US RE29,971 EGrant

Field Effect Semiconductor Device Having an Unsaturated Triode Vacuum Tube Characteristic

Issue Date:1979-04-17

•18 Claims

•3 Drawing Sheets

Abstract

A field effect transistor comprises a semiconductor channel, a source and a drain electrode formed at the opposite ends of the channel and a gate electrode provided on the side of the channel. The channel has a small impurity density and therefore the depletion layer extending from the gate goes deep into the channel to substantially close the conductive portion of the channel even in the absence of a gate voltage. The drain current will not flow where the drain voltage is below a certain threshold voltage, and will flow where the drain .[.volage.]. voltage is above the threshold voltage exhibiting a substantially linear resistance characteristic. This drain-current to drain-voltage characteristic simulates the anode-current to anode-voltage characteristic of the triode vacuum tube very closely.

Metadata

Assignee

Zaidan Hojin Hondotai Kenkyn Shinkokai

Inventors

Jun-ichi Nishizawa
Takeshi Terasaki

Application Information

Application Number:US 5765412

Filing Date:1975-05-12

Priority Date:1971-07-31

Art Unit:254

Classifications

IPC:

H01L 2988H01L 2980H01L 2978

Field of Search:

35722;23;12

Patent Drawings (3 sheets)

Description

Background of the Invention

1. Field of the Invention:

This invention relates to a field effect transistor, and more particularly to a field effect transistor having a drain-current to drain-voltage characteristic similar to the anode-current to anode-voltage characteristic of the triode vacuum tube.

2. Description of the Prior Art:

There are two types in field effect transistors (FET's), i.e., a metal-oxide-semiconductor (MOS) type and a junction-gate (JUG) type. In both cases, the current of the carriers (unipolar) flowing from the source to the drain is effectively controlled by the gate voltage. The gate voltages applied with respect to the source voltage works to control the height of the depletion layer extending from the gate into the channel, which in turn controls the height of the region through which a current is allowed to flow. In a MOS FET, the current flowing through the channel formed beneath the gate electrode and insulated from the gate electrode by an oxide layer is controlled by the electric field established in the channel by the gate voltage. This is due to the variation in the height of the depletion layer extending from the oxide-semiconductor contact. In a JUG FET, a depletion layer formed around the PN junction is varied by the gate voltage and controls the current flowing through the channel. In conventional FET's of the both types, it is arranged that the current channel is open (conductive) when no gate voltage is applied externally and the conducting channel height is varied by the applied gate voltage.

The present inventors have found that various advantages can be obtained by forming an FET in such a fashion that the depletion layers (space charged layers) extending from the gates are substantially contiguous to each other even when no gate voltage is applied. This will be first described referring to a junction type FET proposed in Japanese Pat. application No. 28405/1971 by one of the present inventors, which has triode-like characteristics (unsaturated type) unlike the conventional current saturation type characteristics, and has a reduced series (source to drain) resistance so that the product of the series resistance r.sub.s (this forms a factor for generating negative feedback) and the transconductance G.sub.m is suppressed substantially less than unity.

A typical example of the characteristic curves is shown in FIG. 1, and a schematically illustrated structure which produces the characteristics of FIG. 1 is shown in FIG. 2. Namely, when the gate voltage is absent or small, the drain current I.sub.D increases almost linearly with increasing the drain voltage V.sub.D, as is illustrated by curves 1, 2, and 3. This may be called resistance modulation, since the variation in the gate voltage results in a variation in the resistance between the source and the drain i.e., .delta.V.sub.D /.delta.I.sub.D. When the negative gate voltage is increased in magnitude to suppress the drain current I.sub.D, the drain current I.sub.D first does not begin to flow until the drain voltage V.sub.D reaches a certain value, and then above said certain value rapidly increases more than linearly with increasing drain voltage V.sub.D as is shown by curves 4, 5, and 6. The phenomenon .[.that.]. whereby the drain current I.sub.D increases linearly with increasing voltage V.sub.D as is shown by curves 1, 2 and 3 mainly appears in the case where the depletion layers extending from the gate electrodes G and G' .[.does.]. do not yet touch .[.yet.]. each other, whereas the phenomenon that the drain current I.sub.D .[.do.]. does not begin to flow until the drain voltage V.sub.D reaches a certain positive value and increases rapidly with increasing drain voltage V.sub.D above said certain value mainly appears when the depletion layers extending from the gates have grown large enough by the application of a gate voltage and touch (not touch, to say exactly, but become very close) each other. In the latter case, the applied drain voltage below the certain value is found to be used for decreasing the potential barrier of the pinch-off portion made in the current path by the depletion layers.

