Abstract:
A field emission RF amplifier. The field emission RF amplifier includes one or more RF amplification units on a substrate and held in a vacuum state and facing a reflection electrode. The RF amplification unit includes a cathode electrode, gate electrode, and an anode electrode all formed on the same substrate. The cathode electrode has a CNT emitter. A DC voltages are applied to the cathode and anode electrodes. An RF signal is input at the cathode electrode and is amplified and output at the anode electrode. Capacitors and inductors are arranged to filter out AC and DC components where needed. An improved amplification of RF signals with high electron mobility and good impedance matching abilities result.

Description:
CLAIM OF PRIORITY 
   This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from an application for FIELD EMISSION RF AMPLIFIER earlier filed in the Korean Intellectual Property Office on 14 Feb. 2004 and there duly assigned Serial No. 2004-9839. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a field emission RF (Radio Frequency) amplifier, and more particularly, to a RF amplifier in which a field emission device carries out amplification. 
   2. Description of the Related Art 
   An RF amplifier receives a RF signal through an RF input terminal in the state that a predetermined DC bias voltage is supplied, and amplifies an amplitude of the RF signal to an RF output terminal. For such an RF amplification, the RF amplifier includes an RF amplification unit located between the RF input terminal and the RF output terminal. 
   A transistor can be the RF amplifier. However, since the transistor uses solid silicon, the transistor has poor electron mobility. Accordingly, a vacuum tube allowing electron transfer in a vacuum state can be substituted for the transistor, however, a problem exists that a conventional vacuum tube is too large in volume. 
   Studies for using a field emission device as the amplification unit in a vacuum panel are currently underway. An example of performing RF amplification using a silicon field emitter array is disclosed in “Silicon Field Emitter Arrays with Low Capacitance and Improved Transconductance for Microwave Amplifier Applications”, by D. Palmer, et al., J.Vac.Sci, Techno.B 13(2), Mar/Apr 1995, pp. 576–579. 
   However, since the FEA (Field Emission Array) structure used for RF amplification uses a gate insulating layer with 4 μm thickness consisting of silicon oxide, it is difficult to match standard input/output resistance of 50Ω of an RF device due to the increase of capacitance between the gate and the cathode. Also, such an increase of capacitance reduces an output current, thus deteriorating an amplification effect of an RF signal. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to provide an improved field emission RF amplifier. 
   It is also an object of the present invention to provide an field emission RF amplifier with an improved amplification effect. 
   It is further an object of the present invention to provide a field emission RF amplifier that has improved impedance matching. 
   These and other objects can be achieved by a field emission RF amplifier that facilitates impedance matching with a different RF terminal by arranging a vacuum field emission device as a planar type, and improving an RF amplification effect. The RF amplifier includes an RF amplification unit on a substrate, the RF amplification unit includes a cathode electrode formed on the substrate, a gate electrode formed on the substrate at a side of the cathode electrode and separated from the cathode electrode by a predetermined distance, an anode electrode formed on the substrate at a side of the gate electrode opposite from that of the cathode electrode, an electron emission source located on the cathode electrode, the electron emission source emitting electrons by an electric field, a reflection electrode formed over the substrate and in parallel with the substrate, an RF signal input terminal and a DC cathode bias voltage being electrically connected to the cathode electrode and an RF output terminal and a DC anode bias voltage being electrically connected to the anode electrode. 
   A vacuum space is formed between the reflection electrode and the substrate with the cathode electrode, the gate electrode and the anode electrode formed therein. A negative voltage is applied to the reflection electrode so as to reflect electrons output from the electron emission source toward the anode electrode. The anode electrode, the gate electrode, and the cathode electrode are located on a same surface of the substrate. The electron emission source is CNT (carbon nanotube). 
