Patent Publication Number: US-8120015-B2

Title: Resonant structure comprising wire and resonant tunneling transistor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. §119(a) of Korean Patent Application No. 10-2008-0007186, filed on Jan. 23, 2008, in the Korean Intellectual Property Office, the entire disclosures of both of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a resonant structure comprising a wire, a resonant tunneling transistor, and a method for fabricating the resonant structure. More particularly, the present invention relates to a resonant structure which provides a greater oscillation, a resonant tunneling transistor, and a method for fabricating the resonant structure. 
     2. Description of the Related Art 
     Due to the development of electronic technology, diverse types of microminiaturized portable devices have become widely used. Such microminiaturized portable devices require microminiaturized components. 
     In order to implement microminiaturized and ultra-light components, Micro Electro-Mechanical Systems (MEMS), which include mechanical or electromechanical devices having a microminiaturized structure which can be measured in micrometers, or Nano Electro-Mechanical Systems (NEMS), which include mechanical or electromechanical devices having a microminiaturized structure which can be measured in nanometers, have become increasingly used. NEMS are electromechanical systems which may be one hundred-thousandth the size of a human hair, which transform an electric signal into a mechanical movement, or inversely a mechanical movement into an electric signal. 
     A microminiaturized resonator using MEMS or NEMS technology can be used as a component of a filter or a duplexer in portable communication devices for radio frequency (RF) communication. 
     A conventional MEMS resonator needs a high force constant in order to generate RF signals of over 1 gigahertz, and has difficulty in tuning. 
     A conventional NEMS resonator has low oscillation, so only electric signals of low size are output. Accordingly, additional devices such as amplifiers have to be used along with the conventional NEMS resonator. In addition, the conventional NEMS resonator also has difficulty in tuning. 
     SUMMARY OF THE INVENTION 
     An aspect of embodiments of the present invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of embodiments of the present invention is to provide a microminiaturized resonant structure which provides a higher output property, a resonant tunneling transistor, and a method for fabricating the resonant structure. 
     Another aspect of embodiments of the present invention is to provide a microminiaturized resonant structure which provides a higher output property and is easy for tuning, a resonant tunneling transistor, and a method for fabricating the resonant structure. 
     In order to achieve the above-described and other aspects of embodiments of the present invention, a resonant structure is provided including a first terminal, a second terminal which faces the first terminal, a wire unit which connects the first terminal and the second terminal, a third terminal which is spaced a part at a certain distance from the wire unit, and which resonates the wire unit, and a potential barrier unit which is formed on the wire unit and which provides a negative resistance component. 
     The potential barrier unit may include a plurality of potential barriers which are formed on the wire unit to be spaced apart from each other. 
     The wire unit may be a nano-wire having a section with a circular or polygonal shape. 
     The wire unit may be formed of silicon (Si), and wherein the potential barrier unit may include a plurality of potential barriers which are formed of silicon-germanium (SiGe), and a well area which is formed between the plurality of potential barriers. 
     The resonant structure may further include a magnetic field generation unit which generates a magnetic field around the wire unit. 
     The resonant structure may further include a magnetic substance which is formed on part of the wire unit, and which displaces the wire unit reacting to the magnetic field. 
     In order to achieve the above-described and other aspects of embodiments of the present invention, a resonant tunneling transistor is provided, including a drain part, a source part which faces the drain part, a wire unit which connects the drain part and the source part, a gate part which is spaced apart at a certain distance from the wire unit, and which generates resonant tunneling between the drain part and the source part by resonating the wire unit, and a potential barrier unit which increases an electric current flowing between the drain part and the source part by providing a negative resistance component when the resonant tunneling is generated. 
     The wire unit may be formed of Si, and wherein the potential barrier unit may includes a plurality of potential barriers which may be formed of SiGe, and a well area which is formed between the plurality of potential barriers. 
     The resonant tunneling transistor may further include a magnetic field generation unit which generates a magnetic field around the wire unit, and a magnetic substance which is formed on part of the wire unit, and displaces the wire unit reacting to the magnetic field. 