In the above example, linear characteristics as shown by curves 1, 2, and 3 appeared when the gate voltage was small in magnitude, and characteristics very closely resembling those of a triode vacuum tube as shown by curves 4, 5, and 6 appeared when the gate voltage exceeded a certain value. Further, it is desirable that the value .delta.V.sub.D /.delta.V.sub.G, which corresponds to the amplification factor .mu. of the triode vacuum tube, .[.is desired to.]. be large for obtaining a field effect transistor of a superior efficiency. Thus, it is desired to realize the characteristics corresponding to curves 4, 5, and 6 even in the region of small gate voltage, or in other words without the accompany of the characteristics corresponding to curves 1, 2, and 3, for providing elements of superior characteristics of a good efficiency and of little distortion.

It has been found by the present inventors that the above requirement can be satisfied by forming an FET in such a manner that the depletion layers extending from the gate electrodes are substantially contiguous (very close but not integrally connected) to each other even when no gate voltage is applied.

This can be achieved by using depletion layers due to carrier diffusion-recombination across a PN junction. Namely, the extent of a depletion .[.layers.]. layer across the PN junction is determined by the barrier potential (or contact potential) and the impurity concentration (density) in the crystal. Practically, if the resistivity of the semiconductor crystal substrate is known, an FET having such depletion layers which are formed only by the carrier diffusion-recombination and are contiguous to each other even when no gate voltage is applied can be formed by appropriately selecting the distance between the gate electrodes G and G'. In such a structure, since the depletion layers almost touch each other, the drain current I.sub.D can easily show triode-like characteristics, not showing linear increase of the drain current with increasing drain voltage, even without the application of a large negative gate voltage V.sub.G. Namely, characteristics as shown in FIG. 3 are obtained with a reduction or absence of the linear region indicated by curves 1, 2, and 3 in FIG. 1. These transistors have such advantages that sufficient function can be obtained with a small gate voltage, that a large variation in the drain voltage V.sub.D can be obtained by a small variation in the gate voltage V.sub.G, and that excellent action with less distortion can be performed. In addition to these advantages, capacitances between gate-and-source, and gate-and-drain are reduced and the frequency characteristics are improved.

The above description has been made on a transistor having a reduced series resistance, but it also holds for a conventional transistor having a large series resistance. A conventional FET having a large series resistance and showing pentode-like characteristics can be considered as the above-mentioned FET having a reduced series resistance and showing triode-like characteristics, itself, but now .[.provide wih.]. provided with a negative feed back circuit, or in .[.another word,.]. other words, the FET .[.operating.]. operates in an emitter follower fashion. Therefore, the advantages of the present concept described above can be also applied to such kind of transistors.

Next, description will be made on the state in which the depletion layers respectively extending from the gates touch each other. As is described above, the height of the depletion layer is a function of the barrier potential at the junction or contact and the impurity concentration (density) in the crystal. Usually, the height of a depletion layer is calculated by assuming that no carriers exist in the depletion layer and that only space charges which are perfectly ionized exist in the depletion layer and solving the Poisson's equation.

For example, in a case where a plate shaped PN junction has a stepwise carrier concentration distribution, i.e. the carrier concentration on one side of the PN junction is far larger than that on the other side, so that a depletion layer grows only into the other side, the height of the depletion layer .alpha. is expressed by

where R is a factor dependent on the physical constants of the semiconductor, N.sub.b the impurity concentration (density) in the semiconductor on that side in which the depletion layer grows, and V the applied voltage including the barrier potential. Strictly speaking, it is not that there are no carriers at all in the depletion layer, nor that a clear boundary exists at the edge of the depletion layer between the perfectly ionized region and the non-ionized region. Carriers are distributed according to the Fermi-Dirac distribution even into a depletion layer. The effective extent of a depletion layer is at least three times larger than the width of the depletion layer .alpha. calculated as above assuming that the depletion layer is perfectly ionized. Namely, the calculated height of the depletion layer based on the perfect ionization assumption is much lower than the actual effective height. Therefore, even if such semi-conductor materials in which the calculation with the perfect ionization assumption tells that the depletion layers touch each other only by the barrier potential with a gate-to-gate distance set at 20 micrometers is employed, the actual depletion layers can touch (become very close to) each other with the gate-to-gate distance set at about 60 micrometers.

