Abstract:
A potential sensor, includes a field effect transistor, a power supply and a switching device. The power supply supplies a direct current voltage to a gate electrode of the field effect transistor. The switching device switches between connecting the gate electrode to the power supply and disconnecting the gate electrode from the power supply. When the gate electrode is connected to the power supply, the field effect transistor is in action. When the gate electrode is disconnected from the power supply, the field effect transistor is in action.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a potential sensor for inspecting a liquid crystal panel used in a television, a personal computer or the like. 
     2. Description of the Related Art 
     Typically, as a method for evaluating a liquid crystal panel, there are an operation test, a reliability test, an appearance test and the like. The above-mentioned inspection of the liquid crystal panel is mainly carried out as a lighting operation check test after a product is assembled. The higher speed of the inspection is required in conjunction with the larger size and the higher accuracy of the liquid crystal panel. 
     FIG. 1 is a view showing a conventional method for inspecting a liquid crystal panel. Traditionally, as the method for inspecting the liquid crystal panel, for example, a probe method is employed which makes respective probes  32 ,  31  come into contact with a gate voltage wiring  33  and a source voltage wiring  35  that are connected to a TFT transistor  30 , as shown in FIG. 1, and then inspects a disconnection, a short-circuit or the like of a pixel electrode  2  based on its output voltage. This inspecting method detects a defect of the pixel electrode  2  of a liquid crystal panel  9  by applying an inspection signal to the source voltage wiring  35  and the gate voltage wiring  33  of the liquid crystal panel  9 . 
     However, the above inspecting method has the problems, such as an increase in a contact error or an increase in a maintenance cost of the probes  32 ,  31 . Moreover, a larger amount of display information requires a larger size, a full color and a higher minuteness of a liquid crystal display, which results in the problem of the higher speed of the inspection. 
     Japanese Laid Open Patent Application (JP-A-Heisei, 5-11000) discloses an active matrix array inspection apparatus as described below. The active matrix array inspection apparatus is provided with: a gate signal generator; a source signal generator; a gate signal line selector for selectively switching and connecting each gate signal line of an active matrix array to be inspected, to any of an output terminal, an open terminal and a ground terminal of the gate signal generator; a source signal line selector for selectively switching and connecting each source signal line of the active matrix array to be inspected, to any of an output terminal, an open terminal and a ground terminal of the source signal generator; a non-contact probe for detecting an electric condition of a drain terminal of a thin film transistor connected to the gate signal line and the source signal line at a non-contact condition; and a judging device for judging the acceptance or rejection of the thin film transistor on the basis of the detection output from the non-contact probe. 
     SUMMARY OF THE INVENTION 
     The present invention is accomplished in view of the above mentioned problems. Therefore, an object of the present invention is to provide a potential sensor for detecting a voltage of an inspection target at a non-contact condition to attain a higher speed of an inspection. 
     In order to achieve an aspect of the present invention, a potential sensor, includes: a field effect transistor; a power supply supplying a direct current voltage to a gate electrode of the field effect transistor; and a switching device switching between connecting the gate electrode to the power supply and disconnecting the gate electrode from the power supply, and wherein when the gate electrode connected to the power supply, the field effect transistor is in action, and when the gate electrode is disconnected from the power supply, the field effect transistor is in action. 
     In this case, when the gate electrode is disconnected from the power supply, the field effect transistor is in action with a charge included in an oxide film under the gate electrode. 
     Also in this case, the charge is charged in the oxide film when the gate electrode is connected to the power supply. 
     Further in this case, wherein when the potential sensor is used, the gate electrode is coupled through an air-gap to an inspection target. 
     In this case, a voltage is applied to the inspection target. 
     Also in this case, the air-gap has an interval of approximately 20 μm. 
     Further in this case, when the gate electrode is connected to the power supply, the voltage applied to the inspection target is applied to the switching device not to be outputted in a source electrode of the field effect transistor. 
     In this case, when the gate electrode is disconnected from the power supply, the voltage applied to the inspection target is outputted in a source electrode of the field effect transistor not to be applied to the switching device. 
     Also in this case, the field effect transistor is an enhancement type MOS-FET. 
     In order to achieve another aspect of the present invention, a potential sensing method, includes: (a) providing a field effect transistor; (b) providing a power supply supplying a direct current voltage to a gate electrode of the field effect transistor; (c) providing a switching device switching between connecting the gate electrode to the power supply and disconnecting the gate electrode from the power supply; (d) applying a voltage to an inspection target; (e) coupling the gate electrode through an air-gap to the inspection target; (f) connecting the gate electrode to the power supply; (g) disconnecting the gate electrode from the power supply after the (f); and (h) outputting the voltage applied to the inspection target from a source electrode of the field effect transistor while the (g) is performed. 
     In this cases the (f), (g) and (h) are performed repeatedly. 