     In order to achieve the above-described and other aspects of embodiments of the present invention, a method for fabricating a resonant structure is provided, including forming a plurality of potential barriers by doping a surface of a plurality of semiconductor layers which are sequentially disposed, forming a plurality of terminals which face each other by doping an area of the first surface wherein the plurality of potential barriers are formed between the plurality of terminals, forming a wire unit including the plurality of potential barriers by patterning an area where the plurality of terminals are not formed, and ensuring a space for resonating the wire unit by etching the semiconductor layer under the wire unit. 
     The step of etching the semiconductor layer under the wire unit may include forming a terminal which is spaced apart at a certain distance from the wire unit by etching the semiconductor layer which contacts the wire unit from among the plurality of semiconductor layer. 
     The method may further include forming a magnetic field generation unit on one side of the wire unit. 
     The method may further include coating a magnetic substance on part of the wire unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a mimetic diagram illustrating a resonant structure according to an embodiment of the present invention; 
         FIG. 2  is a mimetic diagram illustrating a resonant tunneling transistor according to an embodiment of the present invention; 
         FIG. 3  is a graph illustrating features of an electric current of a resonant structure; 
         FIGS. 4A to 4F  illustrate a method for fabricating a resonant structure according to an embodiment of the present invention; 
         FIGS. 5A to 5F  are vertically-sectioned view corresponding to  FIGS. 4A to 4F ; and 
         FIG. 6  is a circuit diagram illustrating a circuit using a resonant tunneling transistor according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE PRESENT INVENTION 
     Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures. 
       FIG. 1  is a mimetic diagram illustrating a resonant structure according to an embodiment of the present invention. As shown in  FIG. 1 , the resonant structure includes a plurality of terminals  110 ,  120  and  150 , a wire unit  130 , and a potential barrier unit  140 . 
     Among the plurality of terminals  110 ,  120  and  150 , a first and second terminal  110  and  120  are spaced apart to face each other. One of the first and second terminal  110  and  120  may be connected to a bias power, and the other may be connected to a ground power. For convenience of description, it is assumed that the first terminal  110  is connected to a bias power hereinafter. 
     The wire unit  130  is formed between the first terminal  110  and the second terminal  120 . The wire unit  130  may be implemented as a wire which directly connects the first terminal  110  to the second terminal  120 , in particular, as a nano-wire having a thickness and a length of the nanometer unit. A section of the wire unit  130  may have a circular shape or a polygonal shape such as a quadrangle. 
     The third terminal  150  is disposed at one side of the wire unit  130 . The third terminal  150  and the wire unit  130  are spaced apart at a certain distance, so a space for oscillating the wire unit  130  is ensured. If an external control power (not shown) is provided, the third terminal  150  biases the wire unit  130 , so resonance occurs. 
     The potential barrier unit  140  is formed on the wire unit  130 , and includes a plurality of potential barriers  141   a  and  141   b , and a well area  142  which is formed between the potential barriers  141   a  and  141   b.    
     The potential barrier unit  140  forms a potential well in an energy band gap when the wire unit  130  is resonated. The potential well facilitates resonant tunneling between the first terminal  110  and the second terminal  120 . That is, the potential barrier unit  140  provides a negative resistance component while the wire unit  130  is resonated, so the current flowing between the first terminal  110  and the second terminal  120  sharply increases. As a result, since the transduction efficiency which detects electric signals from oscillation is improved, the problem that a conventional NEMS resonator provides a low output property resulting from low oscillation can be solved. The operation of the potential barrier unit  140  will be described in greater detail below. 
       FIG. 2  illustrates a resonant structure according to another embodiment of the present invention. In this embodiment, in addition to the resonant structure of  FIG. 1 , a magnetic field generation unit  160  is added to one side of the third terminal  150 , and a magnetic substance  170  is added to the wire unit  130 . 
     The resonant structure may be used as a resonant tunneling transistor. In this case, the first terminal  110  may be a drain part, the second terminal  120  may be a source part, and the first terminal  150  may be a gate part. In  FIG. 2 , the resonant structure which is used as a resonant tunneling transistor is described as an example. 
     The gate part  150  supports the drain part  110  and the source part  120 . More specifically, a support layer  151  is disposed to be spaced apart on the gate part  150 , and the drain part  110  and the source part  120  are formed on each of the support layer  151 . 
