DESCRIPTION FIELD EFFECT TRANSISTOR USING OXIDE FILM FOR CHANNEL AND METHOD OF MANUFACTURING THE SAME TECHNICAL FIELD The present invention relates to a field effect transistor including an oxide film as a semiconductor layer, a method of manufacturing the same and a display device. In particular, the present invention relates to a field effect transistor having transistor characteristics which can be applied to a display device or the like, a method of manufacturing the same, and a display device. BACKGROUND ART A field effect transistor (FET) is a three-terminal device having a gate electrode, a source electrode, and a drain electrode. Further, the field effect transistor is an electronic active device in which a current flowing through a channel layer (a current between a source electrode and a drain electrode) is controlled by voltage applied to a gate electrode. . In particular, an FET which uses as a channel layer a thin film is called a thin film FET {rhino film transistor, TFT). The device can be formed on an several substrates formed of ceramic, glass. plastic, or the like. The above-mentioned TFT has an advantage of being easily formed on a substrate having a relatively large area because the TFT uses thin film technology, and is widely used as a driving device in a flat panel display device such as a liquid crystal display device. More specifically, in an active liquid crystal display (ALCD), TFTs formed on a glass substrate are used to turn on and off individual image pixels. Also, in a prospective high performance organic LED display (OLED), it is thought to be effective to current-drive the pixels by TFTs. Further, a higher performance liquid crystal display device is realized in which a TFT circuit having a function of driving and controlling the whole image is formed on a substrate in the vicinity of an image display region. A TFT which is currently most widely used is a metal-insulator-semiconductor field effect transistor (MIS~FET> device using a polycrystalline silicon film or an amorphous silicon film as a channel layer material. An amorphous silicon TFT is practically used for pixel driving and a high-performance polycrystalline silicon TFT is practically used for drive control of the entire image. However, it is difficult to form the amorphous silicon TFT and the polycrystalline silicon TFT on a substrate such as a plastic plate or a plastic film because a high-temperature process is essential for device formation. In recent years, active development has been conducted for realizing a flexible display by using TFTs formed on a substrate such as a polymer plate or a polymer film for a driver circuit of an LCD or OLED. Attention is focused on an organic semiconductor film which is made of a material capable of being formed on a plastic film or the like, which can be formed at a low temperature. For example, pentacene has been under research and development for the organic semiconductor film. The organic semiconductor has an aromatic ring. Large carrier mobility is obtained in the stack direction of the aromatic ring at the time of crystallization. For example, it is reported that, in the case where the pentacene is used for an active layer, the carrier mobility is approximately 0.5 cm’(Vs)"-'-, which is equal to that of an amorphous Si-MOSFET. However, the organic semiconductor such as the pentacene has low thermal stability (it is unstable when temperature exceeds 150°C) and toxicity (carcinogenicity). Therefore, a practical device has not been realized. Attention has been recently focused on an oxide fraternal as a material which can be applied to rhe channel layer of a TFT. For example, a TFT using, as the channel layer thereof, a transparent conductive oxide polycrystalline thin film containing ZnO as a main ingredient has been under active development. The thin film can be formed at a relatively low temperature, and therefore it is possible to form the film on a substrate such as a plastic plate or a plastic film. However, in the case of a compound containing ZnO as a main ingredient, a stable amorphous phase cannot be formed at a room temperature and a polycrystalline phase is formed. Therefore, it is difficult to increase an electron mobility thereof because of scattering of polycrystalline grain boundaries. In addition, it is difficult to achieve reproductively in the characteristics of a TFT device because a polycrystalline grain shape and an interconnection thereof are significantly changed depending on film formation methods. A thin film transistor using an amorphous In-Ga-Zn-0-based oxide has been reported by K. Nomura, et al.. Nature, 432, 488 (2004). The thin film transistor can be formed on a plastic plate or a glass substrate at a room temperature. The device shows a normally-off characteristics with field effect mobility of approximately 6 to 9. There is also an advantage in that the thin film transistor is transparent to visible light. The inventors of the present invention have studied a thin film transistor using an oxide including an amorphous In-Ga-Zn-0 oxide. As a result, there is the case where a variation in transistor characteristic {Id~Vg characteristic) of a TFT occurs, although an extent of the variation is depended on the channel materials or the manufacturing conditions etc. When the TFT is used for, for example, a pixel circuit of a display, the variation in characteristic causes a variation in operation of an organic LED or a liquid crystal element which is to be driven, thereby finally reducing the image quality of the display. DISCLOSURE OF THE INVENTION In view of the above, an object of the present invention is to reduce the above-mentioned variation in characteristic. Examples of a factor of the variation include: 1) a parasitic resistance caused between each of source and drain electrodes and a channel; and 2) a variation in positional relationship among a