Summary of the Invention

An object of the invention is to provide a field effect transistor having triode vacuum-tube-like characteristics.

Another object of the invention is to provide a field effect transistor comprising a semiconductor substrate including a current channel, a source and a drain electrode, and gate electrodes sandwiching the current channel, the depletion layers extending into the channel from the gate electrodes being substantially contiguous to each other even in the absence of a gate voltage.

A further object of the invention is to provide a field effect transistor comprising a semiconductor substrate including a current channel of a low carrier concentration (density) and gate regions of a high carrier concentration, a source and a drain electrode formed on the semiconductor substrate at the both ends of the current channel, and gate electrodes formed on said gate regions.

Brief Description of the Drawings

FIG. 1 is a graphical chart showing the drain-current to drain-voltage characteristics of a field effect transistor of non-saturated current type;

FIG. 2 is a schematic cross sectional view of a field effect transistor having the characteristics as shown in FIG. 1;

FIG. 3 is a graphical chart showing the drain-current to drain-voltage characteristics of a field effect transistor according to the present invention;

FIG. 4 is a schematic cross sectional view of a junction type field effect transistor according to this invention;

FIGS. 5A and 5B are schematic perspective and schematic cross sectional views respectively of another embodiment of a junction type field effect transistor according to the invention;

FIGS. 6A and 6B are schematic perspective and partial cross sectional views, respectively, of a further embodiment of a junction type field effect transistor according to the invention;

FIGS. 7 and 8 illustrate further embodiments of junction type field effect transistors of high output power according to the invention; and

FIGS. 9 to 11 are cross sectional views of embodiments of MOS type field effect transistors according to the invention.

Description of the Preferred Embodiments

Now, this invention will be described on the preferred embodiments referring to the accompanying drawings. Throughout the figures illustrating the embodiments of the invention, the gate voltage is set at zero unless particularly specified.

A silicon FET is shown in FIG. 4 for illustrating the height of the depletion layer. In a semiconductor substrate, gate electrode regions indicated by a hatched area are formed. Provided that the impurity concentration (density) in the gate regions is far larger than that in the channel region and that the impurity concentration in the channel region is uniformly distributed, the voltage V between the channel region and the gate region when the depletion layers extending from the gate regions touch each other is expressed by the equation

on the basis of the assumption of perfect ionization, where q is the electron charge, N.sub.B the impurity concentration in the channel region, .epsilon. the dielectric constant of the semi-conductor, and a the height of the depletion layer (in this case, equal to .alpha. half of the gate-to-gate distance). When no gate voltage is applied, the voltage V is entirely formed by the contact potential (i.e., the barrier potential). Assuming that this contact potential is 0.6 volt, the maximum half distance .alpha. between the gates becomes about 9, 3, and 0.9 micrometer(s) for the impurity concentrations N.sub.B of 10.sup.13 /cm.sup.3, 10.sup.14 /cm.sup.3, and 10.sup.15 /cm.sup.3 respectively. Since these values are calculated on the assumption of perfect ionization, the actual maximum distances between the gates G and G' (two times the height of a depletion layer) become approximately 18.times.3, 6.times.3, and 1.8.times.3 micrometers for the semiconductors having an impurity concentration of 10.sup.13 /cm.sup.3, 10.sup.14 /cm.sup.3, and 10.sup.15 /cm.sup.2 respectively.

FIGS. 5A and 5B show an embodiment of a junction-gate type FET having a circular transverse cross section. An annular gate is provided in the periphery of a cylindrical semiconductor body. In this case, the voltage V when the depletion layer touches itself and closes the current path is expressed by

on the assumption of perfect ionization, where r.sub.a is the radius of the annular gate. Actually, since the width of a depletion layer is about three times as large as the calculated value, the depletion layer becomes contiguous when the radius r.sub.a is about .sqroot.2.times.9.times.3, .sqroot.2.times.3.times.3, and .sqroot.2.times.0.9.times.3 micrometers for the impurity concentration N.sub.B of 10.sup.13 /cm.sup.3, 10.sup.14 /cm.sup.3, and 10.sup.15 /cm.sup.3, respectively.

