MOS capacitor, liquid crystal display, integrated circuit and method of manufacture thereof

A MOS capacitor used in an active matrix liquid crystal display is manufactured by a process comprising the steps of forming capacitor electrodes with a dielectric layer between them in a semiconductor layer, forming a p+ diffused region and an n+ diffused region adjacent to the capacitor electrodes in the semiconductor layer, and making a complementary connection between the regions.

TECHNICAL FIELD

The present invention generally relates to liquid crystal display devices, and more particularly to a liquid crystal display device having a MOS (metal-oxide-semiconductor) capacitor and a fabrication process thereof. Further, the present invention relates to such a MOS capacitor, a semiconductor device having such a MOS capacitor, and a fabrication process of those.

Conventionally, liquid crystal display devices have been used widely in portable information processing apparatuses such as so-called notebook computer, as a low-power-consuming compact information display device.

On the other hand, application of a liquid crystal display device is by no means limited to such a portable information processing apparatus. Today, liquid crystal display devices are used also in desk-top type information processing apparatuses as replacement of conventional CRT display device. Further, liquid crystal display devices are attractive as a display device of high-definition televisions (HDTV). Particularly, application to a projection-type HDTV display device is studied.

In the case of such high-performance, large-area liquid crystal display devices, a simple matrix driving construction used conventionally cannot provide satisfy the required specification in terms of response speed, contrast ratio, color purity, and the like. Thus, in such high-performance, large-area liquid crystal display devices, an active-matrix driving method is used in which each pixel is driven by a corresponding thin-film transistor (TFT). In the liquid crystal display device that employs an active-matrix driving method, it has been practiced to use an amorphous silicon liquid crystal display device in which amorphous silicon is used for the active region of the TFT. On the other hand, amorphous silicon has a drawback of small electron mobility and cannot satisfy the specification required for such a high-performance liquid crystal display device. Thus, there is a need of using a polysilicon TFT as the TFT of a high-performance liquid crystal display device.

Generally, a liquid crystal display device that uses the active matrix driving uses a capacitor for each TFT for retaining a driving voltage applied to a liquid crystal layer. Such a capacitor may be formed by a dielectric film sandwiched by a pair of metal electrodes similarly to ordinary capacitors. On the other hand, in view of the fact that the capacitor is used in cooperation with a highly miniaturized TFT, it is advantageous to construct the capacitor to have a so-called MOS structure.

BACKGROUND ART

FIG. 1 shows the general construction of a conventional active-matrix driven liquid crystal display device.

Referring to FIG. 1 , the liquid crystal display device includes a TFT glass substrate 1 A carrying thereon a number of TFTs and transparent pixel electrodes cooperating thereto and an opposing glass substrate 1 B formed on the TFT substrate 1 A. Between the substrate 1 A and the substrate 1 B, a liquid crystal layer 1 is confined by means of a seal member 1 C. In the illustrated liquid crystal display device, the transparent pixel electrodes are selectively driven via a corresponding TFT and the orientation of liquid crystal molecules is changed selectively in the liquid crystal layer in correspondence to the selected pixel electrode. Further, polarizers not illustrated are disposed at respective outer sides of the glass substrates 1 A and 1 B. Further, a molecular alignment film not illustrated is formed on the inner sides of the glass substrates 1 A and 1 B in contact with the liquid crystal layer 1 . The molecular alignment film thereby restricts the orientation of the liquid crystal molecules.

FIG. 2 shows a part of the TFT glass substrate 1 A in an enlarged scale.

Referring to FIG. 2 , the glass substrate 1 A carries thereon a number of pad electrodes 13 A, to which a scanning signal is supplied, and a number of scanning electrodes 13 extend therefrom, wherein the glass substrate 1 A further carries thereon a number of pad electrodes 12 A, to which a video signal is supplied, and a number of signal electrodes 12 extend therefrom. The scanning electrodes 13 and the signal electrodes 12 extend in such a manner that an elongating direction of a scanning electrode 13 intersects generally perpendicularly to an elongating direction of a signal electrode 12 . Further, TFTs 11 are formed at the intersections of the scanning electrodes 13 and the signal electrodes 12 . Further, the substrate 1 A carries transparent pixel electrodes 14 thereon such that a pixel electrode 14 corresponds to each of the TFTs 11 , and each TFT 11 is selected by a scanning signal on a corresponding scanning electrode 13 . Thereby, the selected TFT 11 drives the cooperating transparent pixel electrode 14 by a video signal on the corresponding signal electrode 12 .

FIG. 3 shows the construction of a liquid crystal cell driving circuit for driving one pixel of the liquid crystal display device of FIG. 2 .

Referring to FIG. 3 , a number of liquid crystal cells 15 are formed in the liquid crystal layer 1 of FIG. 1 in correspondence to the plurality of pixels, and it can be seen that a number of the TFTs 11 are formed on the TFT substrate, which corresponds with the glass substrate 1 A of FIG. 1 , in a row and column formation in correspondence to the liquid crystal cells 15 . Further, it can be seen that the signal lines 12 supplying the video signals to the TFTs 11 extend on the TFT substrate 1 A in a column direction in a substantially parallel relationship with each other. Further, it can be seen that the gate electrodes (scanning electrodes) 13 controlling the TFTs 11 extend substantially parallel with each other. In the illustrated example, a TFT 11 is formed of a pair of serially connected TFTs 11 A and 11 B and drives the corresponding liquid crystal cell 15 via the pixel electrode 14 . Further, a capacitor 16 is connected to the TFT 11 parallel to the liquid crystal cell 15 . The capacitor 16 thereby constitutes an accumulating capacitance holding the driving voltage applied to the liquid crystal cell 15 . In the construction, the capacitor 16 is connected between the pixel electrode 14 and a capacitance line 17 .

As explained before, the accumulating capacitance 16 may be constructed by sandwiching a dielectric film between a pair of metal electrode patterns. In the case of an active-matrix driven liquid crystal display device, however, it is more advantageous to construct the same in the form of a MOS capacitor.

FIG. 4 shows the circuit construction of a conventional liquid display device that has such a MOS capacitor.

Referring to FIG. 4 , the liquid crystal cell is formed of a glass substrate 10 A corresponding to the foregoing TFT substrate 1 A, a polysilicon pattern 10 B formed on the glass substrate 10 A, and an oxide film 10 C formed on the glass substrate 10 A so as to cover the polysilicon pattern 10 B. The TFT 11 is formed of n -type diffusion regions 10 a , 10 b and 10 c formed in the foregoing polysilicon pattern 10 B, a gate electrode 11 a of Al or polysilicon formed on the oxide film 10 C between the foregoing diffusion regions 10 a and 10 b , and a gate electrode 11 b formed similarly of Al or polysilicon on the oxide film 10 C between the diffusion regions 10 b and 10 c . It should be noted that the gate electrode 11 a corresponds to the foregoing TFT 11 A and the gate electrode 11 b corresponds to the foregoing TFT 11 B. Further, the oxide film 10 C constitutes a gate insulation film underneath the gate electrodes 11 a and 11 b . Further, the signal line 12 is connected to the diffusion region 10 a and the gate control line 13 is connected to the gate electrodes 11 a and 11 b.