   According to another aspect of the present invention, there is provided an RF amplifier which includes a plurality of RF amplification units on a substrate, each of the plurality of RF amplification units includes a cathode electrode formed on the substrate, a gate electrode formed on the substrate at a side of the cathode electrode and separated from the cathode electrode by a predetermined distance, an anode electrode formed on the substrate at a side of the gate electrode opposite from the cathode electrode, an electron emission source located on the cathode electrode, the electron emission source emitting electrons by an electric field, and a reflection electrode formed over the substrate and in parallel with the substrate, an RF input signal and a DC cathode bias voltage are electrically connected to the cathode electrode and a DC anode bias voltage electrically connected to the anode electrode and serving as an RF output signal. The RF amplification units are connected in serial to each other with a capacitor is located between the RF amplification units. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
       FIG. 1  is a sectional view illustrating a schematic structure of a RF amplification unit used in a field emission RF amplifier according to a preferred embodiment of the present invention; 
       FIG. 2  is a simulation of an operation of the RF amplification unit of  FIG. 1 ; 
       FIG. 3  is a schematic plan view of an RF amplifier according to an embodiment of the present invention; 
       FIG. 4  is an equivalent circuit diagram of  FIG. 3 ; 
       FIG. 5  is a schematic plan view of an RF amplifier according to another embodiment of the present invention; and 
       FIG. 6  is an equivalent circuit diagram of  FIG. 5 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Turning now to the figures,  FIG. 1  is a sectional view illustrating a schematic structure of an RF amplification unit used in a field emission RF amplifier according to a preferred embodiment of the present invention. Referring to  FIG. 1 , an anode electrode  140 , a gate electrode  130  and a cathode electrode  120  are separated and located by a predetermined distance on a substrate  110  in an RF amplifier. An electron emission source, for example, a CNT (carbon nanotube) emitter  121  is formed on the cathode electrode  120 . A reflection electrode  150  is separated and located a predetermined distance from the substrate  110 . Wall bodies  160  are formed between edges of the reflection electrode  150  and the substrate  110 , so that a vacuum space is formed therein. 
   As a method for vacuum-sealing the vacuum space, a hot outgassing method for a conventional flat panel display can be utilized. Also, it is possible to form a vacuum space by putting a getter material for gas absorption, such as ST 122 , on the substrate  110  and/or the reflection electrode  150 . 
   The substrate  110  can be made of an insulating material, for example, an alumina or a quartz. The anode electrode  140 , the gate electrode  130  and the cathode electrode  120  may be formed with a thickness of 0.25 μm using conductive materials such as ITO (indium tin oxide) or Cr. A gap of 50 μm is formed between the gate electrode  130  and the cathode electrode  120 . Each of the anode electrode  140 , the gate electrode  130  and the cathode electrode  120  can be formed to have a width from 500 to 900 μm. This 500 to 900 micron width allows for impedance matching with an RF microstrip circuit. 
   The CNT emitter  121  can be formed by screen printing and baking. Alternatively, it is possible to form a CNT catalyst metal on the cathode electrode  120 , then flow a carbon-containing gas to grow CNT on the CNT catalyst metal. 
   Turning now to  FIG. 2 ,  FIG. 2  is a simulation view illustrating an operation of the field emission RF amplifier of  FIG. 1 . Referring to  FIGS. 1 and 2 , the anode electrode  140 , the gate electrode  130 , and the cathode electrode  120  are located on the same plane and designed to each have a width of 900 μm, 500 μm and 500 μm respectively. A gap between the anode electrode  140  and the gate electrode  130  and a gap between the gate electrode  130  and the cathode electrode  120  are designed to be 800 μm and 50 μm, respectively. The lengths of the electrodes are designed to be 1 mm. The reflection electrode  150  is separated by 1.1 mm from lower electrodes  120 ,  130 , and  140 . 
   Also, DC voltages of 1.5 kV, −130 V and −140 V are applied respectively to the anode electrode  140 , the cathode electrode  120  and the reflection electrode  150 , and the gate electrode  130  is grounded. Preferably, a negative voltage greater than that applied to the cathode electrode  120  is applied to the reflection electrode  150 . Electrons extracted from the cathode electrode  120  by the gate electrode  130  are curved toward the gate electrode  130  and change their directions by the negative voltage applied to the reflection electrode  150 , causing the electrons to collide with the anode electrode  140  held at a strong voltage. 
   Turning now to  FIG. 3 ,  FIG. 3  is a schematic plan of an RF amplifier according to an embodiment of the present invention. Substantially the same components as those in the embodiment are denoted by the same reference numbers throughout the drawings, and detailed descriptions therefor are omitted. 
   Referring to  FIG. 3 , the RF amplifier includes a RF amplification unit. The RF amplification unit consists of electrodes  120 ,  130  and  140  on a substrate  110 , a reflection electrode  150  located above and over the electrodes  120 ,  130  and  130  in a manner to be opposite to the electrodes  120 ,  130  and  140 , and wall bodies ( 160  of  FIG. 1 ) formed between edges of the reflection electrode  150  and the substrate  110  and used to seal the space between the substrate  110  and the reflection electrode, allowing an inner space of the RF amplification unit to be maintained in a vacuum state. 