gate, a source, and a drain. Specifically, a first object of the present invention is to device a connection between the channel of a transistor and each of the source and drain electrodes thereof, thereby reducing the variation in characteristic. A second object of the present invention is to provide a structure capable of forming the positional relationship among the gate, the source, and the drain with high precision and a manufacturing method thereof, thereby reducing the variation in characteristic. The present invention provides a field effect transistor, including an oxide film as a semiconductor layer, wherein the oxide film includes one of a source part and a drain part which is added with one of hydrogen and deuterium. The present invention also provides a field effect transistor, including an oxide film as a semiconductor layer, wherein the oxide film includes a channel part, a source part, and a drain part; and a concentration of one of hydrogen and deuterium in the source part and the drain part is larger than a concentration of one of hydrogen and deuterium in the channel part. The field effect transistor according to the present invention is used for a display device according to the present invention. Further, the present invention provides a method of manufacturing a field effect transistor including an oxide film as a semiconductor layer, including the steps of: forming the oxide film on a substrate; and adding one of hydrogen and deuterium to a portion of the oxide film to form a source part and a drain part. Furthermore, the present invention provides a method of manufacturing a field effect transistor including an oxide film as a semiconductor layer, including the steps of: forming the oxide film on a substrate; forming a gate electrode on the oxide film thorough a gate insulating film; and adding one of hydrogen and deuterium to the oxide film using a pattern of the gate electrode as a mask to form a source part and a drain part in the oxide film in self-alignment with the pattern of the gate electrode. BRIEF DESCRIPTION OF THE DRAWINGS FIGS. lA and IB are cross sectional views illustrating structural examples of a field effect transistor according to the present invention; FIG. 2 is a diagram illustrating a change in resistivity of amorphous In-Ga-Zn-0 oxide film in a case where hydrogen is added thereto; FIGS. 3A and 3B are cross sectional views illustrating a method of manufacturing a field effect transistor according to the present invention with using a self-alignment method; FIGS. 4A, 4B, 4C and 4D are cross sectional views illustrating a method of manufacturing a field effect transistor according to the present invention; FIGS. 5A and 5B are cross sectional views illustrating structural examples of a field effect transistor according to the present invention; FIG. 6 is a cross sectional view illustrating a structural example of a field effect transistor according to the present invention; FIGS. 7A and 7B are graphs illustrating TFT characteristics of a field eifect transistor according to the present invention; FIGS. 8A and 8B are graphs illustrating a hysteresis characteristic of the field effect transistor according to the present invention; FIG. 9 is a graph illustrating a relationship between electric conductivity of an amorphous In-Ga-Zn-0 oxide film and an oxygen partial pressure during film formation; FIG. 10 is a cross sectional view illustrating an apparatus for forming the amorphous oxide film; FIG. 11 is a cross sectional view illustrating an example of a display device according to the present invention; FIG. 12 is a cross sectional view illustrating another example of a display device according to the present invention; and FIG. 13 is a diagram, illustrating a structure of a display device in which pixels each including an organic EL device and a thin film transistor are two-dimensionally arranged. BEST MODE EOR CARRYING OUT THE INVENTION FIGS. lA and IB are cross sectional views illustrating structural examples of a field effect transistor according to an embodiment of the present invention. FIG. lA illustrates an example of a top-gate structure and FIG. IB illustrates an example of a bottom-gate structure. In FIGS. lA and IB, provided are on a substrate 10, a channel layer (oxide thin film) 11, a gate insulating layer 12, a source electrode 13, a drain electrode 14, a gate electrode 15, a source part 16, a drain part 17, and a channel part 18. The channel layer 11 includes the source part 16, the drain part 17, and the channel part 18. As illustrated in FIG. lA, the gate insulating layer 12 and the gate electrode 15 are formed on the channel layer 11 in mentioned order, thereby obtaining the top-gate structure. As illustrated in FIG. IB, the gate insulating layer 12 and the channel layer 11 are formed on the gate electrode 15 in mentioned order, thereby obtaining the bottom-gate structure. In FIG. lA, the source part and the drain part also serve as the source electrode and the drain electrode, respectively. In FIG. IB, the channel part of the transistor and the source electrode {drain electrode) are connected with each other through the source part (drain part). As illustrated in each of FIGS. lA and IE, in the field effect transistor (FET) according to this embodiment, the oxide thin film which is the channel layer 11 includes the channel part 18, the source part 16, and the drain part 17. The source part 16 and the drain part 17 are added with hydrogen or deuterium to reduce a resistivity thereof. When the channel part 18 contains hydrogen or deuterium, the concentration of hydrogen or deuterium of each of the source part 16 and the drain part 17 is set to a value larger than the concentration of hydrogen or deuterium of the channel part 18. There are the case where hydrogen or deuterium is actively added to the channel part 18 and the case where hydrogen is contained therein