Patent Publication Number: US-2012025187-A1

Title: Transistors, methods of manufacturing transistors, and electronic devices including transistors

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2010-0074404, filed on Jul. 30, 2010, in the Korean Intellectual Property Office (KIPO), the entire contents of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     Example embodiments relate to transistors, methods of manufacturing the transistors and electronic devices including the transistors. 
     2. Description of the Related Art 
     Transistors are widely used as switching devices or driving devices in electronic devices. In particular, because thin film transistors (TFTs) may be formed on glass substrates or plastic substrates, TFTs are generally used in flat panel devices such as liquid crystal display devices, organic light-emitting display devices and/or the like. 
     Commercialized flat panel display devices may include a thin film transistor having a channel layer formed of amorphous silicon (hereinafter referred to as amorphous silicon thin film transistor), and/or a thin film transistor having a channel layer formed of a poly-crystalline silicon layer (hereinafter referred to as poly-crystalline silicon thin film transistor). 
     However, a charge mobility of the amorphous silicon thin film transistor may be low at about 0.5 cm 2 /Vs, and thus it may be difficult to increase an operating speed of a flat panel display device. In addition, because an optical band gap of amorphous silicon is about ˜1.8 eV, a leakage current may be generated and dangling bonds (i.e., chemical bonds that do not join another atom) may be increased due to irradiation of visible light. The characteristics of the amorphous silicon thin film transistor may be deteriorated. 
     In the case of a poly-crystalline silicon thin film transistor, a crystallization process, an impurity implanting process, and an activation process may be required. The manufacturing process thereof may be complicated and expensive compared to an amorphous silicon thin film transistor. In addition, crystal grains of a poly-crystalline silicon layer may not be uniform and when the poly-crystal silicon layer is used as a channel layer of a large size display device the quality of a screen image may decrease. 
     A transistor including a channel layer formed of an oxide layer, referred to as an oxide thin film transistor, is being researched. Oxide thin film transistors may be of large size, low cost, high performance, and high reliability. 
     SUMMARY 
     Example embodiments may include transistors of high and/or increased performance and high and/or increased reliability, methods of manufacturing the transistors, and electronic devices including the transistors. 
     According example embodiments, a transistor may include a channel layer including an oxide, a source and a drain respectively contacting two ends of the channel layer, a gate corresponding to the channel layer, and a gate insulation layer interposed between the channel layer and the gate. The channel layer may include a first layer and a second layer sequentially arranged from the gate insulation layer. The first layer may be a conductor that is depleted according to a gate voltage condition and the second layer may have a lower electric conductivity than that of the first layer. 
     An oxygen vacancy concentration of the first layer may be higher than an oxygen vacancy concentration of the second layer. An electrical conductivity of the first layer may be greater than or equal to 10 3  S/cm. An electrical conductivity of the second layer may be greater than or equal to 10 −8  S/cm and less than 10 3  S/cm. A carrier concentration of the first layer may be greater than or equal to 10 18 /cm 3  and less than or equal to 10 21 /cm 3 . A carrier concentration of the second layer may be greater than or equal to 10 13 /cm 3  and less than 10 18 /cm 3 . The first layer may have a thickness of about 5 nm to about 50 nm. 
     The first layer and the second layer may include the same metal composition. The first layer and the second layer may have the same metal concentration. The oxide of the channel layer may be a ZnO-based oxide or an InO-based oxide. The gate may be disposed under the channel layer. The transistor may further include an etch stop layer formed on the channel layer. The gate may be disposed over the channel layer. According to other example embodiments, a flat panel display device may include the above-described transistor. 