     The wire unit  130  which connects the drain part  110  and the source part  120  is formed on the gate part  150 . As shown in  FIG. 2 , the wire unit  130  is formed to be spaced apart from the gate part  150  by completely removing the support layer  151 . Alternatively, if a space for oscillating the wire unit  130  can be ensured, it is also possible to form the wire unit  130  by etching a part of the support layer  151  in order for the support layer  151  not to contact the wire unit  130  instead of completely removing the support layer  151 . In addition, the support layer  151  is disposed to form a space between the wire unit  130  and the gate part  150 , so if the drain part  110  and the source part  120  have an appropriate thickness, the support layer  151  can be omitted. 
     If the wire unit  130  is formed of silicon (Si) as an example in  FIG. 2 , the entire wire unit  130  consists of a silicon area, a first potential barrier  141   a , a silicon area  142 , a second potential barrier  141   b , a silicon area, a magnetic substance  170 , and a silicon area, in sequence. The first potential barrier  141   a  and the second potential barrier  141   b  are formed of silicon-germanium (SiGe). 
     The silicon area  142  between the first potential barrier  141   a  and the second potential barrier  141   b  operates as a well area, so the silicon area  142  forms the potential barrier unit  140  together with the first potential barrier  141   a  and the second potential barrier  141   b . n +  silicon connection units  131   a  and  131   b  may be formed at both ends of the wire unit  130  to be connected to the drain part  110  and the source part  120 , respectively. 
     If the wire unit  130  has the structure of  FIG. 2 , when a particular drain bias is provided to the drain part  110 , a chemical potential of the source part  120  forms a potential well by being arranged with a quantum energy level of the well area  142  in the potential barrier unit  140  on an energy band gap. A negative resistance caused by the potential well, that is, a negative differential resistance (NDR) is formed. In this case, a sharply current increase/decrease occurs in the NDR peak. 
     Since the wire unit  130  is formed to float in the air, the wire unit  130  is oscillated according to the natural frequency, so the capacity of electrostatic coupling between the gate part  150  and the well area  142  changes, so the location of the NDR peak changes. Therefore, if the bias condition is adapted to area around the NDR peak by adjusting the voltage applied to the gate part  150  and the bias power applied to the drain part  110 , the oscillation of the wire unit  140  increases. As a result, a high output of electric current can be obtained. Therefore, a resonant structure having a high output property without a separate amplifier can be implemented. 
     When the wire unit  130  is located above a wide plate such as the gate part  150  as shown in  FIG. 2 , the gate electrostatic capacity and the quantity of location change are expressed according to the following mathematical function. 
     
       
         
           
             
               
                 
                   
                     
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     In Mathematical function 1, C G  indicates a gate electrostatic capacity, Z indicates the quantity of the change of the wire unit  130 , L indicates the length of the wire unit  130 , and r indicates a distance between the wire unit  130  and the gate part  150 . 
     The change of the gate voltage applied to the gate part  150  and the change of the magnetic field generated by the magnetic field generation unit  160  commonly affect the wire unit  130 , so the oscillation displacement δz and the resultant change of electrostatic capacity δC G  occur as shown in Mathematical function 1. Consequently, the oscillation of the wire unit  130  can be adjusted. 
     In the exemplary embodiment of  FIG. 2 , the magnetic field generation unit  160  and the magnetic substance  170  have been added. The magnetic field generation unit  160  may be implemented as an electromagnet which generates a magnetic field around the wire unit  130 . 
     If the electricity is applied to the magnetic field generation unit  160 , a magnetic field is generated. The magnetic field gives the action or repulsion to the magnetic substance  170  on the wire unit  130 . 
     The magnetic substance  170  on the wire unit  130  reacts to the magnetic field, so the wire unit  130  is displaced. As a result, the wire unit  130  which is being oscillated according to the natural frequency changes to have a higher frequency width due to interaction between the magnetic substance  170  and the magnetic field generation unit  160 , so frequency tuning is performed. The resonant structure according to an embodiment of the present invention generates a magnetic field around the wire unit  130 , so the natural frequency of the wire unit  130  can be tuned simply and efficiently. Therefore, the resonant structure according to an embodiment of the present invention has tuning higher than a conventional MEMS resonator or a conventional NEMS resonator, and so can be applied to a tunable RF system. The resonant structure of  FIG. 2  can be implemented using the any of the MEMS and NEMS technology. 
