Abstract:
Provided is a current controlling device for controlling an electric field emission current in connection with an electric field emission device which emits electrons in response to an applied voltage, the device including: a first current controlling transistor forming a current path in response to a first gate voltage; a second current controlling transistor connected between the field emission device and the first current controlling transistor and forming a current path in response to a second gate voltage; and a control logic controlling the first and second gate voltages, wherein the control logic controls a upper limit of the field emission current by using the first gate voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This U.S. non-provisional patent application claims priority under 35 U.S.C. §119 of Korean Patent Application No. 10-2012-0136141, filed on Nov. 28, 2012, and No. 10-2012-0037876, filed on Apr. 12, 2012, the entire contents of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention disclosed herein relates to a current controlling device and an electric field emission system including the same. 
         [0003]    An electric field emission device includes a cathode where an electric field emitting source (an emitter) emitting electrons is formed. When an electric field is applied to the cathode of the field emission device, the electrons emitted from the emitter are attracted to an anode. The electric field applied to the cathode is determined by an anode voltage in a dipole structure, or a gate voltage in a three-pole structure. 
         [0004]    For stable driving, a current flowing through the electric field emission device is required to be constantly controlled. There is a method of controlling a voltage applied to the electric field emission device in order to control the current of the field emission device. However, the current of the electric field emission device increases exponentially in response to the applied voltage. Also, since a characteristic of the emitter of the electric field emission device may be degraded or activated over time, a current emitted for an identical voltage may decrease or increase. Accordingly, it is typically difficult to control an electric field emission current to be constant by using a voltage applied to an electric field emission device. For stable driving of the electric field emission device, a technique is required to control the field emission current to be constant without controlling an applied voltage. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention provides a current controlling device and an electric field emission system including the same, capable of controlling an electric field emission current of the electric field emission system to be constant. More particularly, the current controlling device according the present invention directly controls a current which flows through a cathode of the field emission device using a plurality of transistors connected in series to the cathode. 
         [0006]    Embodiments of the present invention provide current controlling devices for controlling an electric field emission current in connection with an electric field emission device which emits electrons in response to an applied voltage, the devices including: a first current controlling transistor forming a current path in response to a first gate voltage; a second current controlling transistor connected between the field emission device and the first current controlling transistor and forming a current path in response to a second gate voltage; and a control logic controlling the first and second gate voltages, wherein the control logic controls a upper limit of the field emission current by using the first gate voltage. 
         [0007]    In some embodiments, the current controlling device may be driven under a condition that the applied voltage is provided with a value equal to or greater than a reference voltage; and the reference voltage may induce an electric field emission current equal to or greater than the upper limit of the field emission current from the field emission device. 
         [0008]    In other embodiments, the current controlling device may be driven under a condition that the applied voltage is provided with a value equal to or lower than an upper limit voltage; and the upper limit voltage may be determined on the basis of a characteristic of the field emission device and an allowable drain-source voltage of the second current controlling transistor. 
         [0009]    In still other embodiments, the control logic may control the second gate voltage for the second current controlling transistor to be constantly in a turn-on state. 
         [0010]    In even other embodiments, the control logic may control the second gate voltage to maintain the second gate voltage to be constant and higher than the first gate voltage. 
         [0011]    In yet other embodiments, the control logic may control the second gate voltage to cause the first current controlling transistor to operate in a saturated region. 
         [0012]    In further embodiments, the second current controlling transistor may be a power metal oxide semiconductor field-effect transistor (Power MOSFET). 
         [0013]    In still further embodiments, the first current controlling transistor may be a depletion mode Power MOSFET or an enhance mode Power MOSFET. 
         [0014]    In other embodiments of the present invention, electric field emission systems include: an electric field emission device including a cathode for emitting electrons in response to an applied voltage; and a current controlling device connected to the field emission device and controlling an electric field emission current, wherein the current controlling device includes: a first current controlling transistor forming a current path in response to a first gate voltage; a second current controlling transistor connected between the cathode and the first current controlling transistor and forming a current path in response to a second gate voltage; and a control logic controlling the first and second gate voltages, wherein the control logic controls a upper limit of the field emission current by using the first gate voltage. 
         [0015]    In some embodiments, the field emission device may further include an anode receiving electrons; and the electrons are emitted according to a voltage difference between the anode and the cathode. 
         [0016]    In other embodiments, the anode may include a fluorescent body for generating a light, and may generate the light according to the received electrons. 
