Patent ID: 12211849

DETAILED DESCRIPTIONS

Hereinafter, an embodiment of the present disclosure will be described in detail with reference to the accompanying drawings. In the present disclosure, various changes may be made. The present disclosure may have various forms. Thus, specific embodiments may be illustrated in the drawings and may be described in detail herein. However, the embodiments are not intended to limit the present disclosure to a specific form. It should be understood that the present disclosure may include all changes, equivalents or substitutes included in the spirit and scope of the present disclosure. In illustrating the drawings, like reference numerals have been used for like elements.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the singular forms “a” and “an” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “includes”, and “including” when used in this specification, specify the presence of the stated features, integers, operations, elements, and/or components, but do not preclude the presence or addition of one or greater other features, integers, operations, elements, components, and/or portions thereof.

Unless otherwise defined, all terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this inventive concept belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

FIG.1is a cross-sectional view for illustrating a super-steep switching device according to one embodiment of the present disclosure.

Referring toFIG.1, a super-steep switching device100according to one embodiment of the present disclosure may include a substrate110, a semiconductor channel120, a source electrode130, a drain electrode140, and a gate electrode150.

A type of the substrate110is not particularly limited as long as the substrate may support the semiconductor channel120, the source electrode130, and the drain electrode140thereon. For example, the substrate110may include one selected from a metal having an insulating film formed on a surface thereof, a semiconductor substrate, a ceramic substrate, a polymer substrate, and the like.

The semiconductor channel120may be disposed on the substrate110. The semiconductor channel120may be made of a semiconductor material having impact ionization characteristic. The semiconductor channel120may be made of a p-type semiconductor material or an n-type semiconductor material. For example, the semiconductor channel120is made of a two-dimensional p-type semiconductor material such as black phosphorus (BP), tungsten diselenide (WSe2), boron arsenide (BAs), or etc. or may be made of a two-dimensional n-type semiconductor material such as indium selenide (InSe), molybdenum disulfide (MoS2), indium selenide (In2Se3), zirconium diselenide (ZrSe2), hafnium diselenide (HfSe2), etc.

In one embodiment, the semiconductor channel120may be made of a material having a relatively low bandgap. This is because, as the band gap of the semiconductor channel120is smaller, an intensity of a critical electric field intensity that causes avalanche carrier multiplication, which will be described below, may be reduced. For example, the semiconductor channel120may be made of a semiconductor material having a bandgap of about 0.3 to 1.9 eV.

The source electrode130and the drain electrode140may be in contact with the semiconductor channel120while being disposed on the substrate110and being spaced apart from each other. An area of the semiconductor channels120between the source electrode130and the drain electrode140may constitute a channel through which current may flow. Each of the source and drain electrodes130and140may be made of an electrically conductive material.

The gate electrode150may be disposed on the semiconductor channel120so as to overlap only a portion of the semiconductor channel120, and may receive a voltage so as to generate an electric field in the semiconductor channel120. In one embodiment, the gate electrode150may be disposed so as to overlap only a portion of the semiconductor channel120adjacent to the source electrode130. Further, an insulating layer151may be formed between the gate electrode150and the semiconductor channel120.

In one embodiment, a top surface of the semiconductor channel120may include a first area A overlapping the gate electrode150, and a second area B non-overlapping the gate electrode150. A ratio of a length of the first area A and a length of the second area B may be adjusted to control characteristic of the super-steep switching device.

In one embodiment, in a device structure in which the first area A and the second area B are formed, a voltage for generating an electric field having an intensity greater than a minimum electric field intensity (hereinafter, referred to as a ‘critical electric field intensity’) at which the avalanche multiplication occurs in the first area A may be applied to the drain electrode140.

Further, a voltage may be applied to the gate electrode150simultaneously with the voltage application to the drain electrode140. As the gate voltage gradually increases, the avalanche carrier multiplication phenomenon may occur in the first area A. That is, while the electric field having the intensity above the critical electric field intensity is applied to the first area A under the voltage application to the drain electrode140, the gate voltage is gradually increased to generate the avalanche carrier multiplication, thereby implementing super-steep switching phenomenon at room temperature.