     According to yet other example embodiments, methods of manufacturing transistors may include forming a gate and forming a channel layer corresponding to the gate, the channel layer including an oxide, forming a gate insulation layer between the gate and the channel layer and forming a source and a drain respectively contacting two ends of the channel layer. The forming of the channel layer may include forming a first layer and a second layer that are sequentially formed on a surface of the gate insulation layer. The first layer may be formed as a conductor that is depleted according to a gate voltage condition and the second layer may be formed to have a lower electrical conductivity than that of the first layer. 
     The first layer may be formed at a first oxygen partial pressure and the second layer may be formed at a second oxygen partial pressure that is greater than the first oxygen partial pressure. A reaction gas used in the forming the channel layer may include O 2  and Ar, and the first layer may be formed under a condition that a flow rate of O 2  to Ar (O 2 /Ar) is greater than 0 and less than 0.1, and the second layer may be formed under a condition that a flow rate of O 2  to Ar (O 2 /Ar) is greater than or equal to 0.1. The first layer and the second layer may be formed using an in-situ method. An oxygen vacancy concentration of the first layer may be higher than that of the second layer. 
     An electrical conductivity of the first layer may be greater than or equal to 10 3  S/cm. An electrical conductivity of the second layer may be greater than or equal to 10 −8  S/cm and less than 10 3  S/cm. The first layer may be formed to have a thickness of about 5 nm to about 50 nm. The channel layer may include a ZnO-based oxide or an InO-based oxide. The transistor may be formed to have a bottom gate structure or a top gate structure. 
     According to further example embodiments, a transistor may include a channel layer including first and second layers, the first layer configured to switch between a normally conductive state and a depletion state according to a voltage bias, a conductivity of the second layer less than a conductivity of the first layer in the conductive state, the channel layer including an oxide, a source and a drain on the channel layer, a gate on the channel layer and a gate insulation layer between the first layer and the gate, the first layer between the second layer and the gate insulation layer. 
     According to still further example embodiments, methods of manufacturing transistors may include forming a gate, forming a channel layer including an oxide by sequentially forming first and second layers corresponding to the gate, the first layer formed to be switchable between a conductive state and a depletion state according to a voltage bias, the second layer formed with a conductivity that is less than a conductivity of the first layer in the conductive state, forming a gate insulation layer between the gate and the first layer and forming a source and drain on the channel layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Example embodiments will be more clearly understood from the following brief description taken in conjunction with the accompanying drawings.  FIGS. 1-14  represent non-limiting, example embodiments as described herein. 
         FIGS. 1-5  are cross-sectional diagrams illustrating transistors according to example embodiments; 
         FIGS. 6A-6D  are cross-sectional diagrams illustrating methods of manufacturing transistors according to example embodiments; 
         FIGS. 7A-7D  are cross-sectional diagrams illustrating methods of manufacturing transistors according to other example embodiments; 
         FIG. 8  is a graph illustrating variation in electric characteristics of an oxide layer as a function of O 2 /Ar flow rate in a method of manufacturing transistors according to example embodiments; 
         FIG. 9  is a graph illustrating variation in oxygen concentration of a channel layer as a function of depth into the channel layer for a channel layer formed using a method of manufacturing transistors according to example embodiments; 
         FIGS. 10-12  are graphs illustrating gate voltage (V GS )-drain current (I DS ) characteristics of a transistor according to example embodiments and a comparison example; and 
         FIGS. 13 and 14  are graphs illustrating variation in gate voltage (V GS )-drain current (I DS ) characteristics of transistors due to light irradiation according to example embodiments. 
     
