Patent Publication Number: US-11380766-B2

Title: Transistor structure

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/427,656 filed Feb. 8, 2017, which claims the priority benefit of French Application for Patent No. 1657587, filed on Aug. 5, 2016, and the priority benefit of Greece Application for Patent No. 20160100358, filed on Jul. 5, 2016, the disclosures of which are hereby incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to the field of electronic components, and in particular to a transistor structure. 
     BACKGROUND 
     An N-channel MOS transistor comprises a drain region and a source region, both N-type doped, separated by a channel region covered with an insulated gate. Main electrodes are located on the drain and source regions. A positive voltage is applied between the drain and the source through the main electrodes. The flowing of a current through the transistor between the main electrodes is then controlled by a gate voltage, or control voltage, applied between the gate and the source. An advantage of the MOS transistor is that the control current which flows to the gate is close to zero. 
     When the control voltage is greater than a threshold voltage, the transistor is in a conductive state and a current flows through the transistor between the main electrodes. 
     When the control voltage is decreased from the conductive state, the MOS transistor switches to the off state as soon as the control voltage becomes lower than the threshold voltage. As long as the difference between the control voltage and the threshold voltage is smaller than a few hundreds of millivolts, there remains a leakage current, the transistor then being in a lightly blocked state. The intensity of the leakage current decreases as the control voltage decreases, in a way which depends on temperature. The variation of the leakage current according to the control voltage is characterized by a subthreshold swing value defined by the decrease of the control voltage, which causes a division by 10 of the intensity of the leakage current. At an ambient temperature close to 20° C., a MOS transistor has a subthreshold swing value greater than 60 mV/decade. 
     Due to the high subthreshold swing value, in order for the leakage current to disappear, the control voltage should be much lower than the threshold voltage, for example, distant by more than 500 mV from the threshold voltage. The transistor is then strongly blocked and no significant current flows through the transistor. 
     Similarly to the MOS transistor, a bipolar transistor comprises two N-type doped regions, in contact with main electrodes. The emitter and collector regions are separated by a base region. A positive voltage is applied between the collector and the emitter. 
     The bipolar transistor is in the on state when a control current is conducted from the base region to the emitter region. To achieve this, a control voltage greater than a threshold voltage is applied between the base and the emitter. The control current corresponds to the circulation of holes through the transistor. The presence of such holes enables a main current to flow between the main electrodes. The bipolar transistor then has a gain defined by the ratio of the main current to the control current. An advantage of the bipolar transistor is that it enables a particularly high main current to flow. 
     The control voltage is decreased to block the bipolar transistor. There remains a leakage current similar to that of the MOS transistor between the main electrodes of the bipolar transistor as long as the control voltage is close to the threshold voltage. In the same way as in the MOS transistor, the control voltage should be much lower than the threshold voltage so that the bipolar transistor is strongly blocked and that no significant current flows through the transistor. 
     There is a need for a transistor enabling to combine some of the advantages of MOS transistors and of bipolar transistors and to overcome all or part of their disadvantages. 
     SUMMARY 
     Thus, an embodiment provides a transistor comprising: a quasi-intrinsic region of a first conductivity type covered with an insulated gate and extending between two first doped regions of a second conductivity type, a main electrode being arranged on each of the first regions; and a second doped region of the second conductivity type in contact with the quasi-intrinsic region and at a distance (physically and electrically separate) from the first two regions, a control electrode being arranged on the second region. 
     According to an embodiment, the transistor is arranged on the insulator of a SOI structure. 
     According to an embodiment, the quasi-intrinsic region is strip-shaped, the second doped region comprising two electrically-connected portions arranged on either side of a portion of the quasi-intrinsic region. 
     According to an embodiment, the quasi-intrinsic region has the shape of a fork comprising a portion of connection between a handle and two branches, a portion of the handle extending between the first two regions, the second region extending between the two branches, the second region being in contact with the two branches and with the connection portion. 
     According to an embodiment, the insulator is arranged on a semiconductor substrate of the second conductivity type provided with a bias contact. 
     According to an embodiment, the insulator has a thickness smaller than 30 nm. 
     According to an embodiment, the second region is physically separated from the first regions by a distance shorter than 500 nm. 
     According to an embodiment, the thickness of the quasi-intrinsic region is smaller than 20 nm. 
     An embodiment provides an electronic circuit comprising a transistor such as previously defined and a circuit for controlling the transistor, wherein the first conductivity type is type N and the control circuit is capable of applying between each of the main electrodes and the gate a voltage greater than 0.5 V and between the control electrode and the gate a voltage greater than 1 V. 
