Abstract:
An integrated electronic detector operates to detecting a variation in potential on an input terminal. The detector includes a MOS transistor having a drain forming an output. Variation in drain current is representative of the variation in potential. A bipolar transistor has a base forming the input terminal and a collector electrically connected to the gate of the MOS transistor. The detector has a first configuration in which the bipolar transistor is conducting and the MOS transistor is turned off. The detector has a second configuration in which the bipolar transistor is turned off and the MOS transistor is in a sub-threshold operation. Transition of the detector from the first configuration to the second configuration occurs in response to the variation in potential.

Description:
PRIORITY CLAIM 
       [0001]    This application claims priority from French Application for Patent No. 1651570 filed Feb. 25, 2016, the disclosure of which is incorporated by reference. 
       TECHNICAL FIELD 
       [0002]    Various embodiments of the invention relate to integrated circuits and, more particularly, to integrated circuits capable of detecting small variations in electric potential induced, by way of a non-limiting example, by the detection of target molecules by a specific sensor in a ‘lab-on-a-chip’. 
       BACKGROUND 
       [0003]    A lab-on-a-chip conventionally comprises a molecular sensor connected to an electronic circuit configured for converting the data relating to the content of a fluid (a liquid or a gas) into electrical form. This data may be the presence or absence of a target molecule in the fluid, for example in the framework of a search for a virus in a sample of blood. 
         [0004]    Lab-on-a-chip devices are known that use field-effect transistors, in which a molecular sensor is electrically connected to the gate of a transistor. Thus, the variation in potential induced by the detection of the target molecule of the sensor leads to a variation in the drain current of the transistor. 
         [0005]    For example, for a variation in potential V G  of 0.3 volts on the gate of a conventional field-effect transistor, the value of the drain current I drain  of the transistor may be multiplied by 10 5 . 
         [0006]    The slope factor of a transistor—also known by those skilled in the art under the term “sub-threshold swing”—is a value allowing the characterization of the voltage to be applied to the gate of an MOS transistor in order to make its drain current vary by a decade. 
         [0007]    In this example, the slope factor of the transistor is equal to 60 mV/decade. However, this value is too high for some applications, such as for example the detection of deoxyribonucleic acid (DNA) molecules, which require devices having a slope factor less than 60 mV/decade. 
         [0008]    Lab-on-a-chip devices also exist that use advanced technologies, such as for example silicon nano-wires or carbon nanotubes, and which enable the fabrication of transistors having a lower slope factor. 
         [0009]    However, the fabrication of devices integrating these technologies requires specific and complex methods, which are not yet suitable for industrialization. 
       SUMMARY 
       [0010]    Thus, according to one embodiment, here, an integrated electronic detector is provided having a low value of slope factor, fabricated by conventional technologies and compatible with a technology of the FDSOI type. 
         [0011]    According to one aspect, an integrated electronic detector is provided configured for detecting the appearance of a variation in potential on an input terminal, comprising an MOS transistor whose drain forms an output terminal, and in which the variation of the drain current is representative of the said variation in potential on the input terminal. 
         [0012]    According to a general feature of this aspect, the sensor furthermore comprises a bipolar transistor whose base forms the input terminal and whose collector is electrically connected to the gate of the MOS transistor. 
         [0013]    The detector also has a first configuration in which the bipolar transistor is conducting and the MOS transistor is turned off, and a second configuration in which the bipolar transistor is turned off and the MOS transistor is in a sub-threshold operation, with for example its gate floating. The detector is then configured for going from its first configuration to its second configuration when the said variation in potential occurs. 
         [0014]    As is known to those skilled in the art, the sub-threshold operation of an MOS transistor is an operation in which a current may be measured flowing between the source and the drain of the transistor while the gate-source voltage is below the threshold voltage. 
         [0015]    In the first configuration, the emitter of the bipolar transistor is for example negatively biased, and in the first and the second configurations, the drain of the MOS transistor is for example positively biased. 