    
    
     It should be noted that these figures are intended to illustrate the general characteristics of methods, structure and/or materials utilized in certain example embodiments and to supplement the written description provided below. These drawings are not, however, to scale and may not precisely reflect the precise structural or performance characteristics of any given embodiment, and should not be interpreted as defining or limiting the range of values or properties encompassed by example embodiments. For example, the relative thicknesses and positioning of molecules, layers, regions and/or structural elements may be reduced or exaggerated for clarity. The use of similar or identical reference numbers in the various drawings is intended to indicate the presence of a similar or identical element or feature. 
     DETAILED DESCRIPTION 
     Example embodiments will now be described more fully with reference to the accompanying drawings, in which example embodiments are shown. Example embodiments may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of example embodiments to those of ordinary skill in the art. In the drawings, the thicknesses of layers and regions are exaggerated for clarity. Like reference numerals in the drawings denote like elements, and thus their description will be omitted. 
     It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. As used herein the term “and/or” includes any and all combinations of one or more of the associated listed items. Other words used to describe the relationship between elements or layers should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” “on” versus “directly on”). 
     It will be understood that, although the terms “first”, “second”, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of example embodiments. 
     Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “includes” and/or “including,” if used herein, specify the presence of stated features, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof. 
     Example embodiments are described herein with reference to cross-sectional illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of example embodiments. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, example embodiments should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, an implanted region illustrated as a rectangle will, typically, have rounded or curved features and/or a gradient of implant concentration at its edges rather than a binary change from implanted to non-implanted region. Likewise, a buried region formed by implantation may result in some implantation in the region between the buried region and the surface through which the implantation takes place. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of example embodiments. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, such as those defined in commonly-used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
       FIG. 1  is a cross-sectional diagram illustrating a transistor according to example embodiments. The transistor may be a bottom gate type thin film transistor in which a gate G 1  is under a channel layer C 1 . Referring to  FIG. 1 , a gate G 1  may be on a substrate SUB 1 . The substrate SUB 1  may be a plastic substrate and/or another substrate that may be used in a semiconductor device process (e.g., a glass substrate and/or a silicon substrate). The gate G 1  may be of a general electrode material (e.g., a metal, a conductive oxide and/or the like). A gate insulation layer GI 1  covering the gate G 1  may be on the substrate SUB 1 . The gate insulation layer GI 1  may include, for example, a silicon oxide layer and/or a silicon nitride layer. The gate insulation layer GI 1  may include, for example, a high-k dielectric layer with a higher dielectric constant than that of a silicon nitride layer. The gate insulation layer GI 1  may be a single layer or a multi-layer structure. 
     A channel layer C 1  may be on the gate insulation layer GI 1 . The channel layer C 1  may be over the gate G 1 . A width of the channel layer C 1  along an X-axis direction may be relatively greater than a width of the gate G 1 . The channel layer C 1  may be an oxide and may be a multi-layer structure. For example, the channel layer C 1  may be a double-layer structure including a first oxide layer (hereinafter referred to as a first layer  10 ) and a second oxide layer (hereinafter referred to as a second layer  20 ). The first layer  10  and the second layer  20  may be sequentially stacked. The first layer  10  may be nearer to the gate G 1  than the second layer  20 . The first layer  10  may be between the gate insulation layer GI 1  and the second layer  20 . The first layer  10  may contact the gate insulation layer GI 1  and the second layer  20  may be separated from the gate insulation layer GI 1 . A thickness of the first layer  10  may be, for example, about 5 nm to about 50 nm. A thickness of the second layer  20  may be, for example, about 10 nm to about 1000 nm. 
     A source electrode S 1  and a drain electrode D 1  respectively contacting two ends of the channel layer C 1  may be on the gate insulation layer GI 1 . The source electrode S 1  and the drain electrode D 1  may be single layer or multi-layer structures. The source electrode S 1  and the drain electrode D 1  may be of the same or similar material as that of the gate G 1  and/or other materials. A passivation layer P 1  covering the channel layer C 1 , the source electrode S 1 , and the drain electrode D 1  may be on the gate insulation layer GI 1 . The passivation layer P 1  may be, for example, a silicon oxide layer, a silicon nitride layer and/or an organic layer (e.g., a structure including at least two of these layers). Thicknesses of the gate G 1 , the gate insulation layer GI 1 , the source electrode S 1 , and the drain electrode D 1  may be from about 50 nm to about 300 nm, from about 50 nm to about 400 nm, from about 10 nm to about 200 nm, and from about 10 nm to about 200 nm, respectively. 