A further embodiment of a junction-gate type FET is shown in FIGS. 6A and 6B, in which a plurality of cylindrical gate regions are formed on a line with an interval of 2d. The pinch-off voltage in this case takes a little more complicated form and is expressed as

where r.sub.j indicates the radius of one cylindrical gate region. At the interval about three times as large as the interval 2d calculated from the above equation, the depletion layer can be considered as contiguous.

For example in the embodiment of FIG. 5, the series resistance increases with increasing longitudinal length L of the gate electrode and decreases with decreasing length L. Thus, an FET of a large output power can be formed by connecting a large number of such channels.

FIG. 7 shows an embodiment of a large output FET along the above line.

Alternatively, a large output FET having a planar structure as shown in FIG. 8 may be formed. In this case, the distance 2a between the adjacent gates is also arranged considering the impurity concentration so that the current channel is interrupted by the contiguous depletion layers. The gates and the sources are respectively connected in parallel for a large power transistor.

Various alterations and modifications are possible within the spirit and scope of the present invention.

If the impurity concentration in the channel region is not uniform due to the employment of a diffusion process, etc., the calculation of the height of a depletion layer becomes complicated, but a value three times as large as the calculated value on the basis of the perfect ionization assumption also holds for the actual situation.

The present invention is not limited to junction-gate type FET's, but is also applicable to MOS FET's. The gist of the present invention lies in the depletion layers contiguous to each other. In a MOS FET, a space charge region is usually formed under an oxide film beneath the gate electrode. The dimension of the space charge region differs according to the properties of the oxide film but can be given by the Debye length which is dependent on the impurity concentration in the substrate. Thus, structures in which depletion layers touch each other even in the absence of a gate voltage can also be realized in MOS structure by utilizing the internal potential at an insulator-semi-conductor contact corresponding to the barrier potential at a junction.

FIGS. 9, 10, and 11 show embodiments of MOS FET's according to the invention. In FIG. 9 a source and a drain electrode is formed on the opposite surfaces and a gate electrode is formed around the source to effectively extend the depletion layer. The radius of the gate electrode is selected less than the Debye length so that the current channel from the source electrode is closed by the depletion layer even in the absence of a gate voltage. FIG. 10 shows an embodiment in which an electrically isolated region is formed in one surface of a semiconductor substrate, and a source, an annular gate and an annular drain electrode are formed on said region.

FIG. 11 shows another embodiment which is intended to provide a high output power by alternatively forming source and drain electrodes respectively connected in parallel.

In the above embodiments, the shape of the source and/or drain and/or gate electrode may be rectangular or comb form. The gate electrodes are insulated from the semi-conductor substrate by an insulator film such as an SiO.sub.2 film.

The present invention is applied to silicon elements in the above embodiments but is also applicable to other semiconductor materials such as GaAs. Further, with the use of a hetero junction, a space charged region not only due to the carrier concentration but also due to the difference in the band structures can be utilized.

The present invention is described on various structures, but is most effective to those having a reduced series resistance to have a small output resistance. If such elements are assembled in an integrated circuit, there can be provided superior switching characteristics which are made more effective by the smallness of the accompanying capacitances.

When the gate-to-gate distance is further reduced, the standing-up point of the drain current shifts to higher drain voltage side and suitable circuit designs based on the thus obtained characteristics are possible. Therefore, this invention gives the upper limit for the gate-to-gate distance.

Claims

We claim:

1. A field effect semiconductor device capable of exhibiting unsaturable drain voltage versus drain current characteristics comprising: a first and second spaced gate region formed of a relatively high impurity doped semiconductor material and having a first conductivity type; a semiconductor region located adjacent to said first and second spaced regions and having an opposite conductivity type, said .[.second.]. semiconductor region being formed of a substantially intrinsic semiconductor material of a low carrier concentration, said semiconductor region having a portion forming a current channel between said first and second gate regions, said channel including the semiconductor region disposed between respective junctions formed with said first and second gate regions defining the sides of said channels and including a depletion layer incident to each of said junctions, said depletion layers being contiguous or overlapping each other to achieve a pinch-off condition within said channel in the absence of a reverse bias voltage applied to said first and second gate regions; a source region located along said channel for supplying a current thereto; a drain region spaced from said source region along said channel for providing an output current, said source region and drain region comprising semiconductor regions having the same conductivity type as said channel region; gate biasing means for applying a reverse bias voltage to said first and second gate regions to increase the effective length of the overlapping depletion layers along said current channel with an increase in magnitude of the applied reverse bias voltage to thereby determine a threshold drain voltage, wherein the application of a drain voltage greater than said threshold voltage allows the carriers from the source region to travel toward said drain region in the depletion layer existing along the current channel in the same manner as that exhibited by a triode vacuum tube.