In the construction of FIG. 4 , it can be seen that the diffusion region 11 c extends in the right direction in the drawing and forms an n -type diffusion region 10 d . Further, an electrode 11 c of Al or polysilicon is formed on the oxide film 10 C in correspondence to the diffusion region 10 d similarly to the gate electrodes 11 a and 11 b , wherein the electrode 11 c thus formed constitutes a capacitor electrode of the foregoing capacitor 16 .

In the liquid crystal display device of such a construction, the TFTs 11 A and 11 B are turned on in response to the selection signal on the gate bus line 13 and the capacitor 16 is charged by the video signal on the signal line 12 via the diffusion region 10 d . As a result, the potential of the pixel electrode 14 connected to the diffusion region 10 c and the diffusion region 10 d is maintained at a predetermined driving level until the next selection signal comes in.

On the other hand, such a conventional liquid crystal display device has a drawback in the point that, while it allows a self-aligned formation of the diffusion regions 10 a , 10 b and 10 c by using the gate electrodes 11 a and 11 b as a mask, does not allow a self-aligned formation for the diffusion region 10 d . Thus, in order to form the diffusion region 10 d , it is necessary to conduct an ion implantation process separately to the diffusion regions 10 a - 10 c by using a separate mask.

However, the use of such a separate mask process and separate ion implantation process for forming the diffusion region 10 d increases the number of fabrication steps significantly. Further, there is a risk of mask alignment error, which may lead to the problems such as variation of the threshold value or increase of number of the defective devices. In the construction of FIG. 2 , it is possible to abandon the use of self-alignment process for forming the diffusion regions 10 a - 10 c and use the same mask process used for forming the diffusion region 10 d . However, such a process has a drawback, due to the fact that the formation of the oxide film 10 C is conducted after the ion implantation process, that the surface of the polysilicon pattern 10 B is tend to be contaminated by impurity elements. In the case of fabricating a semiconductor integrated circuit, such impurity elements can be eliminated by conducting a cleaning process. In the case of a liquid crystal display device that uses a glass substrate, on the other hand, thorough cleaning process cannot be used and the impurity elements tend to remain on the polysilicon pattern once the impurity element has caused a contamination.

FIG. 5 shows the construction of another conventional liquid crystal display device that eliminates the problem of the liquid crystal display device of FIG. 4 . In FIG. 5 , those parts corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted.

Referring to FIG. 5 , the illustrated liquid crystal display device includes an n -type diffusion region 10 e in the polysilicon pattern 10 B in addition to the n -type diffusion regions 10 a - 10 c constituting the TFTs 11 A and 11 B, by a self-alignment process that uses the gate electrodes 11 a , 11 b and the capacitor electrode 11 c as a mask. Thus, the problems of increased number of fabrication steps and the contamination of the polysilicon pattern 10 B by impurity element are avoided. In the construction of FIG. 3 , a predetermined voltage is applied to the electrode 11 c via the capacitance line 17 and a surface accumulation layer is inducted in the polysilicon pattern 10 B in the intrinsic or low-doped region 10 f between the diffusion regions 10 c and 10 e . It should be noted that the region 10 f has an impurity concentration similar to the channel region formed in the polysilicon pattern 10 B between the diffusion regions 10 a and 10 b or between the diffusion regions 10 c and 10 d.

Thus, the construction of FIG. 5 can avoid the problem pertinent to the construction of FIG. 4 . On the other hand, the construction of FIG. 5 requires another power source for driving the capacitance line 17 and for inducing a surface accumulation layer in the region 10 f , and the construction of the driving circuit of the liquid crystal becomes complex. Thereby, the problem of increase of the manufacturing cost cannot be avoided. Further, as can be seen from the circuit diagram of FIG. 3 , the capacitance line 17 thus applied with a high voltage crosses the signal line on the TFT substrate 10 F. As there exists only a thin interlayer insulation film between the capacitance line 17 and the signal line 12 , there is a risk that a leakage current or breakdown of insulation occurs. It should be noted that the voltage applied to the capacitance line 17 is a voltage much higher than the voltage used commonly in an ordinary semiconductor integrated circuit. Further, in view of the fact that the high voltage is applied continuously to the capacitance line 17 , the gate oxide film 10 C tends to degrade more than a gate oxide film used in an ordinary MOS transistor. Thus, the capacitor 16 of FIG. 5 has a drawback in terms of reliability.

Further, the construction of FIG. 5 has a drawback, associated with the application of high voltage to the capacitance line 17 , in that there tend to be formed domains in the liquid crystal cell in correspondence to the capacitance line and other wiring or TFT. In order to avoid disturbance of representation associated with such a domain formation, it is necessary to provide an opaque mask along the capacitance line with a substantial width. However, formation of such a wide opaque mask causes a decrease of aperture ratio of the liquid crystal display device.

DISCLOSURE OF THE INVENTION

Accordingly, it is a general object of the present invention to provide a novel and useful MOS-type capacitance device, a liquid crystal display device, a semiconductor device, and a fabrication process thereof, wherein the foregoing problems are eliminated.

Another and more specific object of the present invention is to provide a MOS-type capacitance device having a simple construction for easiness of fabrication, a liquid crystal display device having such a MOS-type capacitance device, and a fabrication process thereof.

Another object of the present invention is to provide a MOS-type capacitance device, characterized by:

a substrate,

a semiconductor layer formed on the substrate,

an insulating film formed on the semiconductor layer,

an electrode formed on the insulating film,

a first diffusion region formed in the semiconductor layer adjacent to the electrode, and

a second diffusion region formed in the semiconductor layer adjacent to the electrode,

the first diffusion region being doped to a first conductivity type, the second diffusion region being doped to a second, opposite conductivity type.

Another object of the present invention is to provide a MOS-type capacitance device, characterized by:

a substrate,

an electrode formed on the substrate,

an insulating film formed on the substrate so as to cover the electrode,

a semiconductor layer formed on the insulating film,

a first diffusion region formed in the semiconductor layer adjacent to a first edge of the electrode,

a second diffusion region formed in the semiconductor layer adjacent to another edge of the electrode,

the first diffusion region being doped to a first conductivity type, the second diffusion region being doped to a second, opposite conductivity type.

