Steep subthreshold swing bipolar transistor based on ferroelectric nano-voids and preparation method thereof

Abstract

The present invention discloses a steep subthreshold swing bipolar transistor based on ferroelectric nanogaps and a preparation method thereof. The transistor comprises, arranged from bottom to top, a substrate, a buffer layer, a conductive film layer, a ferroelectric dielectric film layer, and a gate, as well as a top gate control signal unit and a source-drain signal input unit. One end of the top gate control signal unit is connected to the gate, and the other end is connected to a series branch. The two ends of the source-drain signal input unit are respectively connected to the source and drain of the conductive film layer to form a series branch. The source-drain signal input unit is used to read the current of the steep subthreshold swing bipolar transistor. The top gate control signal unit is used to output a voltage to generate a vertical electric field to control the opening and closing of the nanogaps in the conductive film layer. The steep subthreshold swing bipolar transistor proposed in the present invention has the advantages of near-zero off-current, large on-current, steep subthreshold swing, low power consumption, and compatibility with traditional CMOS processes.

Description

Technical Field

[0001] The present invention belongs to the field of microelectronics technology, and more specifically, relates to a steep subthreshold swing bipolar transistor based on ferroelectric nanogaps and a method for fabricating the same, providing a new low-power transistor paradigm for the post-Moore era. Background Art

[0002] In the post-Moore era, integrated circuits are facing problems such as large leakage current and difficulty in further reducing operating voltage due to device size reduction, which directly limits the further reduction of circuit power consumption. In recent years, in traditional complementary metal oxide semiconductor (CMOS) technology, although the scaling of traditional device geometry has been ongoing, the operating voltage has not been significantly reduced. This is due to the fundamental bottleneck in subthreshold swing (SS). The subthreshold swing SS of a field effect transistor is usually defined as the source-drain current (I ds ) changes by one order of magnitude, the gate-source voltage (V gs ) changes. Due to the influence of channel capacitance and gate dielectric capacitance, the minimum SS value of metal oxide semiconductor field effect transistor (MOSFET) devices at room temperature is 60mV / decade. This is mainly determined by its drift-diffusion working mechanism and is called the "Boltzmann limit". Therefore, it is urgent to seek transistors with new working mechanisms to break the limitation and reduce the SS value to below 60mV / decade. At present, many types of steep subthreshold swing transistors have been proposed, such as tunneling field effect transistor (TFET), Dirac source field effect transistor (DCFET), negative capacitance field effect transistor (NCFET), nanoelectromechanical field effect transistor (NEM-FET), etc.

[0003] Tunneling field-effect transistors utilize quantum band-to-band tunneling (BTBT) to improve switching behavior. Unlike single-carrier tunneling, in which electrons or holes participate in tunneling, BTBT involves electrons and holes tunneling from the valence band to the conduction band, which can effectively reduce the transmission factor. A low subthreshold swing is achieved by efficiently controlling the current generated by the quantum tunneling effect through the gate voltage. However, carrier injection through band-to-band quantum tunneling may limit the drive current density, which conflicts with the requirement that logic transistors have both a low subthreshold swing and a high on-state current.

[0004] In a Dirac source field-effect transistor, a Dirac material with linear energy dissipation near the Dirac point, such as monolayer graphene, is used as the source material. A Dirac source can achieve a more localized electron distribution, thereby reducing the transfer factor. However, achieving extremely small subthreshold swings in this way is difficult. Furthermore, the performance of a Dirac source field-effect transistor strictly depends on the quality of the Dirac material, and the stable preparation of Dirac materials and device construction pose some challenges for further applications.

[0005] Negative capacitance field-effect transistors (NFC) primarily utilize ferroelectric materials in the gate to achieve a lower subthreshold swing. By leveraging the negative capacitance of the ferroelectric material to amplify the gate voltage, a low subthreshold swing is achieved. However, in this configuration, the reduced subthreshold swing leads to increased hysteresis in the transfer characteristic curve. Improving the performance of this transistor requires balancing the subthreshold swing with its hysteresis. This is currently the focus of much research.

[0006] The abrupt switch from on to off state can be easily found in nanoelectromechanical field-effect transistors, which control a mechanically movable gate via electrostatic forces. NEMFETs can achieve rapid on-off switching for very small changes in gate voltage. However, as devices scale, precisely controlling nanogap switching via electrostatic forces may become challenging. Relatively large contact resistance may also reduce the on-current. In addition, the nanogap at the heart of NEMFETs is typically formed by the final “release” etching step of a highly rigorous manufacturing process, greatly increasing the complexity of manufacturing.