     The first layer  10  and the second layer  20  of the channel layer C 1  may have different electrical characteristics. The first layer  10  may be a conductor with an electrical conductivity of, for example, 10 3  S/cm or greater. A depletion region may be induced in the first layer  10  according to a voltage applied to the gate G 1  (e.g., a gate voltage). For example, the first layer  10  may be depleted based on a gate voltage. The second layer  20  may be a semiconductor with an electrical conductivity that is less than that of the first layer  10 . For example, the electrical conductivity of the second layer  20  may be about 10 −8  S/cm or greater. The electrical conductivity of the second layer  20  may be less than about 10 3  S/cm. 
     The electrical conductivities of the first and second layers  10  and  20  may be related to carrier concentrations thereof. A carrier concentration of the first layer  10  may be greater than or equal to about 10 18 /cm 3  and less than or equal to about 10 21 /cm 3 . A carrier concentration of the second layer  20  may be greater than or equal to about 10 13 /cm 3  and less than about 10 18 /cm 3 . An electrical resistance of the first layer  10  may be about 10 Ωcm or less, and an electrical resistance of the second layer  20  may be greater than about 10 Ωcm. An oxygen vacancy concentration of the first layer  10  may be different from that of the second layer  20 . For example, the oxygen vacancy concentration of the first layer  10  may be higher than that of the second layer  20 . A large amount of oxygen vacancies may indicate that relatively few oxygen ions are present. An oxygen ion concentration of the first layer  10  may be less than an oxygen ion concentration of the second layer  20 . 
     The first layer  10  may be a conductor that is electrically conductive when no external factors influence the first layer  10  (e.g., normally conductive). An external factor may influence the first layer  10  and the first layer  10  may lose its conductivity. For example, an external factor may induce characteristics similar to an insulator in the first layer  10 . For example, when a negative (−) voltage is applied to the gate G 1 , as carriers (e.g., electrons) of the first layer  10  transfer to the second layer  20 , the first layer  10  may be depleted (e.g., become a depletion region). In an OFF state where a negative (−) voltage is applied to the gate G 1 , the first layer  10  may have insulating characteristics. Because the first layer  10  is conductive in an ON state, a current between the source electrode S 1  and the drain electrode D 1  may mostly flow through the first layer  10 . 
     Because the first layer  10  which has characteristics of a conductor may obtain the characteristics of an insulator through an external factor, the first layer  10  may function as a channel of a transistor for ON/OFF switching. If the first layer  10  is excessively thick, it may be difficult to form a depletion region in the first layer  10  even when a negative (−) voltage is applied to the gate G 1 . If the first layer  10  is excessively thick, it may be difficult to induce insulating characteristics in the first layer  10 . The first layer  10  may be relatively thin. For example, the thickness of the first layer  10  may be from about 5 nm to about 50 nm. The second layer  20  may be a thickness of about 10 nm to about 1000 nm. 
     The first layer  10  may be an oxide (e.g., metal oxide) of the same group as the second layer  20 . Metal compositions of the first layer  10  and the second layer  20  may be the same or similar. Metal element concentrations of the first layer  10  and the second layer  20  may be the same or similar. The first layer  10  and the second layer  20  may be an oxide based on the same material, but the electrical characteristics thereof may be determined by oxygen vacancy concentrations (or oxygen ion concentrations) or carrier concentrations thereof. When the first layer  10  and the second layer  20  are formed of an oxide based on the same material, the first layer  10  and the second layer  20  may be formed using an in-situ method using one sputter target, thereby simplifying a manufacturing process. 
     For example, the first layer  10  and the second layer  20  may include a ZnO-based oxide and/or an InO-based oxide. When the first layer  10  and the second layer  20  include a ZnO-based oxide, they may include at least one of a group 13 element (e.g., In, Ga, and/or Al), a group 14 element (e.g., Sn and/or Si), a group 4 element (transition metal) (e.g., Zr, Hf, and/or Ti), a group 2 element (e.g., Mg), a group 3 element (transition metal) (e.g., Y and/or La), a group 11 element (transition metal) (e.g., Cu), and/or other transition metals (e.g., Ta and/or Cr). The first layer  10  and the second layer  20  may include an InZnO-based oxide (IZO-based oxide), for example, GaInZnO, HfInZnO, ZrInZnO, MgInZnO, LaInZnO, AIInZnO, SiInZnO, and/or CuInZnO, and/or a ZnSnO based oxide (ZTO-based oxide), for example, GaZnSnO, HfZnSnO, ZrZnSnO, MgZnSnO, LaZnSnO, AlZnSnO, SiZnSnO, and/or CuZnSnO. The first layer  10  and the second layer  20  may include ZnO and/or InO. 
     The channel layer C 1  may be a plurality of oxide layers, for example, the first and second layers  10  and  20 , which are of different electrical characteristics, and the operating characteristics and/or reliability of the transistor may be improved. 
       FIG. 2  is a cross-sectional diagram illustrating a transistor according to example embodiments. Referring to  FIG. 2 , a source electrode S 1 ′ and a drain electrode D 1 ′ spaced apart from each other may be on the gate insulation layer GI 1 . A channel layer C 1 ′ contacting the source electrode S 1 ′ and the drain electrode D 1 ′ may be formed on the gate insulation layer GI 1  between the source electrode S 1 ′ and the drain electrode D 1 ′. The source electrode S 1 ′ and the drain electrode D 1 ′ may contact two ends of a lower surface of the channel layer C 1 ′. The channel layer C 1 ′ may include first and second layers  10 ′ and  20 ′ (e.g., sequentially stacked layers). The first and second layers  10 ′ and  20 ′ may be of the same or similar material as that of the first and second layers  10  and  20  described with respect to  FIG. 1 . The transistor of  FIG. 2  may be the same or similar in structure to the transistor of  FIG. 1 , except the positions of the channel layer C 1 ′, the source electrode S 1 ′, and the drain electrode D 1 ′. 