         [0016]    Thus, in the first configuration, which is the initial state, the base-emitter voltage of the bipolar transistor is for example slightly higher than its threshold voltage, which allows a conducting state of the transistor and hence the gate of the MOS transistor to be biased at substantially the same potential as the emitter of the bipolar transistor. Since this potential is negative, the MOS transistor is turned off. 
         [0017]    A small variation in potential on the base of the bipolar transistor allows the bipolar transistor to be turned off, rendering the gate of the MOS transistor floating. 
         [0018]    Thus, the stray field generated by the positive bias of the drain is no longer compensated by the negative bias of the gate, which leads to a large variation of the drain current, of the order of several decades. It should be noted that, since the MOS transistor is operating under its threshold voltage, this drain current remains lower than the drain current flowing in the MOS transistor in the conducting state. 
         [0019]    The MOS transistor can be fabricated using a technology of the silicon-on-insulator type and comprising a buried secondary control electrode (commonly denoted by those skilled in the art under the term “back gate”), situated under the insulator and electrically coupled to the emitter of the bipolar transistor. 
         [0020]    According to one embodiment, the detector comprises: a carrier substrate with a first type of conductivity comprising, in a first region, a first semiconductor well of a second type of conductivity; a buried insulating layer situated between the first well and a semiconductor film, the MOS transistor being situated within and on the semiconductor film, and the secondary control electrode situated under the buried insulating layer. 
         [0021]    The carrier substrate comprises, in a second region, a second semiconductor well of the first type of conductivity forming the base of the bipolar transistor, a first region of the first type of conductivity, a second region of the second type of conductivity forming the collector of the bipolar transistor, an extension of the first semiconductor well forming the emitter of the bipolar transistor, and a metallization connecting the collector of the bipolar transistor to the gate of the MOS transistor. 
         [0022]    The first type of conductivity is for example a conductivity of the P type and the second type of conductivity is for example a conductivity of the N type. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    Other advantages and features of the invention will become apparent upon examining the detailed description of non-limiting embodiments of the invention and from the appended drawings in which: 
           [0024]      FIGS. 1 and 2  illustrate an embodiment of a detector circuit. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]      FIG. 1  illustrates an integrated circuit IC here generally comprising a substrate of the fully-depleted silicon-on-insulator type (or FDSOI, according to the acronym well known of those skilled in the art) conventionally comprising a carrier substrate  1 , a buried insulating layer  9 , and an intrinsic semiconductor film  10 . 
         [0026]    The integrated circuit IC here incorporates a detector DIS. 
         [0027]    The carrier substrate  1  is a substrate of a first type of conductivity, for example here a substrate made of silicon with a light doping of the P type, and in which two regions Z 1  and Z 2  may be defined. 
         [0028]    In the first region Z 1 , which is here bounded by two insulating regions, for example a first and a second deep trench isolation (known by the acronym DTI)  13  and  14 , the detector comprises a first semiconductor well  3  having a second type of conductivity, for example the N type of conductivity, which is located under a part of the buried insulating layer  9 . 
         [0029]    The detector also comprises an MOS transistor, here an NMOS transistor  11 , formed in this case within and on the film of fully-depleted silicon. 
         [0030]    The source S and drain D regions are raised up and formed in a conventional manner within and on the film  10 , by silicon epitaxy. 
         [0031]    The gate G of the transistor conventionally comprises a layer of insulating material underneath a gate material. 
         [0032]    This configuration in which the NMOS transistor  11  is situated on a semiconductor well of the same type of conductivity as the transistor, here a conductivity of the N type, is known by those skilled in the art under the term “Flip well”. 
         [0033]    In a second region Z 2  of the substrate bounded by the first isolation trench  13  and a third isolation trench  15 , the buried insulating layer  9  has been eliminated. 
         [0034]    In this second region Z 2 , the upper portion of the first well  3  comprises an extension  8  insulated from the MOS transistor  11  by the trench  13 . Furthermore, the detector DIS comprises, in this region Z 2 , a second buried semiconductor well  2 , with P-type doping, a first upper portion of which is separated from the well extension  8  by a fourth deep isolation trench  4 . 