When the electric field is applied to the first area A, the charge carriers are accelerated in the first area A. In general, the velocity of the charge carriers does not increase indefinitely. Rather, due to impact thereof with the lattice, the velocity of the charge carriers is saturated at a constant level. However, when a sufficiently strong electric field, that is, an electric field having the intensity larger than the critical electric field intensity is applied, the charge carriers sufficiently accelerated under the electric field collide with the lattice to cause an electron in the valence band to transfer to the conduction band, such that a new electron-hole pair is created. The new electron-hole pair again acquires high energy to continuously generate additional electron-hole pairs, and thus the carrier density may be greatly increased. In the present disclosure, the avalanche multiplication means that carriers are multiplicated under the impact ionization as described above, and the critical electric field intensity indicates the minimum electric field intensity at which the avalanche multiplication occurs.

In one embodiment, regarding dark current change characteristic based on the electric field to the first area A, when the electric field having an intensity below the critical electric field intensity is applied to the first area A, a magnitude of the dark current generated in the first area A increases linearly based son the intensity of the applied electric field. To the contrary, when an electric field having an intensity larger than the critical electric field intensity is applied thereto, the magnitude of the dark current generated in the first area A increases in a superlinear manner due to the avalanche multiplication. Accordingly, the critical electric field intensity may refer to an electric field intensity at a point where behavior of the dark current changes from a linear manner to the superlinear manner in the dark current change characteristic based son the electric field for the first area A.

In this regard, when the intensity of the electric field applied to the first area A greater than the critical electric field intensity continuously increases, breakdown of the first area A occurs. Accordingly, the drain electrode140may apply, to the semiconductor channel120, an electric field having an intensity that is greater than the critical electric field intensity and is lower than the intensity of the electric field at which breakdown of the semiconductor channel120occurs.

In one example, as described above, according to the present disclosure, the ratio of the length of the first area A and the length of the second area B may be controlled to implement the super-steep switching device that exhibits a low SS (Subthreshold Swing) value even at room temperature. In this regard, the SS (Subthreshold Swing) means a VGSvalue required to increase a drain-source current IDSby 10 times in a field effect transistor.

In one embodiment, the second area B may have a length greater than 5 nm so that a tunneling phenomenon does not occur therein, and may have the length smaller than 400 nm so that steep switching may be possible. The second area B may have a length smaller than 400 nm so that a steep-switching phenomenon does not occur therein. For an optimized operation of the super-steep switching device according to this embodiment, the ratio of the length of the first area A and the length of the second area B may be in a range of 1:0.1 to 0.4.

When the ratio of the length of the second area to the length of the first area A is smaller than 0.1, an off current increases such that an on/off ratio decreases as, and thus the SS value increases, and thus, probability of occurrence of impact ionization decreases. On the contrary, when the ratio of the length of the second area B to the length of the first area A exceeds 0.4, the steep-switching does not occur.

However, in the super-steep switching device of this embodiment, a length of a gate area of the semiconductor channel120having impact ionization characteristic may be adjusted as described above, thereby increasing the probability of occurrence of the avalanche carrier multiplication phenomenon occurring in the first area A, and thus, significantly increasing the number of charge carriers generated in the semiconductor channel120. As a result, the super-steep switching device having a very low (5 mV/dec or lower) SS (Subthreshold Swing) value even at room temperature and having an optimized On/Off ratio may be implemented.

FIG.2is a schematic diagram and a circuit diagram of an inverter device according to one embodiment of the present disclosure.

Referring toFIG.2, an inverter device20according to one embodiment of the present disclosure may include the super-steep switching device100and a pull-up transistor200.

The super-steep switching device100includes the configuration as described above. Thus, the description thereof will be omitted.

The pull-up transistor200may be connected in series with the super-steep switching device100and may operate in a complementary manner to the super-steep switching device100.

In one embodiment, a gate electrode210of the pull-up transistor200and the gate electrode150of the super-steep switching device100are electrically connected to each other. Thus, the same gate voltage may be applied thereto.

