|Publication number||USRE42530 E1|
|Application number||US 11/234,816|
|Publication date||Jul 12, 2011|
|Filing date||Sep 23, 2005|
|Priority date||Sep 17, 2001|
|Also published as||US6624463, US20030054615|
|Publication number||11234816, 234816, US RE42530 E1, US RE42530E1, US-E1-RE42530, USRE42530 E1, USRE42530E1|
|Inventors||Hyun-Tak Kim, Kwang-Yong Kang|
|Original Assignee||Electronics And Telecommunications Research Institute|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (13), Non-Patent Citations (3), Classifications (15), Legal Events (2)|
|External Links: USPTO, USPTO Assignment, Espacenet|
1. Field of the Invention
The present invention relates to a field effect transistor, and more particularly, to a switching field effect transistor (FET) using abrupt metal-insulator transition.
2. Description of the Related Art
Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) have been widely used as micro and super-speed switching transistors. A MOSFET has two pn junction structures showing linear characteristics at a low drain voltage as a basic structure. However, when a channel length is reduced to about 50 nm or below as the degree of integration of a device increases, an increase in a depletion layer changes the concentration of a carrier and an decrease of the depth of the gate insulator remarkably makes current flowing between a gate and a channel.
To overcome this problem, IBM institute has performed Mott FETs using the Mott-Hubbard insulator in a channel layer in reference “D. M. Newns, J. A. Misewich, C. C. Tsuei, A. Gupta, B. A. Scott, and A. Schrott, Appl. Phys. Lett. 73, 780 (1998)”. The Mott-Hubbard insulator undergoes a transition from an antiferromagnetic insulator to an metal. This transition is called the Mott-Hubbard metal-insulator transition in reference “J. Hubbard, Proc. Roy. Sci. (London) A276, 238 (1963), A281, 401 (1963)”. This is a continuous (or second order) phase transition. Unlike MOSFETs, Mott FETs perform an ON/OFF operation according to metal-insulator transition and do not have a depletion layer, thereby remarkably improving the degree of integration of a device and achieving a higher-speed switching characteristic than MOSFETs.
Since Mott-Hubbard FETs use continuous metal-insulator transition, charges used as carriers should be continuously added until the best metallic characteristics reach. Accordingly, the added charges must have a high concentration. Generally, charges N per unit area can be expressed by Equation (1).
Here, “∈” denotes the dielectric constant of a gate insulator, “e” denotes a basic charge, “d,” denotes the thickness of the gate insulator, and “Vg” denotes a gate voltage.
For example, in the case of La2CuO4, which is one of the materials falling under the group Mott-Hubbard insulator, when holes are added to La2CuO4, the characteristics of La2-xSrxCuO4(LSCO) appear, and a metal having best hole carriers at x=0.15 (15%) is obtained. Here, the added holes become carriers. Generally, x=0.15 is a high concentration, so if the N value increases, the dielectric constant of the gate insulator increases, the thickness of the gate insulator decreases, or the gate voltage increases. However, when the dielectric constant is too great, the fatigue characteristics of a dielectric sharply worsens during a high-speed switching operation, thereby reducing the life of a transistor. Moreover, there is a limit in decreasing the thickness of the gate insulator due to limitations in fabrication processes. In addition, when the gate voltage increases, power consumption also increases, which makes it difficult to be the transistor with a low power.
To solve the above-described problems, it is an object of the present invention to provide a switching field effect transistor using abrupt metal-insulator transition so that the field effect transistor shows metallic characteristics even if holes of a low concentration are added thereto.
To achieve the above object of the invention, there is provided a field effect transistor including a substrate; a Mott-Brinkman-Rice insulator formed on the substrate, the Mott-Brinkman-Rice insulator undergoing abrupt metal-insulator transition when holes add therein; a dielectric layer formed on the Mott-Brinkman-Rice insulator, the dielectric layer adding holes into the Mott-Brinkman-Rice insulator when a predetermined voltage is applied thereto; a gate electrode formed on the dielectric layer, the gate electrode applying the predetermined voltage to the dielectric layer; a source electrode formed to be electrically connected to a first portion of the Mott-Brinkman-Rice insulator; and a drain electrode formed to be electrically connected to a second portion of the Mott-Brinkman-Rice insulator.
Preferably, the substrate is formed of SrTiO3.
Preferably, the Mott-Brinkman-Rice insulator is formed of LaTiO3, YTiO3, Ca2RuO4, Ca2IrO4, V2O3, (CrxV1-x)2O3, CaVO3, SrVO3 and YVO3.
Preferably, the dielectric layer is formed of Ba1-xSrxTiO3 or dielectric materials.
Preferably, the source electrode and the drain electrode are separated from each other by the dielectric layer.
The above object and advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings in which:
Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. The present invention is not restricted to the following embodiments, and many variations are possible within the spirit and scope of the present invention.
The following description concerns the operating principle of a field effect transistor (FET) according to the present invention.
If holes are added to the Mott-Brinkman-Rice insulator 100 at a very low concentration, the Mott-Brinkman-Rice insulator 100 is abruptly transformed into a metal due to a decrease in the Coulomb interaction and is thus changed into a non-uniform metallic system having both a metal phase and an insulator phase. Such abrupt transition, that is, first-order transition is well described in the reference “Hyun-Tak Kim in Physica C 341-348, 259 (2000); http://xxx.lanl.gov/abs/cond-mat/0110112”. (2000).” Here, the Mott-Brinkman-Rice insulator 100 is changed into a non-uniform metal system because the number of electrons becomes less than the number of atoms due to the addition of holes.