without adding hydrogen actively. As described later, the electric conductivity of the source part (drain part) can be increased by the addition of hydrogen or deuterium thereto. In addition, when the concentration of hydrogen or deuterium of the source part [drain part) is increased to a value larger than the concentration of hydrogen or deuterium of the channel part 18, the electric conductivity of the source part (drain part) can be set to a value larger than the electric conductivity of the channel part, According to the structure, the channel part and the source (drain) electrode can be electrically connected with each other with high reliability, whereby a thin film transistor having small variation can be realized. In particular, according to this embodiment, the source part and the drain part are formed in the oxide film. Therefore, stable electrical connection can be made in comparison with the case of a conventional structure in which the source electrode and the drain electrode are directly formed on the oxide film. In this embodimentf a top-gate structure, a bottom-gate structure, a staggered structure, or a coplanar structure can be arbitrarily used as the structure of the field effect transistor. In view of the stable electrical connection, the coplanar structure illustrated in FIG. lA may be used. When the coplanar structure is used, the source and drain electrodes are directly connected with an interface between the gate insulating layer and the channel layer, and high-reliability electrical connection can be attained. The transistor according to this embodiment may have a structure in which the gate electrode and the source (drain) part are formed in self-alignment. That is, as described later, hydrogen is added to the oxide film by using a pattern of the gate electrode as a mask, whereby the source part and the drain part in self-alignment with respect to the pattern of the gate electrode are formed in the oxide film. When the self-alignment method is used, a transistor can be realized in which the overlap between the source (drain) part and the gate electrode is small and uniform. As a result, a parasitic capacitance of the transistor which is caused in an overlap portion between the gate electrode and the drain part can be reduced and made uniform. Because of the small parasitic capacitance, high-speed operation can be realized. Because of the uniform parasitic capacitance, a transistor excellent in uniform characteristics is can be realized. (Source and Drain Parts) As described earlier, the source part 16 and the drain part 17 are added with hydrogen or deuterium to reduce resistivity thereof. The inventors of the present invention found that, when hydrogen (or deuterium) is added to an amorphous In-Ga-Zn-0 thin film, the electric conductivity of the oxide thin film becomes larger. In the case that the channel part 18 contains hydrogen or deuterium, the concentration of hydrogen or deuterium of each of the source part and the drain part is increased to a value larger than the concentration of hydrogen or deuterium of the channel part, so electrical connecting can be improved. FIG- 2 is a characteristic graph illustrating an example of a relationship between a hydrogen ion implantation amount and resistivity. FIG. 2 illustrates a change in electric conductivity to nhe hydrogen ion implantation amount in the case where ions are implanted into an InGaZnOj thin film having a film thickness of approximately 500 nm. The abscissa (x-axis) is a logarithmic representation of the hydrogen ion implantation amount per unit area and the ordinate (y-axis) is a logarithmic representation of the resistivity. Therefore, hydrogen can be added to the amorphous oxide film to control the electric conductivity. When hydrogen or deuterium is added to the source part and the drain part, the electric conductivity thereof can be increased. When the channel part contains hydrogen or deuterium, the concentration of hydrogen of each of the source part and the drain part is increased to a value larger than the concentration of hydrogen of the channel part. Therefore, the electric conductivity of each of the source part and the drain part can be set to a value larger than the electric conductivity of the channel part. As described above, when each of the source part and the drain part is made of a material substantially identical (except for the hydrogen concentration) to that of the channel part, satisfactory electrical connection between the channel part and each of the source electrode and the drain electrode can be realized. That is, the source (drain) electrode is connected with the channel part through the source (drain) part, thereby realizing the satisfactory electrical connection. In this embodiment, any resistivity that is smaller than the resistivity of the channel part can be used as the resistivity of each of the source part and the drain part. The resistivity of each of the source part and the drain part may be equal to or smaller than 1/10 of the resistivity of the channel part. When the resistivity of each of the source part and the drain part becomes equal to or smaller than 1/1000 of the resistivity of the channel part, the source (drain) part can be used as the source (drain) electrode. The amount of change in resistivity to a change in hydrogen concentration depends on a composition of the oxide film, film quality thereof, or the like. For example, when hydrogen ions of approximately 10"‘'‘ (1/cm’) per volume are implanted into an In-Ga-Zn-0 thin film having approximately 1000 Qcm, the resistivity thereof can be reduced to approximately several 50 Qcm. When hydrogen ions of approximately lO'''‘ il/cm’) are implanted, the resistivity can be reduced to approximately 0.5 Qcm. A concentration range of the hydrogen added to each of the source part and drain part depends on a structure of the oxide film, but a