[0007] The current steep subthreshold swing transistors still have some limitations in materials, device principles, preparation methods and performance, such as limited driving current density, high requirements for material quality and manufacturing process, difficulty in obtaining extremely small subthreshold swing, increased hysteresis and other technical problems, which need further improvement. Summary of the Invention

[0008] In response to the above-mentioned defects or improvement needs of the prior art, the present invention provides a steep subthreshold swing bipolar transistor based on ferroelectric nanogaps and a preparation method, thereby solving the technical problems of existing steep subthreshold swing transistors, such as limited driving current density, high material quality requirements, difficulty in obtaining extremely small subthreshold swing, and increased hysteresis.

[0009] To achieve the above objectives, according to one aspect of the present invention, a steep subthreshold swing bipolar transistor based on ferroelectric nanogaps is provided, comprising, arranged from bottom to top, a substrate, a buffer layer, a conductive film layer, a ferroelectric dielectric film layer, and a gate, as well as a top gate control signal unit and a source-drain signal input unit;

[0010] One end of the top gate control signal unit is connected to the gate, and the other end is connected to the series branch;

[0011] The two ends of the source-drain signal input unit are respectively connected to the source and drain of the conductive film layer to form a series branch; the source-drain signal input unit is used to read the current of the steep subthreshold swing bipolar transistor;

[0012] The top gate control signal unit is used to apply cyclic voltage pulses to generate nano-gaps in the conductive film layer, and generate a vertical electric field by outputting voltage to control the opening and closing of the nano-gaps in the conductive film layer.

[0013] Furthermore, when the top gate control signal unit outputs zero voltage, the nano-gap in the conductive film layer is in a closed state. At this time, the source-drain signal input unit is used to read the on-current of the steep subthreshold swing bipolar transistor, and the transistor is in an on state.

[0014] Furthermore, when the absolute value of the voltage output by the top gate control signal unit is higher than the threshold voltage, the nanogap in the conductive film layer is in an open state. At this time, the source-drain signal input unit is used to read the off current of the steep subthreshold swing bipolar transistor, and the transistor is in a closed state.

[0015] Furthermore, when the top gate control signal unit outputs a positive voltage and the positive voltage reaches a positive threshold voltage, the nanogap in the conductive film layer opens instantaneously, and the nanogap remains open while the voltage is greater than the positive threshold voltage.

[0016] Furthermore, when the top gate control signal unit outputs a negative voltage and the negative voltage reaches a negative threshold voltage, the nanogap in the conductive film layer opens instantaneously, and when the voltage is less than the negative threshold voltage, the nanogap remains open.

[0017] Furthermore, the thickness of the ferroelectric dielectric thin film layer is less than or equal to 500 nm, and the threshold voltage of the transistor is less than 1V.

[0018] According to another aspect of the present invention, a method for manufacturing a steep subthreshold swing bipolar transistor is provided, comprising:

[0019] A buffer layer is grown on the substrate by an atomic layer deposition process, a conductive thin film layer is then grown on the buffer layer by magnetron sputtering or electron beam evaporation, and a ferroelectric dielectric thin film layer is then grown on the conductive thin film layer by an atomic layer deposition process; and a gate is again grown on the ferroelectric dielectric thin film layer by magnetron sputtering or electron beam evaporation;

[0020] Ion beam etching is used to remove all materials above the source and drain in the conductive film layer to expose the source and drain positions. The two ends of the source-drain signal input unit are respectively connected to the source and drain of the conductive film layer through aluminum wires bonded by a wire bonding machine to form a series branch. One end of the top gate control signal unit is connected to the gate through aluminum wires bonded by a wire bonding machine, and the other end is connected to the series branch.

[0021] According to another aspect of the present invention, there is provided a steep subthreshold swing bipolar transistor based on ferroelectric nanogaps, comprising: a first metal electrode, a second metal electrode, a first conductive film, a second conductive film, a ferroelectric material, a side-gate control signal unit, and a source-drain signal input unit;

[0022] The first conductive film and the second conductive film are both located above the ferroelectric material, the first metal electrode is located above the first conductive film, the second metal electrode is located above the second conductive film, the first metal electrode and the first conductive film have the same size, the second metal electrode and the second conductive film have the same size, the second metal electrode and the second conductive film form an electrode, the first metal electrode and the first conductive film form a side gate, and the distance between the electrode and the side gate is 300nm-1μm;

[0023] The two ends of the source-drain signal input unit are respectively connected to the source and drain of the electrode to form a series branch, and the source-drain signal input unit is used to read the current of the steep subthreshold swing bipolar transistor;

[0024] One end of the side gate control signal unit is connected to the side gate, and the other end is connected to the series branch, which is used to apply cyclic voltage pulses to generate nano gaps in the electrode, and generate a vertical electric field by outputting voltage to control the opening and closing of the nano gaps in the electrode.