According to the present invention, the MOS-type capacitance device shows substantially the same capacitance with regard to a positive voltage or a negative voltage, or with regard to a low-frequency signal or a high-frequency signal, by forming a complementary connection. The MOS-type capacitance device thereby performs as an effective capacitor. Further, the MOS-type capacitance device of the present invention can be formed simultaneously with the fabrication process of other MOS transistors, without adding fabrication steps. In the MOS-type capacitance device of the present embodiment, the type diffusion region and the p -type diffusion region are formed by conducting an ion implantation process after covering the semiconductor layer by the insulating film. Thereby, the problem of contamination of the semiconductor layer by impurity elements occurring in the conventional art is eliminated. Associated with this, the problem of variation of the threshold voltage or other operational characteristics, caused in the transistors that are formed on the semiconductor layer simultaneously to the MOS-type capacitance device, as a result of the contamination of the impurity elements, is eliminated. When the MOS-type capacitance device of the present invention is used for driving a liquid crystal display device, the capacitor electrode may be simply held at a common potential level. Thereby, the stress applied to the capacitor insulation film or other interlayer insulation film is reduced and the degradation of display characteristics arising from the stress is avoided.

Another object of the present invention is to provide a liquid crystal display device, characterized by:

a first glass substrate, a second glass substrate opposing the first glass substrate,

a liquid crystal layer confined between the first glass substrate and the second glass substrate,

a signal electrode extending on the first glass substrate,

a scanning electrode extending on the first glass substrate,

a common potential line extending on the first glass substrate,

a thin-film transistor formed at an intersection of the signal electrode line and the scanning electrode,

a pixel electrode electrically connected to the thin-film transistor,

and an accumulating capacitance connected parallel to the pixel electrode,

the thin-film transistor being formed in a semiconductor layer formed on the first glass substrate,

the accumulating capacitance comprising:

an insulating film formed on the semiconductor layer,

a capacitor electrode formed on the insulating film,

a first diffusion region formed in the semiconductor layer adjacent to the capacitor electrode, and

a second diffusion region formed in the semiconductor layer adjacent to the capacitor electrode,

the first diffusion region being doped to a first conductivity type, the second diffusion region being doped to a second, opposite conductivity type.

According to the present invention, it is possible to reduce the manufacturing cost of the liquid crystal display device by using the MOS-type capacitance device in the liquid crystal display device. Further, the liquid crystal display device has an advantageous feature of improved reliability and high yield of production due to reduction of stress applied to a gate insulation film, the capacitor insulation film or other interlayer insulation films.

Another object of the present invention is to provide a projection-type liquid crystal display device, characterized by:

an optical source,

a liquid crystal panel disposed in an optical path of an optical beam produced by the optical source for spatial modulation, and

a projecting optical system projecting the optical beam that has been spatially modulated by the liquid crystal panel,

the liquid crystal panel comprising:

a first glass substrate,

a second glass substrate opposing the first glass substrate,

a liquid crystal layer confined between the first glass substrate and the second glass substrate, a signal electrode extending on the first glass substrate,

a scanning electrode extending on the second glass substrate,

a common potential line extending on the first glass substrate,

a thin-film transistor formed at an intersection of the signal electrode line and the scanning electrode,

a pixel electrode electrically connected to the thin-film transistor, and

an accumulating capacitance connected parallel to the pixel electrode,

the thin-film transistor being formed in a semiconductor layer formed on the first glass substrate,

the accumulating capacitance comprising:

an insulating film formed on the semiconductor layer,

a capacitor electrode formed on the insulating film,

a first diffusion region formed in the semiconductor layer adjacent to the capacitor electrode, and

a second diffusion region formed in the semiconductor layer adjacent to the capacitor electrode,

the first diffusion region being doped to a first conductivity type, the second diffusion region being doped to a second, opposite conductivity type.

According to the present invention, it becomes possible to manufacture a projection-type liquid crystal display device using a MOS-type capacitance device at low cost without increasing the number of manufacturing steps. The liquid crystal display device has an advantageous feature of improved reliability and high yield of production due to reduction of stress applied to a gate insulation film, the capacitor insulation film or other interlayer insulation films.

Another object of the present invention is to provide a semiconductor integrated circuit device having a capacitor, the capacitor comprising a substrate, an insulating film formed on the substrate, an electrode formed on the insulating film, a first diffusion region formed in the substrate adjacent to the electrode, and a second diffusion region formed in the substrate adjacent to the electrode, characterized in that the first diffusion region is doped to a first conductivity type and the second diffusion region is doped to a second, opposite conductivity type.

According to the present invention, various semiconductor integrated circuits can be manufactured by using the MOS-type capacitance device.

Another object of the present invention is to provide a method of fabricating a MOS-type capacitance device, characterized by the steps of:

forming a semiconductor film on a substrate,

forming an insulating film on the semiconductor film,

forming a gate electrode on the insulating film,

introducing an impurity element of a first conductivity type into the semiconductor film at a first side of the gate electrode while using the gate electrode as a mask, and

introducing an impurity element of a second, opposite conductivity type into the semiconductor film at another side of the gate electrode while using the gate electrode as a mask.

Another object of the present invention is to provide a method of fabricating a liquid crystal display device having a MOS-type capacitance, characterized in that the MOS-type capacitance is fabricated according to the steps of:

forming a semiconductor film on a glass substrate,

forming an insulating film on the semiconductor film,

forming a gate electrode on the semiconductor film,

introducing an impurity element of a first conductivity type into the semiconductor film at a first side of the gate electrode while using the gate electrode as a mask, and

introducing an impurity element of a second, opposite conductivity type into the semiconductor film at another side of the gate electrode while using the gate electrode as a mask.

Another object of the present invention is to provide a method of fabricating a liquid crystal display device having a MOS-type capacitance device, characterized in that the MOS-type capacitance device is fabricated according to the steps of:

forming a capacitor electrode on a glass substrate,

forming an insulating film on the glass substrate so as to cover the capacitor electrode,

forming a semiconductor film on the insulating film,

introducing an impurity element of a first conductivity type into the semiconductor film at a first side of the gate electrode, and

introducing an impurity element of a second, opposite conductivity type into the semiconductor film at another side of the gate electrode.

Another object of the present invention is to provide a method of fabricating a semiconductor integrated circuit device having a MOS-type capacitance device, characterized in that the MOS-type capacitance device is fabricated according to the steps of:

forming an insulating film on the semiconductor substrate,

forming a gate electrode on the insulating film,

introducing an impurity element of a first conductivity type into the semiconductor substrate at a first side of the gate electrode while using the gate electrode as a mask, and

introducing an impurity element of a second conductivity type into the semiconductor substrate at another side of the gate electrode while using the gate electrode as a mask.

According to the present invention, it becomes possible to fabricate a MOS-type capacitance device, or a liquid crystal display device using the same, or a semiconductor integrated circuit device using the same.

BEST MODE OF IMPLEMENTING THE INVENTION

Principle

FIG. 6A is a diagram showing the principle of the MOS-type capacitance device according to the present invention, while FIG. 6B shows an equivalent circuit diagram thereof. In the drawings, those parts corresponding to the parts explained previously are designated by the same reference numerals and the description thereof will be omitted.

Referring to FIG. 6A , it can be seen in the MOS-type capacitance device that a diffusion region 10 h of n -type is formed in the semiconductor layer 10 B in correspondence to a first edge of the capacitor electrode 11 c and a diffusion region 10 i of p -type is formed in the semiconductor layer 10 B in correspondence to another edge of the gate electrode 11 c.