       FIG. 3  is a cross-sectional diagram illustrating a transistor according to example embodiments. Referring to  FIG. 3 , the etch stop layer ES 1  may be on the channel layer C 1 . A width of the etch stop layer ES 1  along an X-axis direction (see the coordinates of  FIG. 1 ) may be less than a width of the channel layer C 1 . Two ends of the channel layer C 1  may not be covered by the etch stop layer ES 1 . The source electrode S 1  may cover first ends of the etch stop layer ES 1  and the channel layer C 1 , and the drain electrode D 1  may cover second ends of the etch stop layer ES 1  and the channel layer C 1 . During an etch process for forming the source electrode S 1  and the drain electrode D 1 , the etch stop layer ES 1  may prevent the channel layer C 1  from being damaged by the etch process and/or reduce such damage. The etch stop layer ES 1  may include, for example, a silicon oxide, a silicon nitride, and/or an organic insulation material. Whether to use the etch stop layer ES 1  may be determined in consideration of the material of the channel layer C 1  and the material of the source electrode S 1  and the drain electrode D 1 . The structure of the transistor of  FIG. 3  may be the same or similar to that of the transistor of  FIG. 1 , except for the etch stop layer ES 1 . 
       FIG. 4  is a cross-sectional diagram illustrating a transistor according to example embodiments. The transistor may be a top gate type thin film transistor in which a gate G 2  is over a channel layer C 2 . Referring to  FIG. 4 , the channel layer C 2  may be on a substrate SUB 2 . The channel layer C 2  may be an inverse structure of the channel layer C 1  of  FIG. 1 . The channel layer C 2  of  FIG. 4  may include a second layer  22  corresponding to the second layer  20  of  FIG. 1  and a first layer  11  corresponding to the first layer  10  of  FIG. 1 . The second layer  22  and the first layer  11  may be sequentially stacked on the substrate SUB 2 . The first layer  11  may be a conductor and the second layer  22  may be a semiconductor. A source electrode S 2  and a drain electrode D 2  may be on the substrate SUB 2  so as to contact two ends of the channel layer C 2 , respectively. A gate insulation layer GI 2  covering the channel layer C 2 , the source electrode S 2 , and the drain electrode D 2  may be on the substrate SUB 2 . The gate G 2  may be formed on the gate insulation layer GI 2 . The gate G 2  may be over the channel layer C 2 . A passivation layer P 2  covering the gate G 2  may be formed on the gate insulation layer GI 2 . 
     Materials and thicknesses of the substrate SUB 2 , the first layer  11 , the second layer  22 , the source electrode S 2 , the drain electrode D 2 , the gate insulation layer GI 2 , the gate G 2 , and the passivation layer P 2  of  FIG. 4  may be the same or similar to those of the substrate SUB 1 , the first layer  10 , the second layer  20 , the source electrode S 1 , the drain electrode D 1 , the gate insulation layer GI 1 , the gate G 1 , and the passivation layer P 1 , respectively, of  FIG. 1 . The function of the first layer  11  and the second layer  22  of  FIG. 4  may be the same or similar to that of the first layer  10  and the second layer  20  of  FIG. 1 , respectively. 
       FIG. 5  is a cross-sectional diagram illustrating a transistor according to example embodiments. Referring to  FIG. 5 , a source electrode S 2 ′ and a drain electrode D 2 ′ spaced apart from each other may be on the substrate SUB 2 . A channel layer C 2 ′ contacting the source electrode S 2 ′ and the drain electrode D 2 ′ may be on the substrate SUB 2  between the source electrode S 2 ′ and the drain electrode D 2 ′. The source electrode S 2 ′ and the drain electrode D 2 ′ may respectively contact two ends of a lower surface of the channel layer C 2 ′. The channel layer C 2 ′ may include a second layer  22 ′ and a first layer  11 ′ on the substrate SUB 2  (e.g., sequentially stacked on the substrate SUB 2 ). The first layer  11 ′ and the second layer  22 ′ may be, for example, of the same material as the first layer  11  and the second layer  22  of  FIG. 4 . The first layer  11 ′ may be a conductor, and the second layer  22 ′ may be a semiconductor. The structure of the transistor of  FIG. 5  may be the same or similar to that of the transistor  FIG. 4 , except the positions of the channel layer C 2 ′, the source electrode S 2 ′, and the drain electrode D 2 ′. 
       FIGS. 6A-6D  are cross-sectional diagrams illustrating methods of manufacturing transistors according to example embodiments. Reference numerals in  FIGS. 6A-6D  that are the same as reference numerals in  FIG. 1  may denote like elements. Referring to  FIG. 6A , a gate G 1  may be formed on a substrate SUB 1 , and a gate insulation layer GI 1  may be formed to cover the gate G 1 . The gate insulation layer GI 1  may be formed to include, for example, a silicon oxide and/or a silicon nitride. The gate insulation layer GI 1  may be formed to include, for example, a high-k electric material with a higher dielectric constant than that of a silicon nitride. The gate insulation layer GI 1  may be a single layer or a multi-layer structure. 