[0025] Furthermore, the ferroelectric material is a ferroelectric material with strain volatile properties, and the ferroelectric domains in the ferroelectric material with strain volatile properties undergo volatile flipping between in-plane and out-of-plane directions under the action of a vertical electric field.

[0026] According to another aspect of the present invention, a method for manufacturing a steep subthreshold swing bipolar transistor is provided, comprising:

[0027] Conductive films and metal electrodes are grown on ferroelectric materials by magnetron sputtering or electron beam evaporation; then parallel distributed electrodes and side gates are prepared by electron beam exposure and ion beam etching; the two ends of the source-drain signal input unit are respectively connected to the source and drain of the electrode by bonding aluminum wires with a wire bonding machine to form a series branch, and one end of the side gate control signal unit is connected to the side gate by bonding aluminum wires with a wire bonding machine, and the other end is connected to the series branch.

[0028] In general, the above technical solutions conceived by the present invention can achieve the following beneficial effects compared with the prior art:

[0029] (1) The switching of the steep subthreshold swing bipolar transistor based on ferroelectric nano-voids of the present invention is controlled by a quasi-mechanical method. Specifically, the electric field generated by the gate (top gate and side gate) control signal unit causes the ferroelectric domains in the ferroelectric material to flip unevenly. The local large strain generated during the domain flipping process generates nano-voids in the conductive film. The open and closed state of the nano-voids can be controlled by the gate voltage. When the nano-void is open, the air gap can completely block the current in the conductive film, and the source and drain are in a completely insulating state; when the nano-void is closed, the conductive path returns to the ohmic contact low resistance state. This principle of controlling the switching of transistors by a quasi-mechanical method can get rid of the Boltzmann limit in traditional CMOS, and can complete state switching instantly under small voltage changes, achieving a subthreshold swing of less than 60mV / decade. Experimental results show that the average SS is 31mV / dec and the minimum SS is 13.23mV / dec under current changes across 5 orders of magnitude. Therefore, the steep subthreshold swing bipolar transistor proposed in the present invention has the advantages of nearly zero off-current, large on-current, no limit on driving current density, low requirements on material quality and manufacturing process, steep subthreshold swing and low power consumption.

[0030] (2) The steep subthreshold swing bipolar transistor of the top gate structure of the present invention provides support for the buffer layer, the conductive film layer, the ferroelectric dielectric film layer and the gate through the substrate. The buffer layer is located on the surface of the substrate. Since the substrate has a very high Young's modulus relative to the conductive film layer, it is difficult to deform. In order to avoid the constraint of the conductive film layer when it is in direct contact with the substrate, a buffer layer is added between the conductive film layer and the substrate. The ferroelectric dielectric film layer is located above the conductive film layer. Under the action of the electric field, the ferroelectric domains in the ferroelectric material will undergo uneven flipping, which may cause local stress release and generate nano-voids in the conductive film layer.

[0031] (3) The present invention can effectively control the state of the nano-gap by applying voltage. When the gap is open, the source and drain are in a completely insulated state; when the gap is closed, the source and drain are in an ohmic contact low resistance state. When the transistor is in the on state, the conduction current (I ON ) is the current of the conductive film in the ohmic low resistance state. At this time, the transistor can achieve a large on-current. The off-current of the transistor (I OFF ) is read when the nanogap is fully open. The current path is completely blocked by the air gap, so the transistor has a near-zero off-current (I OFF ). At the same time, low source-drain voltage can achieve about 10 6All of this provides a theoretical basis for the realization of low-power transistors. Because the state change of the transistor is controlled by a quasi-mechanical method, a tiny voltage change can instantly cut off the current path. In principle, a sudden switching starting from the subthreshold region can be achieved. Therefore, the requirements of low subthreshold swing and high on-current density can be met simultaneously, which are the key indicators for logic transistors to benefit from steep slopes.

[0032] (4) In a top-gate steep subthreshold swing bipolar transistor, simulation results show that when the thickness of the ferroelectric dielectric layer is reduced to 500nm, the transistor can be controlled by a gate voltage below 1V. At the same time, the fabrication process of the top-gate transistor is compatible with conventional CMOS processes. Compared with side-gate transistors, top-gate transistors are expected to achieve higher integration density.