In this case, as shown in
In the metal region M of
Here, k<1 is satisfied, and abrupt metal-insulator transition occurs at a value within a range between k=1 and a certain value close to k=1. This theoretical equation is introduced in the reference “W. F. Brinkman, T. M. Rice, Phys. Rev. B2, 4302 (1970)”. The theory about a strong correlation was introduced in the reference “N. F. Mott, Metal-Insulator Transition, Chapter 3, (Taylor & Frances, 2nd edition, 1990) for the first time.
Meanwhile, the effective mass m*/m of a carrier in the entire metal system of
Here, ρ is a band filling factor and can be expressed by a ratio of the number of electrons (or carriers) to the number of atoms. In this case, when k=1, there occurs a abrupt transition from a value close to ρ=1 to ρ=1. This theory is well described in the reference “Hyun-Tak Kim in Physica C 341-348, 259 (2000); http://xxx.lanl.gov/abs/cond-mat/0110112”. (2000).”
For example, in the case of a material of Sr1-xLaxTiO3 (SLTO), La+3 is substituted for Sr+2 in an insulator of SrTiO3(STO) when the material is doped with electrons, and in contrast, Sr+2 is substituted for La+3 in a Mott-Brinkman-Rice insulator of LaTiO3(LTO) when the material is doped with holes.
As shown in
It can be concluded from the results of the tests shown in
A gate electrode 430 is formed on the dielectric layer 420 to apply a predetermined voltage to the dielectric layer 420. A source electrode 440 is formed on a first portion of the Mott-Brinkman-Rice insulator 410, and a drain electrode 450 is formed on a second portion of the Mott-Brinkman-Rice insulator 410. The source electrode 440 and the drain electrode 450 are separated by the dielectric layer 420.
The following description concerns the operations of the FET. A predetermined voltage is applied to the source electrode 440 and the drain electrode 450, thereby generating a predetermined potential on the surface of the Mott-Brinkman-Rice insulator 410 of LaTiO3(LTO). Next, a gate voltage is applied to the gate electrode 430 so that Sr+2 holes can flow from the dielectric layer 420 of Ba1-xSrxTiO3 (BSTO) into a Mott-Brinkman-Rice insulator 410 at low concentration. Then, the Mott-Brinkman-Rice insulator 410 undergoes abrupt metal-insulator transition, and the conductive channel 415 is formed. As a result, current flows between the source electrode 440 and the drain electrode 450 through the conductive channel 415.
When the concentration of holes is 5%, that is, ρ=0.95, the number of electrons in a metal region formed due to the abrupt metal-insulator transition is about 4×1014/cm2. This number of electrons is about at least 100 times of the number of electrons (about 1012/cm2) in a channel of a usual Metal Oxide Semiconductor Field Effect Transistor (MOSFET), so a high current amplification can be achieved.
According to circumstances, electrons can be added to the Mott-Brinkman-Rice insulator 410. However, the addition of electrons increases power consumption more than the addition of holes. In other words, when a gate voltage Vg is 0.12 volts, the dielectric constant ∈ of the dielectric layer 420 is 200, and the thickness “d” of the dielectric layer 420 is 50 nm, then the number Ncharge of static hole charges corresponding to a low concentration ρ=0.95 is about 4×1014/cm2 (Ncharge=Vg∈/ed). Accordingly, if a hole concentration Nhole is set to about 4×1014/cm2, and other variables, i.e., the dielectric constant ∈ and the thickness “d” of the dielectric layer 420 are adjusted to the conditions of transistor fabrication, the gate voltage Vg can be significantly decreased, so power consumption can be decreased.
However, when static electrons corresponding to a high concentration ρ=0.95 are added to the Mott-Brinkman-Rice insulator 410, the number Nelectron of electrons is more than the number Nhole of holes. Accordingly, even if the dielectric constant ∈ and the thickness “d” of the dielectric layer 420 are properly adjusted, the gate voltage Vg becomes greater than in the case of adding holes. As a result, power consumption increases compared to the case of adding holes at a low concentration. In this specification, a transistor according to the present invention is referred to as a Mott-Gutzwiller-Brinkman-Rice-Kim (MGBRK) transistor in order to discriminate it from a Mott or Mott-Hubbard (MH) FET.
As described above, a FET according to the present invention provides the following effects. First, since a depletion layer does not exist, there is no limit in the length of a channel. Therefore, the degree of integration of a device and a switching speed can be greatly increased. Second, since a dielectric layer having a properly high dielectric constant is used as a gate insulator, an appropriate concentration of holes for doping can be obtained with a low voltage without greatly reducing the thickness of the dielectric layer. Third, when holes are added to a Mott-Brinkman-Rice insulator at a low concentration to provoke abrupt metal-insulator transition, a high current gain and a low power consumption can be achieved.
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|1||A field effect transistor based on the Mott transition in a molecular layer by C. Zhou; Applied physics letters Feb. 3, 1997 pp. 598-600.|
|2||*||Application of Gutzwiller's Variational Method to the Metal-Insulator Transition. W. F. Brinkman and T. M. Rice Bell Telephone Laboratories, Murray Hill, New Jersey 07974 Apr. 16, 1970.|
|3||Mott Transition field effect transistor by DM Newns; Applied physics letters Aug. 10, 1998 pp. 780-782.|
|U.S. Classification||257/43, 257/194, 257/192, 257/310|
|International Classification||H01L31/0336, H01L31/072, H01L31/0328, H01L29/12, H01L31/109, H01L29/786, H01L49/00, H01L29/78, H01L29/772|
|May 1, 2015||REMI||Maintenance fee reminder mailed|
|Sep 23, 2015||LAPS||Lapse for failure to pay maintenance fees|