     An embodiment provides a transistor such as previously defined and a circuit for controlling the transistor, wherein the first conductivity type is type P and the control circuit is capable of applying between the gate and each of the main electrodes a voltage greater than 0.5 V and between the gate and the control electrode a voltage greater than 1 V. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, wherein: 
         FIGS. 1A to 1D  are simplified views illustrating an embodiment of a transistor; 
         FIG. 2  shows the variation of a current according to a voltage in a transistor according to the embodiment of  FIGS. 1A to 1D ; 
         FIG. 3  shows an electronic circuit comprising a transistor according to the embodiment of  FIGS. 1A to 1D ; 
         FIG. 4  is a simplified cross-section view illustrating another embodiment of a transistor; and 
         FIG. 5  is a simplified top view illustrating another embodiment of a transistor. 
     
    
    
     DETAILED DESCRIPTION 
     For clarity, the same elements have been designated with the same reference numerals in the various drawings and, further, the various drawings are not to scale. For clarity, only those steps and elements which are useful to the understanding of the described embodiments have been shown and are detailed. 
     In the following description, unless otherwise specified, when reference is made to terms qualifying a relative position, such as term “above” and “under”, reference is made to the orientation of the concerned element in the cross-section views. Unless otherwise specified, expression “in the order of” means to within 10%, preferably to within 5%. 
     In the present disclosure, expression “quasi-intrinsic” designates an intrinsic semiconductor (non-doped) or a semiconductor having a doping level lower than 10 15  atoms/cm 3 . A quasi-intrinsic semiconductor may thus be slightly N-type or P-type doped. Further, “heavily-doped” qualifies a semiconductor having a doping level greater than 5*10 18  atoms/cm 3 . 
       FIGS. 1A to 1D  illustrate an embodiment of a transistor  1 .  FIG. 1A  is a simplified perspective view where the insulating material of insulating trenches is not shown. Views  1 B to  1 D correspond to cross-section views along respective planes B, C, and D shown in  FIG. 1A . 
     Transistor  1  is located on a portion of an insulating layer  3  of a structure of semiconductor-on-insulator type, SOI, for example, silicon on insulator. In the SOI structure, insulating layer  3  covers a P-type doped semiconductor substrate  5 , for example, silicon. Transistor  1  comprises a strip-shaped region  7  made of P-type quasi-intrinsic semiconductor (Pi). Quasi-intrinsic strip  7  is entirely covered with a gate  9  insulated by an insulator  11  located between the gate and the strip. Transistor  1  comprises a conduction structure  13  in association with a portion  14  of strip  7  and a control structure  15  in association with a portion  16  of strip  7 . A portion  17  of strip  7 , shown in  FIG. 1B , is located between portions  14  and  16  and interconnects conduction and control structures  13  and  15 . 
     Conduction structure  13 , shown in  FIG. 1C , corresponds to the structure of a MOS transistor. Conduction structure  13  comprises heavily N-type doped regions  18  and  19  arranged opposite each other on either side of portion  14  of strip  7  and in contact with the large sides of strip  7 . Region  18  corresponds to a drain region and region  19  corresponds to a source region. A main electrode  22  connected to a node  23  of application of a potential VC is in contact with N+ region  18 . A main electrode  24  connected to a node  25  of application of a potential VE is in contact with N+ region  19 . Gate  9  is connected to a node  27  of application of a potential VG. 
     Control structure  15 , shown in  FIG. 1D , comprises heavily N-type doped N+ regions  29  and  31 , physically and electrically separate from N+ regions  18  and  19 , arranged opposite each other on either side of portion  16  of strip  7  and in contact, for example, by abrupt junctions with the large sides of strip  7 . Regions  29  and  31  are connected together to a node  35  of application of a potential VB via electrodes  37 . 
     Each of electrodes  22 ,  24 , and  37  associated with one of N+ regions  18 ,  19 ,  29 , and  31  may be a conductive layer portion partly or totally covering the N+ region or may be a conductive via arranged on the N+ region. An N+-type doped epitaxial semiconductor portion, not shown, may cover each of N+ regions  18 ,  19 ,  29 , and  31 , and the associated electrode then covers the epitaxial portion. Substrate  5  is connected by a contact area  39  to a node of application of a bias potential VBG. Insulating trenches  41  delimit and insulate transistor  1  and contact area  39 . 
     Transistor  1  is used according to an operating mode different from that of a MOS transistor. In this operating mode, the MOS transistor corresponding to conduction structure  13  is strongly blocked by a gate voltage distant, for example, by more than 0.5 V, from the threshold voltage of the MOS transistor. A positive voltage VC-VE is applied between drain and source regions  18  and  19 , and the gate voltage then corresponds to voltage VG-VE between the gate and source region  19 . 
     Although the MOS transistor is strongly blocked, control structure  15  enables to control the flowing of a main current between the main electrodes of conduction structure  13 . To achieve this, a voltage VB-VG is applied between, on the one hand, N+ regions  29  and  31  and, on the other hand, gate  9 . 