         [0035]    The isolation trench  4  has a depth chosen so as to allow a PN junction to subsist under it which is going to form part of a bipolar transistor, here of the NPN type, as will be explained hereinafter. 
         [0036]    In this regard, although a space has been shown in the schematic  FIG. 3  for the sake of clarity, under the isolation trench  4 , the wells  2  and  3  are adjacent or sufficiently close to form an effective PN junction. 
         [0037]    The detector comprises, in a second upper portion of the second well  2 , a first highly doped region  5  of the N type and a second highly doped region  6  of the P type allowing a first contact C 1  to be made enabling the second well  2  to be biased. 
         [0038]    The two doped regions  5  and  6  are mutually separated by a fifth deep isolation trench  7 . 
         [0039]    A contact C 2  allows the extension  8  of the first well  3  and the first well  3  itself to be biased. 
         [0040]    It should be noted here that the second contact C 2  allows the NMOS transistor  11  to be biased via its back face. 
         [0041]    The MOS transistor therefore has a buried secondary control electrode or second gate, referred to as “back gate” BG, situated under the buried insulating layer  9 . 
         [0042]    The integrated circuit IC conventionally comprises an interconnection part situated on top of the substrate and comprising several metallization levels and vias. This interconnection part is commonly denoted by those skilled in the art as “BEOL” (for Back End Of Line). 
         [0043]    The first highly doped region  5  is here electrically connected to the gate G of the NMOS transistor  11  via an electrical link comprising a metal track ML and vias V 1  and V 2  situated within the said interconnection part. 
         [0044]    A schematic representation of the circuit from an electrical standpoint is illustrated in  FIG. 2 . 
         [0045]    The NMOS transistor  11  is shown in the figure, comprising its drain D, source S and gate G regions. 
         [0046]    The junctions between the second well  2 , the first highly doped region  5 , and the first well  3  form a bipolar transistor  12 , here an NPN transistor, whose first doped region  5  is the collector C, whose second well  2  is the base B, and whose extension of the first well  3  is the emitter E. 
         [0047]    The emitter E is therefore connected to the back gate BG of the MOS transistor  11 , and the collector C to the main gate G of the NMOS transistor  11 . 
         [0048]    Thus, the first contact C 1  allows the base B of the bipolar transistor  12  to be biased, and the second contact C 2  allows the emitter E of the bipolar transistor  12  and the back gate BG of the NMOS transistor  11  to be simultaneously biased. 
         [0049]    In operation, the circuit may advantageously be used in a lab-on-a-chip. In this case, the first contact C 1 , in other words the base of the bipolar transistor, is used as an input terminal of the sensor, and the drain D of the NMOS transistor  11  is used as an output terminal. 
         [0050]    For example, the first contact C 1  may be electrically connected to a molecular sensor, comprising for example and conventionally a functionalized electrode comprising a functionalized metal layer comprising a material capable of attaching itself to a target molecule, and optionally a dielectric layer under the said metal layer. 
         [0051]    When a molecule attaches itself to the sensor, an electrical charge is generated in the sensor making the potential of the base B vary. This variation in potential depends notably on the nature of the molecule to be detected. 
         [0052]    The drain D of the NMOS transistor  11  may for example be connected to a read circuit. 
         [0053]    In this example, the first contact C 1 , in other words the base B of the bipolar transistor, is initially at a potential V C1  of zero, which corresponds to the absence of a molecule on the sensor. 
         [0054]    The drain D of the MOS transistor is biased, for example at a potential of 0.6 volts. 
         [0055]    This bias of the drain D generates a stray field within the channel, and the drain then acts as a spurious gate. This phenomenon, known to those skilled in the art by the term “fringing field”, is conventional in MOS transistors of the silicon-on-insulator type. 