     Also in this case, when each of the (f) and (g) is performed, the field effect transistor is in action. 
     Further in this case, when the (f) is performed, a charge from the power supply is charged in an oxide film under the gate electrode. 
     In this case, the when the (g) is performed, the field effect transistor is in action with the charge included in the oxide film. 
     Also in this case, the (e) includes setting the air-gap to have an interval of approximately 20 μm. 
     Further in this case, when the (f) is performed, the voltage applied to the inspection target is applied to the switching device not to be outputted from the source electrode. 
     In this case, when the (g) is performed, the voltage applied to the inspection target is not applied to the switching device. 
     Also in this case, the inspection target is a pixel electrode of a liquid crystal panel. 
     Further in this case, the field effect transistor is an enhancement type MOS-FET. 
     In this case, the (h) is performed in a condition that the gate electrode is not in mechanical contact with the inspection target. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view showing a method for inspecting a liquid crystal panel according to a conventional example; 
     FIG. 2 is a circuit block of a potential sensor using a field effect transistor according to an embodiment of the present invention; 
     FIG. 3A is a view showing an alternating voltage signal applied to a pixel electrode of an inspection target, in an embodiment of the present invention; 
     FIG. 3B is a view showing an ON/OFF state of an analog switch, in an embodiment of the present invention; 
     FIG. 3C is a view showing a signal outputted from an output terminal of an enhancement type MOS-FET, in an embodiment of the present invention; and 
     FIG. 4 is a section view showing a detection principle of a potential sensor according to an embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of a potential sensor in the present invention will be described below in detail with reference to the attached drawings. 
     It should be noted that there is a copending U.S. patent application Ser. No. 09/789,543, entitled “Apparatus and method for testing electrode structure for thin display device using FET function”, claiming a priority based on Japanese patent application No. Heisei 10-225968, invented by Shinichi Murakawa, Takashi Doi, Yoshio Egashira and Shigeo Ueda who are four inventors other than Tadashi Rokkaku of five inventors of the present application, and assigned to an assignee who is an assignee of the patent application. The content of the copending U.S. application is incorporated herein by reference. 
     FIG. 2 is a circuit block of a potential sensor using a field effect transistor according to an embodiment of the present invention. FIG. 3A is a timing chart showing an alternating voltage signal applied to a pixel electrode of an inspection target. FIG. 3B is a timing chart showing an ON/OFF state of an analog switch. FIG. 3C is a timing chart showing a signal outputted from an output terminal of an enhancement type MOS-FET. And, FIG. 4 is a section view showing the detection principle of the potential sensor using the field effect translator. By the way, the same symbols are given to the same members in FIGS. 2 to  4 . 
     In FIG. 2, a gate (G) terminal of the enhancement type MOS-FET (enhancement type field effect transistor) (FET potential sensor)  1  is coupled or connected through an air-gap  7  to a pixel electrode  2  in a liquid crystal panel  9 . Also, the gate (G) terminal of the enhancement type MOS-FET  1  is connected through an analog switch circuit  3  having a switch ON/OFF switching terminal  10  to a V GS  (gate-bias) direct current power supply  5 . 
     By the way, the liquid crystal panel  9  is provided with the pixel electrode  2  and a glass  8 . A pixel voltage  6  is applied to the pixel electrode  2 . Also, a V DS  (drain voltage) direct current power supply  4  is connected to a drain (D) terminal of the enhancement type MOS-FET  1 . An output terminal  11  of the enhancement type MOS-FET  1  is connected to a source (S) terminal of the enhancement type MOS-FET  1 . 
     The potential sensor using the field effect transistor in this embodiment includes the enhancement type MOS-FET  1  and the analog switch circuit  3 , as illustrated in the circuit shown in FIG.  2 . So, the disconnection or the short-circuit of the pixel electrode  2  in the liquid crystal panel  9  of the inspection target is inspected by detecting the signal of the pixel voltage  6  when the analog switch circuit  3  is turned ON and OFF, in the enhancement type MOS-FET  1  through the air-gap  7 . 
     The inspection principle of the liquid crystal panel will be described below with reference to FIGS. 2 and 4. The liquid crystal panel is inspected by using the potential sensor in which the enhancement type field effect transistor (FET) is applied. That is, the disconnection or the short-circuit of the pixel electrode  2  in the liquid crystal panel  9  is detected by the enhancement type MOS-FET  1  through the air-gap  7  by turning ON and OFF the analog switch circuit  3 . 
     As shown in FIG. 4, there is an interval d of, for example, 20 μm between the liquid crystal panel  9  and the enhancement type MOS-FET  1 . The enhancement type MOS-FET  1  inspects the disconnection or the short-circuit of the pixel electrode  2 . Also, the pixel electrode  2  in the liquid crystal panel  9  is arrayed as pixels of, for example, 640×480. 