     Referring to  FIG. 6B , an oxide thin film for forming a channel with a multi-layer structure may be formed on the gate insulation layer GI 1 . For example, a first oxide layer (hereinafter referred to as a first layer  10 ) and a second oxide layer (hereinafter referred to as a second layer  20 ) may be formed (e.g., sequentially formed). The first layer  10  and the second layer  20  may be deposited, for example, using a physical vapor deposition (PVD) method (e.g., a sputtering method and/or an evaporation method). The first layer  10  and the second layer  20  may be formed of an oxide (e.g., metal oxide) based on the same group (material). The first layer  10  and the second layer  20  may be of the same metal composition. The first layer  10  and the second layer  20  may be of the same or similar metal element concentrations. For example, the first layer  10  and the second layer  20  may be formed of a ZnO-based oxide and/or an InO-based oxide. When the first layer  10  and the second layer  20  are formed of a ZnO-based oxide, the ZnO-based oxide may include, for example, at least one of a group 13 element (e.g., In, Ga, and/or Al), a group 14 element (e.g., Sn and/or Si), a group 4 element (transition metal) (e.g., Zr, Hf, and/or Ti), a group 2 element (e.g., Mg), a group 3 element (transition metal) (e.g., Y and/or La), a group 11 element (transition metal) (e.g., Cu), and/or other transition metals (e.g., Ta and/or Cr). The first layer  10  and the second layer  20  may include an InZnO-based oxide (IZO-based oxide), for example, GaInZnO, HfInZnO, ZrInZnO, MgInZnO, LaInZnO, AlInZnO, SiInZnO, and/or CuInZnO, and/or a ZnSnO based oxide (ZTO-based oxide), for example, GaZnSnO, HfZnSnO, ZrZnSnO, MgZnSnO, LaZnSnO, AlZnSnO, SiZnSnO, and/or CuZnSnO. The first layer  10  and the second layer  20  may include ZnO and/or InO. 
     When the first layer  10  and the second layer  20  are formed of an oxide based on the same material, they may be formed at different oxygen partial pressures. The first layer  10  may be formed at a first oxygen partial pressure, and the second layer  20  may be formed at a second oxygen partial pressure that is greater than the first oxygen partial pressure. When forming the first layer  10  and the second layer  20 , a reaction gas including O 2  and Ar may be used. In this case, the oxygen partial pressures may be adjusted by varying a flow rate of O 2  gas and Ar gas. The first layer  10  may be formed under a condition that a flow rate of O 2  to Ar (O 2 /Ar) is greater than 0 and smaller than 0.1. The second layer  20  may be formed under a condition that a flow rate of O 2  to Ar (O 2 /Ar) is 0.1 or greater. 
     By forming the first layer  10  and the second layer  20  at different oxygen partial pressures, the first layer  10  and the second layer  20  may formed with different electrical characteristics. According to the above-described method, the first layer  10  may be formed as an electrical conductor with an electrical conductivity of about 10 3  S/cm or greater. The second layer  20  may be formed as a semiconductor of less electrical conductivity than that of the first layer  10 . For example, the electrical conductivity of the second layer  20  may be greater than or equal to about 10 −8  S/cm and less than about 10 3  S/cm. 
     A carrier concentration of the first layer  10  may be greater than or equal to about 10 18 /cm 3  and less than or equal to about 10 21 /cm 3 . A carrier concentration of the second layer  20  may be greater than or equal to about 10 13 /cm 3  and less than about 10 18 /cm 3 . An electrical resistance of the first layer  10  may be about 10 Ωcm or less, and an electrical resistance of the second layer  20  may be greater than about 10 Ωcm. An oxygen vacancy concentration of the first layer  10  may differ from that of the second layer  20 . For example, the oxygen vacancy concentration of the first layer  10  may be higher than that of the second layer  20 . An oxygen ion concentration of the first layer  10  may be less than that of the second layer  20 . The first layer  10  may be formed to a thickness of about 5 nm to about 50 nm. The second layer  20  may be formed to a thickness of about 10 nm to about 1000 nm. 
     When the first layer  10  and the second layer  20  are formed of an oxide based on the same material, the first layer  10  and the second layer  20  may be formed using an in-situ method using one sputter target by differentiating process conditions. The manufacturing process may be simplified. Using the methods according to example embodiments, mass production of transistors may be facilitated and transistors with a large surface area may be manufactured. The first layer  10  and the second layer  20  may be, for example, formed at different oxygen partial pressures. The first layer  10  and the second layer  20  with different properties may be formed, for example, by varying a chamber pressure and/or a source power. The first layer  10  and the second layer  20  may be formed at varying oxygen partial pressures or at a fixed oxygen partial pressure, while varying a chamber pressure and/or source power. The first layer  10  and the second layer  20  may be formed of an oxide based on the same material. The first layer  10  and the second layer  20  may be formed of oxides based on different materials. 
     Referring to  FIG. 6C , a channel layer C 1  may be formed by patterning the first layer  10  and the second layer  20 . The patterned first layer  10  may be the same or similar to the first layer  10  of  FIG. 1 , and the patterned second layer  20  may be the same or similar to the second layer  20  of  FIG. 1 . Referring to  FIG. 6D , a source electrode S 1  and a drain electrode D 1  respectively contacting two ends of the channel layer C 1  and exposing a portion of an upper surface of the channel layer C 1  may be formed on the gate insulation layer GI 1 . The source electrode S 1  and the drain electrode D 1  may be a single layer or a multi-layer structure. The exposed portion of the channel layer C 1  that is not covered by the source electrode S 1  and the drain electrode D 1  may be treated with plasma containing oxygen. 