concentration may be equal to or larger than 10"‘' /cm/. In particular, when a concentration equal to or larger than approximately 10"‘‘ /cm’ is set, the electric conductivity of each of the source part and the drain part becomes larger, so the source part and the drain part can be used as the source electrode and the drain electrode. As described above, depending on a film formation condition, the.oxide film may contain hydrogen without active hydrogen addition in some cases. Therefore, there is the case where the channel part contains hydrogen without adding hydrogen actively. Even in such a case, in order to form the source part and the drain, part, hydrogen is added in a post-process such that a hydrogen amount which exceeds the amount of hydrogen contained in the channel part is introduced into the source part and the drain part. Therefore, the structure and the effect as described above can be realized. A method of locally reducing a hydrogen amount in a portion of the oxide film may also be employed, to use the portion as the channel part. The concentration of hydrogen can be evaluated by the measurement using SIMS (secondary ion mass spectrometry). Depending on an evaluation apparatus, a detection limit is approximately lO'‘‘ /cm"‘. A value equal to or smaller than the detection limit can be indirectly calculated by an extrapolation based on a linear relationship between a hydrogen addition process parameter (oxygen partial pressure at the time of film formation or ion implantation amount as described later) and the amount of hydrogen contained in the thin film. In each of FIGS. lA and IB, the single source part and the single drain part are formed. As illustrated in FIG. 6, multiple source parts 16a and 16b and multiple drain parts 17a and 17b may be provided. The source parts 16a and 16b have different electric conductivities. The drain parts 17a and 17b have different electric conductivities. The electric conductivity may increase in order of the channel part 18, the source part 16a, and the source part 15b. Further, the electric conductivity may increase in order of the channel part 18, the drain part 17a, and the drain part l7b. To obtain such a structure, it is only necessary to increase the additional amount of hydrogen ion in order of the channel part 18, the source part 16a, and the source part 16b and to increase the additional amount of hydrogen in order of the channel part 18, the drain part 17a, and the drain part 17b. (Channel Layer; Oxide Film) Any miaterial that is oxide can be used as the material of the channel layer {oxide film). Examples of the material include In oxide and Zn oxide from which a large mobility can be obtained. Further, the channel layer may be mad' " morphous oxide. When the following amorphous oxide film is added with hydrogen, the electric conductivity can be effectively increased. In particular, components of the channel layer made of amorphous oxide are represented by [ (Sni-xM4J02]a- [ (Ini-vM3y) 2O3] b- [ (Zni_zM2,0) ] c where 0 having an atomic number smaller than that of In, and M2 is a group II element (Mg or Ca) having an atomic number smaller than that of Zn. In particular, [ (Ini-yGay) 203]b' [ (ZnO) ] c (where 0< lO""‘ /cm’, and the estimated electron mobility was approximately 2 cm’/V-sec. From light absorption spectrum analysis, forbidden band energy width of the manufactured amorphous oxide film was approximately 3 eV. Next, the gate insulating layer 12 was formed by patterning using a photolithography method and a lift¬off method. The gate insulating layer was obtained by forming a Y2O3 film at a thickness of 150 nm using an electron beam deposition method. A relative dielectric constant of the Y2O3 film was approximately 15. Further, the gate electrode 15 was formed using a photolithography method and a lift-off method. The channel length thereof was 40 ]m[ while the channel width thereof was 200 lam. The electrode was made of Au and had a thickness of 30 nm. Next, hydrogen {or deuterium) ion was implanted to the amorphous oxide thin film (FIG. 3A) to form the source part and the drain part in the channel layer (FIG. 3B) . During the ion implantation, as illustrated in FIG. 3A, hydrogen ions were implanted into the channel layer through the gate insulating layer. According to such a manner, the gate electrode was used as a mask and the source part and the drain part were arranged in self-alignment corresponding to the pattern of the gate electrode. In the ion implantation, H"‘ (proton) was used as an ion species and an accelerating voltage was 20 kV. An ion irradiation amount per area was able to be set to approximately 1x10"‘ (1/cm’) to IxlO" (1/cm’) . A specimen to which deuterium ions D'‘ are implanted was separately prepared as in the above-mentioned case. Composition analysis was performed using a SIMS to evaluate a hydrogen content. A hydrogen concentration of a thin film of a specimen irradiated with ions at 1 X lO"‘ (1/cm.’) was approximately 2 " 10'''‘ {l/cm"‘). Therefore, for example, in the case of a specimen in which an ion irradiation amount was 1 " 10"'"‘ d/cm") , a hydrogen concentration was not able to be measured because it was equal to or smaller than the detection limit. However, it was able to be evaluated that the hydrogen concentration was approximately 2 =< 10-’ (1/cm’) . The hydrogen ion irradiation amount of each of the source part and the drain part in the thin film transistor according to this example was set to IxlO''"‘ (1/cm’) . The hydrogen concentration was evaluated to be approximately 2 >< 10’° (1/cm"‘). Electric conductivity of a specimen separately prepared was evaluated. The electric conductivity was approximately 80 S/cm. In this example, each of the source part and the drain part had sufficient high electric conductivity, so that the structure illustrated in FIG. lA with omitting a source electrode and a drain electrode