     By the way, the enhancement type MOS-FET  1  is the FET that is typically used. Its content is explained in “Analog Electronic Circuit in Integrated Circuit Era”, by Nobuo Fujii, published by Shokodo K. K, Vol. 13, pp41-42, dated May 10, 1990. That is, a direct, current power supply V GS  is applied between a gate (G) and a source (S) in a condition that the direct current power supply V DS  is applied between a drain (D) and a source (S) of the FET. This causes a drain (D)-to-source (S) current I D  to flow between them, and makes the enhancement type MOS-FET  1  active. 
     Here, the analog switch circuit  3  is turned ON by applying a positive direct current voltage (or, a negative direct current voltage) to the switch ON/OFF switching terminal  10  of the analog switch circuit  3 . As a result, the V GS  direct current power supply  5  is applied to the gate (G) portion of the enhancement type MOS-FET  1 , and the FET  1  is made active. 
     At this time, the alternating voltage signal of the pixel voltage  6  applied to the pixel electrode  2  is sent, coupled or induced to the gate (G) portion of the enhancement type MOS-FET  1  through the air-gap  7 . In this case, since the analog switch circuit  3  is at the ON-state, the pixel voltage  6  is passed through the analog switch circuit  3  and the V GS  direct current power supply  5 . Thus, as shown in FIGS. 3A to  3 C, the pixel voltage  6  is not outputted to the output terminal  11  of the enhancement type MOS-FET  1 . Hence, the operational flow is at a stage of preparing for a measurement, at the ON-state of the analog switch circuit  3 . 
     Next, the analog switch circuit  3  is turned OFF by applying the negative direct current voltage (or, the positive direct current voltage) to the switch ON/OFF switching terminal  10  of the analog switch circuit  3 . 
     At this time, the alternating voltage signal of the pixel voltage  6  applied to the pixel electrode  2  is sent to the gate (G) portion of the enhancement type MOS-FET  1  through the air-gap  7 . Moreover, since the analog switch circuit  3  is at the OFF-state, the pixel voltage  6  is sent to the enhancement type MOS-FET  1 . As shown in FIGS. 3A to  3 C, the pixel voltage  6  is outputted to the output terminal  11  of the enhancement type MOS-FET  1 . 
     However, since the analog switch circuit  3  is Originally at the OFF-state, the V GS  direct current power supply  5  is not applied to the gate (G) portion of the enhancement type MOS-FET  1 . For this reason, the FET  1  is not at the active state. Thus, the pixel voltage  6  should not be outputted to the output terminal  11  of the FET  1 . However, the state of the switch in the analog switch circuit  3  is repeated such as ON, OFF, ON, OFF, . . . . Hence, the analog switch circuit  3  is always at the ON-state in the switch immediately before the switch of the analog switch circuit  3  is at the OFF-state. 
     Here, when attention is paid to the sectional structure of the gate (G) portion in the enhancement type MOS-FET  1  in FIG. 4, an oxide film  12  exists just beneath the gate (G). This is the feature of the MOS-FET. The enhancement type MOS-FET  1  is made active since the voltage (charge) of the V GS  direct current power supply  5  is transiently accumulated in this oxide film  12 . 
     Thus, when the switch of the analog switch circuit  3  is turned OFF from the ON-state, if a period of the OFF-state is short (about several milliseconds although it depends on the quality of the oxide film  12  itself), the enhancement type MOS-FET  1  is made active in a condition equal to the condition that the V GS  direct current power supply  5  is applied to the gate (G) portion of the enhancement type MOS-FET  1  and in the condition that the analog switch circuit  3  is at a high impedance state since the switch is at the OFF-state). Thus, the signal of the pixel voltage  6  applied to the pixel electrode  2  is outputted to the output terminal  11  of the enhancement type MOS-FET  1  without any output to the analog switch circuit  3 . 
     The above-mentioned method can inspect the disconnection or the short-circuit of the pixel electrode  2  at the high speed in the nor-contact condition by detecting the pixel voltage  6  applied to the pixel electrode  2  in the liquid crystal panel  9  and accordingly judging the detected signal. 
     The potential sensor in this embodiment is the potential sensor for detecting the voltage of the inspection target under the combination of the field effect transistor and the switching circuit. The gate-to-source voltage is applied to the gate terminal of the field effect transistor, at the ON-state, on the basis of the operation at which the switching circuit is turned ON or OFF. So, the field effect transistor is made active. Then, at the OFF-state, the switching circuit is made at the high impedance state, and the field effect transistor is made active. 
     By the way, the present invention is not limited to the above-mentioned embodiment. Various modifications may be made thereto, without departing from the spirit and scope of the invention. 
     The present invention can provide the potential sensor which detects the voltage of the inspection target at the non-contact condition to attain the higher speed of the inspection.