         [0017]    In still other embodiments, the anode may generate X-rays according to the received electrons. 
         [0018]    In even other embodiments, the field emission device may further includes: an anode receiving electrons; and a gate located between the anode and the cathode and inducing electron emission, and wherein the electrons may be emitted according to a voltage difference between the gate and the cathode. 
         [0019]    In yet other embodiments, the field emission device may further include a focusing electrode focusing the electrons emitted from the cathode, and the focusing electrode may be located between the gate and the anode. 
         [0020]    In further embodiments, the applied voltage may be provided to be equal to or higher than a reference voltage; and the reference voltage may induce an electric field emission current equal to or greater than the upper limit of the field emission current from the field emission device. 
         [0021]    In still further embodiments, the applied voltage may be provided to be equal to or lower than an upper limit voltage; and the upper limit voltage may be determined on the basis of a characteristic of the field emission device and an allowable drain-source voltage of the second current controlling transistor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The accompanying drawings are included to provide a further understanding of the present invention, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present invention and, together with the description, serve to explain principles of the present invention. In the drawings: 
           [0023]      FIG. 1  is a block diagram illustrating a electric field emission system; 
           [0024]      FIG. 2  is a graph illustrating a method of controlling an electric field emission current of a electric field emission system; 
           [0025]      FIG. 3  is a graph re-illustrating the graph of  FIG. 2  in consideration of an operation of an actual current controlling transistor; 
           [0026]      FIG. 4  is a block diagram illustrating an improved filed emission system, according to the present invention ; 
           [0027]      FIG. 5  is a graph illustrating a method of controlling an electric field emission current of the electric field emission system of  FIG. 4 ; 
           [0028]      FIG. 6  is a graph illustrating operation results of the electric field emission system of  FIG. 1 ; 
           [0029]      FIG. 7  is a graph illustrating operation results of the electric field emission system of  FIG. 4 ; 
           [0030]      FIG. 8  is a circuit diagram illustrating an electric field emission display to which a electric field emission system according to the present invention is applied; 
           [0031]      FIG. 9  is a view illustrating an embodiment that a electric field emission system according to the present invention is applied to an electric field emission X-ray source; and 
           [0032]      FIG. 10  is a graph illustrating operations of field emission X-ray sources to which the present invention is applied and the present invention is not applied. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0033]    Preferred embodiments of the present invention will be described below in more detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be constructed as 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 scope of the present invention to those skilled in the art. 
         [0034]    In the drawings, the dimensions of layers and regions are exaggerated for clarity of illustration. Like reference numerals refer to like elements throughout. 
         [0035]    Hereinafter, it will be described about an exemplary embodiment of the present invention in conjunction with the accompanying drawings. 
         [0036]      FIG. 1  is a block diagram illustrating an electric field emission system. Referring to  FIG. 1 , an electric field emission system  10  includes an electric field emission device  11  and a current controlling transistor  12 . 
         [0037]    The field emission device  11  includes a cathode for emitting electrons. The field emission device  11  is provided with an applied voltage V a  for generating an electric field. In the field emission device  11  having a dipole structure, an applied voltage V a  may be applied to an anode. Alternatively, in the field emission device  11  having a triple pole structure, the applied voltage V a  may be applied to a gate. 
         [0038]    When a voltage difference equal to or greater than a predetermined value is generated between an anode, or a gate in a case of the three-pole structure, and an emitter, electrons are emitted from the emitter of the cathode by tunneling. A voltage difference between the applied voltage and the cathode, which is necessary for the electrons to be emitted from the cathode, is defined as an electric field emission voltage V ac . 
         [0039]    The current controlling transistor  12  is connected to the cathode of the field emission device  11  and directly controls an electric field emission current of the field emission device  11 . The current controlling transistor  12  may be a metal oxide semiconductor field-effect transistor (MOSFET). 
         [0040]    Referring to  FIG. 1 , the drain of the current controlling transistor  12  is connected to the cathode of the field emission device  11 , and the source is connected to the ground. The gate of the current controlling transistor  12  is provided with a gate voltage V g . 
         [0041]    A drain-source current of the current controlling transistor  12  may be controlled by the gate voltage V g . The same current as the drain-source current of the current controlling transistor  12  has to flow through the field emission device  11  connected in series with the current controlling transistor  12 . Accordingly, when the drain-source current is controlled by the current controlling transistor  12 , a potential of the cathode voltage V c  of the field emission device  11  changes and the current emission current may be controlled. 