   An RF signal is received through an RF input terminal  210  and is input to one end of the cathode electrode  120 . A capacitor C 1  is used to filter out and prevent any DC voltage component from the RF input terminal  210  from reaching the cathode electrode  120 . Also, an external DC bias cathode voltage −Vc is applied to the cathode electrode  120  via an inductor L 1 . The inductor L 1  filters out and prevents any alternating component of the −Vc voltage source from reaching the cathode electrode  120 . As illustrated in  FIG. 3 , the gate electrode  130  is electrically grounded. 
   A DC bias anode voltage +Va is applied to the anode electrode  140  via inductor L 3 . The inductor L 3  filters out any alternating component from the +Va voltage source from reaching the anode electrode  140 . An RF signal amplified by the anode electrode  140  is input to an RF output terminal  220  through a capacitor C 3 . The capacitor C 3  prevents any DC voltage component from flowing to the RF output terminal  220  from the anode electrode  140 . Meanwhile, capacitors C 4  and C 5  are blocking capacitors for preventing an RF signal transferred from the RF input terminal  210  from leaking out. The capacitors C 4 , C 5  and inductors L 1 , L 3  corresponding to the capacitors C 4 , C 5  respectively form low-frequency filters. 
   Operations of the RF amplifier with the above-described structure will now be described. First, an RF signal is received through the RF input terminal  210  and is transferred to the cathode electrode  120  and mixed with a DC bias cathode voltage −Vc at the cathode electrode  120 . Then, electrons are emitted from a CNT emitter  121  on the cathode electrode  120  by the gate electrode  130 . The path of the emitted electrons is curved toward the gate electrode  130 . 
   At this time, the electrons are reflected (or pushed away) from the reflection electrode  150  by a negative voltage applied to the reflection electrode  150 , and the electrons then proceed to the anode electrode  140 . At this time, a current variation width of the RF signal at the anode electrode  140  increases according to a voltage difference between the cathode voltage −Vc and a gate voltage (ground). That is, if the gate electrode  130  is grounded and a DC voltage of −120 through −140 V is applied to the cathode electrode  120 , a current variation width ΔI of the RF signal at the anode electrode  140  increases by the electrons colliding at the anode electrode  140 , so that the RF signal has a voltage variation width ΔV as a multiplication of the current variation width ΔI by impedance of the anode electrode  140  being a metal strip. That is, since the voltage variation width of the RF signal received through the RF input terminal  210  increases at the anode electrode  140 , the RF signal is amplified. 
   The capacitor C 1  removes any DC component of the RF signal received through the RF input terminal  210 . Meanwhile, inductors L 1  and L 3  allow the DC bias voltage from the cathode bias voltage −Vc and the anode bias voltage +Va to pass through while blocking any alternating component. The capacitors C 4  and C 5  block the RF signal at the cathode electrode  120  and the anode electrode  140  respectively from leaking out through the inductors L 1  and L 3  respectively. 
   Turning now to  FIG. 4 ,  FIG. 4  is an equivalent circuit diagram of  FIG. 3 . Referring to  FIG. 4 , reference numerals Z 1  and Z 2  indicate impedances created by capacitance and inductance of capacitor and inductor located at transmission paths of bias DC voltage connected to the corresponding electrode, respectively. The impedance can be created to approach standard impedance required for impedance matching of the RF circuit due to the reduction of capacitance. 
     FIG. 5  is a schematic plan view of an RF amplifier according to another embodiment of the present invention. The substantially same components as those in the embodiment are denoted by the same reference numbers throughout the drawings, and detailed descriptions therefor are omitted. 
   Referring to  FIG. 5 , the RF amplifier includes two RF amplification units (first RF amplification unit and second RF amplification unit). The first amplification unit has a cathode electrode  120   a,  a gate electrode  130   a  and an anode electrode  140   a.  The second amplification unit also has a cathode  120   b,  a gate electrode  130   b  and an anode electrode  140   b.  These six electrodes are on a substrate  110  with a reflection electrode  150  separated from substrate  110  by wall bodies ( 160  of  FIG. 1 ) for sealing which are formed between edges of the reflection electrode  150  and the substrate  110 . The inner space of each RF amplification unit is maintained in a vacuum state. 
   A capacitor C 2  is placed between the two RF amplification units to filter out any DC bias voltage from the RF signal output from the first RF amplification unit, passing only the alternating component to the second RF amplification unit. 