[0033] (5) The ferroelectric domains in the ferroelectric material below the side-gate electrode of the present invention undergo uneven flipping under the action of a vertical electric field. To more effectively control domain flipping, the spacing between the electrode and the side-gate can be appropriately reduced to achieve a large vertical electric field, provided that the side-gate control signal unit outputs the same voltage.

[0034] (6) Ferroelectric materials are ferroelectric materials with strain-volatile properties, and transistors exhibit bipolar characteristics. This is presumably because the in-plane domains within them can be transformed into out-of-plane domains under either a vertical upward electric field or a vertical downward electric field. The similar local strain generated at this time will cause the nanovoids to transform into an open state.

[0035] (7) The preparation process of the transistors with top-gate structure and side-gate structure in the present invention is easy to implement and has no special requirements for the preparation materials, which reduces the preparation cost and difficulty, and greatly reduces the complexity of the steep subthreshold swing bipolar transistor. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 Schematic diagram of the structure of a ferroelectric nano-gap-based side-gate steep subthreshold swing bipolar transistor provided by an embodiment of the present invention;

[0037] Figure 2 This is a scanning electron microscope image of a ferroelectric nano-gap-based side-gate steep subthreshold swing bipolar transistor provided by an embodiment of the present invention;

[0038] Figure 3 This is a scanning electron microscope image of a nanovoid in a side-gate steep subthreshold swing bipolar transistor based on a ferroelectric nanovoid provided by an embodiment of the present invention;

[0039] Figure 4(a) is an n-type transfer characteristic curve of a ferroelectric nano-gap-based side-gate steep subthreshold swing bipolar transistor provided by an embodiment of the present invention;

[0040] Figure 4 (b) is a p-type transfer characteristic curve of a side-gate steep subthreshold swing bipolar transistor based on a ferroelectric nano-void provided by an embodiment of the present invention;

[0041] Figure 5 The graph is a curve showing the relationship between the subthreshold swing and the source-drain current of the steep subthreshold swing bipolar transistor with a side-gate structure based on ferroelectric nano-voids provided by an embodiment of the present invention;

[0042] Figure 6 Schematic diagram of the structure of a ferroelectric nano-gap-based top-gate steep subthreshold swing bipolar transistor provided by an embodiment of the present invention;

[0043] Figure 7 The relationship between the switching voltage and the thickness of the ferroelectric dielectric layer of the bipolar transistor with a steep subthreshold swing and a top gate structure based on ferroelectric nanovoids provided by an embodiment of the present invention obtained by simulation;

[0044] Throughout the drawings, the same reference numerals are used to denote the same elements or structures, wherein:

[0045] 101 is the first metal electrode, 102 is the first conductive film, 103 is the second metal electrode, 104 is the second conductive film, 105 is the ferroelectric material, 106 is the side gate control signal unit, 107 is the source-drain signal input unit, 601 is the substrate, 602 is the buffer layer, 603 is the conductive film layer, 604 is the ferroelectric dielectric film layer, 605 is the gate, 606 is the top gate control signal unit, and 607 is the source-drain signal input unit. DETAILED DESCRIPTION

[0046] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0047] like Figure 1As shown, a ferroelectric nano-void-based side-gate steep subthreshold swing bipolar transistor includes: a first metal electrode 101, a second metal electrode 103, a first conductive film 102, a second conductive film 104, a ferroelectric material 105, a side-gate control signal unit 106, and a source-drain signal input unit 107. The first and second conductive films are both located above the ferroelectric material, with the first metal electrode located above the first conductive film, and the second metal electrode located above the second conductive film. The first metal electrode and the first conductive film have the same dimensions, and the second metal electrode and the second conductive film have the same dimensions. 103 and 104 form the first electrode, and 101 and 102 form the second electrode (side-gate). The first and second electrodes are separated by a distance of approximately 300 nm to 1 μm. Cyclic voltage pulses are applied by the side-gate control signal unit, causing the ferroelectric domains in the ferroelectric material to repeatedly flip under the action of the electric field. The strain generated by the domain flipping creates a nano-void 108 on the first electrode, and the nano-void can be reversibly opened and closed under the control of the side-gate control signal unit.

[0048] The two ends of the first electrode are defined as the source and drain of the transistor, respectively, and are connected to the two ends of the source-drain signal input unit. The source-drain signal input unit is used to read the source-drain working current (I DS ), the output voltage is 50mV.