         [0042]      FIG. 2  is a graph illustrating a method for controlling an electric field emission current of the electric field emission system of  FIG. 1 . In  FIG.2 , a horizontal axis denotes a voltage and a vertical axis denotes a current. 
         [0043]    Referring to the block diagram of  FIG. 1 , the applied voltage V a  of the field emission device  11  is distributed to an electric field emission voltage V ac  of the field emission device  11  and a drain-source voltage V ds  of the current controlling transistor (see reference numeral  12  of  FIG. 1 ). Since V a  has a constant value, V ac  and V ds  have a negative correlation. 
         [0044]    An initial field emission current I e1  of the field emission device (see reference numeral  11  of  FIG. 1 ) for the field emission voltage V ac  of the field emission device  11  is the same as illustrated. The initial field emission current I e1  increases exponentially when V ac  becomes equal to or greater than a predetermined threshold voltage. 
         [0045]    In a state where the gate voltage V g  of the current controlling transistor  12  is constant, an ideal drain-source current I ds  of the current controlling transistor  12  for V ac  is the same as illustrated. 
         [0046]    A saturated current I sat  of the drain-source current I ds  is determined on the basis of the gate voltage V g . 
         [0047]    Since the field emission device  11  and the current controlling transistor  12  are connected in series, an initial field emission current I e1  and the drain-source current I ds  are necessary to have the same value. 
         [0048]    Accordingly, the field emission current of the electric field emission system  10  becomes the saturated current I sat  of the drain-source current I ds , and the value may be adjusted using the gate voltage V g . 
         [0049]    Meanwhile, as described above, when the emitter of the field emission device  11  becomes degraded, an electric field emission current function for V ac  may change and exhibit a curve shape of a degraded field emission current I e2 . 
         [0050]    However, due to the saturation characteristic of the current controlling transistor  12 , the degraded field emission current I e2  also becomes to have a value of the saturated current I sat  of the drain-source current I ds . Accordingly, the electric field emission system  10  of  FIG. 1  may maintain the field emission to be current constant despite of the degradation of the field emission device  11 . 
         [0051]    However, the graph of  FIG. 2  is about an ideal case. An actual saturated current I sat  of the current controlling transistor  12  is not maintained to be constant. 
         [0052]    Hereinafter, more detailed description is provided. 
         [0053]      FIG. 3  is a graph re-illustrating the graph of  FIG. 2  in consideration of the operation of an actual current controlling transistor  12 . The same as  FIG. 2 , the horizontal axis denotes a voltage and the vertical axis denotes a current. 
         [0054]    The initial field emission current I e1  and the degraded field emission current I e2 , of the field emission device for V ac  are the same as ones of  FIG. 2 . The initial field emission current I e1  and the degraded field emission current I e2  increase exponentially when V ac  becomes equal to or greater than a predetermined threshold voltage. 
         [0055]    In a state where the gate voltage V g  of the current controlling transistor  12 , the drain-source current I ds  of the current controlling transistor  12  for V ac  is the same as illustrated. When V ds  increases, the drain-source current I ds  is not saturated and is increased as illustrated. 
         [0056]    Accordingly, when the field emission current function of the field emission device  11  changes I e1  to I e2 , the current emission current also changes I 1  to I 2  and is not maintained to be constant. Also, since the electric field emission system  10  is controlled with a single current controlling transistor  12 , a saturation region of the current controlling transistor  12  may not be generated in a necessary current region. 
         [0057]      FIG. 4  is a block diagram illustrating an improved electric field emission system according to the present invention. Referring to  FIG. 4 , the electric field emission system  100  includes an electric field emission device  110  and a current controlling device  120 . 
         [0058]    The electric field emission system  100  may maintain the field emission current to be constant using a plurality of transistors which are connected in series and included in the current controlling device  120 , even though an electric field emission current function changes. In addition, the electric field emission system  100  may adjust an electric field emission current level to a desired current level using the current controlling device  120 . 
         [0059]    The current emission device  110  includes a cathode for emitting electrons. The current emission device  110  are the same in a configuration and operations as the field emission device  11  of  FIG. 1 . 
