in the following detailed description , only certain exemplary embodiments of the present invention have been shown and described , simply by way of illustration . as those skilled in the art would realize , the described embodiments may be modified in various different ways , all without departing from the spirit or scope of the present invention . accordingly , the drawings and description are to be regarded as illustrative in nature and not restrictive . in addition , when an element is referred to as being “ on ” another element , it can be directly on the another element or be indirectly on the another element with one or more intervening elements interposed therebetween . hereinafter , like reference numerals refer to like elements , fig1 a through 1g are cross - sectional views illustrating a method of manufacturing a thin film transistor ( tft ) according to an embodiment of the present invention . first , as illustrated in fig1 a , an active layer 12 is formed on a buffer layer 11 that is formed on a substrate 10 . the substrate 10 may be glass or other various plastic substrates such as acryl . when a tft is formed on the substrate 10 and a display device is formed on the tft such that light passes through the tft and is output to the outside , a transparent substrate may be used . alternatively , a reflective substrate such as metal may be used as the substrate 10 after forming a tft on the substrate 10 and a display device on the tft . in this case , various suitable modifications depending on various purposes can be made , for example , a modification can be made such that light passes through the tft between the display device and the substrate to enable optical resonance . a patterned active layer 12 is formed on the buffer layer 11 . the patterned active layer 12 may be formed of a semiconductor oxide , specifically , of a material including zno such as zno , inzno , znsno or zningao . however , the present invention is not limited thereto , and other materials having characteristics of a semiconductor can be used to form the patterned active layer 12 . the patterned active layer 12 can be formed in a pattern as illustrated in fig1 a by using a deposition method using a mask or by forming a semiconductor material corresponding to the entire upper surface of the substrate 10 and then patterning the semiconductor material . after forming the patterned active layer 12 , a gate insulating layer 13 is formed on the active layer 12 and the buffer layer 11 as illustrated in fig1 b . the gate insulating layer 13 can be formed using various suitable materials such as an organic material like parylene or epoxy having insulation characteristics . also the gate insulating layer 13 may be formed of an inorganic material , and if the gate insulating layer 13 needs to be relatively dense , the gate insulating layer 13 may be formed using silicon oxide or silicon nitride . then , a gate electrode 14 and a bottom electrode 15 a of a capacitor 15 are formed on the gate insulating layer 13 as illustrated in fig1 c . the gate electrode 14 and the bottom electrode 15 a of the capacitor may be formed of a conductive material such as al , mo , w , cr , ni or compounds thereof . in one embodiment , if light needs to pass through other elements besides the active layer 12 of the tft , the gate electrode 14 and the bottom electrode 15 a of the capacitor 15 may be formed using various transparent conductive materials such as indium tin oxide ( ito ) and / or indium zinc oxide ( izo ). the gate electrode 14 and the bottom electrode 15 a of the capacitor 15 can be formed as a single layer structure or a multi - layer structure . after forming the gate electrode 14 , hydrogen ions can be implanted into the active layer 12 using the gate electrode 14 as a self - aligned mask as illustrated in fig1 d . then , as the gate electrode 14 covers a channel region of the active layer 12 , a source region 12 b and a drain region 12 c of the active layer 12 can be doped with ions while hydrogen ions are implanted as illustrated in fig1 e . in one embodiment of the present embodiment , the hydrogen ion dose is in the range from 10 16 / cm 2 to about 10 18 / cm 2 , and the hydrogen ion injection energy is between 50 kev to 150 kev . in one embodiment , when the hydrogen ion dose is lower than 10 16 / cm 2 , the improvement of contact resistance is low . in another embodiment , when the hydrogen ion dose is higher than 10 18 / cm 2 , the solubility limit is exceeded , thereby reducing doping efficiency . in one embodiment , when the hydrogen ion injection energy is lower than 50 kev , the injection depth of the dose from the surface is decreased . in another embodiment , when the hydrogen ion injection energy is higher than 150 kev , the cost of a suitable ion injection equipment increases . accordingly , as illustrate in fig1 e , the source region 12 b and the drain region 12 c of the active layer 12 can be made of n + layer for forming an ohmic contact . accordingly , the hydrogen concentration of the source region 12 b and the drain region 12 c of the active layer 12 may be set to be in the range from 10 18 / cm 3 to 10 21 / cm 3 . in one embodiment , when the hydrogen concentration of the source region 12 b and the drain region 12 c of the active layer 12 is lower than 10 18 / cm 3 , the improvement of contact resistance is low . in another embodiment , when the hydrogen concentration is higher than 10 21 / cm 3 , the solubility limit is exceeded , thereby reducing doping efficiency . according to the embodiment of fig1 e , no additional activation process is needed after the process for hydrogen ion implanting of the active layer 12 as described above . accordingly , the present invention can be applied to a plastic substrate to which a high temperature heat treatment process cannot be applied . in one embodiment of the present invention , during an activation process after implanting hydrogen ions into the active layer 12 , a low temperature heat treatment at a temperature of 200 ° c . or lower on the active layer 12 is sufficient for obtaining an activation effect . accordingly , despite the heat treatment , this embodiment of the present invention can be applied to a plastic substrate . after the hydrogen ion injection process as described above , an interlayer insulating layer 16 is formed to cover the gate electrode 14 and the bottom electrode 15 a of the capacitor 15 as illustrated in fig1 