[0049] The second electrode is defined as a side gate. One end of the side gate control signal unit is connected to the first electrode, and the other end is connected to the second electrode, for controlling the on and off of the steep subthreshold swing bipolar transistor based on the ferroelectric nano-gap side gate structure.

[0050] The ferroelectric material is a barium titanate (BaTiO3) single crystal or other ferroelectric materials with strain-volatile properties. The ferroelectric domains in the barium titanate single crystal can undergo volatile flipping between in-plane and out-of-plane directions under the action of a perpendicular electric field.

[0051] The ferroelectric domains in the ferroelectric material below the first electrode undergo uneven flipping under the influence of a vertical electric field. To more effectively control domain flipping, the spacing between the first and second electrodes can be appropriately reduced to achieve a large vertical electric field, while maintaining the same voltage across the side-gate control signal unit. Here, the spacing between the first and second electrodes ranges from 300nm to 1μm.

[0052] The conductive film is made of MnPt alloy, and the metal electrode is made of Pt film. Other suitable materials are also applicable, such as TiN, NiFe, NiPt, FePt, MnAl, Au, Cu, Ag, etc.

[0053] By applying cyclic voltage pulses through the side-gate control signal unit, the uneven flipping of domains causes localized large strain, which generates nano-voids on the first electrode. These voids can be reversibly opened and closed under the control of the side-gate control signal unit. When the voids are open, the source and drain are completely insulated; when the voids are closed, the source and drain form an ohmic contact with low resistance.

[0054] When the side gate control signal unit applies zero voltage, the nanogap in the first electrode is in a closed state. At this time, the current read by the source-drain signal input unit is the on-state current of the transistor (I ON ), the transistor is in the on state.

[0055] When the positive voltage applied by the side gate control signal unit reaches the positive threshold voltage (about 30V), the nanogap in the first electrode opens instantly. During the period when the voltage is greater than the threshold voltage, the nanogap remains open. At this time, the current read by the source-drain signal input unit is the off current of the transistor (I OFF ), the transistor is in the off state.

[0056] When the negative voltage applied by the side gate control signal unit reaches the negative threshold voltage (about -30V), the nanogap in the first electrode opens instantly. During the period when the voltage is less than the negative threshold voltage, the nanogap remains open. At this time, the current read by the source-drain signal input unit is the off current of the transistor (I OFF ), the transistor is also in the off state.

[0057] When the output voltage of the side gate control signal unit returns to zero voltage from the positive and negative high voltage, the nanogap in the first electrode returns to the closed state, and the current read by the source-drain signal input unit returns to the on-current of the transistor (I ON ), the transistor returns to the on state. Therefore, the nanovoids, caused by the localized large strain generated by the uneven flipping of ferroelectric domains, exhibit volatile on-off behavior. This rapid mechanical flipping escapes the Boltzmann limit and, similar to mechanically switched nanoelectromechanical devices, allows instantaneous state switching with tiny voltage changes, achieving a subthreshold swing of less than 60mV / decade. It is also clear that the transistor exhibits bipolar characteristics.

[0058] The on-state current of this transistor (I ON ) is the current of the conductive film in the ohmic contact state, so a large on-current can be achieved. And by choosing a conductive material with lower resistivity, such as Au, the on-current (I ON ).

[0059] The off-state current (I OFF) is read when the nanogap is fully open. The first electrode path is completely blocked by the air gap, so the transistor has an off-current (I OFF ).

[0060] The preparation method of a side-gate structure transistor is as follows: a conductive film and a metal electrode of appropriate thickness are grown on a ferroelectric material substrate by coating methods such as magnetron sputtering or electron beam evaporation; a first electrode and a second electrode distributed in parallel are then prepared by electron beam exposure (or photolithography) and ion beam etching; the electrical contact of the electrodes is achieved by bonding aluminum wires with a wire bonding machine, and then connected to the corresponding input ports of the side-gate control signal unit and the source-drain signal input unit.

[0061] like Figure 2 Figure 2 shows a scanning electron microscope image of a side-gate steep subthreshold swing bipolar transistor based on a ferroelectric nanovoid. The first and second electrodes are separated by 409 nm. The ferroelectric material here is a (100)-BaTiO3 single crystal, which is in the T phase at room temperature and has a thickness of 500 μm. The conductive film here is a MnPt alloy, and the metal electrode is Pt.