FIG. 6B shows the equivalent circuit diagram of the MOS-type capacitance device of FIG. 6 A.

Referring to FIG. 6B , the MOS-type capacitance device includes a capacitance Co corresponding to the capacitance of the oxide film 10 C connected to the gate electrode 11 c and diodes D n and D p respectively correspond to the diffusion region 10 h and the diffusion region 10 i . Further, junction capacitance C jn and C jp respectively corresponding to the diffusion regions 10 h and 10 i are included in the MOS-type capacitance device.

FIGS. 7A and 7B show the capacitance-voltage characteristic (C-Vg characteristic) of the MOS-type capacitance device 10 of FIGS. 6A and 6B respectively for the case in which a positive and negative d.c. bias voltages Vg are applied. It should be noted that FIG. 7A shows the construction for measuring the capacitance, wherein the capacitance value of the MOS-type capacitance device is obtained by measuring an impedance Z across output terminals OUT. FIG. 7B shows the capacitance-voltage characteristic thus obtained.

Referring to FIG. 7A , a complementary MOS-type capacitance device 10 is formed from the MOS-type capacitance device 10 of FIGS. 6A and 6B , by using the capacitor electrode 11 c as a first terminal and forming a second terminal by commonly connecting the n -type diffusion region 10 h and the p -type diffusion region 10 i . The value of the capacitance C is then obtained by measuring the impedance Z at the foregoing output terminals OUT while applying a minute a.c. signal higher than 100 kHz in frequency from an a.c. signal source and simultaneously applying a positive or negative d.c. bias voltage Vg across the first and second terminals from a d.c. source 22 .

FIG. 7B shows the capacitance of the MOS-type capacitance device 10 obtained from the test as a function of the d.c. bias voltage Vg. In FIG. 7B , it should be noted that the vertical axis represents a normalized capacitance value C/Co of the obtained capacitance value C normalized by the capacitance value Co of the oxide film 10 C. The horizontal axis, on the other hand, shows the d.c. bias voltage Vg. Referring to the C-Vg characteristic of FIG. 7B , the broken line represents the case in which the capacitance between the capacitor electrode 11 c and the n -type diffusion region 10 h is obtained in the MOS-type capacitance device 10 without using the complementary construction of FIG. 7A , while the one-dotted line represents the case in which the capacitance between the capacitor electrode 11 c and the p -type diffusion region 10 i is obtained without using the foregoing complementary construction. As represented by the broken line, the capacitance between the capacitor electrode 11 c and the n -type diffusion region 10 h takes a value comparable to the capacitance value Co during the interval in which a positive voltage Vg is applied to the capacitor electrode 11 c , due to the formation of an electron accumulation layer on the surface of the semiconductor layer 10 B right underneath the electrode 11 c in continuation to the n -type diffusion region 10 h . On the other hand, in the case a negative voltage Vg is applied to the capacitor electrode 11 c , no such electron accumulation layer is formed and the capacitance value becomes very small. Similarly, as represented by the one-dotted line, the capacitance between the capacitor electrode 11 c and the p -type diffusion region 10 i takes a value comparable to the capacitance value Co during the interval in which the negative voltage Vg is applied to the capacitor electrode 11 c , due to the formation of a hole accumulation layer on the surface of the semiconductor layer 10 B right underneath the electrode 11 c in continuation to the p -type diffusion region 10 i . On the other hand, in the case the positive voltage Vg is applied to the capacitor electrode 11 c , no such a hole accumulation layer is formed and the capacitance becomes very small. Further, in the case the complementary construction is not used, the C-Vg characteristic changed depending on the frequency as represented in FIG. 7 B. In FIG. 7B , Low-frequency C-V Characteristic designates a C-Vg characteristic for a low frequency range, while High-frequency C-V Characteristic designates a C-Vg characteristic for a high-frequency range.

In the case the complementary construction shown in FIG. 7A is used, on the other hand, a C-Vg characteristic represented in FIG. 7B by a continuous line is obtained. Thus, the MOS-type capacitance device 10 having such a complementary construction shows a generally constant capacitance value irrespective of whether the applied d.c. bias voltage is positive or the applied d.c. bias voltage is negative. Further, the C-Vg characteristic of the MOS-type capacitance device 10 does not substantially depend on the frequency when the MOS-type capacitance device 10 has the complementary construction. This is because there occurs no formation of depletion layer in the semiconductor layer 11 B right underneath the capacitor electrode 11 c , irrespective of whether the polarity of the voltage applied to the capacitor electrode 11 c is positive or negative and an electron accumulation layer or hole accumulation layer is formed continuously to the n -type diffusion region 11 h or p -type diffusion region. In such a carrier accumulation layer, it should be noted that the carriers inducted on the surface of the semiconductor layer 11 B can follow the voltage applied to the capacitor electrode 11 c with high response speed.

Thus, the MOS-type capacitance device 10 having the device of FIGS. 6A and 6B in a complementary connection as represented in FIG. 7A shows a generally the same capacitance value to both of positive and negative voltages.

FIGS. 8A and 8B show the capacitance characteristic of the MOS-type capacitance device 10 for the case in which a symmetric high-frequency a.c. signal, such as the one used when driving a liquid crystal display device, is applied while eliminating the d.c. bias source 22 from the test apparatus of FIG. 7 A. FIG. 8A shows the waveform of the high-frequency a.c. signal, while FIG. 8B shows the capacitance of the MOS-type capacitance device 10 corresponding to the waveform of FIG. 8 A.

Referring to FIG. 8A , the driving signal used in a liquid crystal display device is a symmetric rectangular wave signal having an amplitude between a minimum level V min and a maximum level V max . In the case the complementary connection is used, it can be seen from FIG. 8B that the MOS-type capacitance device of FIGS. 6A and 6B shows a generally constant capacitance irrespective of the polarity or amplitude of the driving signal. In FIG. 8B , it should be noted that the vertical axis represents a normalized capacitance C/Co normalized by the capacitance Co of the insulating film 10 C, while the horizontal axis represents the voltage applied to the capacitor electrode 11 c by the signal of FIG. 8 A.

Thus, by using the complementary connection as represented in FIG. 7A , the MOS-type capacitance device of the present invention shown in FIGS. 6A and 6B shows a substantially constant capacitance to any of positive and negative voltages of any of low-frequency signal and high-frequency signal and performs as an effective capacitor. The MOS-type capacitor of the present invention can be formed simultaneously to the fabrication process of other MOS transistors without adding a fabrication step, and the manufacturing cost of the electronic apparatus using such a MOS-type capacitance device, such as a liquid crystal display device, is reduced. In the MOS-type capacitance device of the present embodiment, the n -type diffusion region 11 h and the p -type diffusion region 11 i are formed by conducting an ion implantation process after the semiconductor layer 10 B is covered with the insulating film 10 C. Thereby, the problem of contamination of the semiconductor layer 10 B by the impurity element as in the case of the conventional example of FIG. 4 does not occur. Associated with this, the problem of variation of the threshold voltage or other operational characteristics of the MOS transistors formed on the semiconductor layer 10 B simultaneously to the MOS-type capacitance device, caused as a result of contamination by the impurity element, is eliminated. In the case the MOS-type capacitance device of the present invention is used for driving a liquid crystal display device, it is sufficient to maintain the capacitance electrode at the common voltage level and the stress applied to the insulating film 10 C or other interlayer insulation film is reduced. Associated with this, the degradation of representation caused by such stress is avoided.