e , and then contact holes 16 a of the interlayer insulating layer 16 are formed as illustrated in fig1 f . hence , a source electrode 17 a and a drain electrode 17 b are formed on the interlayer insulating layer 16 as illustrated in fig1 g . while the source electrode 17 a is formed , a top electrode 15 b of the capacitor 15 is formed as one unit with the source electrode 17 a . the source electrode 17 a , the drain electrode 17 b , and the top electrode 15 b of the capacitor 15 can be formed of various suitable conductive materials , such as al , mo , w , cr , ni or compounds thereof and / or various suitable transparent conductive materials , such as ito and / or izo , as a single layer structure or a multi - layer structure . after manufacturing the tft , an organic light emitting device ( or organic light emitting diode ) is further stacked to contact the drain electrode 17 b to form an organic light emitting display device . in this case , since the active layer 12 of the tft is transparent , the tft can be applied to a bottom emitting device in which light is emitted toward ( or in a direction of ) the substrate 10 . fig2 a through 2h are cross - sectional views illustrating a method of manufacturing a tft according to another embodiment of the present invention . first , as illustrated in fig2 a , a gate electrode 14 ′ and a buffer layer 11 ′ are both formed on the substrate 10 ′ in a manner that is substantially the same as described above with reference to fig1 a , 1 b , and / or 1 c . after forming the gate electrode 14 ′, a gate insulating layer 13 ′ is formed on the gate electrode 14 ′ and the buffer layer 11 ′ as illustrated in fig2 b . in fig2 b , the gate insulating layer 13 ′ is illustrated to cover the gate electrode 14 ′; however , the method of manufacturing the tft according to the current embodiment is not limited thereto . after forming the gate insulating layer 13 ′, a patterned active layer 12 ′ is formed on the gate insulating layer 13 ′ as illustrated in fig2 c . the patterned active layer 12 ′ can be formed of an oxide semiconductor material that is substantially the same as described above with reference to the active layer 12 . after forming the patterned active layer 12 ′, a photoresist layer 30 ′ is formed on the patterned active layer 12 ′ to expose a source region 12 b ′ and a drain region 12 c ′ of the patterned active layer 12 ′. in more detail , as illustrated in fig2 d , the photoresist layer 30 ′ is formed on an entire surface of the substrate 10 ( e . g ., on the patterned active layer 12 ′ and the gate insulating layer 13 ′), and then the photoresist layer 30 ′ is patterned to expose the source region 12 b ′ and the drain region 12 c ′ of the patterned active layer 12 ′ as illustrated in fig2 e . the photoresist layer 30 can be patterned using a suitable photolithography method . then , as illustrated in fig2 f , by covering a channel region 12 a ′ of the patterned active layer 12 ′ with the patterned photoresist layer 30 ′ that acts as a mask , hydrogen ions are implanted into the exposed source region 12 b ′ and the drain region 12 c ′ of the patterned active layer 12 ′. in the present embodiment , an interlayer insulating layer is further formed before forming the photoresist layer 30 ′, and then the photoresist layer 30 ′ may be formed and patterned as described above . the hydrogen ions implanting is the same ( or substantially the same ) as described above and thus will not be described again . then , a conductive layer 17 ′ corresponding to an entire surface of the substrate 10 ′ is formed as illustrated in fig2 g . due to the presence of the patterned photoresist layer 30 ′, the conductive layer 17 ′ is not formed as one unit so as to correspond to the entire surface of the substrate 10 ′; however , the conductive layer 17 ′ is formed on the patterned photoresist layer 30 ′ and on the exposed source region 12 b ′, the drain region 12 c ′ and the channel region 12 a ′ of the patterned active layer 12 ′. the conductive layer 17 ′ that is formed on the exposed source region 12 b ′ and the drain region 12 c ′ of the patterned active layer 12 ′, and the conductive layer 17 ′ formed on the patterned photoresist layer 30 ′ is separated as illustrated in fig2 g due to a step difference of the patterned photoresist layer 30 ′. the conductive layer 17 ′ may be formed of various suitable conductive materials , such as al , mo , w , cr , ni or compounds thereof and / or various suitable transparent conductive materials , such as ito or izo , as a single layer structure or a multi - layer structure . then , the patterned photoresist layer 30 ′ is removed using a lift - off method in order to pattern the conductive layer 17 ′ as illustrated in fig2 h . in other words , a source electrode 17 a ′ contacting the source region 12 b ′ of the patterned active layer 12 ′ and a drain electrode 17 b ′ contacting the drain region 12 c ′ of the patterned active layer 12 ′ are formed . in view of foregoing and according to an embodiment of the present invention , a photoresist layer is used for implanting a source region and a drain region of an active layer ( e . g ., the active layer 12 , 12 ′) with hydrogen ions as described above , and the photoresist layer is also consequently used for forming a source electrode and a drain electrode of a conductive layer . thus , unlike a conventional tft manufacturing method , which requires a doping process on semiconductor layers and a patterning process or mask process for forming a source electrode and a drain electrode , only one patterning process or one mask process , that is , a patterning of the photoresist layer , is needed according to the embodiment of the present invention . thus , manufacturing costs are reduced and yield is increased . in addition , according to a method of manufacturing a tft according to an embodiment of the present invention as described above , the tft is provided to have a source electrode , a drain electrode , and an active layer ; and in which an ohmic contact can be formed between the source electrode and the active layer , or between the drain electrode and the active layer . while the present invention has been described in connection with certain exemplary embodiments , it is to be understood that the invention is not limited to the disclosed embodiments , but , on the contrary , is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims , and equivalents thereof .