     The main current flows when voltage VB-VG is positive and exceeds a tunnel effect threshold voltage which may be greater than 1 V, for example, in the order of 1.5 V. 
     Indeed, in this case, valence electrons of region  7 , located close to one of N+ regions  29  and  31  leave region  7  by tunnel effect and become conduction electrons in N+ regions  29  or  31 . The leaving of these electrons corresponds to the appearing of holes in region  7 . A tunnel current which flows from N+ regions  29  and  31  to region  7  is thus obtained. The tunnel current is favored by the abruptness of the junctions between region  7  and N+ regions  29  and  31 . 
     Due to the quasi-intrinsic character of region  7 , the holes originating from the tunnel current may propagate along a significant distance without disappearing by recombination. Thus, the holes originating from the tunnel current propagate in portion  14  of region  7  located in conduction structure  13 . The distance between portions  14  and  16  of region  7 , or width of portion  17  (in the gate width direction), may be limited, for example, to a value smaller than 500 nm. This enables to ease the propagation of holes in portion  14  of region  7 . Further, due to the strip shape of region  7 , the holes are channeled and may simply access the entire region  7 . Here again, this enables to ease the propagation of holes in portion  14  of region  7 . 
     In conduction structure  13 , quasi-intrinsic portion  14  with biased gate  9  has a function similar to that of a base region of a bipolar transistor. The bipolar transistor comprises region  19 , which forms an emitter region, and region  18 , which forms a collector region. Due to the high value of the tunnel effect threshold voltage, voltage VB, greater than voltage VE, is sufficient for the holes originating from the tunnel current to have an energy level which enables them to penetrate into emitter region  19 . A control current then flows through the bipolar transistor. Accordingly, a main current flows in the bipolar transistor between collector region  18  and emitter region  19 . 
     A property of control structure  15  is that, when control voltage VB-VG decreases from the tunnel effect threshold voltage, the tunnel current particularly rapidly becomes negligible. Further, the ratio of the tunnel current to the main current corresponds to the gain of the bipolar transistor. The tunnel current is thus amplified by the transistor. 
     Thus, according to an advantage, the main current in transistor  1  is particularly high and decreases particularly rapidly when control voltage VB-VG decreases from the tunnel effect threshold voltage. When control voltage VB-VG is smaller than the tunnel effect threshold voltage, the subthreshold swing value, defined by the decrease of the control voltage which causes a division by 10 of the intensity of the main current, is smaller than 60 mV/decade. 
       FIG. 2  shows examples of curves of the variation of main current I CE  according to potential VB in a transistor of the type of transistor  1  of  FIGS. 1A to 1D . These curves have been obtained by digital simulation. Curves  43 ,  45 , and  47  correspond to fixed negative values respectively equal to −0.9 V, −0.8 V, and −0.5 V of gate potential VG. Potentials VE and VBG are zero and a positive 100 mV voltage is applied between the collector region and the emitter region. 
     The fixed values of potential VG used in the examples of  FIG. 2  guarantee that the MOS transistor is strongly blocked. The subthreshold swing values are approximately 40 mV. 
     In the example of  FIGS. 1A to 1D , substrate  5  provided with contact area  39  may form a rear gate associated with transistor  1 . The values of the MOS transistor threshold or tunnel effect threshold voltages may be controlled by the application to the rear gate of a selected value of potential VBG. Such a control is made easier when the thickness of insulating layer  3  is small, for example, smaller than 30 nm. 
     As an example, N+ regions  29  and  31  are doped to a level greater than 10 20  atoms/cm 3 . As an example, region  7  has a thickness smaller than 20 nm, which corresponds to a FDSOI-type structure. The distance separating regions  18  and  19 , or gate length, may be in the range from 20 to 40 nm. The width of portion  14  may be in the range from 0.5 to 2 μm. The width of portion  16  may be in the range from 200 to 400 nm. Insulated gate  9  may extend on regions  18  and  19  along a distance smaller than 5 nm. 
     It should be noted that, in the embodiment of transistor  1 , regions  18  and  19  may be arranged symmetrically with respect to quasi-intrinsic region  7 . Thus, although, in the described embodiment, voltage VC-VE is positive, other operating modes are possible, where voltage VC-VE is negative, region  18  corresponding to a source or emitter region, and region  19  corresponding to a drain or collector region. 
     Although operating modes where the MOS transistor of conduction structure  13  is in the strongly blocked state have been described, other operating modes are possible where the MOS transistor is in the conductive state or in the lightly blocked state. The operation of the MOS transistor controlled by gate voltage VG-VE can then be associated with the operation of the bipolar transistor of conduction structure  13 . For this purpose, a control voltage VB-VG enabling to inject holes into conduction structure  13  while applying a voltage VG-VE allowing by itself the flowing of a main current between the main electrodes may be applied. 