       FIG. 3  is a graph illustrating V DS -I feature of a resonant structure or a resonant tunneling transistor according to an embodiment of the present invention. 
     Referring to  FIG. 3 , the current I changes according to the voltage V DS  between the drain part  110  and the source part  120 . The current I increases sharply and peaks at a particular voltage V DS , decreases, and gradually increases again. 
     If the potential of the well area  142  changes by changing the gate voltage V G  applied to the gate part  150 , the quantum energy level changes, so the location of the drain bias where the NDR peak occurs changes. Therefore, when a particular gate voltage is fixed at the voltage V G , the output current I can peak by adjusting V DS  to V DS1 . 
     Furthermore, as the gate voltage V G  applied to the gate part  150  changes to V G +ΔV G   1 , V G +ΔV G   2 , or V G +ΔV G   3 , the waveform of the output current I changes. Accordingly, if the current bias voltage V DS  is fixed at V DS3 , the output current I can peak by adjusting the gate voltage V G  to V G +ΔV G   2 . 
     The features of the current and voltage of the resonant tunneling transistor can be expressed according to the following mathematical function of the drain voltage V DS  and the gate electric charge Q G .
 
 I   DS   =I   DS ( V   DS   , Q   G )  [Mathematical function 2]
 
     In Mathematical function 2, Q G  indicates C G V G , C G  indicates the gate electrostatic capacity, and V G  indicates the gate voltage. 
     The quantity of the change of the output current I DS  can be expressed according to the following mathematical function 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     In Mathematical function 3, δV G  indicates the input RF or MW signal, δI DS  indicates the output RF or MW signal, and δV DS  indicates another output RF or MW) signal for frequency mixing. Comparing a general resonant structure having a semiconductor wire which is similar to  FIG. 2  with the resonant structure of  FIG. 2  having the potential barrier unit  140 , the change δC G  of the gate electrostatic capacity due to mechanical resonance in the resonant structure of  FIG. 2  has the same value. However, as shown in  FIG. 3 , since the value ∂I DS /∂Q G  of the resonant structure of  FIG. 2  is higher than that of the general resonant structure, the entire output signal becomes higher. 
       FIGS. 4A to 4F  illustrate a method for fabricating a resonant structure according to an embodiment of the present invention, and  FIGS. 5A to 5F  are vertically-sectioned view corresponding to  FIGS. 4A to 4F . 
     Referring to  FIGS. 4A and 5A , a plurality of semiconductor layers are sequentially disposed. From the top, a first layer  100 , a support layer  151 , and a second layer  150  are disposed. The first layer  100  forms a first terminal  110 , a second terminal  120 , and a wire unit  130 . The second layer  150  forms a third terminal  150  by patterning process. 
     Subsequently, doping is performed on one surface of the plurality of semiconductor layers. In more detail, a plurality of potential barriers  141   a  and  141   b  are formed by doping one surface of the first layer  100 . The first layer  100  is formed of a silicon, and the potential barriers  141   a  and  141   b  are doped with SiGe. A silicon area between the potential barriers  141   a  and  141   b  forms a well area  142 . 
     Next, as shown in  FIGS. 4B and 5B , a mask layer  180  is formed on the first layer  100  by lithography process. The mask layer  180  is formed on the surface of the first layer  100  excluding the first terminal  110  and the second terminal  120 . 
     Subsequently, as shown in  FIGS. 4C and 5C , doping is performed using n + . Only part of the first layer  100  on which the mask layer  180  is not formed is doped, so the first terminal  110  and the second terminal  120  are symmetrically formed. 
     Then, as shown in  FIGS. 4D and 5D , a mask layer  190  is formed to cover the first terminal  110  and the second terminal  120  by lithography process. In this case, the mask layer  190  is formed to have a pattern corresponding to the wire unit  130  between the mask layer  190  covering the first terminal  110  and the mask layer  190  covering the second terminal  120 . 