FIRST EMBODIMENT

FIGS. 9A-9E show the fabrication process of a MOS-type capacitance device 30 according to a first embodiment of the present invention.

Referring to FIG. 9A , a semiconductor pattern of polysilicon or amorphous silicon is formed on a substrate 31 and a dielectric film 33 of SiO 2 , and the like, is formed on the substrate 31 in the step of FIG. 9B so as to cover the semiconductor pattern 32 . The substrate 31 may be a glass substrate of a liquid crystal display device or an insulating substrate for other purpose. The substrate 31 may be a single crystal Si substrate. Further, the semiconductor pattern 3 may be a single crystal Si pattern.

Next, in the step of FIG. 9C , a conductive film of Al or conductive polysilicon is deposited on the dielectric film 33 in the step of FIG. 9C , and a capacitor electrode 34 is formed by patterning the conductive film. Further, in the step of FIG. 9D , an n-type impurity element such as As or P is introduced into the semiconductor pattern 32 through the dielectric film 33 while using the capacitor electrode 34 as a self-aligned mask. By applying a thermal annealing process thereafter, a diffusion region 32 of n -type is formed at a first side of the capacitor electrode 34 . During the ion implantation process of the foregoing n-type impurity element, the part of the semiconductor pattern 32 located at the other side of the capacitor electrode 34 is covered by a resist mask.

Next, in the step of FIG. 9E , the resist mask is removed and the part of the semiconductor pattern 32 located at the first side of the capacitor electrode 34 is covered by another resist mask. By introducing a p-type impurity element such as BF into the semiconductor pattern 32 through the dielectric film 33 , followed by a thermal annealing process, a diffusion region 32 B of p -type is formed at the other side of the capacitor electrode 34 .

In the fabrication process of the MOS-type capacitance device 30 according to the present embodiment, the ion implantation process is conducted in the step ( FIG. 9D ) and in the step ( FIG. 9E ) after the semiconductor pattern 32 is covered by the dielectric film 33 . Thereby, the problem of the surface of the semiconductor pattern 32 being contaminated by the impurity element is avoided. Particularly, it should be noted that the process of FIGS. 9A-9E is completely compatible with the fabrication process of the top-gate type TFT used in liquid crystal display devices. Thus, according to the present embodiment, it is possible to form other MOS transistors on the semiconductor pattern 32 simultaneously to the formation of the MOS-type capacitance device 30 .

In the case of forming a top-gate type n-channel TFT adjacent to the MOS-type capacitance device 30 , it is possible to form a gate electrode in the step of FIG. 9C on the semiconductor pattern 32 or on a similar semiconductor pattern simultaneously to the capacitor electrode 34 and form a source region and a drain region of n -type at both sides of the gate electrode in the step of FIG. 9D simultaneously to the formation of the diffusion region 34 . In the case the TFT to be formed is a p-channel TFT, source and drain regions of p -type are formed at both sides of the gate electrode in the step of FIG. 9E , simultaneously to the diffusion region 32 B.

The MOS-type capacitance device 30 thus formed has a desirable capacitance characteristic explained previously with reference to FIG. 7B or FIG. 8 B.

FIG. 10A shows a MOS-type capacitance device 30 A according to a modification of the MOS-type capacitance device 30 of FIG. 9 E.

Referring to FIG. 10A , the n -type diffusion region 32 A is formed in the semiconductor pattern 32 with a separation from the capacitor electrode 34 in the case of the MOS-type capacitance device 30 A, and an LDD region 32 a of n -type is formed therebetween. Similarly, the p -type diffusion region 32 B is formed with a separation from the capacitor electrode 34 , and an LDD region 32 b of p -type is formed therebetween. The formation of the LDD region 32 a or 32 b is achieved by forming a side wall insulation film on the capacitor electrode 34 . Alternatively, a separate mask process may be used. By forming the LDD regions 32 a or 32 b , it is possible to increase the breakdown voltage of the MOS-type capacitance device 30 A.

FIG. 10B shows a MOS-type capacitance device 30 B according to a modification of the MOS-type capacitance device 30 A of FIG. 10 A.

Referring to FIG. 10B , a part of the LDD region of the MOS-type capacitance device 30 A of FIG. 10A , such as the LDD region 32 b , is omitted in the MOS-type capacitance device 30 B. In such a construction, too, it is possible to increase the MOS-type capacitance device.

FIG. 10C shows a MOS-type capacitance device 30 C according to a further modification of the MOS-type capacitor 30 A of FIG. 10 A.

Referring to FIG. 10C , the n -type diffusion region 32 A is formed in the semiconductor pattern 32 with a separation form the capacitor electrode 34 and there is formed an offset region 32 c therebetween. Similarly, the p -type diffusion region 32 B is formed with a separation from the capacitor electrode 34 and an offset region 32 d is formed therebetween. By forming the offset region 32 c or 32 d as such, it is possible to increase the breakdown voltage of the MOS-type capacitance device 30 C.

FIG. 11 shows the MOS-type capacitance device 30 of FIG. 9E in a plan view.

Referring to FIG. 11 , it can be seen that the capacitor electrode 34 covers a central part of the semiconductor pattern 32 and the part of the semiconductor pattern 32 exposed at the first side of the capacitor electrode 34 is doped to n -type and forms the diffusion region 32 A. Further, the part exposed at the other side is doped to p -type and forms the diffusion region 32 B. Further, an ohmic contact 32 A is formed in the n -type diffusion region 32 A, and an ohmic contact 32 B is formed in the p -type diffusion region 32 B.

FIG. 12A shows a MOS-type capacitance device 30 D according to a modification of the MOS-type capacitance device 30 of FIG. 11 in a plan view.

Referring to FIG. 12A , the semiconductor pattern 32 is exposed only at the first side of the capacitor electrode 34 and the n -type diffusion region 32 A and the p -type diffusion region 32 B are formed in the foregoing exposed part adjacent with each other. In such a construction, the ohmic contacts 32 A and 32 B can be formed respectively on the diffusion regions 32 A and 32 B in the form of a single continuous ohmic contact. Thereby, the complementary connection of the n -type diffusion region 32 A and the p -type diffusion region 32 B can be formed easily.

FIG. 12B shows the construction of a MOS-type capacitance device 30 E according to a modification of the MOS-type capacitance device 30 D of FIG. 12A in a plan view.