In one embodiment, a semiconductor layer220of the pull-up transistor200may be made of an n-type semiconductor material capable of performing a complementary operation with the super-steep switching device. For example, the semiconductor layer220of the pull-up transistor200may be made of an n-type semiconductor material such as molybdenum disulfide (MoS2), indium selenide (In2Se3), rhenium disulfide (ReS2), or molybdenum diselenide (MoSe2).

In one embodiment, the semiconductor layer220of the pull-up transistor200may be made of a p-type semiconductor material capable of performing a complementary operation with the super-steep switching device. For example, the semiconductor layer220of the pull-up transistor200may be made of a p-type semiconductor material such as black phosphorus (BP), tungsten diselenide (WSe2), boron arsenide (BAs), or tellurium.

The inverter device20according to the present disclosure may exhibit a high inverter gain and ideal noise margin characteristic based on the super-steep switching phenomenon, due to a simple series connection circuit configuration between the super-steep switching device100and the pull-up transistor200that may operate in a complementary manner to the super-steep switching device100.

Hereinafter, Examples of the present disclosure will be described in detail. However, the following Examples are only some examples of the present disclosure, and the scope of the present disclosure is not limited to the following Examples.

Example 1: Characteristics of Super-Steep Switching Device

(a) inFIG.3shows a schematic diagram of a WSe2homojunction transistor-based super-steep switching device according to one embodiment of the present disclosure, and a band structure diagram that changes based on a drain voltage and a top gate voltage.

Referring to (a) inFIG.3, it may be identified that when sufficiently high drain voltage (Avalanche bias) and top gate voltage are applied, the impact ionization occurs in the second area (Ungated I2region) non-overlapping the gate electrode.

Further, (b) and (c) inFIG.3show output characteristic (ID-VD) and transfer characteristic (ID-VG), respectively, exhibiting a rapid increase in current based on the impact ionization phenomenon. (c) inFIG.3shows, as an inset, an enlarged graph of an area where super-steep switching takes place. Based on the graph, it may be identified that the device according to the present disclosure overcomes the thermionic limit of a CMOS device of 60 mV/dec and rather exhibits an SS value of 2.73 mV/dec.(d) inFIG.3is a contour diagram of a channel current changing based on the drain voltage and the top gate voltage and shows a steep-switching result measured via the impact ionization. Referring toFIG.3(d), it may be identified that a critical top gate voltage required for the steep-switching changes based on change in the applied drain voltage (higher than or equal to VBRdetermined based on a length of the ungated-region). On the contrary, it may be identified when the drain voltage smaller than VBRis applied, the channel current stays at a saturated current.

Example 2: Changes in Characteristic Based on Gate Area Length Adjustment of Super-Steep Switching Device

(a) inFIG.4is a graph showing change in critical voltage VBRbased on change in the length of the first area (Gated region) overlapping the gate electrode and in the length of the second area (Ungated I2region) non-overlapping the gate electrode.

As shown (a) in inFIG.4, it may be identified that as both the length of the first area (Gated region) overlapping the gate electrode and the length of the second area (Ungated I2region) which does not overlap the gate electrode are smaller, the critical voltage VBRdecreases.

However, since device characteristic must be based on the critical voltage as well as the On/Off ratio, and the SS value, the probability of occurrence of impact ionization, the On/Off current ratio, and the SS value, based on the ratio of the length of the first area (Gated region) and the length of the second area (Ungated I2region) which does not overlap the gate electrode were measured and were shown in (b) to (d) inFIG.4respectively.

First, referring to (b) to (d) inFIG.4, it may be identified that the off current increases as the length of the second area (Lungated) decreases, and the On current increases as the length of the first area (Lgated) decreases. Considering these results, the length of the first area (Lgated) is fixed to 1 μm, and the length of the second area (Lungated) changes. Thus, it may be identified that when the length of the second area (Lungated) is in a range of 100 to 400 nm, there is little change in the On current and Off current, the SS value is a remarkably low value of 5 mV/dec or smaller.

