Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1999-05-04
2001-03-27
Jackson, Jr., Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S402000, C257S407000, C257S414000, C257S421000, C257S422000, C257S659000
Reexamination Certificate
active
06208000
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a semiconductor device having a transistor structure, and more particularly to a semiconductor device having a charge accumulating layer under a gate electrode.
Recently, a floating-gate memory device has been proposed as a structure in which the “single electron phenomenon” is applied to a MOS semiconductor device (IBM, S. Tiwari, IEDM95, p. 521). In this device, a record, which indicates whether or not electrons flowing through the channel of the substrate are accumulated in semiconductor particles in the gate oxide film, will appear in current/voltage characteristics between its source and drain. Therefore, this device is expected to be applied as a memory device.
FIG. 1
is a sectional view illustrating the device structure proposed in the above document. In the figure, reference numeral
1
denotes an Si substrate, reference numeral
2
a tunnel oxide film, reference numeral
4
an SiO
2
film, reference numeral
5
a gate electrode, reference numeral
6
a source region, reference numeral
7
a drain region, reference numeral
8
an inversion layer, and reference numeral
21
Si fine particles. This device is characterized in that Si fine particles
21
with a size of about 5 nm are provided on the tunnel oxide film
2
with a thickness of 2 nm or less, and the gate electrode
5
is provided on the resultant structure.
In this device, electrons contained in the inversion layer
8
directly tunnel into the Si fine particles
21
on the tunnel oxide film
2
when a gate voltage has been applied. When electrons have tunneled into the Si fine particles, the electron distribution of a conduction band in the inversion layer
8
located under the Si fine particles
21
varies. As a result, the threshold voltage of a gate having its channel connected to the band changes. Since the change in the threshold voltage is about 0.36V, the state of electrons in the Si fine particles
21
can be sensed by sensing a change in the current flowing through the inversion layer
8
with respect to the gate voltage.
FIGS. 2A
,
2
B and
2
C are views illustrating changes in the conduction band of the above-described device. When a positive voltage has been applied to the gate, electricity is transmitted and accumulated into the Si fine particles from the inversion layer
8
via the tunnel oxide film
2
, as is shown in
FIG. 2A
(“Write” state). Even if the application of the voltage to the gate electrode
5
is stopped, the electricity is retained in the Si fine particles
21
, as is shown in
FIG. 2B
(“Store” state). In this state, the threshold voltage of the transistor increases. On the other hand, when a negative voltage has been applied to the gate, the electricity accumulated in the Si fine particles is discharged to the substrate side via the tunnel oxide film
2
, as is shown in FIG.
2
C. In this state, the threshold voltage returns to its original value (“Erase” state).
As described above, electricity can be transmitted into, retained in and discharged from the Si fine particles
21
, and the threshold voltage of the device varies depending upon whether or not electricity is accumulated in the Si fine particles
21
. This being so, this device can be used as a memory device.
Elements of this kind, however, have the following problems; in the conventional floating-gate device using the single electron phenomenon, the distance between a quantum dot and the channel layer of the substrate is as close as about 2 nm. Accordingly, the threshold voltage, which is disadvantageous, if high, in the case of a flash memory, can be suppressed. However, it is highly possible that electrons will return into the substrate. This being so, the retention time, i.e. the time required for the electricity in the quantum dot to discharge into the substrate side, is as short as several months. This is rather shorter than the retention time, i.e. several years, in the case of a usual flash memory which does not use the single electron effect.
Although as described above, the floating-gate device using the single electron effect can have a low threshold voltage but cannot retain electricity for a long time. This is a factor which makes it difficult to use the device as a memory device.
BRIEF SUMMARY OF THE INVENTION
It is the object of the invention to provide a floating-gate semiconductor device using the single electron effect, which has a low threshold voltage and can retain electricity for a sufficiently long time.
(Structure)
According to a first aspect of the invention, there is provided a semiconductor device comprising: a charge accumulating layer containing a magnetic substance and formed directly on a semiconductor substrate; a gate insulating film formed on the charge accumulating layer; a gate electrode formed on the gate insulating film; and source and drain regions formed in surface portions of the semiconductor substrate such that the gate electrode is interposed therebetween.
According to a second aspect of the invention, there is provided a semiconductor device comprising: a first gate insulating film formed on a semiconductor substrate; a charge accumulating layer containing a magnetic substance and formed on the first gate insulating film; a second gate insulating film formed on the charge accumulating layer; a gate electrode formed on the second gate insulating film; and source and drain regions formed in surface portions of the semiconductor substrate such that the gate electrode is interposed therebetween.
According to a third aspect of the invention, there is provided a semiconductor device comprising: a first charge accumulating layer containing a magnetic substance and directly formed on a semiconductor substrate; a first gate insulating film formed on the first charge accumulating layer; at least one second charge accumulating layer containing a magnetic substance and formed on the first gate insulating film; a second gate insulating film formed on the second charge accumulating layer; a gate electrode formed on the second gate insulating film; and source and drain regions formed in surface portions of the semiconductor substrate such that the gate electrode is interposed therebetween.
According to a fourth aspect of the invention, there is provided a semiconductor device comprising: a first gate insulating film formed on a semiconductor substrate; a first charge accumulating layer containing a magnetic substance and formed on the semiconductor substrate; a second gate insulating film formed on the first charge accumulating layer; at least one second charge accumulating layer containing a magnetic substance and formed on the second gate insulating film; a third gate insulating film formed on the second charge accumulating layer; a gate electrode formed on the third gate insulating film; and source and drain regions formed in surface portions of the semiconductor substrate such that the gate electrode is interposed therebetween.
The following features are preferable for the embodiments of the invention:
(1) The charge accumulating layer contains magnetic fine particles or is formed of a magnetic film;
(2) The semiconductor substrate is formed of an Si substrate or an SOI substrate;
(3) The charge accumulating layer contains Co, Fe, FeNi, Ni or CoPt;
(4) The magnetic fine particles have a size of 1-50 nm; and
(5) Where two or more charge accumulating layers are provided, a low resistance semiconductor such as polysilicon is used as the material of a charge accumulating layer as a second or any other layer except a first layer.
(Effect)
The effect of this invention will be described.
This invention is characterized by a magnetic substance used as the material of a charge accumulating layer, and in particular by a magnetic substance used as the material of a quantum dot formed in a gate insulating film. Further, the invention is characterized in that a magnetic substance is directly provided on a semiconductor substrate. This uses the fact that a Schottky barrier is formed between magnetic fine particles as metal particles and a semi
Fujita Shinobu
Inomata Koichiro
Tanamoto Tetsufumi
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Jackson, Jr. Jerome
Kabushiki Kaisha Toshiba
Ortiz Edgardo
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