         [0060]    The current controlling device  120  includes a first current controlling transistor  121 , a second current controlling transistor  122  and a control logic  123 . The first current controlling transistor  121  and the second current controlling transistor  122  are connected to the cathode of the current emission device  110  in series. The first current controlling transistor  121  and the second current controlling transistor  122  may be a Power MOSFET for tolerating a high voltage. In particular, the first current controlling transistor  121  may be a depletion mode MOSFET or an enhancement mode MOSFET. However, the first current controlling transistor  121  and the second current controlling transistor  122  according to the present invention are not limited thereto. 
         [0061]      FIG. 4  illustrates only the first current controlling transistor  121  and the second current controlling transistor  122 , but the number of current controlling transistors included in the current controlling device  120  is not limited. For example, the current controlling device  120  may include  3  or more current controlling transistors connected in series. 
         [0062]    The control logic  123  controls each node voltage of the first and second current controlling transistors  121  and  122 . The control logic  123  may adjust or limit a current level of the field emission current using the first current controlling transistor  121 . Also, the control logic  123  may maintain the field emission current to be constant using the first and second current controlling transistors  121  and  122  together. At this time, the voltage V a  applied to the field emission device is necessary to be sufficiently high so that a level of the current to be emitted is equal to or higher than a desired current level. 
         [0063]    The control logic  123  provides a first gate voltage V g1  to the gate of the first current controlling transistor  121 . The source of the first current controlling transistor  121  is connected to the ground and the drain thereof is connected to source of the second current controlling transistor  122 . The drain-source current of the first current controlling transistor  121  is determined in response to the gate voltage V g1  and drain voltage V d1  of the first current controlling transistor  121 . 
         [0064]    The control logic  123  provides a second gate voltage V g2  to the gate of the second current controlling transistor  122 . The source of the second current controlling transistor  122  is connected to the drain of the first current controlling transistor  121  and the drain thereof is connected to the cathode of the field emission device  110 . The drain-source current of the second current controlling transistor  122  is determined in response to the gate-source voltage V g2 -V d1  and the drain-source voltage V c -V d1  of the second current controlling transistor  122 . 
         [0065]    The control logic  123  may control an amount of the field emission current using the first gate voltage V g1 . In addition, the control logic  123  may control a upper limit of the drain node of the first current controlling transistor  121  using the second gate voltage V g2 . 
         [0066]    Hereinafter, a current control method of the electric field emission system  100  by the control logic  123  will be described in detail with reference to  FIG. 5 . 
         [0067]      FIG. 5  is a graph illustrating the field emission current control method of the electric field emission system of  FIG. 4 . In  FIG. 5 , the horizontal axis denotes a voltage, and the vertical axis denotes a current. 
         [0068]    Referring to the block diagram of  FIG. 4 , the applied voltage V a  of the field emission device  110  is distributed to an electric field emission voltage (hereinafter V ac ) of the field emission device  110 , the drain-source voltage (hereinafter V ds2 ) of the second current controlling transistor  122  and the drain-source voltage (hereinafter V ds1 ) of the first current controlling transistor  121 . 
         [0069]    The initial field emission current I e1  and the degradation field emission current I e2  of the field emission device  110 , with respect to the field emission voltage V ac  of the field emission device  110  (see  FIG. 4 ) of  FIG. 5 , are the same as the ones of  FIGS. 2 and 3 . The initial field emission current I e1  and degradation field emission current I e2  increase exponentially when V ac  becomes equal to or greater than a predetermined threshold voltage. 
         [0070]    Since the field emission device  110  and the first and second current controlling transistors  121  and  122  are connected in series, the field emission current and the drain-source currents I ds1  and I ds2  of the first and second current controlling transistors are necessary to have the same value. 
         [0071]    Meanwhile, the drain-source current of the serially connected first and second current controlling transistors  121  and  122  exhibits a form identical to an ideal drain-source current of a single current controlling transistor. Accordingly, the field emission current may be maintained to be constant despite of degradation of the field emission device  110  of the electric field emission system  100  of  FIG. 4 . Hereinafter, an operation of the electric field emission system  100  at the time of degradation of the field emission device  110  will be described in detail. 
         [0072]    When the field emission device  110  is degraded, V ac  is necessary to increase in order to provide an identical electrical emission current. Since the applied voltage V a  is provided constantly, the cathode voltage V c  is necessary to be lowered in order to provide an identical electrical emission current. 
         [0073]    The cathode voltage V c  is distributed to the drain-source voltages of the first and second current controlling transistors  121  and  122 . Since the drain voltage of the first current controlling transistor  121  is maintained to be lower than the second gate voltage V g2  due to the second current controlling transistor  122 , most of the cathode voltage V c  is distributed to the drain-source voltage of the second current controlling transistor  122 . 