     A passivation layer P 1  covering the exposed portion of the channel layer C 1  and the source electrode S 1  and the drain electrode D 1  may be formed on the substrate SUB 1 . The passivation layer P 1  may be a silicon oxide layer, a silicon nitride layer, and/or an organic layer, for example, a structure including at least two of these layers. The transistor formed as described above may be thermally annealed. Transistors with the structure of  FIGS. 2 and 3  may be formed by modified examples of the method illustrated in  FIGS. 6A-6D . One of ordinary skill in the art will understand how to manufacture transistors with the structure of  FIGS. 2 and 3  with knowledge of the methods illustrated in  FIGS. 6A-6D , thus descriptions thereof will be omitted. 
       FIGS. 7A-7D  are cross-sectional diagrams illustrating methods of manufacturing transistors according to example embodiments. Reference numerals in  FIGS. 7A-7D  that are the same as reference numerals in  FIG. 4  may denote like elements. Referring to  FIG. 7A , an oxide thin film for forming a channel with a multi-layer structure may be formed on a substrate SUB 2 . For example, a second oxide layer (hereinafter referred to as a second layer  22 ) and a first oxide layer (hereinafter referred to as a first layer  11 ) may be sequentially formed. The first layer  11  and the second layer  22  may be the same or similar to the first layer  10  and the second layer  20  of  FIG. 6B . The material, the fabrication method, and the thicknesses of the first layer  11  and the second layer  22  may be the same or similar to those of the first layer  10  and the second layer  20  of  FIG. 6B . 
     A channel layer C 2  as illustrated in  FIG. 7B  may be formed by patterning the first layer  11  and the second layer  22 . The channel layer C 2  may have an inverse structure of the channel layer C 1  of  FIG. 6C . Referring to  FIG. 7C , a source electrode S 2  and a drain electrode D 2  respectively contacting two ends of the channel layer C 2  may be formed on the substrate SUB 2 . An exposed portion of the channel layer C 2  that is not covered by the source electrode S 2  and the drain electrode D 2  may be treated with, for example, plasma containing oxygen. A gate insulation layer GI 2  covering the exposed portion of the channel layer C 2  and the source electrode S 2  and the drain electrode D 2  may be formed on the substrate SUB 2 . The gate insulation layer GI 2  may be formed of the same or similar material to that of the gate insulation layer GI 1  of  FIG. 6A . The gate insulation layer GI 2  may be formed as a same or similar structure to that of the gate insulation layer GI 1  of  FIG. 6A . The gate insulation layer GI 2  may be formed as an inverse structure of the gate insulation layer GI 1  of  FIG. 6A . 
     Referring to  FIG. 7D , a gate G 2  may be formed on the gate insulating layer GI 2 . The gate G 2  may be over the channel layer C 2 . A passivation layer P 2  may be formed on the gate insulating layer GI 2  to cover the gate G 2 . The passivation layer P 2  may be formed of the same or similar material and the same or similar stacked structure as the passivation layer P 1  of  FIG. 6D . A transistor formed as described above may be thermally annealed. A transistor with the structure of  FIG. 5  may be formed by modifying the manufacturing method illustrated in  FIGS. 7A-7D . One of ordinary skill in the art will understand how to manufacture transistors with the structure of  FIG. 5  with knowledge of the methods illustrated in  FIGS. 7A-7D , thus descriptions thereof will be omitted. 
       FIG. 8  is a graph illustrating variation in electric characteristics of an oxide layer as a function of O 2 /Ar flow rate during deposition of an oxide layer. The oxide layer may be for forming a channel in a method of manufacturing transistors according to example embodiments. The oxide layer may be HfInZnO. Referring to  FIG. 8 , as the O 2 /Ar flow rate increases, a carrier concentration of the oxide layer may be reduced and electrical resistance thereof may be increased. When the O 2 /Ar flow rate is less than about 0.1, an oxide layer with conductor characteristics may be formed. When the O 2 /Ar flow rate is about 0.1 or greater, an oxide layer with semiconductor characteristics may be formed. With respect to  FIGS. 6B and 7A , the first layers  10  and  11  may be formed at a O 2 /Ar flow rate that is greater than 0 and less than about 0.1, and the second layers  20  and  22  may be formed at a O 2 /Ar flow rate that is about 0.1 or greater. 
       FIG. 9  is a graph illustrating variation in oxygen concentration of a channel layer as a function of depth into the channel layer for a channel layer formed using a method of manufacturing transistors according to example embodiments. The channel layer may be formed of HfInZnO. In  FIG. 9 , a region A may correspond to a semiconductor region (e.g., the second layer  20  of  FIG. 1 ) and a region B may correspond to a conductor region (e.g., the first layer  10  of  FIG. 1 ). In  FIG. 9 , an X-axis may denote an acquisition time in seconds (s). The longer the acquisition time, the greater the measured depth of the channel layer. Referring to  FIG. 9 , an oxygen concentration of the region B, the conductor region, may be relatively lower than that of the region A, the semiconductor region. The difference in the oxygen concentrations may be one of the factors that generate the difference in the electrical characteristics of the region A (e.g., the second layer  20  of  FIG. 1 ) and the region B (e.g., the first layer  10  of  FIG. 1 ). 
       FIGS. 10-12  are graphs illustrating gate voltage (V GS )-drain current (I DS ) characteristics of a transistor according to example embodiments and a comparison example. The structure of the transistor according to example embodiments may be the structure of  FIG. 3  and may include a double-layer channel. The structure of the transistor according to the comparison example may be similar to the structure of  FIG. 3  and may include a single layer channel. Material for the channel layers of both transistors may be HfInZnO. The characteristics (carrier concentration), thicknesses, and formation conditions (O 2 /Ar flow rate) of the channel layers may be shown in Table 1 below. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Carrier 
                   