However, it is identified that when the ratio of the length of the second area (Lungated) to the length of the first area (Lgated) is smaller than 0.1 (that is, the length of the second area (Lungated) is smaller than 100 nm), the Off current suddenly increases such that the On/Off ratio decreases. Thus, the SS value increases.

Further, as shown in (b) inFIG.4, it may be identified that when the length of the second area is in a range of 100 to 400 nm based on the length of 1 μm of the first area, the probability of occurrence of impact ionization is high. However, it may be identified that when the length of the second area (Lungated) is smaller than 100 nm, the probability of occurrence of impact ionization is reduced. It may be identified that when the length of the second area (Lungated) exceeds 400 nm, the steep-switching does not occur.

From these results, it may be identified that when the ratio of the length of the first area and the length of the second area is in a range of 1:0.1 to 0.4 in accordance with the present disclosure, the On/Off ratio of the device and the SS value of the device may be optimized.

FIG.5is a graph showing a change in breakdown voltage (VBR) according to a change in a length (Lungated, nm) of the second area under the same VGScondition (−3 V).

As shown inFIG.5, it can be seen that a lower breakdown voltage may be secured by reducing the length of the second area.

FIG.6is a graph showing IDSaccording to VGSunder a condition of having VDSof 3 V and a second area length (Lungated) of 45 nm.

As shown inFIG.6, it can be seen that an SS value is 2.9 mV/dec and a VTHvalue is −0.38 V. In addition, it can be seen that VTH(VGS) decreases as the length of the second area decreases as compared with (d) ofFIG.3.

FIG.7is a graph showing IDSaccording to VDSunder each of a condition of having VDSof 3 V and a second area length (Lungated) of 40 nm and a condition of having VDSof 3 V and a second area length (Lungated) of 45 nm.

As shown inFIG.7, it can be seen that VBR(VDS) decreases as the length of the second area decreases. In addition, it can be seen that VBR(VDS) decreases as the length of the second area decreases as compared with (c) ofFIG.3.

FIG.8is a graph showing a change in threshold voltage according to a change in the length of the second area.

As shown inFIG.8, it can be seen that a threshold voltage (VGS) also decreases as the length of the second area decreases from 300 nm to 30 nm.

Example 3: Implementation and Analysis of Impact Ionization Characteristic of Two-Dimensional Semiconductor

(a) and (b) inFIG.9are respectively graphs showing transfer characteristic (ID-VG) and output characteristic (ID-VD) of WSe2having impact ionization characteristic.

Referring to (a) and (b) ofFIG.9, the impact ionization phenomenon due to a sharp increase in current under a high voltage (avalanche bias) of VDS<−15 V may be identified.(c) inFIG.9is a graph showing a result of analyzing the impact ionization characteristic of WSe2.

The impact ionization characteristic was analyzed based on a following equation:

M=(1/(1-(EECR)n))<Equ⁢a⁢t⁢i⁢on>where M is a multiplication factor, E is an electric field intensity, ECRis a critical electric field intensity, and n is an index related to an ionization scattering cross section).

Referring to (c) ofFIG.9, it may be identified that WSe2exhibits a high multiplication factor value of up to 5000 and a low critical electric field intensity (Critical E-field) value of 25 kV/cm, and thus WSe2is an optimized material having the impact ionization.(d) inFIG.9shows changes in critical voltage VBRand critical electric field intensity ECRbased on the length of the semiconductor channel according to the present disclosure. As shown in (d) inFIG.9, it may be identified that an operation voltage may be lowered as the semiconductor channel length is reduced.(e) inFIG.9shows the change in the critical electric field intensity ECRand the multiplication factor based on a thickness of the semiconductor channel according to the present disclosure. As shown in (e) inFIG.9, it may be identified that as the thickness of the semiconductor channel decreases, the energy bandgap of the channel increases, and accordingly, the critical electric field intensity ECRalso increases. Further, for comparison, regarding the multiplication factor under the same electric field condition (E=52 kV/cm), it may be identified that the multiplication factor decreases as the thickness of the semiconductor channel decreases.(f) inFIG.9shows the change in the impact ionization characteristic based on a temperature. Only a small change of about 2.5 KV/cm may be observed based on the temperature change of 100 to 300 K. This corresponds to 2.5×10−4V/mm. Thus, the change in the operation voltage is 0.075 V at a channel length of about 300 nm. This supports the reason why the super-steep switching device according to the present disclosure operates even at room temperature.