         [0074]    When the second gate voltage V g2  is provided in a constant level, the drain voltage of the first current controlling transistor  121  is fixed to a constant level by the second gate voltage V g2 , even though the cathode voltage V c  is changed due to degradation of the field emission device  110 . At this time, the second gate voltage V g2  is provided for the second current controlling transistor  122  to operate in a full ON state. 
         [0075]    Accordingly, since the drain voltage of the first current controlling transistor  121  is fixed, the drain-source currents of the first and second current controlling transistors  121  and  122  are limited by the first current controlling transistor  121  and the value thereof depends on the gate voltage V g1  of the first current controlling transistor  121 . 
         [0076]    As described above, when the field mission device  110  is degraded, the field emission current function for V ac  changes and the cathode voltage V c  may be changed. However, due to the saturation characteristics of the first and second current controlling transistors  121  and  122 , the field emission current may be maintained a constant value I std  by the first current controlling transistor  121 . Accordingly, the electric field emission system  100  of  FIG. 4  may maintain the field emission current to be constant despite of degradation of the field emission device  110 . In an embodiment, when the second gate voltage is set so that the first current controlling transistor operates in a saturated region, the current control characteristic may be further enhanced 
         [0077]    At this time, the applied voltage V a  is necessary to be sufficiently high so that the field emission voltage V ac  increases to a level in which the above-described operation is possible. Since a difference between the applied voltage V a  and the field emission voltage V ac  is distributed mostly to the second current controlling transistor  122 , an upper limit of the applied voltage V a  may be determined by an allowable drain-source voltage of the second current controlling transistor  122 . 
         [0078]    The source of the first current controlling transistor  121  is connected to the ground in  FIG. 4 . This is an exemplary case, but a bias condition according to the present invention is not limited thereto. For example, a negative voltage may be applied to the source of the first current controlling transistor  121 . In response to this, the gates of the first and second current controlling transistors  121  and  122  will be provided with a voltage higher than the source voltage. 
         [0079]      FIG. 6  is a graph illustrating operation results of the current controlling transistor  12  in  FIG. 1 .  FIG. 7  is a graph illustrating operation results of the field control device  120  in  FIG. 4 . In  FIGS. 6 and 7 , the horizontal axis denotes a voltage and the vertical axis denotes a current. 
         [0080]    The graphs of  FIG. 6  represent a drain-source current vs. a drain voltage when the gate voltage is 3.5V, 3.6V and 3.7V. As illustrated, the lower the drain voltage provided to the current controlling transistor is, the field emission current value is maintained to be stable despite of a change of the drain voltage. However, when the voltage applied to the field emission device increases to a predetermined level or more, the drain-source current is not maintained to be constant and increases rapidly. 
         [0081]    The graphs of  FIG. 7  exhibit an electric field emission current with respect to the applied voltage when the gate voltage of the first current controlling transistor is 3.8V, 3.9V, 2.95V, 4V and 4.2V. When the drain voltage grater than the one in  FIG. 1  is provided, the drain-source current is maintained to be constant, even though the applied voltage increases to 1000V or more. In addition, the constant value may be confirmed to be controlled using the gate voltage of the first current transistor. 
         [0082]    When the drain voltage has a predetermined level or more in  FIG. 7 , the field emission current slightly increases. This is a phenomenon caused by heating of the first and second current controlling transistors. When a heat sink is attached, more stable characteristics may be exhibited. 
         [0083]      FIG. 8  is a block diagram illustrating an embodiment where the electric field emission system according to the present invention is applied to an electric field emission display. The field emission display  200  includes an electric field emission display module  210  and a current controlling device  220 . 
         [0084]    The field emission display module  210  includes a cathode  211  and an anode  212 . The field emission display module  210  generates a light according to a voltage difference between the cathode  211  and the anode  212 . The field emission display module  210  of the present embodiment is illustrated as a dipole structure, but is not limited thereto. For example, the field emission display module  210  may further include a gate and generate a light according to a voltage difference between the gate and the cathode. 
         [0085]    The anode  212  includes a fluorescent body for generating a light. When the fluorescent body of the anode  212  collides with electrons, electrons in the fluorescent body are separated from and the light is generated. 
         [0086]    The anode  212  is provided with the applied voltage V a  in order to induce the electrons. The applied voltage V a  has a greater positive value than a voltage of the cathode  211 . 