   An RF signal received through the RF input terminal  210  is input to the cathode electrode  120   a  of the first amplification unit. The amplified RF signal from the first RF amplification unit is input to the cathode electrode  120   b  of the second amplification unit. A capacitor C 1  is used to filter out any DC component from the received signal at the RF input terminal  210  before the signal is sent to the cathode terminal  120   a  of the first amplification unit. The capacitor C 2  filters out any DC bias from the anode electrode  140   a  of the first amplification unit before it reaches the cathode electrode  120   b  of the second amplification unit. Also, an external DC bias cathode voltage −Vc is applied to each cathode electrode  120   a ,  120   b  through inductors L 1  and L 2  respectively. The inductors L 1  and L 2  filter out any alternating component from the cathode voltage source −Vc before it reaches a cathode electrode. 
   As can be seen in  FIG. 5 , each gate electrode  130   a  and  130   b  is grounded. Also, a DC bias anode voltage Va is applied to each the anode electrode  140  through inductors L 3  and L 4 . The inductors L 3  and L 4  prevent an alternating component from being input to the anode electrode  140 . An RF signal passing through the second RF amplification unit and amplified at the anode electrode  140   b  is input to an RF output terminal  220  via a capacitor C 3 . The capacitor C 3  prevents a DC bias voltage from flowing out to the RF output terminal. 
   Meanwhile, capacitors C 4  and C 5  are blocking capacitors that prevent the RF signal received through the RF input terminal  210  or the RF signal input to the second amplification unit from leaking out. The capacitors C 4 , C 5  and corresponding inductors L 1 , L 2 , L 3 , and L 4  form low-frequency filters. 
   The operation of the RF amplifier with the above-described structure will now be described. First, an RF signal received from the RF input terminal  210  is transferred to the cathode electrode  120   a  and mixed with a DC bias cathode voltage −Vc at the cathode electrode  120   a . Successively, electrons are emitted from a CNT emitter  121   a  on the cathode electrode  120   a  by the gate electrode  130   a  and the emitted electrons are curved toward the gate electrode  130   a.  At this time, the electrons are reflected from the reflection electrode  150  by a negative voltage applied to the reflection electrode  150 , and proceed to the anode electrode  140   a.  At this time, a current variation width of the RF signal in the anode electrode  140   a  increases according to a voltage difference between the cathode voltage −Vc and a gate voltage. That is, if the gate voltage  130   a  is grounded and a DC voltage of −120 through −140 V is applied to the cathode electrode  120   a,  a current variation width ΔI of the RF signal in the anode electrode  140   a  increases by electrons colliding at the anode electrode  140   a,  so that the RF signal has a voltage variation width ΔV as a multiplication of the current variation width ΔI by impedance of the anode electrode  140   a  being a metal strip. That is, since the voltage variation width of the RF signal received through the RF input terminal  210  increases at the anode electrode  140   a,  the RF signal is amplified. 
   Successively, the amplified RF signal at the anode electrode  140   a  passes the capacitor C 2  so that any DC component is blocked. Accordingly, a first amplified RF signal is input to the cathode electrode  120   b  of the second RF amplification unit. The RF signal input to the second RF amplification unit is amplified by the second RF amplification unit in the above-described manner and input to the capacitor C 3 . Therefore, the RF signal whose DC component is removed after passing through the capacitor C 3  is output to the RF output terminal  220 . 
   The capacitor C 1  removes a DC component of the RF signal received from the RF input terminal  210 . Meanwhile, inductors L 1  through L 4  pass a DC bias voltage of the cathode bias voltage −Vc or the anode bias voltage +Va and block any alternating component. Capacitors C 4  and C 5  block an RF signal at the cathode electrode  120  and the anode electrode  140  from leaking out through the inductors. 
   Turning now to  FIG. 6 ,  FIG. 6  is an equivalent circuit diagram of  FIG. 5 . Referring to  FIG. 6 , reference numbers Z 1  through Z 4  indicate impedances formed by capacitances and inductances of capacitors and inductors located at transmission paths of bias DC voltages connected to the corresponding electrode, respectively. The impedance can be made to approach standard impedance required for impedance matching of the RF circuit due to the reduction of capacitance. 
   In the above embodiment, an RF amplifier including two RF amplification units has been described, however, the present invention is not limited thereto. That is, an RF amplifier including three or more RF amplification units can be implemented. 
   As described above, since a field emission RF amplifier according to the present invention is a planar type field emission device in a vacuum state as an RF amplification unit, it is possible to increase electron mobility in the amplification unit and improve an amplification effect. Also, by arranging electrodes forming a field emission device on a plane, it is possible to reduce capacitance between a gate electrode and a cathode electrode and accordingly easily create standard impedance. 
   While the present invention has been particularly illustrated and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.