Example 4: Inverter Device Having High Inverter Gain and Ideal Noise Margin

(a) inFIG.10is a schematic diagram and a circuit diagram of an inverter device according to one embodiment of the present disclosure in which a super-steep switching device and a pull-up transistor capable of performing a complementary operation therewith are connected in series with each other. In this regard, VIndenotes the top gate voltage and Voutdenotes an output voltage between two transistors.(b) inFIG.10is a graph showing the transfer characteristic (ID-VG) of the inverter device according to one embodiment of the present disclosure. It may be identified that the inverter device has two stable logic state areas based on the complementary operation, based on a gate voltage of a green dot.(c) inFIG.10shows the Vout−Vin, curve of the inverter device. Based on the curve, the inverter device operation in the series connection circuit structure of the two transistors may be identified. Further, referring to (c) inFIG.10, it may be identified that the inverter device has the high inverter gain value (Peak gain˜73) and the ideal noise margin (NML+NMH˜98.65% of VDD) resulting from the super-steep switching phenomenon.

Example 5: Identification of Materials Usable as a Semiconductor Channel

FIG.11is a graph showing an IDSmeasured according to VGSfor a device having a semiconductor channel material MoS2, a semiconductor channel length of 2.5 μm, a semiconductor channel width of 5 μm, and a second area (Ungated) length of 390 nm.

FIG.12is a graph showing an IDSmeasured according to VGSfor a device having a semiconductor channel material InSe, a semiconductor channel length of 5 μm, a semiconductor channel width of 3.5 μm, and a second area (Ungated) length of 370 nm.

FIG.13is a graph showing an IDSmeasured according to VGSfor a device having a semiconductor channel material InSe, a semiconductor channel length of 5 μm, a semiconductor channel width of 2.5 μm, and a second area (Ungated) length of 100 nm.

FIG.14is a graph showing an IDSmeasured according to VGSfor a device having a semiconductor channel material BP, a semiconductor channel length of 5.5 μm, a semiconductor channel width of 1 μm, and a second area (Ungated) length of 170 nm.

FIG.15is a graph showing an IDSmeasured according to VGSfor a device having a semiconductor channel material ZrSe2, a semiconductor channel length of 1 μm, a semiconductor channel width of 0.5 μm, and a second area (Ungated) length of 180 nm.

FIG.16is a graph showing an IDSmeasured according to VGSfor a device having a semiconductor channel material ZrSe2, a semiconductor channel length of 1 μm, a semiconductor channel width of 0.5 μm, and a second area (Ungated) length of 180 nm.

FromFIGS.11to16, an SS value and an on/off ratio value were measured for each device, and the measured results are summarized through <Table 1> below.

TABLE 1ChannelChannelUngated-SSOn/off ratioMateriallengthwidthregion length(mV/dec)(10×)MoS22.5 μm5 μm390 nm30.441InSe5 μm3.5 μm370 nm11.4715 μm2.5 μm100 nm13.321BP5.5 μm1 μm170 nm34.572.5ZrSe21 μm0.5 μm180 um18.1661 μm0.5 μm180 um27.715WSe23 μm2.5 μm300 nm2.736

As shown inFIGS.11to16and <Table 1>, it can be seen that I2FET may be implemented using various two-dimensional materials.

However, in the case of MoS2and InSe, there is a problem in that the on/off ratio is low, and in the case of BP and ZrSe2, there is a problem in that on-current is low. However, in the case of WSe2, it can be seen that both of the above-described problems may be solved and a low operating voltage may be achieved.

Although the above description has been made with reference to a preferred embodiment of the present disclosure, those skilled in the art may variously modify and change the present disclosure without departing from the spirit and scope of the present disclosure as described in the claims below.