         [0087]    The current controlling device  220  includes a first current controlling transistor  221 , a second current controlling transistor  222  and a control logic  223 . 
         [0088]    As described above, the control logic  223  may control the gate voltages of the first and second current controlling transistors  221  and  222 . The control logic  223  may adjust or limit a current level of the field emission current using the first gate voltage V g1  provided to the first current controlling transistor  221 . In addition, the control logic  223  may maintain the field emission current to be constant using the first and second current controlling transistors  221  and  222 . 
         [0089]    The current controlling device  220  according to the present invention may adjust or limit a current level of the field emission current using the control logic  223 . Also, the current controlling device  220  may control the intensity of the light generated in the field emission display module  210  by maintaining the level of the field emission current to be constant. 
         [0090]      FIG. 9  is a block diagram illustrating an embodiment where the electric field emission system according to the present invention is applied to an X-ray source. The field emission X-ray source  300  includes an electric field emission X-ray device  310  and a current controlling device  320 . 
         [0091]    The field emission X-ray device  310  includes a cathode  311 , an anode  312 , a gate  313 , a first focusing electrode  314   a  and a second focusing electrode  314   b.    
         [0092]    The cathode  311  includes a Carbon Nanotube (CNT) emitter. When a great voltage difference occurs between the cathode  311  and the anode  212 , or the cathode  311  and the gate  312 , electrons are emitted from the CNT emitter. The electrons emitted from the cathode  311  pass the gate  313 , the first focusing electrode  314   a  and the second focusing electrode, and then are focused on the anode  312 . 
         [0093]    The gate  313  is a plate having a mesh type where a plurality of gate holes are formed. The gate  313  induces electron emission from the cathode  311 . Also, the first focusing electrode  314   a  and the second focusing electrode  314   b  prevent diffusion of the electrons emitted from the cathode  311  and induce the electrons to be focused on the anode  312 . In the anode  312 , an X-rays is generated by the focused electrons which are emitted from the cathode  313  and focused. 
         [0094]    When the field emission X-ray source  300  operates, it is important to maintain a magnitude of the generated X-ray to be constant. However, on operation of the field emission X-ray source  300 , a distance between the cathode  312  and the gate  313  is not maintained to be constant and varied due to physical vibration of the gate  313  of the field emission X-ray device  310 . The magnitude of the X-ray varies in response to the variation of the distance between the cathode  312  and the gate  313 . 
         [0095]    The field emission X-ray source  300  according to the present invention may control the field emission current flowing through the field emission X-ray module  310  using the current controlling device  320 . The field emission X-ray source  300  may maintain magnitude of the X-ray generated from the field emission X-ray device  310  to be constant by maintaining the field emission current to be constant. 
         [0096]    The current controlling device  320  includes a first current controlling transistor  321 , a second current controlling transistor  322  and a control logic  323 . 
         [0097]    As described above, the control logic  323  may control the gate voltages of the first and second current controlling transistors  321  and  322 . The control logic  323  may adjust or limit a current level of the field emission current using the first gate voltage V g1  provided to the first current controlling transistor  321 . Also, the control logic  323  may maintain the field emission current to be constant using the first and second current controlling transistors  321  and  322 . 
         [0098]    The current controlling device  320  according to the present invention may adjust or limit the field emission current. Also, the current controlling device  320  may maintain the magnitude of the X-ray generated in the field emission X-ray source  310  to be constant by maintaining a level of the field emission current to be constant. 
         [0099]      FIG. 10  is a graph illustrating operation results of the field emission X-ray source  300  to which the present invention is applied and an electric field emission X-ray source to which the present invention is not applied. Referring to  FIG. 10 , it can be seen that the magnitude of X-ray of a typical field emission X-ray source vibrates as the time changes, while the magnitude of X-ray of the X-ray emission X-ray source according to the present invention is maintained to be constant. 
         [0100]    According to the current controlling device and the electric field emission system including the same according to the present invention, the field emission current can be maintained to be constant. Also, a level of the field emission current, which is maintained to be constant, can be set to a desired value. 
         [0101]    The above-disclosed subject matter is to be considered illustrative, and not restrictive, and the appended claims are intended to cover all such modifications, enhancements, and other embodiments, which fall within the true spirit and scope of the present invention. Thus, to the maximum extent allowed by law, the scope of the present invention is to be determined by the broadest permissible interpretation of the following claims and their equivalents, and shall not be restricted or limited by the foregoing detailed description.