was employed. (Comparative Example 1) In a comparative example, a device having a structure in which a source electrode and a drain electrode were directly formed on an oxide film was produced. An amorphous oxide layer was formed on a substrate. After that, the source electrode, the drain electrode, a gate insulating layer, and a gate electrode were formed by patterning. The self-alignment method was not used. The formation of each layer was performed based on Example 1. An Au electrode having a thickness of 30 nm was used as each of the source electrode and the drain electrode. Evaluation of Characteristics of TFT Device FIGS. 7A and 7B illustrate exemplary current-voltage characteristics of the TFT device measured at a room temperature. FIG. 7A illustrates the Id-Vd characteristics, while FIG. 7B illustrates the Id-Vg characteristics. As illustrated in FIG. 7A, when predetermined gate voltage Vg was applied and the dependence of the source-drain current Id on the drain voltage Vd along with variation of Vd was measured, behavior of a typical semiconductor transistor, i.e., saturation (pinch off) at a time when Vd was approximately 6V, was exhibited. With regard to the gain characteristics, when Vd of 4 V was applied, the threshold value of the gate voltage VG was approximately -0.5 V. When Vg was 10 V, the current Id of approximately 1.0 >‘ 10"‘ A flew. The on/off ratio of the transistor was 10’ or more. Further, when the field effect mobility was calculated from the output characteristics, the field effect mobility of approximately 8cm’{Vs)~''" was obtained in a saturation region. The manufactured device was irradiated with visible light and the same measurement was made. No change in the transistor characteristics vjas observed. A variation in characteristics of a plurality of the devices produced on the same substrate was evaluated. The variation in this example was smaller than that in the comparative example. For example, a variation in on-currents was evaluated. The variation was approximately ±15% in the comparative example. In contrast to this, the variation was approximately ±10% in this example. In the field effect transistor according to this example, the channel layer (oxide thin film) included the channel part, and the source part and the drain part, each of which had a hydrogen concentration larger than that of the channel part. Therefore, it is expected that stable electrical connection was able to be made between the channel part and each of the source electrode and the drain electrode, thereby improving the uniformity and reliability of the device. The TFT according to this example had hysteresis smaller than that of the TFT of the comparative example. FIGS. 8A and 8B illustrate the Id-Vg characteristics in this example and the comparative example for comparison therebetween. FIG. 8A illustrates the comparative example and FIG. 8B illustrates the example of the TFT characteristics in this example. As shown in the drawings, vjhen hydrogen was added to the channel layer, the hysteresis of the TFT was able to be reduced. That is, according to this example, the satisfactory electrical connection which was resistant to trap charges was able to be realized between each of the source and drain electrode and the channel, so the thin film transistor having small hysteresis was able to be realized. Next, the dynamic characteristic of the top-gate thin film transistor was evaluated. A voltage of 5 V was applied between the source and the drain. Voltages of +5 V and -5 V which were applied to the gate electrode, each of which had a pulse width of 30 psec. and a period of 30 msec, were alternately switched to measure a .response of a drain current. In this example, current rising was excellent and a variation in rising times between devices was small. That is, according this example, the positional relationship among the gate, the source, and the drain was able to be realized with high precision by the self-alignment method. Therefore, high-speed operation was possible and the device having high uniformity was able to be realized. A large characteristic difference was not observed between the case of the hydrogen ion implantation and the case of the deuterium ion implantation. It can be expected that the field effect transistor having a relatively large field effect mobility according to this example be used for, for example, a circuit for operating an organic light emitting diode. (Example 2} In this example, a structure and a manufacturing method were based on Example 1. However, a hydrogen implantation amount was controlled such that the hydrogen concentration of each of the source part and the drain part became approximately IxlO"*"‘ (1/cm''), In this example, the electric conductivity of each of the source part and the drain part was insufficient, and therefore the on-current was slightly smaller than that in Example 1. Electric conductivity of a specimen separately prepared at the above-mentioned hydrogen concentration was evaluated to obtain an electric conductivity of approximately 0.01 3/cm. When the relatively low hydrogen concentration was applied to the source part and the drain part, the insulating layer 19, the source electrode, and the drain electrode were further provided as illustrated in FIG. 5A, so that the satisfactory transistor characteristics was able to be realized as in the case of Example 1. The hysteresis characteristic, the uniformity, and the high-speed operation performance were also preferable. (Example 3) This example is an example in which the bottom-gate TFT device having the coplanar structure as illustrated in FIG. IB was manufactured. In this exam.ple, the device was produced using the manufacturing method illustrated in FIGS. 4A to 4D. The self-alignment method was not used. A channel layer made of amorphous In-Ga-Zn-0 oxide was formed using a PLD method. The source part and