2. The field effect semiconductor device of claim 1, in which the extent of the depth of said depletion layers is determined by the contact potential at said junction and by the impurity concentration of the semiconductor material of the channel region, wherein the gate-to-gate distance is determined.

3. The field effect semiconductor device of claim 1, in which the output current at said drain region exhibits an unsaturated non-linear characteristic with respect to the applied drain voltage when said drain voltage is above a threshold determined by a given gate bias voltage.

4. The field effect semiconductor device of claim 1, in which said channel constituting the semiconductor region is formed of a plate elongated in the source-to-drain direction, and said first and second gate regions are formed on the two principal surfaces of said plate sandwiching therebetween said current channel, the depletion layers extending from said gates and contacting or overlapping each other.

5. The field effect semiconductor device of claim 4, in which each of said gate electrodes has a dimension in the longitudinal direction along said current channel small enough to reduce the series resistance of the channel.

6. The field effect semiconductor device of claim 1, in which said source and drain regions and said gate regions are disposed on one surface of the semiconductor material constituting the current channel.

7. The field effect semiconductor device of claim 1, in which said channel constituting semiconductor material is silicon having an impurity concentration below the order of 10.sup.15 /cm.sup.3.

8. The field effect semiconductor device of claim 4, in which said current channel between the gate regions has a diametrical dimension of at most three times the total width of the total depletion layers calculated from the assumption of perfect ionization in the depletion layers.

9. A field effect transistor according to claim .[.1.]. 19, wherein said semiconductor substrate is cylindrical and the gate electrode has a hollow cylindrical shape formed on the side surface of said cylindrical substrate surrounding the current channel extending therewithin.