Referring to FIG. 12B , the capacitor electrode 34 is included in the semiconductor pattern in the plan view, and a first exposed part of the semiconductor pattern 32 is doped to n -type and forms the diffusion region 32 A. On the other hand, the other exposed part of the semiconductor pattern 32 is doped to p -type and forms the diffusion region 32 B. Similarly to the embodiment of FIG. 12A , the ohmic contact 32 A of the diffusion region 32 A and the ohmic contact 32 B of the diffusion region 32 B are formed in continuation with each other and a single ohmic contact is formed. With this, the present embodiment can realize a complementary connection connecting the n -type diffusion region 32 A and the p -type diffusion region 32 B easily.

SECOND EMBODIMENT

FIGS. 13A-13E show the fabrication process of a MOS-type capacitor 40 according to a second embodiment of the present invention wherein the process of the present embodiment is compatible with the fabrication process of a TFT of the bottom gate construction.

Referring to FIG. 13A , a capacitor electrode pattern 42 of conductive amorphous silicon and the like, is formed on an insulating substrate 41 such as a glass substrate. Next, in the step of FIG. 13B , a dielectric film 43 of an SiO 2 film or the like is formed on the insulating substrate 41 so as to cover the capacitor electrode pattern 42 . In the step of FIG. 13B , an amorphous silicon film 44 is deposited further on the dielectric film 43 .

Next, in the step of FIG. 13C , the amorphous silicon film 44 is patterned to form a semiconductor pattern 44 P, and an ion implantation process of As or P is conducted into a part of the semiconductor pattern 44 P located at a first side of the capacitor electrode pattern 42 . As a result, a diffusion region 44 A of n -type is formed.

Further, in the step of FIG. 13E , a part of the semiconductor pattern 44 P located at the other side of the capacitor electrode pattern 42 is subjected to an ion implantation process of BF and a p -type diffusion region 44 B is formed. The ion implantation process of FIGS. 13D and 13E may be conducted after covering the semiconductor pattern 44 P by an insulating film such that the ion implantation process is made through the insulating film.

The MOS-type capacitance device 40 of the present embodiment can be formed simultaneously with bottom-gate-type TFTs in an active matrix driven liquid crystal display device.

THIRD EMBODIMENT

FIGS. 14A and 14B show an example of applying the MOS-type capacitance device 30 explained before to the liquid crystal cell driving circuit of an active-matrix-type liquid crystal display device of FIG. 3 , as a driving circuit 50 according to a third embodiment of the present invention, wherein those parts of FIGS. 14A and 14B explained previously are designated by the same reference numerals and the description thereof will be omitted.

Referring to FIG. 14A , the driving circuit 50 of the present embodiment includes the TFT 11 a formed on the semiconductor layer 10 B of polysilicon and the like and a the MOS-type capacitance device 30 formed on the semiconductor layer 10 B adjacent to the TFT 11 a . Similarly to the construction explained before with reference to FIG. 5 , the TFT 11 a includes the n -type diffusion regions 10 a and 10 b formed in the semiconductor layer 10 B, and the gate electrode 11 a is formed on the insulating film 10 C between the diffusion regions 10 a and 10 b . Thereby, the insulating film 10 C constitutes a gate insulation film right underneath the gate electrode 11 a.

On the other hand, the MOS-type capacitance device 30 has a construction shown in FIG. 6A or FIG. 9 E and includes the diffusion region 10 b in the semiconductor layer 10 B as the n -type diffusion region 10 h of FIG. 6A or the diffusion region 32 A of FIG. 9 E and further the p -type diffusion region 10 i as the diffusion region 32 A of FIG. 9 E. Further, the capacitor electrode 11 c is formed on the insulating film 10 C between the diffusion regions 10 b and 10 i.

To the gate electrode 11 a , a control signal VG shown in FIG. 14B is supplied via the signal line 13 . Referring to FIG. 14B , the control signal VG has a level of Vg 1 in the ordinary state and causes a transition to the level of Vg 1 only when the TFT 11 a is selected. Further, a video signal VS shown in FIG. 14B is supplied to the diffusion region 10 a . The video signal is forwarded to the diffusion region 10 b via the channel region of the TFT 11 a and held in the MOS-type capacitance device 30 . The video signal VS is a symmetric a.c. signal having a frame period T as represented in FIG. 14 B and changes the value alternately between Vmin and Vmin in the interval of minimum signal level. Further, the video signal VS changes the value alternately between Vmax and Vmax in the interval of maximum signal level. Further, in the interval of intermediate signal level, the value of the signal changes alternately positive and negative with a level intermediate between Vmax and Vmin. Further, the capacitor electrode 11 c is held at a common potential level (Vcom), which is applied to a transparent opposinit e 4 lectrode on the opposing substrate 1 B (see FIG. 1 ). The capacitor electrode 11 c is connected to the capacitance line 17 of FIG. 3 , while in the present embodiment, the foregoing common potential level Vcom is supplied to the capacitance line.

The video signal VS held in the MOS-type capacitance device 30 is applied to the liquid crystal cell 15 from the n -type diffusion region 10 b via the pixel electrode 14 (see FIG. 2 ).

In the present embodiment, the MOS-type capacitance device 30 has a characteristic explained previously with reference to FIG. 7B or FIG. 8 B and holds the positive and negative video signals VS stably.

As explained before, the MOS-type capacitance device 30 is completely compatible with the fabrication process of the TFT 11 a and thus can be formed simultaneously with the formation of the TFT 11 a.

In an active matrix driven liquid crystal display device, the use of the MOS-type capacitance device 30 of the present invention in combination with the TFT 11 a achieves a stabilization of voltage applied to the liquid crystal cell 15 and a high-quality and high-stability displaying becomes possible. Further, the present embodiment, which uses the same voltage supplied to the transparent opposing electrode for the voltage supplied to the capacitance line 17 , can eliminate the use of a separate driving power source for driving the capacitance line 17 .

FOURTH EMBODIMENT

FIG. 15 shows the construction of a liquid crystal panel 60 of a direct view type liquid crystal display device that uses the liquid crystal cell driving circuit 50 of FIGS. 14A and 14B .

Referring to FIG. 15 , the liquid crystal panel 60 is formed of the TFT substrate 1 A and the opposing substrate 1 B explained with reference to FIG. 1 and the liquid crystal layer 1 confined therebetween. On the TFT substrate 1 A, the liquid crystal cell driving circuits 50 (not shown) of FIG. 14 are arranged in a row and column formation in correspondence to a row and column arrangement of the pixel electrodes 14 (see FIG. 2 ). Further, a gate-side peripheral circuit 1 G for selecting a gate control line 13 and a signal-side peripheral circuit 1 V for selecting a signal line 12 are formed on the TFT substrate 1 A so as to surround the array of the pixel electrodes 14 and the liquid crystal cell driving circuits 50 .