the drain part were formed using hydrogen plasma processing. First, the gate electrode 15 was patterned using a photolithography method and a lift-off method on a glass substrate.10 (manufactured by Corning Incorporated, 1737). The electrode was made of Ta and had a thickness of 50 nm. Next, the gate insulating layer 12 was formed by patterning using a photolithography method and a lift¬off method. The gate insulating layer was obtained by forming an Hf02 film in a thickness of 150 nm using a laser deposition method. Further, an amorphous In-Zn-Ga-0 oxide film which is the channel layer was formed by patterning using a photolithography method and a lift-off method. An amorphous In-Zn-Ga-0 oxide film was deposited by a PLD method using a KrF excimer laser. The amorphous In-Zn-Ga-0 oxide film, was deposited with using a polycrystalline sintered material having a composition of InGaOslZnO); as the target. The oxygen partial pressure when the film was formed was 7 Pa. It should be noted that the power of the KrF excimer laser was 1.5 ‘ 10"‘ rriJ/cm’/pulse, the pulse width was 20 nsec, and the repetition frequency was 10 Hz. Further, the temperature of the substrate was 25°C. As a result of X-ray fluorescence (XRF) spectroscopy, the compositional ratio of metals in the thin film was Iii:Ga:Zn = 0.97:1.01:4. Further, as a result of pattern analysis using spectroscopic ellipsometry, it was found that the root-mean-square roughness (Rrms) of the thin film was approximately 0.6 nm and the film thickness thereof was approximately 100 nm. With regard to the obtained film, when glance angle X-ray diffraction (thin film method, incident angle = 0.5 degrees) was carried out approximately on the surface of the film, no clear diffraction peak was detected, which shows that the manufactured In-Zn-Ga-0 film, was an amorphous film. Next, a resist mask 20 having the same pattern as that of the gate electrode was formed by patterning (FIG. 4A). After that, hydrogen was added to the amorphous In-Ga-2n-0 thin film which was the channel layer by hydrogen plasma processing using a plasma processing apparatus. The hydrogen plasma processing can be performed using a parallel-plate type plasma CVD apparatus or an RIE type plasma etching apparatus (FIG. 4B) . A specimen to be processed (substrate obtained after the above step) was stored in the apparatus which was evacuated. After that, a gas containing hydrogen was introduced from a reactive gas introduction port and radio frequency power was introduced into a processing container by a radio frequency (RF) power source, thereby generating plasma. For example, an electrode interval was set to 5 cm, a substrate temperature was set to 100°C, an H2 gas flow rate was set to 500 seem, and an inner pressure of the chamber was set to 1 Torr. A hydrogen content of the thin film subjected to the hydrogen plasma processing increased and a resistivity thereof reduced. Subsequently, the drain electrode 14 and the source electrode 13 were formed by patterning. Each of the electrodes was made of gold and had a thickness of 30 nm (FIG. 4C>. Finally, the mask 20 was etched to form a thin film transistor illustrated in FIG. 4D. A channel length was 50 pm and a channel width was 180 ]im. (Comparative Example 2) A specimen not subjected to the above-mentioned hydrogen plasma processing was prepared. That is, the channel layer had a substantially uniform hydrogen concentration over the entire film region thereof and did not include the source part and the drain part. Other structures and manufacturing methods were based on Example 2. Characteristic Evaluation of TFT Device The thin film transistor according to this embodiment exhibited a behavior of a typical semiconductor transistor which saturated (pinched off) at Vd = 5 V. An on/off ratio of the transistor was 10’ or more and a field effect mobility thereof was approximately 7 cm’ (Vs)'-’. When a plurality of devices were produced, a variation in characteristics of the TFTs according to Example 3 was smaller than that of the TFTs according to Comparative Example 2. The hysteresis characteristic and the high-speed operation performance of the devices of Example 3 were also preferable. In the field effect transistor according to this example, the channel layer (oxide thin film) included the channel part, and the source part and the drain part, each of which had a hydrogen concentration larger than that of the channel part. Therefore, it is expected that stable electrical connection was able to be made between the channel part and each of the source electrode and the drain electrode, thereby improving the uniformity and reliability of the device. It can be expected that the field effect transistor having a relatively large field effect mobility according to this example be used for, for example, a circuit for operating an organic light emitting diode. (Example 4) This example is an example in which a top-gate TFT device as illustrated in FIG. 5B was formed on a plastic substrate. A polyethylene terephthalate (PET) film was used as the substrate. First, a channel layer was formed on the substrate by patterning. Further, in this example, in formation of the channel layer, a polycrystalline sintered material having a composition of In203/Z.nO having a size of two inches was used as the target (material source}, and the applied RF power was 100 W. The total pressure of the atmosphere when the film was formed was 0.4 Pa, where the gas flow ratio was Ar; O2 = 100:2. The film formation rate was 12 nm/min, and the temperature of the substrate was 25°C. With regard to the obtained film, when glance angle X-ray diffraction (thin film method, incident angle =0.5 degrees) was carried out approximately on the surface of the film, no clear diffraction peak was detected, which shows that the manufactured In-Zn-0 film was an amorphous film.. As a result of X-ray fluorescence (XRF) spectroscopy, the compositional ratio of metals was In:Zn = 1.1:0.9. Next, a gate insulating layer and a gate electrode were stacked. The gate insulating layer and the gate electrode are formed to be the same pattern. The gate electrode is a transparent conductive film composed of 1’203 :Sn, Next, hydrogen plasma processing was performed as in the case of Example 3. The source part 16 and the drain part 17 were formed in self-alignment using the gate electrode as a mask. A source electrode and a drain electrode were formed by patterning. A transparent conductive film composed of 10203:Sn was used as each of the source electrode and the drain electrode and a thickness thereof was 100 nm. Characteristic Evaluation of TFT Device The TFT formed on the PET film was measured at a room temperature. An on/off ratio of the transistor is 10’ or more. Calculated field effect mobility was approximately 3 cm’{Vs}""'. As in the case of Exainple 1, the variation in characteristics between devices, the hysteresis characteristic, and the high-speed operation performance were preferable. While the device formed on the PET film was bent at a curvature radius of 30 mm, the transistor characteristic was m.easured in the sam.e manner. A large change in transistor characteristic was not observed. The same measurement was performed with visible irradiation. A change in transistor characteristic was not observed. The thin film transistor produced in this embodiment was transparent to visible light and formed on a flexible substrate. (Example 5) In this example, a display device using the field effect transistor illustrated in FIG. 12 will be described. In the field effect transistor, an island of an ITO film serving as the drain electrode was extended from a short side thereof up to 100 \im. A portion corresponding to 90 ijm of an extended portion was left to ensure wirings for the source electrode and the gate electrode. Then, the TFT was coated with an insulating layer. A polyimide film was applied onto the insulating layer and subjected to a rubbing step. On the other hand, a plastic substrate in which an ITO film and a polyimide film were formed thereon and a rubbing step was performed was prepared. The plastic substrate was opposed to the substrate on which each field effect transistor was formed at a gap of 5 um and then nematic liquid crystal was injected thereto. A pair of polarizing plates were provided on both sides of such a structure. When a voltage was applied to the source electrode of the field effect transistor and a voltage applied to the gate electrode thereof was adjusted, light transmittance was changed only in a region of 30 pm x 90 ]im which was the portion of the island of the ITO film which was extended from the drain electrode. The transmittance was able to be continuously changed by a source-drain voltage during the application of the gate voltage at which the field effect transistor became an on-state. Therefore, a display device using a liquid crystal cell as a display element as illustrated in FIG. 12 was produced. In another example, a white plastic substrate was used as the substrate on which each TFT was formed. A material of each of the electrodes of the TFT was replaced by gold. The polyimide film and the polarizing plates were omitted. A gap between the white plastic substrate and the transparent plastic substrate was filled with capsules in which particles and a fluid were coated with an insulating coating. In the case of a display device having such a structure, a voltage between the extended drain electrode and the ITO film located thereabove was controlled by the field effect transistor, so that the particles in the capsules move in the longitudinal direction. Therefore, the reflectance of the extend drain region as viewed from the transparent substrate side was able to be controlled for display. In another example, a plurality of field effect transistor were formed adj acent to one another to produce, for example, a normal current control circuit including four transistors and a capacitor. Therefore, the TFT illustrated in FIG. 11 was able to be used as one of final-stage transistors to drive an EL element. For example, the field effect transistor using tHe ITO film as the drain electrode was employed. An organic electroluminescence element including a charge injecting layer and a light emitting layer was formed in the region of 30 \im ‘ 90 jim which was the portion of the island of the ITO film which was extended from the drain electrode. Thus, the display device using the EL element was able to be produced. (Example 6) Display elements and field effect transistors, each of which corresponds to that in Example 5, were two-dimensionally arranged. For example, 7425 x 1790 pixels, each of which included the display element such as the liquid crystal cell or the EL element and the field effect transistor in Example 5 and had an area of 30 ]im X 115 pm., were arranged in a square shape at a pitch of 40 ]im. in a short-side direction and at a pitch of 120 ]im in a long-side direction. Then, 1790 gate wirings passing through the gate electrodes of 7425 field effect transistors in the long-side direction were provided and 7425 signal wirings passing through portions of the source electrodes of 1790 TFTs which protrude from amorphous oxide semiconductor film islands by 5 iim in the short-side direction were provided. The wirings were connected v:ith a gate driver circuit and a source driver circuit. In the case of the liquid crystal display element, when color filters, each of which was equal to the liquid ctystal display element in size and aligned therewith, were provided on the surfaces of the devices such that red (R), green (G), and blue (B) filters were repeated in the long-side direction, an active matrix color image display device (approximately 211 ppi and A4 size) was able to be constructed. In the case