment of Fig. 1 3 because the second insulating layer 514-2 is relatively thick and this reduces or prevents short circuits.

As described above, the active matrix liquid crystal display devices of Figs. 1 2 and 1 3 are such that breakdown voltage at the intersection of the source line and the gate line in each picture element is made higher than that of the gate insulating film of the thin film transistor by making the insulating layer thicker where the source line and gate line intersect than elsewhere. Damage to the active matrix liquid crystal display device due to static electricity can be reduced or prevented to only involve a single picture element and not produce a line fault. Accordingly, manufacturing yield is much improved and this aids mass production. In addition, the capacitance of the capacitor can be made relatively large so improving writing characteristics of a data signal. As a result, display characteristics of the active matrix liquid crystal display device as a whole are much improved.

In the embodiments of the present invention described in relation to Figs. 1 2 and 1 3 the electrodes of the capacitor are separate but it is clear that the invention is applicable to an active matrix liquid crystal display device where one electrode of the capacitor and the gate line of the adjacent picture element are common.

Claims (14)

1. A thin film transistor formed on an insulating substrate comprising a source electrode connected to a source region, a drain electrode connected to a drain region, a channel region between the source region and the drain region, said channel region being substantially covered by an extension of the source electrode or drain electrode.

2. A thin film transistor as claimed in claim 1 in which the source region is substantially covered by the source electrode.

3. A thin film transistor as claimed in claim 1 or 2 in which the drain region is substantially covered by the drain electrode.

4. A thin film transistor comprising a source region, a drain region and a gate electrode, the drain region or source region being in direct contact with a transparent electrode.

5. A thin film transistor as claimed in claim 4 in which the transparent electrode is made of indium oxide, tin oxide or indium tin oxide.

6. A thin film transistor as claimed in claim 4 or 5 in which a metallic layer overlies the region of direct contact between the source region or drain region and the transparent electrode.

7. A thin film transistor as claimed in any of claims 4 to 6 in which the source region and drain region are formed in a polycrystalline thin film.

8. An active matrix liquid crystal display device comprising a matrix of picture ele ments each including a thin film transistor as claimed in any preceding claim.

9. An active matrix liquid crystal display device having a matrix of picture elements each of which is composed of a driving electrode, a capacitor for storing a data signal, a MOS transistor having a drain electrode, a source electrode and a gate electrode which is covered by a gate insulating layer, the MOS transistor being arranged selectively to apply the data signal to the capacitor, a gate line connected to said gate electrode, a source line orthogonal to the gate line and connected to said source electrode, and a further insulating layer between the gate line and the source line, the breakdown voltage of the further insulating layer being higher than the breakdown voltage of the gate insulating layer.

10. An active matrix liquid crystal display device as claimed in claim 9 in which the thickness of the further insulating layer at least in the region of the section of each gate line with each source line is greater than in other regions.

11. An active matrix liquid crystal display device as claimed in claim 9 or 10 in which in each picture element at least a portion of said gate line or said MOS transistor is covered by the respective driving electrode, an insulating layer being disposed therebetween.

1 2. An active matrix liquid crystal display device as claimed in any of claims 9 to 11 in which the picture elements are formed on a glass substrate.

1 3. A thin film transistor substantially as herein described with reference to and as shown in Fig. 4 or Fig. 9 or Fig. 10 of the accompany drawings.

14. An active matrix liquid crystal display device substantially as herein described with reference to and as shown in Fig. 1 2 or Fig.

1 3 of the accompanying drawings.

1 5. A thin film transistor formed on an insulating substrate comprising a source electrode, a drain electrode and a gate electrode, either of said source electrode or said drain electrode being prolonged for covering a channel region between a source region and a drain region of said thin film transistor.

1 6. A thin film transistor for a transparent type liquid crystal display panel comprising a source region, a drain region and a gate electrode, said source region or said drain region being brought into contact with a transparent electrode, said transparent electrode being made of indium oxide, tin oxide or indium tin oxide.

1 7. A liquid crystal display device having a plurality of picture elements arranged in a matrix, said picture element composed of driving electrode for liquid crystal, a capacitor for storing data signal, MOS field transistor for apply said data signal to said capacitor and driving electrode, a plurality of gate lines and a plurality of source lines are orthogonally arranged and connected to a gate and a source of said transistor respectively, wherein the breakdown voltage of an insulating layer fabricated between said source line and gate line is higher than a breakdown voltage of a gate insulating layer of said thin film transistor.