Dual gate field effect transistor utilizing Mott transition...

Details Dual gate field effect transistor utilizing Mott transition... Dual gate field effect transistor utilizing Mott transition...

1999-11-19

2002-04-02

Thomas, Tom (Department: 2811)

Active solid-state devices (e.g., transistors, solid-state diode

Semiconductor is an oxide of a metal or copper sulfide

C257S410000, C257S289000

Reexamination Certificate

active

06365913

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to semiconductor switches, particularly to the field effect transistor (FET) type, which are based upon the Mott metal-insulator phenomenon.
2. Brief Description of the Prior Art
Existing computer circuits, both logic and dynamic random memory (DRAM) are dominated by field effect transistors (FET). FETs have been more widely used lately. Of these, the metal oxide field effect semiconductors MOSFETS (metal oxide semiconductor field effect transistors) have been the leading choice of designers. In this type of transistor, a conducting channel of either electrons or holes is established between two regions, called “source” and “drain” to which ohmic low resistance contacts are made. Control of the “source” current to the “drain” current is achieved by applying a potential to the gate electrode, which affects the width of the conducting channel and/or the number of mobile carriers in the channel. Power gain is achieved between the gate source input terminal and the drain source output because of the high input impedance.
A bipolar junction transistor is formed using three doped semiconductor regions separated by two PN junctions. The device is called a bipolar junction transistor (BJT) because the current flow involves carriers of both polarities (holes and electrons). A BJT can be fabricated either as a PNP transistor or a NPN transistor. The central region is called the base. One of the junctions is forward-biased and is called the emitter-base junction. The other junction is reverse biased and is called the collector-base junction. The three terminals are referred to as emitter, base and collector terminals. The regions are doped either lightly or heavily depending upon the use to which the BJT is to be put.
The FET has an extremely high input impedance as noted above, and has a better frequency response than the BJT. The small signal gain of a FET is not as high as that of a BJT; however the FET is a good alternative to the BJT in those areas of application where high impedance and a better frequency response is required. FETs can also be used as constant current sources and voltage controlled resistors in certain special applications. The important areas of use of FETs are digital circuits and systems.
In general, the basic structure of a FET has a main body which is a single continuous length of N-type semiconductor material. However, there is a small section of P-type material placed on either side of the main N-type section. Both of these P-type sections are connected together electrically. The lead connected to these sections is the gate. The other two leads (source and drain) are connected to either end of the N-type material piece.
The semiconductor channel is backed by a substrate of the opposite type of semiconductor. For example, if the channel is made up of an N-type semiconductor, a P-type semiconductor is used as the substrate. MOSFETs are also characterized as depletion or enhancement mode FETs. Depletion mode FETs reduce the current flow by increasing the negative voltage applied to the gate (assuming an N-type channel). In enhancement mode FETs, assuming an N-channel device, a positive voltage is applied between the gate and the source. The higher the value of the voltage, the greater the number of holes drawn from the N-type source into the P-type substrate. These holes then traverse into the N-type region, drawn by the voltage applied between the drain and the source. Thus increasing the voltage on the gate, results in an increase in the channel current (i.e., the current from source to the drain).
The MOSFET is approaching the intrinsic physical limits in channel length, due to, inter alia, doping and double depletion effects. Thus the exponential increase in circuit density on a chip predicted by Moore's Law is not expected to be maintained by Si based devices. The concept of exploiting the Mott metal-insulator transition to make a FET-like switching device which allows functionality for channel lengths on the scale of 10 nm is known. The approach used herein is to replace the silicon channel by a layer of Mott insulator material. The present invention relates to a FET using oxide materials said FET operating at room temperature.
A non Si-based structure similar to that of the present invention has been explored in the literature in the work on “Superconducting FET” (SuFETS) such as are described in J. Mannhart, J. G. Bednorz, K. Muller and D. G. Schlom,
Z. Phys
. 13 Condensed Matter, 83, 307 (1991); A. Levy, J. P. Falck, M. A. Kastner and R. J. Birgeneau,
Phys. Rev
. B 51, 648 (1995); E. H. Taheri, J. W. Cochrane and G. J. Russell,
J Appl. Phys
., 77 761, (1995); J. Mannhart,
Supercond. Sci. Technol
. 9, 49, (1996) and references cited in each, the contents of which are all incorporated by reference herein.
The typical SuFET device comprises a channel of superconducting material, (such as fully oxygenated YBCO in the superconducting state) with source and drain contacts on the superconducting material, a gate insulator (such as strontium nitrate) and a gate electrode. In the most common implementation of the SUFET as disclosed in the references noted above, the device is operated near the superconducting transition temperature and a gate field is applied to induce a transition from a superconducting state to an insulating state. A fundamental difficulty with such SUFET devices is the extremely short electric field screening length of superconducting materials which limits the ability of the gate field to modulate the bulk of the channel.
There have also been attempts (e.g., See Taheri, et al. above) to induce superconductivity in an initially insulating channel by applying a gate electric field. This approach is difficult because of the extremely high fields needed to induce a sufficient density of carriers in the channel to undergo insulator superconductor transition. It must be kept in mind however, that this work has been aimed entirely on attempts to take a device with a channel which can undergo a transition between superconducting and insulating states in the channel, and therefore it must be operated at a temperature near the superconducting transition temperature. The present invention is distinct in that a metal insulator transition is utilized in oxide materials to make a device which can operate at room temperature, without the need for the superconducting state.
To more specifically describe the typical prior art enhancement mode FET that uses an oxide channel, reference is made to FIG.
1
. Enhancement mode oxide channel FET
100
has a source electrode
101
, drain electrode
102
, gate electrode
103
, gate insulator
104
and channel
105
.
It is well known that copper compounds, specifically, cuprates, form a class of materials which demonstrate Mott metal insulator transition. This property makes cuprates suitable for use as the molecular layer in the transistor. In addition, cuprates are well suited for integration with high dielectric oxides such as strontium titanate (SrTiO
3
) and (Ba
1−x
, Sr
x
, TiO
3
), which are all materials suitable as the material used to comprise gate insulator
104
.
In cuprate conductors, the conduction band is formed from well-defined atomic or molecular orbitals. In a cuprate semiconductor for example, this role is the result of d
x2-y2
symmetry orbitals on the Cu sites. In another example, K
n
C
60
, the threefold degenerate set of lowest unoccupied molecular orbitals (LUMO) of C
60
play an analogous role. The simplest model to describe such materials is the Hubbard model described by J. Hubbard, in
Proc. Roy. Sci
. (London) A276, 238(1963); A277, 237(1963); A281, 401(1963) which are incorporated by reference herein.
In an essentially ordered system, such as a cuprate CuO
2
plane, there are found to be two possible global states of the system: insulator and metal. These states are separated by the Mott Transition as described by N. Mott in
Metal Insulator Transitions
, Taylor & Francis, London, 1990 wh

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