                 O 2 /Ar 
               
               
                   
                 concentration (/cm 3 ) 
                 Thickness (nm) 
                 flow rate 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Comparison 
                 A 
                 10 19   
                 40 
                 0.01 
               
               
                 example 
                 B 
                 10 14   
                 40 
                 1 
               
               
                 (single layer 
                 C 
                 10 15   
                 40 
                 5 
               
               
                 channel) 
               
               
                 Example 
                 A/B 
                 10 19 /10 14   
                 10/30 
                 0.01/1 
               
               
                 embodiments 
                 A/C 
                 10 19 /10 15   
                 10/30 
                 0.01/5 
               
               
                 (double-layer 
               
               
                 channel) 
               
               
                   
               
            
           
         
       
     
     Referring to  FIG. 10 , according to the comparison example including a channel A, the whole channel layer may be a conductor and no switching characteristics are displayed. Referring to  FIG. 11 , according to example embodiments in which a channel A/B is included, a higher ON current and a lower OFF current may be shown than the comparison example including a channel B. This indicates that the mobility of the transistor including the channel A/B may be higher that the comparison example including the channel B, and the on/off current ratio of the transistor including the channel A/B may be higher than the transistor of the comparison example including the channel B. Referring to  FIG. 12 , according to example embodiments including a channel A/C, a higher ON current and a lower OFF current may be achieved than the comparison example including a channel C. This is similar to the result of  FIG. 11 . 
     The results of  FIGS. 10-12  may be summarized as shown in Table 2 below. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Mobility (cm 2 /Vs) 
                 Threshold voltage (V) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Comparison 
                 A 
                 (conductor) 
                   
               
               
                 example 
                 B 
                 6.57 
                 −0.27 
               
               
                 (single layer 
                 C 
                 4.47 
                 −1.79 
               
               
                 channel) 
               
               
                 Example 
                 A/B 
                 21.18 
                 −2.90 
               
               
                 embodiments 
                 A/C 
                 18.48 
                 −3.11 
               
               
                 (double-layer 
               
               
                 channel) 
               
               
                   
               
            
           
         
       
     
     As described above, according to example embodiments, an oxide thin film transistor with excellent and/or improved operating characteristics, for example, high and/or improved mobility and high and/or improved ON/OFF current ratio, may be manufactured. The increased mobility may be due to the conductive characteristics of the first layer  10 . 
       FIGS. 13 and 14  are graphs illustrating variation in gate voltage (V GS )-drain current (I DS ) characteristics of a transistor due to light irradiation according to example embodiments. The channel layers of these example embodiments may be the same as those of the example embodiments of Table 1. By irradiating light of about 120 lux to the transistors and respectively applying voltages of −20 V, 10 V, and 0 V to a gate, a drain electrode, and a source electrode of the transistors, variation in the characteristics of the transistors according to time may be measured. 
     Referring to  FIGS. 13 and 14 , variation in the characteristics of the transistor due to light irradiation may be small and/or decreased. In  FIG. 13 , ΔV — 1 nA is about 1.11 V. In  FIG. 14 , ΔV — 1 nA is about 0.18 V. The reference character “ΔV — 1 nA” may denote a difference in ‘V — 1 nA’ before and after light irradiation, calculated as [V — 1 nA(after)−V — 1 nA(before)]. The term ‘V — 1 nA’ may denote a gate voltage that allows a current of 1 nA to flow between the source and drain electrodes. The higher the absolute value of “ΔV — 1 nA”, the greater the variation in the characteristics of the transistors due to light irradiation. The results of  FIGS. 13 and 14  may indicate that the light reliability (photo reliability) of the transistors of example embodiments is good and/or improved. Referring to  FIG. 14 , ΔV — 1 nA may be almost 0. With respect to a conventional transistor including a channel layer formed of an oxide layer, because an oxide may be sensitive to light, variation in the characteristics of the transistor may be generated by light irradiation. 
     According to example embodiments, a plurality of oxide layers with different characteristics may be used and a decrease in the reliability of the general oxide transistor may be prevented and/or reduced. An oxide thin film transistor according to example embodiments may exhibit high and/or improved performance, and high and/or improved reliability. An oxide thin film transistor according to example embodiments may exhibit high and/or improved mobility, and high and/or improved on/off current ratio. An oxide thin film transistor according to example embodiments may exhibit reduced variation in the characteristics thereof due to light. 
     The transistors according to example embodiments may be used as switching devices or driving devices of flat panel display devices, for example, liquid crystal display devices and/or organic light emitting display devices. Transistors according to example embodiments may show small variation in characteristics due to light and good and/or improved operating characteristics. When the transistors, are used in flat panel display devices, the reliability and performance of the flat panel display devices may be improved. The structures of the liquid crystal display devices and the organic light emitting display devices are well known in the art, and thus description thereof will be omitted. The transistors according to example embodiments may be applied not only to flat panel display devices but also to other various electronic devices, for example, memory devices and logic devices, for various purposes. 
     While example embodiments have been particularly shown and described, it will be understood by one of ordinary skill in the art that variations in form and detail may be made therein without departing from the spirit and scope of the claims. For example, it will be understood by those skilled in the art that various changes in form and details of the elements and structure of the transistors of  FIGS. 1-5  of example embodiments may be made. The channel layers C 1 , C 1 ′, C 2 , and C 2 ′ may have a multi-layer structure including at least three layers, or oxygen vacancy concentrations thereof (or oxygen ion concentrations) may gradually vary in a direction of deposition, that is, a thickness direction thereof. The transistors according to example embodiments may have a double-gate structure. The methods illustrated in  FIGS. 6A-6D  and in  FIGS. 7A-7D  may be modified in various ways. In addition, the concepts of example embodiments of the present invention may also be applied to other transistors instead of the oxide thin film transistors.