On the opposing surface of the opposing substrate 1 B facing the substrate 1 A, on the other hand, RGB tri-color filters are formed in a row and column formation in correspondence to each of the pixels, and the foregoing opposing surface is further covered uniformly by a transparent opposing electrode (not shown) such that the transparent opposing electrode covers the foregoing tri-color filters. To the foregoing transparent opposing electrode, the same common potential level Vcom supplied to the capacitor electrode 11 c of the MOS-type capacitance device 30 is supplied at opposing electrode contacts 1 Bc formed at four corners of the substrate 1 B.

FIFTH EMBODIMENT

FIG. 16 shows the construction of the liquid crystal panel 70 used for a projection-type liquid crystal display device that uses the liquid crystal cell driving circuit 50 of FIGS. 14A and 14B , wherein those parts explained previously are designated by the same reference numerals and the description thereof will be omitted.

Referring to FIG. 16 , the liquid crystal panel 70 is formed of the TFT substrate 1 A of FIG. 1 , the opposing substrate 1 B and the liquid crystal layer 1 confined therebetween, and the liquid cell driving circuits 50 of FIG. 14A are arranged on the TFT substrate 1 A in a row and column formation in correspondence to the pixel electrodes 14 (see FIG. 2 ) arranged in row and column formation. Further, the gate-side peripheral circuit 1 G selecting a gate electrode 13 and the signal-side peripheral circuit 1 V selecting a signal line 12 are disposed on the TFT substrate 1 A so as to surround the array of the pixel electrodes 14 and the liquid crystal cell driving circuits 50 .

On the other hand, a transparent opposing electrode (not shown) is formed on the opposing surface of the opposing substrate 1 B facing the substrate 1 A uniformly. Further, the common electric potential level Vcom identical with the one supplied to the capacitor electrode 11 c of the MOS-type capacitance device 30 , is supplied to the foregoing transparent opposing electrode at the opposing electrode contacts 1 Bc, similarly to the liquid crystal panel 70 of FIG. 15 . Further, an opaque pattern BM is formed on the opposing substrate 1 B so as to cover the circuits 1 G or 1 V on the TFT substrate 1 A. A similar opaque patterns is formed, although not illustrated, also on each of the liquid crystal cell driving circuits 50 arranged in row and column formation.

FIG. 17 shows the construction of a projection-type liquid crystal display device 80 that uses the liquid crystal panel 70 of FIG. 16 .

Referring to FIG. 17 , the projection type liquid crystal display device 80 includes a powerful optical source 81 of a metal halide lamp and the like, a dichroic mirror 91 disposed in an optical path of an optical beam 82 emitted from the optical source 81 through a ultra-violet cut filter 81 A constituting a part of the optical source 80 , the dichroic mirror allowing to pass a blue color component and reflecting other lights, a dichroic mirror disposed in an optical path of an optical beam reflected by the dichroic mirror 91 , the dichroic mirror 92 reflecting a red color component and allowing to pass other lights, thus a green optical component, and a mirror 93 disposed in an optical path of a blue color beam passed through the dichroic mirror 91 for reflecting the same, and the blue color beam B passed through the dichroic mirror impinges upon a light valve 93 B formed of the liquid crystal panel 70 of FIG. 16 after being reflected by the mirror 93 and passed through an incoming-side polarizing element 90 B.

The blue color beam B passed through the liquid crystal panel 93 B is then subjected to a spatial modulation by an exit-side polarizing element 93 B disposed with a crossed Nicol state with respect to the incoming-side polarizing element 90 B.

Similarly, the red optical beam separated by the dichroic mirror 22 is caused to pass an incoming-side polarizer 90 R and experiences a spatial modulation by an exit-side polarizing element 94 R after passed through a liquid crystal panel 93 R. Further, the red optical beam spatially modulated by the output-side polarizing element 94 R is added with the blue optical beam spatially modulated by the exit-side polarizing element 94 B at a dichroic mirror 94 and enters another dichroic mirror 96 .

Similarly, the red optical beam separated by the dichroic mirror 92 is caused to pass through an incoming-side polarizing element 90 G and experiences a spatial modulation at an exit-side polarizing element 94 G after passing through a liquid crystal panel 93 G. The green optical beam thus spatially modulated at the exit-side polarizing element 94 G then impinges upon said another dichroic mirror 96 by a mirror 95 . Thereby, the blue optical beam and the red optical beam thus spatially modulated are added, and the optical beams thus added are projected on a screen 98 by a projecting optical system 97 .

SIXTH EMBODIMENT

FIG. 18 shows the construction of a semiconductor integrated circuit 100 that uses the MOS-type capacitance device 10 or 30 according to the present invention.

Referring to FIG. 18 , the semiconductor integrated circuit 100 is formed on a p-type Si substrate 101 covered with a thermal oxide film 102 having a thickness of typically 10-0 nm. Further, a device isolation structure is formed on the substrate 101 by a field oxide film 102 A between a region 100 A on which an active device such as a MOS transistor is to be formed and a region 100 B on which the MOS-type capacitance device is to be formed. Further, n -type diffusion regions 101 A and 101 B are formed on the surface of the Si substrate 101 in correspondence to the region 100 A, while an n -type diffusion region 101 C and a p -type diffusion region 101 D are formed on the region 100 B. Further, a gate electrode 103 A of Al, polysilicon or WSi is formed on the thermal oxide film 102 in a part between the diffusion regions 101 A and 101 B, and a capacitor electrode 103 B is formed between the diffusion regions 101 C and 101 D from the same material as the gate electrode 103 A. Further, the diffusion regions 101 C and 101 D are connected commonly in the region 100 B and a MOS-type capacitance device having a complementary connection similar to the one explained with reference to the first embodiment is formed in the foregoing region 100 B.

SEVENTH EMBODIMENT

FIG. 19 shows the construction of a transfer gate circuit 110 using the complementary-connected MOS-type capacitance device 100 B of FIG. 18 according to a seventh embodiment of the present invention.

Referring to FIG. 19 , a signal supplied to an input terminal 111 is held in a capacitor C 1 corresponding to the complementary-connected MOS-type capacitance device 100 B of FIG. 18 in the form of electric charges, while the electric charges thus held in the capacitor C 1 are transferred to a similar capacitor C 2 at an output side via a MOS transistor Tr which causes a conduction in response to a control signal supplied to the input terminal 113 . With this, an output signal appears on the output terminal 112 connected to the capacitor C 2 .

EIGHTH EMBODIMENT

FIG. 20 shows the construction of a transfer gate circuit 115 according to an eighth embodiment of the present invention wherein the transfer gate circuit 115 of the present embodiment is a modification of the transfer gate circuit 110 of FIG. 19 .

Referring to FIG. 20 , the present embodiment replaces the transistor Tr of the circuit of FIG. 19 with a construction in which a p-channel MOS transistor Tr 1 and an n-channel MOS transistor Tr 2 are connected parallel. Thereby, the transistor Tr 1 is controlled for the conduction by a first control signal supplied to an input terminal 113 , while the transistor Tr 2 is controlled for the conduction by a second control signal supplied to an input terminal 114 .

Other features of the circuit 112 are the same as those of the circuit 110 and description thereof will be omitted.

