Abstract:
A method for operating a sensor for biomolecules or charged ions, the sensor comprising a first field effect transistor (FET) and a second FET, wherein the first FET and the second FET comprise a shared node includes placing an electrolyte containing the biomolecules or charged ions on a sensing surface of the sensor, the electrolyte comprising a gate of the second FET; applying an inversion voltage to a gate of the first FET; making a first electrical connection to an unshared node of the first FET; making a second electrical connection to unshared node of the second FET; determining a change in a drain current flowing between the unshared node of the first FET and the unshared node of the second FET; and determining an amount of biomolecules or charged ions contained in the electrolyte based on the determined change in the drain current.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional of U.S. application Ser. No. 12/756,628, filed on Apr. 8, 2010 the disclosure of which are incorporated herein by reference in their entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    This disclosure relates generally to the field of sensing of biomolecules and charged ions in an electrolyte solution. 
       DESCRIPTION OF RELATED ART 
       [0003]    A field effect transistor (FET), comprising a source, a drain, and a gate, may be used as a sensor for various types of biomolecules, including but not limited to charged ions, such as H+ or Ca++, proteins, glucose, or viruses, by using an electrolyte containing the biomolecules as the FET gate (see P. Bergveld, Sensors and Actuators B 88 (2003) 1-20, for further information). In operation, a voltage may be applied to the FET gate electrolyte by immersing an electrode into the electrolyte, and connecting the electrode to a voltage source. The presence of the electrode may cause the sensor to have a relatively cumbersome setup, and may limit miniaturization and automation of the sensor. The electrode, which may comprise a silver wire coated with a silver chloride layer, may also cause reliability issues in the sensor over time, due to chemical changes in the electrode material that may occur with prolonged use. 
         [0004]    A FET based-sensor that does not require an electrode immersed in the electrolyte may comprise a back-gated silicon nanowire FET structure (See E. Stern et al, Nature, Vol. 445, page 519 (2007) for further information). A back gated FET uses a layer of buried oxide as the gate dielectric. The buried oxide may be relatively thick, resulting in a relatively large sub-threshold slope (greater than 300 mV/decade) and high threshold voltages, and as result, the sensitivity of the sensor may be degraded and the sensing voltage is high. In order to improve sensitivity, the silicon nanowire diameters may be made increasingly thin; however, a relatively thin silicon nanowire may lead to yield issues in sensor fabrication. In order to lower the sensing voltage, the thickness of the buried oxide may be made thinner and the fixed charge density in the buried oxide layer may be reduced. The fabrication processes for thin silicon nanowires and thin buried oxide layer with reduced fixed charge density may be relatively complex and costly compared to fabrication process for regular FETs. 
       SUMMARY 
       [0005]    In one aspect, a method for operating a sensor for biomolecules or charged ions, the sensor comprising a first field effect transistor (FET) and a second FET, wherein the first FET and the second FET comprise a shared node includes placing an electrolyte containing the biomolecules or charged ions on a sensing surface of the sensor, the electrolyte comprising a gate of the second FET; applying an inversion voltage to a gate of the first FET; making a first electrical connection to an unshared node of the first FET; making a second electrical connection to unshared node of the second FET; determining a change in a drain current flowing between the unshared node of the first FET and the unshared node of the second FET; and determining an amount of biomolecules or charged ions contained in the electrolyte based on the determined change in the drain current. 
         [0006]    Additional features are realized through the techniques of the present exemplary embodiment. Other embodiments are described in detail herein and are considered a part of what is claimed. For a better understanding of the features of the exemplary embodiment, refer to the description and to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0007]    Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
           [0008]      FIG. 1  illustrates an embodiment of a dual FET sensor. 
           [0009]      FIG. 2  illustrates an embodiment of a dual FET sensor. 
           [0010]      FIG. 3  illustrates an embodiment of a dual FET sensor. 
           [0011]      FIG. 4  illustrates an embodiment of a method of operating a dual FET sensor. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Embodiments of systems and methods for a dual FET sensor for biomolecules and charged ions are provided, with exemplary embodiments being discussed below in detail. A FET-based sensor structure may comprise two serially connected n-type or p-type metal oxide field effect transistors (MOSFETs, or FETs), where the first FET is a control FET and the second FET is a sense FET having an electrolyte as the gate. The control FET and the sense FET may share a node. The gate dielectric surface of the sense FET may be functionalized such that the surface of the gate dielectric specifically binds the type of biomolecules that the dual FET sensor is used to detect. The biomolecules in the electrolyte bind to the functionalized gate dielectric surface of the sense FET, causing a change in a drain current of the sensor. An amount of biomolecules that are present in the electrolyte may be determined based on the change in the drain current. Use of a dual FET sensor eliminates the need to immerse an electrode in the electrolyte. 
         [0013]      FIG. 1  illustrates an embodiment of a dual FET sensor  100 , comprising a control FET and a sense FET. The sense FET comprises shared node  102  and sensor node  101 , which act as the sense FET source/drain, and a gate comprising the electrolyte  108 . The surface of gate dielectric  105  that is in contact with the electrolyte  108  is functionalized to form sensing surface  106 . The control FET comprises shared node  102  and control node  103 , which act as the control FET source/drain, and control gate  107 . The dual FET sensor  100  is built on a substrate  104 ; sensor node  101 , shared node  102 , and control node  103  are formed in substrate  104 . An insulating material  109  may be located over the substrate  104 . Gate dielectric  105  is located over substrate  104 , sensor node  101 , shared node  102 , and control node  103 . In operation, electrical connections are made to sensor node  101 , control node  103 , and control gate  107 , which may comprise metal lines (not shown), and a drain current (I d ) flows through dual FET sensor  100  between sensor node  101  and control node  103 . 