       FIG. 3  shows an electronic circuit  50  comprising transistor  1  described in relation with  FIGS. 1A to 1D . Transistor  1  is schematically illustrated in top view at a different scale. The elements of electronic circuit  50  other than transistor  1  are represented by a short-circuit  52  (CTRL). Sub-circuit  52  may comprise transistors similar to transistor  1 . 
     When circuit  50  is operating, sub-circuit  52  respectively applies potentials VC, VE, VB, and VG to respective nodes  23 ,  25 ,  35 , and  27  of transistor  1 , as well as a potential VBG to rear gate contact area  39 , these potentials corresponding to the operating modes described in relation with  FIGS. 1A to 1D . Sub-circuit  52  thus plays the role of a circuit for controlling transistor  1 . 
     Although the embodiment of transistor  1  described in relation with  FIGS. 1A to 1D  comprises two N+ regions  29  and  31  arranged on either side of quasi-intrinsic region  7 , other embodiments are possible where one of N+ regions  29  or  31  is omitted. 
       FIG. 4  is a simplified top view of an embodiment of a transistor  60  where a control structure comprises a single N+ region. An insulated gate  9 A entirely covers a quasi-intrinsic P-type region  7 A having a rectangular shape in top view. N+ regions  18 A and  19 A, heavily N-type doped, are arranged on either side of the large sides of the rectangle and in contact with region  7 A. Regions  18 A and  19 A are connected by main electrodes  22  and  24  to nodes  23  and  25  of application of potentials VC and VE. An N+ region  29 A physically and electrically separate from regions  18 A and  19 A is in contact with one of the small sides of region  7 A. Although no rear gate contact area is shown in  FIG. 4 , transistor  60  may be associated with a rear gate. 
     Although the previously-described embodiments comprise a quasi-intrinsic region having a specific shape, the quasi-intrinsic region may have any other shape comprising a first portion associated with a MOS transistor structure and a second portion in contact with a heavily N-type doped region, this shape being adapted to the circulation of holes between the first portion and the second portion. 
       FIG. 5  is a simplified top view of an embodiment of a transistor  70 . Transistor  70  comprises a quasi-intrinsic region  7 B entirely covered with an insulated gate  9 B. Quasi-intrinsic region  7 B has in top view the shape of a fork with two branches, that is, it comprises a handle which extends in a strip along an axis, the strip being continued, on either side at one end and orthogonally to this axis, by portions of connection to branches of rectangular shape. The branches extend parallel to the axis away from the handle. Transistor  70  comprises a conduction structure  13 B associated with a portion of the handle and comprising two heavily N-type doped N+ regions  18 B and  19 B. Regions  18 B and  19 B are arranged on either side of the handle and separated from the connection portions. Transistor  70  further comprises a control structure comprising a heavily-doped N-type region  29 B arranged between the branches, in contact therewith and with the connection portion. The junctions between region  29 B and region  7 B are for example abrupt. Transistor  70  is arranged on the surface of an insulating layer  3  covering a support  5 . 
     In operation, potentials VC, VE, VB, and VG are respectively applied to regions  18 B,  19 B,  29 B, and on gate  9 B, according to the above-described embodiments. 
     The configuration of control structure  15 B enables, for a given tunnel current intensity supplied by the control structure, to limit the capacitances of stray capacitive elements formed, on the one hand, between quasi-intrinsic region  7 B and gate  9 B through insulator  11  and, on the other hand, between quasi-intrinsic region  7 B and support  5  through insulating layer  3 . Thereby, control structure  15 B provides a fast transistor capable of operating at high frequencies. 
     Specific embodiments have been described. Various alterations, modifications, and improvements will occur to those skilled in the art. In particular, although the described embodiments comprise a single control structure, other embodiments are possible where a quasi-intrinsic region comprises a central portion and two end portions, the central portion being comprised in a conduction structure and each of the end portions being comprised in a control structure. The two control structures can then be arranged symmetrically on either side of the conduction structure. The heavily-doped N-type regions of the two control structures may be connected to a same node of application of a potential VB. 
     Although, in the described embodiments, the transistor is associated with a rear gate, other embodiments are possible where the transistor is not associated with a rear gate. 
     In the described embodiments, transistor  1  comprises a conduction structure corresponding to a MOS transistor. The MOS transistor may also comprise LDD-type (lightly-doped drain) regions. 
     Although the described embodiments comprise heavily N-type doped regions in contact with a quasi-intrinsic P-type region arranged under a gate, the N and P conductivity type may be exchanged. One thus obtains an embodiment of a transistor comprising: first heavily P-type doped regions arranged on either side of a quasi-intrinsic N-type region and in contact therewith; and at least one second heavily P-type doped region in contact with the quasi-intrinsic region. The operating modes are similar to the described operating modes where the applied voltages are replaced with voltages of opposite signs. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.