     Subsequently, as shown in  FIGS. 4E and 5E , a pattern of the wire unit  130  including the plurality of potential barriers  141   a  and  141   b  is formed by performing etching process using the mask layer  190 . During this process, part of the support layer  151  which is not covered by the first terminal  110 , the second terminal  120 , and the pattern of the wire unit  130  is also etched, so the third terminal  150  is exposed upwards. 
     In the process of  FIGS. 4D and 5D , the first terminal  110  and the second terminal  120  may partially be exposed towards the wire unit  130  instead of being completely covered by the mask layer  190 . In this case, if the etching process is performed as shown in  FIGS. 4E and 5E , both ends of the pattern of the wire unit  130  are formed of the same material as the first terminal  110  and the second terminal  120 . That is, n +  silicon connection units  131   a  and  131   b  are formed at both ends of the pattern of the wire unit  130 . 
     Subsequently, as shown in  FIGS. 4F and 5F , the support layer  151  under the wire unit  130  is etched, so the wire unit  130  is spaced apart from the third terminal  150 . Consequently, a space  152  for oscillating the wire unit  130  can be ensured. 
     Following this method for fabricating a resonant structure, the magnetic field generation unit  160  and the magnetic substance  170  as shown in  FIG. 2  can also be formed. In the process of  FIGS. 4E and 5E , the third terminal  150  can be divided in two by etching part of the third terminal  150 . One of them can function as the magnetic field generation unit  160 . Alternatively, after completing the process of  FIGS. 4F and 5F , the magnetic field generation unit  160  can separately be formed on one side of the wire unit  130 . 
     In addition, the magnetic substance  170  can be formed by coating part of the first layer  100  with a magnetic material during the process of  FIGS. 4A and 5A , or can also be formed by doping process. 
     In the resonant structure fabricated in the method for fabricating a resonant structure as shown in  FIGS. 4A to 4F  and  FIGS. 5A to 5F , the wire unit  130  has the same thickness as the first terminal  110  and the second terminal  120 . However, in embodiments as shown in  FIGS. 1 and 2 , the wire unit  130  has the thickness thinner than the first terminal  110  and the second terminal  120 . This is the structural difference due to those embodiments. 
     For example, during the etching process of  FIGS. 4F and 5F , if a lower part of the wire unit  130  is etched with the support layer  151 , the wire unit  130  can be formed thinly as shown in  FIGS. 1 and 2 . Alternatively, in the process of disposing the plurality of semiconductor layers, the wire unit  130  can be formed thinner than the first terminal  110  and the second terminal  120  by disposing a sacrifice layer under an area where the wire unit  130  is formed and etching the sacrifice layer later. 
     The lithography process and etching process used in this method for fabricating a resonant structure are conventionally used processes. 
       FIG. 6  is a circuit diagram illustrating a circuit using a resonant tunneling transistor according to an embodiment of the present invention. 
     Referring to  FIG. 6 , a drain part  110  is connected to a power V DS , and a gate part  150  is connected to powers V G  and V in1  together with an inductor and a capacitor. The source part  120  is connected to an output terminal and a grounding terminal. First end of the magnetic field generation unit  160  is connected to the direct current (DC) power V B  and V in2 . Accordingly, I DS +i DS  is output by the resonant transistor. Since the frequency of the output current i DS  can be tuned diversely using multiple RF inputs V in1  and V in2 , diverse output current I DS +i DS  can be obtained. 
     As can be appreciated from the above description, such a resonant structure or a resonant tunneling transistor which is implemented using the resonant structure can be applied to high-speed digital integrated circuits (ICs), such as a D-flip flop, a frequency divider, or a multiplexer. 
     Moreover, such a resonant structure and such a resonant tunneling transistor can be applied to MEMS components or NEMS components which need to transform mechanical signals into electric signals. In particular, such a resonant structure and such a resonant tunneling transistor can be applied to RF systems such as RF filters, frequency combiners, or frequency generators since an RF MEMS which operates in a high frequency requires a high trans-efficiency. In addition, such a resonant structure and such a resonant tunneling transistor can be microminiaturized and increase the output current, and so can be applied to diverse low-power communication systems which will be developed in the future. Furthermore, tuning can be easily performed. 
     While the invention has been shown and described with reference to certain embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.