         [0014]      FIG. 2  illustrates an alternate embodiment of a dual FET sensor  200 . Dual FET sensor  200  also comprising a control FET and a sense FET. The sense FET comprises shared node  202  and sensor node  201 , which act as the sense FET source/drain, and a gate comprising the electrolyte  208  on sense gate dielectric  205 A. The surface of gate dielectric  205 A that is in contact with the electrolyte  208  is functionalized to form sensing surface  206 . The control FET comprises shared node  202  and control node  203 , which act as the control FET source/drain, and control gate  207  on control gate dielectric  205 B. The dual FET sensor  200  is built on a substrate  104 ; sensor node  201 , shared node  202 , and control node  203  are formed in substrate  204 . An insulating material  209  may be located over the substrate  204 . In operation, electrical connections, which may comprise metal lines (not shown), are made to sensor node  201 , control node  203 , and control gate  207 , and a drain current (I d ) flows through dual FET sensor  200  between sensor node  201  and control node  203 . 
         [0015]      FIG. 3  illustrates an alternate embodiment of a dual FET sensor  200 . Dual FET sensor  300  also comprising a control FET and a sense FET. The sense FET comprises sense shared node  302 A and sensor node  301 , which act as the sense FET source/drain, and a gate comprising the electrolyte  308  on gate dielectric  305 . The surface of gate dielectric  305  that is in contact with the electrolyte  308  is functionalized to form sensing surface  306 . The control FET comprises control shared node  302 B and control node  303 , which act as the control FET source/drain, and control gate  307  on gate dielectric  205 . Sense shared node  302 A is connected to control shared node  302 B. The dual FET sensor  300  is built on a substrate  304 ; sensor node  301 , shared nodes  302 A-B, and control node  303  are formed in substrate  304 . An insulating material  309  may be located over the substrate  304 . In operation, electrical connections, which may comprise metal lines (not shown), are made to sensor node  301 , control node  303 , and control gate  307 , and a drain current (I d ) flows through dual FET sensor  300  between sensor node  301  and control node  303 . 
         [0016]    The control gate ( 107 ,  207 , and  307 ) may comprise polysilicon or a metal in some embodiments. The sensor node ( 101 ,  201 ,  301 ), shared node ( 102 ,  202 ,  302 A-B), and control node ( 103 ,  203 ,  303 ), each have the same doping type (n+-type or p+-type) in some embodiments. Substrate  104 ,  204 , and  304  may comprise bulk silicon or silicon-on-insulator, and may have a doping type (n-type or p-type) that is opposite the doping type of the nodes ( 101 - 103 ,  201 - 203 ,  301 - 303 ) in some embodiments. The gate dielectric ( 105 ,  205 A-B,  305 ) may comprise SiO 2 , SiON, a high-k material, or a bilayer of SiO2 and high k with an equivalent oxide thickness (EOT) greater than 20 angstroms (A) in some embodiments. 
         [0017]      FIG. 4  illustrates an embodiment of a method of operating a dual FET sensor  100 .  FIG. 4  is discussed with respect to  FIG. 1 ; method  400  may also be used in conjunction with the dual sensor FETs  200  and  300  shown in  FIGS. 2 and 3 . In block  401 , gate dielectric surface of the sense FET is functionalized to form the sensing surface  106 . Functionalizing of the surface of gate dielectric  105  to form the sensing surface  106  may comprise coating the gate dielectric surface of the sense FET with antibodies or an appropriate chemical that may specifically bind to the particular biomolecules that the sensor is being used to detect in some embodiments. In block  402 , electrolyte  108  is placed on sensing surface  106 . In block  403 , electrical connections are made to control gate  107 , sensor node  101 , and control node  103 . The electrical connections may be made via metal lines connected to each of control gate  107 , sensor node  101 , and control node  103 . A gate voltage is applied to control gate  107  that is sufficient to turn on the control FET. The gate voltage may comprise a constant inversion voltage, and may be between about 11.01 volts (V) and 11.51 V in some embodiments. The control node  103  may be held at a constant voltage V d , which may be about 0.1 V in some embodiments. The sensor node  101  may be held at about 0 V in some embodiments. The shared node  102  and the sense FET gate comprising electrolyte  108  are left floating. In block  404 , biomolecules in electrolyte  108  bind to the sensing surface  106 . The biomolecules bound to sensing surface  106  causes a change in the workfunction at the interface between the sensing surface  106  and the electrolyte  108 , which in turn causes a change in the I d  that flows between the sensor node  101  and the control node  103 . In block  405 , the change I d  is determined. The change in I d  is determined with respect to a drain current that flows through the sensor  100  in the absence of biomolecules. In block  406 , an amount of biomolecules present in electrolyte  108  is determined from the change in I d . 
         [0018]    The control FET has a channel length, which is the distance between the control node  103  and the shared node  102 . The sense FET also has a channel length, which is the distance between the sensor node  101  and the shared node  102 . In some embodiments of dual FET sensor  100 , a channel length of the control FET may be shorter than a channel length of the sense FET. Further, the control FET has a channel width, which is the width of the conduction channel of the control FET measured alongside control node  103  in a direction perpendicular to the direction of the drain current flow. The sense FET also has a channel width, which is the width of the conduction channel of the sense FET measured alongside sense node  101  in a direction perpendicular to the direction of the drain current flow. In some embodiments of dual FET sensor  100 , a channel width of the control FET may be shorter than a channel width of the sense FET. 
         [0019]    The technical effects and benefits of exemplary embodiments include detection of biomolecules or ions in an electrolyte without the need to immerse an electrode in the electrolyte. 
         [0020]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. 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” and/or “comprising,” when used in this specification, 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. 
         [0021]    The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.