NINTH EMBODIMENT

FIG. 21 shows an example of a sampling circuit 120 according to a ninth embodiment of the present invention that uses the complementary connected MOS-type capacitance device 100 B of FIG. 18 .

Referring to FIG. 21 , an input signal supplied to an input terminal 121 is forwarded to a complementary MOS-capacitance device C 3 via a MOS transistor Ti, which causes a conduction in response to a control signal supplied to a control signal terminal 122 , and is held in the MOS-type capacitance device C 3 in the form of electric charges. The electric charges held in the capacitance device C 3 cause a conduction in the MOS transistor T 3 . Thus, by causing a conduction in the MOS transistor T 2 connected in series to the transistor T 3 in response to a sampling signal supplied to a control signal terminal 123 of the transistor T 2 , the electric charges in the capacitance device C 3 are supplied to an output terminal 124 via the transistor T 2 .

TENTH EMBODIMENT

FIG. 22 shows the construction of an opto-electric conversion circuit 130 according to a tenth embodiment of the present invention that uses the complementary-connected MOS-type capacitance device 100 B of FIG. 18 .

Referring to FIG. 22 , the opto-electric conversion circuit 130 includes a photodiode D 1 biased by a bias voltage supplied to a bias power terminal 131 , and the bias voltage on the bias power terminal 131 is supplied to a capacitor C 4 via a MOS transistor T 4 , which causes a conduction in response to a control signal supplied to a control signal terminal 132 , when the photodiode D 1 has caused a conduction by an incoming optical signal. In response thereto, the capacitor C 4 is charged up. The capacitor C 4 has a construction corresponding to the complementary connected MOS-type capacitance device 100 B of FIG. 18 .

In the circuit of FIG. 22 , the electric charges thus held in the capacitor C 4 are read out by an amplifier 133 and an output signal is obtained at an output terminal 134 . The circuit 130 includes a MOS transistor T 5 that discharges the capacitor c 4 and the transistor T 5 causes a conduction in response to a reset signal supplied to a reset terminal 135 .

ELEVENTH EMBODIMENT

FIG. 23 shows the construction of an opto-electric conversion circuit 140 according to an eleventh embodiment of the present invention that uses the complementary-connected MOS-type capacitance device 100 B of FIG. 18 .

Referring to FIG. 23 , the opto-electric circuit 140 includes a capacitor Ct corresponding to the complementary-connected MOS-type capacitance device 100 B of FIG. 18 , wherein the capacitor Ct is charged via a MOS transistor T 6 , which causes a conduction in response to a reset signal supplied to a reset terminal 141 . The photodiode D 1 is connected parallel to the capacitor Ct.

When an optical signal comes in to the photodiode Dl, the capacitor Ct is discharged, and a MOS transistor T 7 supplied with a supply voltage and cooperating with the capacitor Ct is turned off. It should be noted that a transistor T 7 is inserted between the transistor T 7 and a ground potential in series with transistors T 8 and T 9 , which are urged by a bias circuit 142 to a conduction state. Thus, when the transistor T 7 turns off, the potential of the node 143 between the transistors T 7 and T 8 goes low. When the transistor T 10 causes a conduction in response to the control signal supplied to the control terminal 144 in this state, a capacitor CL corresponding to complementary-connected MOS-type capacitance device 100 B connected to the output-side of the transistor T 10 experiences a discharging via the transistor T 10 and the transistors T 8 and T 7 . Thereby, the amplifier 145 detects the potential change of the capacitor CL and supplied to an output terminal 146 as a low level output signal.

On the other hand, the capacitor Ct is in a charged-up state in the circuit 140 of FIG. 23 when the optical signal does not comes in to the photodiode D 1 and the transistor T 7 causes a conduction as a result. Thereby, charge up of the capacitor CL is conducted via the transistor T 10 . In this state, a high-level output signal is obtained at the output terminal 146 .

TWELFTH EMBODIMENT

FIGS. 24A and 24B show the construction of a liquid crystal cell driving circuit 150 according to a twelfth embodiment of the present invention respectively in a cross-sectional view and in a plan view.

Referring to FIGS. 24A and 24B , the p -type diffusion region 10 i extends, in the driving circuit 50 of FIG. 14A , to a part underneath the capacitor electrode 11 c and forms a p -type diffusion region 10 j continuing to the n -type diffusion region 10 c.

In the construction of FIGS. 24A and 24B , the capacitance line 17 is held at the ground level and the selection signal supplied to the control line 13 changes between the ground level and the supply voltage level VDD as represented in FIG. 24 B. Thereby, the voltage applied across the control line (scanning electrode) 13 and the capacitance line 17 takes a maximum level of VDD, and the stress applied to an insulating film or interlayer insulation film in the liquid crystal display device is reduced.

Further, the present invention is not limited to specific preferred embodiments described heretofore, but various variations and modifications may be made without departing from the scope of the invention described in the form of claims.

INDUSTRIAL APPLICABILITY

According to a first aspect of the present invention, the MOS-type capacitance device of the present invention shows a substantially identical capacitance to a positive voltage or negative voltage, or to a low-frequency signal or a high-frequency signal by making a complementary connection, and functions as an effective capacitor. Further, the MOS-type capacitance device of the present invention can be fabricated simultaneously with a fabrication process of other MOS transistors, without adding steps. As the type diffusion region and the p -type diffusion region are formed by an ion implantation process after covering the semiconductor layer by an insulating film in the MOS-type capacitance device of the present embodiment, the problem pertinent to prior art of contamination of the semiconductor layer by an impurity element does not occur. Associated with this, the problem of variation of the threshold voltage or other operational characteristic of the transistor formed on the semiconductor layer simultaneously to the MOS-type capacitance device, caused by the contamination by the impurity element, is eliminated. Further, when the MOS-type capacitance device of the present invention is used for driving a liquid crystal display device, it is sufficient to hold the capacitor electrode at the common potential level, and the stress applied to the capacitor insulation film or other interlayer insulation film is reduced and the problem of degradation of display characteristic caused by the stress is avoided.

According to a second aspect of the present invention, it is possible to reduce the manufacturing cost of the liquid crystal display device by using the MOS-type capacitance device in the liquid crystal display device. Such a liquid crystal display device has an improved reliability due to decreased stress in the gate insulation film, capacitor insulation film or other interlayer insulation film and can be manufactured with improved yield.

According to a third aspect of the present invention, it is possible to fabricate a projection-type liquid crystal display device using the MOS-type capacitance device of the present invention without increasing the number of steps and with a low cost. The liquid crystal display device has an improved reliability due to reduced gate insulation film, capacitor insulation film or other interlayer insulation film, and can be manufactured with improved yield.

Further, according to a fourth aspect of the present invention, various semiconductor integrated circuits can be fabricated by using the MOS-type capacitance device.

Further, according to a fifth aspect of the present invention, it is possible to make a MOS-type capacitance device of the present invention, or a liquid crystal display device using the same, or a semiconductor integrated circuit using the same.