         [0056]    Adjusting the potential of the second contact C 2 , in other words of the emitter of the bipolar transistor  12  and of the back gate BG of the NMOS transistor  11 , in such a manner that the base-emitter voltage is initially slightly higher than the threshold voltage of the bipolar transistor so as to render it conducting, while at the same time only allowing a small variation in potential on the base of the bipolar transistor  12 , leads to a change of its state. 
         [0057]    Thus, since the threshold voltage of the bipolar transistor  12  is in this example 0.6V, the potential of the second contact V C2  is here fixed at −0.65V volts. Since the potential of the base is initially zero, the base-emitter voltage is 0.65V and therefore above the threshold voltage. The bipolar transistor  12  is therefore initially in a conducting state, and the gate G of the transistor  11  is therefore negatively biased. 
         [0058]    Initially, the detector is therefore in a first configuration in which the bipolar transistor  12  is conducting and the NMOS transistor  11  is turned off owing to the negative bias on its gate G. 
         [0059]    Only a very low leakage current flows in the drain, for example here a current I drain  with a value of 10 10  amps. 
         [0060]    It should be noted here that, since the emitter E of the bipolar transistor  12  is electrically connected to the back gate BG of the MOS transistor, the latter is also negatively biased at a potential of −0.65V. However, the effect of the bias of the back gate on the channel with respect to that of the bias of the main gate is negligible. 
         [0061]    When there is a variation of the potential V C1  on the base B of the bipolar transistor  12 , for example a variation of +0.1 volts, due to the attachment of one or more molecules onto the sensor, the base-emitter voltage of the bipolar transistor  12  goes from 0.65 volts to 0.55 volt. The base-emitter voltage is therefore lower than the threshold voltage of the bipolar transistor. 
         [0062]    The detector therefore goes into a second configuration in which the bipolar transistor  12  is turned off, and the gate G of the MOS transistor  11  is no longer biased. The gate of the MOS transistor therefore becomes floating. 
         [0063]    Thus, in the absence of a bias on the main gate, the fringing field generated by the biasing of the drain leads to a significant increase in the conduction of the MOS transistor. The MOS transistor is therefore here in a sub-threshold operation. 
         [0064]    In this example, the drain current I drain  reaches a value of 10 −4  amp, this being an increase of 6 decades. 
         [0065]    Thus, for a small variation of the potential V B  on the base of the bipolar transistor, a large variation in drain current I drain  is obtained. Here, the slope factor of the circuit is 11 mV/decade, which is much lower than the value obtained with devices of the prior art fabricated with conventional technologies. 
         [0066]    It should be noted that the embodiments presented here are non-limiting. 
         [0067]    The fact that the emitter of the bipolar transistor is electrically connected to the back gate of the MOS transistor is due here to the method of fabrication of the integrated circuit and does not influence the operation of the sensor. It would therefore be perfectly possible to envisage a detector in which the emitter and the back gate are mutually isolated, or else a detector in which the MOS transistor were formed using a “bulk” technology with the emitter of the bipolar transistor isolated from the substrate of the MOS transistor. 
         [0068]    Furthermore, although an embodiment has been described here in which the potential of the second contact C 2  is fixed in order to detect a variation in potential on the first contact C 1 , an embodiment in which the potential of the second contact C 2  might be varied in order to search for the value for which the device goes from the first configuration to the second configuration could also be envisaged. 
         [0069]    In this case, depending on the value of potential for which this change of configuration were observed, in other words a significant increase in the drain current I drain , the presence or the absence of molecules on the sensor would be deduced. 
         [0070]    It would also be possible to have an integrated circuit in which the carrier substrate  1  would be more highly doped, with a doping identical to that of the second well  2 . Thus, it would be possible to make the contact with the first contact C 1  on the carrier substrate, for example on its back face. 
         [0071]    Lastly, although the invention is advantageously applicable to the field of lab-on-a-chip devices, its use in any other field requiring the detection of the appearance of small variations in potential could very well be envisaged, by for example connecting the contact Cl to a different sensor other than a molecular sensor.