10. A field effect transistor according to claim .[.1.]. 19, wherein said gate electrode is annular and surrounds the source electrode. .[.11. A field effect transistor according to claim 1, wherein said semiconductor substrate is cylindrical and said gate electrode is formed on an intermediate portion of the side surface of said cylinder..]. 12. A field effect transistor according to claim .[.1.]. 19 wherein a plurality of said gate regions are formed in and traversing the current channel. 13. A field effect semiconductor comprising a semiconductor region formed of a substantially intrinsic semiconductor material having a low carrier concentration, at least one source region of a predetermined conductivity type adjacent to said semiconductor region, at least one drain region of said predetermined conductivity type adjacent to said semiconductor region, removed from said source region, thereby constituting in said semiconductor region a current path from said source to said drain, and at least one gate structure connected to said semiconductor region to form a depletion layer in said semiconductor region extending from said gate which substantially occupies a whole cross-section of said current path in the absence of a gate voltage, said depletion layer being of minimum effective size in the absence of said gate voltage. 14. A field effect semiconductor device comprising a semiconductor region formed of a substantially intrinsic semiconductor material having a low carrier concentration, at least one source region adjacent to said semiconductor region, at least one drain region adjacent to said semiconductor region, removed from said source region, thereby constituting in said semiconductor region a current path from said source to said drain, said source and drain regions being heavily doped and of the same conductivity type, and at least one gate structure connected to said semiconductor region to form a depletion layer in said semiconductor region extending from said gate which substantially occupies a whole cross-section of said current path in the absence of a gate voltage, wherein the application of a drain voltage greater than a threshold voltage forces the carriers from the source to travel toward said drain along said current path through said depletion layer to thereby provide a current-voltage characteristic similar to that of a vacuum tube triode. 15. A field effect semiconductor device comprising a semiconductor region formed of a substantially intrinsic semiconductor material having a low carrier concentration, said carrier concentration being less than 10.sup.15 /cm.sup.3, at least one source adjacent to said semiconductor region, at least one drain adjacent to said semiconductor region removed from said source region, thereby constituting in said semiconductor region a current path from said source to said drain, said source and drain regions being of the same conductivity type, and at least one gate connected to said semiconductor region to form a depletion layer in said semiconductor region extending from said gate which substantially occupies a whole cross-section of said current path in the absence of a gate voltage, wherein upon the application of a reverse bias gate voltage, a threshold voltage is defined above which a drain voltage forces carriers from the source toward said drain through said depletion layer occupying a whole cross-section of said current path to thereby provide a current-voltage characteristic similar to that of a vacuum triode. 16. A field effect semiconductor device capable of exhibiting unsaturable drain voltage versus drain current characteristics comprising: a substantially intrinsic semiconductor region; source and drain semiconductor regions, said source and drain regions being highly doped and of the same conductivity type, and being disposed contiguous to said intrinsic region, said source and drain regions defining a current path therebetween through a portion of said intrinsic region; and gate means for controlling current through said current path in accordance with a gate voltage applied thereto, carriers flowing along said path from said source to said drain when the voltage at said drain exceeds a predetermined threshold value; said gate means including contact potential producing means contiguous to said intrinsic region for generating, in the absence of a gate voltage applied to said gate means, a potential at the junction of said potential producing means with said intrinsic region, said potential producing a depletion layer extending across substantially an entire cross-section of said current path such that said carriers flowing along said current path must pass through said depletion layer, said depletion layer increasing in effective size in response to increasing magnitude of said applied gate voltage to determine thereby said threshold drain voltage value. 17. The device of claim 16 wherein said gate means comprises: a layer of insulator material, one surface thereof adjacent said intrinsic region; and a metallic electrode disposed on the surface of said insulator layer opposite said one surface. 18. A field effect semiconductor device capable of exhibiting unsaturable drain voltage versus drain current characteristics comprising: a gate region formed of a relatively high impurity doped semiconductor material and having a first conductivity type; a semiconductor region located adjacent to said gate region and having a second conductivity type opposite said first conductivity type, said semiconductor region being formed of a substantially intrinsic semiconductor material of a low carrier concentration; a source region formed of a relatively high impurity doped semiconductor material and connected to said semiconductor region; a drain region formed of a relatively high impurity doped semiconductor material and connected to said semiconductor region; said source and drain regions defining a current path therebetween through said semiconductor region, current flowing from said source to said drain along said current path in response to a drain voltage in excess of a predetermined threshold voltage; said semiconductor region including a depletion layer incident to said gate region, said depletion layer forming a pinched-off area within said path in the absence of a reverse bias voltage applied to said gate region; means for applying a reverse bias voltage to said gate region to increase the effective length of the depletion layer forming said pinched-off area along said current path in accordance with an increase in magnitude of the applied reverse bias voltage, to define thereby said threshold drain voltage whereby the application of a drain voltage greater than said threshold voltage forces carriers from the source region to travel toward said drain region through said depletion layer. 19. A field effect semiconductor device capable of exhibiting unsaturable drain voltage versus drain current characteristics comprising: at least one gate region formed of a relatively high impurity doped semiconductor material having a first conductivity type; a semiconductor region located adjacent to said gate region and having a second conductivity type which is opposite to said first conductivity type, said semiconductor regions being formed of a substantially intrinsic semiconductor material of a low carrier concentration; a source region formed of a relatively high impurity doped semiconductor material and connected to said semiconductor region; a drain region formed of a relatively high impurity doped semiconductor material and connected to said semiconductor region; said source and drain regions defining a current path therebetween through said semiconductor region, current flowing from said source to said drain along said current path when the voltage at said drain region exceeds a threshold drain voltage; said semiconductor region having a portion forming a current path, said path including the semiconductor region disposed between the junction formed with said at least one gate region and including a depletion layer incident to said junction, said depletion layer forming a pinched-off area within said current path in the absence of a reverse bias voltage applied to said gate region; and gate biasing means for applying a reverse bias voltage to said at least one gate region to increase the effective length of said depletion layer along said current path in accordance with an increase in magnitude of the applied reverse bias voltage, to determine thereby said threshold drain voltage, whereby the application of a drain voltage greater than said threshold voltage forces carriers from the source region to travel toward said drain region through said depletion layer in the same manner as that exhibited by a triode vacuum tube.

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Abstract

Description