[0062] like Figure 3 A scanning electron microscope image of a nanovoid in a ferroelectric nanovoid-based side-gated steep subthreshold swing bipolar transistor is shown. The white arrow points to the nanovoid. The nanovoid is in its closed state with zero gate voltage.

[0063] like Figure 4 (a) and Figure 4 Figure (b) shows the transfer characteristic curves of a side-gate steep subthreshold swing bipolar transistor based on a ferroelectric nanovoid, for n-type (a) and p-type (b). Here, the source-drain signal input unit outputs 50mV to read the transistor's source-drain current I DS . At the gate voltage V G When it is zero, the nano-gap is closed, the source and drain are in ohmic contact, and the corresponding transistor on-state current I ON About 20μA / μm. When V G When the voltage drops to the negative threshold (about -30V), the current will suddenly switch from the on state to the off state. The current at this time is on the order of tens of picoamperes. This order of magnitude of current can be explained as circuit noise, and the current has reached the test limit of the instrument. G When the negative threshold voltage is increased from -36V, the current suddenly returns to the on state. This behavior is similar to that of n-type MOSFET. G When the positive threshold voltage (about 30V) is reached, the current will suddenly switch from the on state to the off state. GWhen the current drops below the positive threshold voltage from +36V, it suddenly returns to the on state. This behavior is similar to that of a p-type MOSFET. At a source-drain voltage of 50mV, the source-drain current exhibits a 10-fold increase in the transistor's on and off states. 6 At the same time, a sudden switch occurs starting from the subthreshold region.

[0064] Figure 5 The relationship curve between the subthreshold swing and source-drain current of the steep subthreshold swing bipolar transistor with a side-gate structure based on ferroelectric nanovoids (SS-I DS ). This curve is from Figure 4 Extracted transfer characteristic curve of the n-type transistor in (a). The shaded area is a square, and the shaded area indicates the portion where the SS is less than 60mV / dec. It can be seen that the average SS across five orders of magnitude of current variation is 31mV / dec, and the minimum subthreshold swing is 13.23mV / dec.

[0065] like Figure 6 As shown, a top-gate steep subthreshold swing bipolar transistor based on a ferroelectric nanovoid structure includes: a substrate 601, a buffer layer 602, a conductive film layer 603, a ferroelectric dielectric film layer 604, a gate 605, a top-gate control signal unit 606, and a source-drain signal input unit 607. The substrate provides support for the buffer layer, the conductive film layer, the ferroelectric dielectric film layer, and the gate. The buffer layer, located on the substrate surface, relieves stress constraints imposed by the substrate on the conductive film layer, allowing the nanovoids in the conductive film to open and close freely. The conductive film layer, located above the buffer layer, serves as the source and drain of the transistor at its ends. The ferroelectric dielectric film layer, located above the conductive film layer, provides strain to control the opening and closing of the nanovoids. The gate is fabricated on the upper layer of the ferroelectric dielectric film and is connected to the top-gate control signal unit. Cyclic voltage pulses applied by the top-gate control signal unit cause the ferroelectric domains in the ferroelectric dielectric film to flip repeatedly under the action of the electric field. The strain generated by domain reversal will generate a nano-void 608 on the first electrode, and the void can be reversibly opened and closed under the control of the top gate control signal unit.

[0066] The substrate is a single crystal silicon (Si) wafer, which can provide support for the buffer layer, conductive film layer, ferroelectric dielectric film layer, and gate. The buffer layer is located on the surface of the substrate. A buffer layer with a low Young's modulus and moderate brittleness and ductility, such as a loose aluminum oxide (Al2O3) film with a thickness of 40nm-300nm, is added between the conductive film layer and the substrate. The conductive film layer is located above the buffer layer, with its two ends serving as the source and drain of the transistor. It is necessary to select a material with moderate brittleness and ductility, such as MnAl, FePt, FeNi, MnPt, etc. The thickness is approximately 5nm-10nm. The ferroelectric dielectric film layer is located above the conductive film layer. Under the action of an electric field, the ferroelectric domains in the ferroelectric material will undergo uneven flipping, which may lead to local stress release. A barium titanate (BaTiO3) film (with a thickness range of 60nm-300nm) is selected as the ferroelectric dielectric film layer. According to the current preparation process, it is generally necessary to first prepare a layer of strontium titanate (SrTiO3) film (thickness of about 10nm) as a buffer layer, and then grow a BaTiO3 film on the SrTiO3. Similarly, hafnium zirconium oxide (Hf x Zr 1-x O2, HZO) thin film can also be used as a ferroelectric dielectric thin film layer with a thickness of about 15nm.