of the EL element, among two field effect transistors included in an EL element, the gate electrode of a first field effect transistor was connected with a gate line and the source electrode of a second field effect transistor was connected with signal line. The light emission wavelengths of the EL elements were repeated in the order of R, G, and B lights in the long-side direction. Therefore, a light-emission color image display device having the same resolution was able to be constructed. A driver circuit for driving an active mattix circuit may be constructed using the TFT according to this embodiment which is identical to the field effect transistor of the pixel or constructed using an existing IC chip. The field effect transistor according to the present invention can be formed on the flexible material including the PET film. That is, svjitching can be performed with a bending state. In addition. the field effect transistor is transparent to visible light and infrared light which have a wavelength of 400 nm or more, so that the field effect transistor according to the present invention can be applied as a switching element of an LCD or an organic EL display. The field effect transistor according to the present invention can be widely applied to a flexible display, a see-through display, an IC card, an ID tag, and the like. According to the field effect transistor of the present invention, the channel layer (oxide film) includes the source part and the drain part which are added with hydrogen or deuterium. Alternatively, the channel layer {oxide film) includes the channel part containing hydrogen or deuterium and the source part and the drain part which have the hydrogen concentration larger than that of the channel part. Therefore, the stable electrical connection can be made between the channel part and each of the source electrode and the drain electrode, thereby improving the uniformity and reliability of the device. The satisfactory electrical connection resistant to trap charges can be realized between each of the source and drain electrode and the channel, so that the field effect transistor having small hysteresis and excellent characteristic stability can be realized. According to the present invention, when the field effect transistor is to be manufactured, hydrogen is added to the oxide film using the pattern of the gate electrode as the mask. Therefore, the source part and the drain part can be formed in self-alignment with the pattern of the gate electrode, with the result that the positional relationship among the gate, the source, and the drain can be realized with high precision• This application claims priority from Japanese Patent Application No. 2006-074630 filed March 17, 2006, which is hereby incorporated by reference herein. CLAIMS 1. A field effect transistor comprising an oxide film as a semiconductor layer, wherein the oxide film includes one of a source part and a drain part to which one of hydrogen and deuterium is added. 2. A field effect transistor, comprising an oxide film as a semiconductor layer, wherein the oxide film includes a channel part, a source part, and a drain part, and wherein a concentration of one of hydrogen and deuterium in the source part and the drain part is larger than a concentration of one of hydrogen and deuterium in the channel part. 3. A field effect transistor according to claim 1, wherein the source part and the drain part are disposed in self-alignment with a gate electrode, and have a coplanar structure. 4. A field effect transistor according to claim 1, wherein a resistivity of one of the source part and the drain part is 1/10 or less of a resistivity of the channel part. 5. A field effect transistor according to claim 1, wherein the oxide film, is formed of an amorphous oxide represented by; a+b+c =1; M4 is a group VI element selected from the group consisting of Si, Ge and 2r smaller in atomic number than Sn; M3 is Lu or a group III element selected from the group consisting of B, Al, Ga and Y smaller in atomic number than In; and M2 is a group II element selected from the group consisting of Mg and Ca smaller in atomic number than Zn. 6. A display device, comprising: a display element including an electrode; and a field effect transistor according to claim 1, wherein one of a source part and a drain part of the field effect transistor is electrically connected with the electrode of the display device. 7. A display device according to claim 6, wherein a plurality of display elements and a plurality of field effect transistors are two-dimensionally arranged on a substrate. 8. A method of manufacturing a field effect transistor including an oxide film as a semiconductor layer, comprising the steps of: forming the oxide film on. a substrate; and adding one of hydrogen and deuterium to a portion of the oxide film to form a source part and a drain part. 9. A method of manufacturing a field effect transistor including an oxide film as a semiconductor layer, -comprising the steps of: forming the oxide film on a substrate; forming a gate insulating film on the oxide film; forming a gate electrode on the gate insulating film; and adding one of hydrogen and deuterium to the oxide film with using a pattern of the gate electrode as a mask to form a source part and a drain part in the oxide film in self-alignment with the pattern of the gate electrode. 10. A field effect transistor comprising: an oxide semiconductor layer including a channel region; a gate electrode; a gate insulator; a drain electrode; and a source electrode, wherein the oxide semiconductor layer further includes a pair of doped regions adjacent to the source or drain electrode, each of the doped region having at least one of hydrogen and deuterium, and wherein a concentration of one of hydrogen and deuterium in the doped region is larger than a concentration of one of hydrogen and deuterium in the