[0067] The gate is made on the upper layer of the ferroelectric dielectric film and is connected to the top gate control signal unit. The gate material can be selected from Au, Cr, Ti, etc., with a thickness of about 50nm-100nm. The top gate control signal unit is connected to the gate at one end and to one end of the conductive film layer at the other end. The source and drain signal input unit is connected to the two ends (source and drain) of the conductive film layer at both ends. By applying a cyclic voltage pulse through the top gate control signal unit, the ferroelectric domains in the ferroelectric dielectric film layer will undergo uneven flipping under the action of the electric field. The resulting local large strain will produce nano-gaps 608 in the conductive film layer, and the state of the nano-gaps can be effectively controlled by the gate voltage. When the gap is in the open state, the source and drain ends are in a completely insulated state; when the gap is in the closed state, the source and drain ends are in an ohmic contact low-resistance state.

[0068] When the top gate control signal unit applies zero voltage, the nano-gap in the conductive film layer is in a closed state. At this time, the current read by the source-drain signal input unit is the on-current of the transistor (I ON ), the transistor is in the on state.

[0069] When the top gate control signal unit applies a positive (or negative) voltage that reaches the positive (or negative) threshold voltage, the nanogap in the conductive film layer opens instantly. During the period when the absolute value of the gate voltage is greater than the absolute value of the threshold voltage, the nanogap remains open. At this time, the current read by the source-drain signal input unit is the off current of the transistor (I OFF), the transistor is in the off state. The absolute value of the threshold voltage is approximately (0.5V-5V).

[0070] When the output voltage of the top gate control signal unit returns from positive (or negative) high voltage to zero voltage, the nano-gap in the conductive film layer returns to a closed state, and the current read by the source-drain signal input unit returns to the on-current of the transistor (I ON ), the transistor returns to the on state.

[0071] The operating principle of top-gate transistors is based on the nanovoids caused by the uneven reversal of ferroelectric domains, resulting in localized large strain. Top-gate transistors also have the advantage of steep subthreshold swings. They also offer lower gate control voltages (less than 1V), compatibility with traditional CMOS processes, and the potential for higher integration density.

[0072] The fabrication method for a top-gate transistor includes: growing a buffer layer (Al2O3) on a supporting substrate using a process such as atomic layer deposition (ALD); then growing a conductive thin film layer of appropriate thickness on the buffer layer using a coating method such as magnetron sputtering or electron beam evaporation; then growing a ferroelectric thin film layer on the conductive thin film layer using a process such as atomic layer deposition (ALD); and finally growing a metal gate on the ferroelectric thin film layer using a coating method such as magnetron sputtering or electron beam evaporation; then pattern transfer using electron beam exposure or photolithography; and finally, using ion beam etching to precisely remove all material above the source and drain conductive thin film layers, exposing the source and drain locations. Insulation between devices and between electrodes is handled appropriately based on actual conditions. Electrical contact between the electrodes is achieved by bonding aluminum wires using a wire bonder, which are then connected to the corresponding input ports of the top-gate control signal unit and the source / drain signal input unit.

[0073] like Figure 7 Figure 2 shows the relationship between the switching voltage and the thickness of the ferroelectric dielectric layer for a top-gate steep subthreshold swing bipolar transistor based on a ferroelectric nanovoid. Based on experimental test data (at a thickness of 500μm), electric field simulations were performed using simulation software. It was found that the switching threshold voltage decreases with decreasing ferroelectric dielectric layer thickness. When the ferroelectric dielectric layer thickness is reduced to 500nm, the transistor state can be controlled with a gate voltage below 1V.

[0074] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A steep subthreshold swing bipolar transistor based on ferroelectric nanogaps, characterized in that: The device comprises a substrate (601), a buffer layer (602), a conductive film layer (603), a ferroelectric dielectric film layer (604) and a gate (605) arranged in sequence from bottom to top, as well as a top gate control signal unit (606) and a source-drain signal input unit (607); One end of the top gate control signal unit (606) is connected to the gate (605), and the other end is connected to the series branch; The two ends of the conductive film layer (603) serve as the source and drain of the transistor respectively, and the two ends of the source-drain signal input unit (607) are respectively connected to the source and drain at the two ends of the conductive film layer (603) to form a series branch; the source-drain signal input unit (607) is used to read the current of the steep subthreshold swing bipolar transistor; The top gate control signal unit (606) is used to apply a cyclic voltage pulse to generate a nano-gap (608) in the conductive film layer (603), and to generate a vertical electric field by outputting a voltage to control the opening and closing of the nano-gap (608) in the conductive film layer (603); When the top gate control signal unit (606) outputs zero voltage, the nano-gap (608) in the conductive film layer (603) is in a closed state. At this time, the source-drain signal input unit (607) is used to read the on-current of the steep subthreshold swing bipolar transistor, and the transistor is in an on state. When the absolute value of the voltage output by the top gate control signal unit (606) is higher than the threshold voltage, the nano-gap (608) in the conductive film layer (603) is in an open state. At this time, the source-drain signal input unit (607) is used to read the off current of the steep subthreshold swing bipolar transistor, and the transistor is in an off state.

2. The steep subthreshold swing bipolar transistor according to claim 1, wherein: When the top gate control signal unit (606) outputs a positive voltage and the positive voltage reaches a positive threshold voltage, the nano-gap (608) in the conductive film layer (603) is instantly opened, and the nano-gap remains open while the voltage is greater than the positive threshold voltage.

3. The steep subthreshold swing bipolar transistor according to claim 1, wherein: When the top gate control signal unit (606) outputs a negative voltage and the negative voltage reaches a negative threshold voltage, the nano-gap (608) in the conductive film layer (603) opens instantaneously, and when the voltage is less than the negative threshold voltage, the nano-gap remains open.

4. The steep subthreshold swing bipolar transistor according to any one of claims 1 to 3, wherein: The thickness of the ferroelectric dielectric thin film layer is less than or equal to 500 nm, and the threshold voltage of the transistor is less than 1V.

5. The method for preparing a steep subthreshold swing bipolar transistor according to any one of claims 1 to 4, wherein: include: A buffer layer is grown on the substrate by an atomic layer deposition process, a conductive thin film layer is then grown on the buffer layer by magnetron sputtering or electron beam evaporation, and a ferroelectric dielectric thin film layer is then grown on the conductive thin film layer by an atomic layer deposition process; and a gate is again grown on the ferroelectric dielectric thin film layer by magnetron sputtering or electron beam evaporation; Ion beam etching is used to remove all materials above the source and drain in the conductive film layer to expose the source and drain positions. The two ends of the source-drain signal input unit are respectively connected to the source and drain of the conductive film layer through aluminum wires bonded by a wire bonding machine to form a series branch. One end of the top gate control signal unit is connected to the gate through aluminum wires bonded by a wire bonding machine, and the other end is connected to the series branch.

6. A steep subthreshold swing bipolar transistor based on ferroelectric nanogaps, characterized in that: include: a first metal electrode, a second metal electrode, a first conductive film, a second conductive film, a ferroelectric material, a side-gate control signal unit, and a source-drain signal input unit; The first conductive film and the second conductive film are both located above the ferroelectric material, the first metal electrode is located above the first conductive film, the second metal electrode is located above the second conductive film, the first metal electrode and the first conductive film have the same size, the second metal electrode and the second conductive film have the same size, the second metal electrode and the second conductive film form an electrode, the first metal electrode and the first conductive film form a side gate, and the distance between the electrode and the side gate is 300nm-1μm; The two ends of the electrode are respectively defined as the source and drain of the transistor, and the two ends of the source-drain signal input unit are respectively connected to the source and drain at the two ends of the electrode to form a series branch. The source-drain signal input unit is used to read the current of the steep subthreshold swing bipolar transistor; One end of the side gate control signal unit is connected to the side gate, and the other end is connected to the series branch, which is used to apply cyclic voltage pulses to generate nano gaps in the electrode, and generate a vertical electric field by outputting voltage to control the opening and closing of the nano gaps in the electrode.

7. The steep subthreshold swing bipolar transistor according to claim 6, wherein: The ferroelectric material is a ferroelectric material with strain volatile characteristics. The ferroelectric domains in the ferroelectric material with strain volatile characteristics undergo volatile flipping between in-plane and out-of-plane directions under the action of a vertical electric field.

8. The method for preparing a steep subthreshold swing bipolar transistor according to claim 6 or 7, wherein: include: growing conductive films and metal electrodes on ferroelectric materials by magnetron sputtering or electron beam evaporation; Then, parallel distributed electrodes and side gates are prepared by electron beam exposure and ion beam etching; the two ends of the source-drain signal input unit are connected to the source and drain of the electrode respectively through aluminum wires bonded by a wire bonding machine to form a series branch, and one end of the side gate control signal unit is connected to the side gate through aluminum wires bonded by a wire bonding machine, and the other end is connected to the series branch.

Images