Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
1996-12-12
2001-12-25
Chaudhuri, Olik (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S015000, C257S017000
Reexamination Certificate
active
06333516
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a quantum effect device which utilizes a quantum effect such as tunnel effect.
2. Description of the Related Art
In recent years, it has been proposed that a logic circuit be assembled without using three-terminal transistors such as bipolar transistors. (See Craig S. Lent et al.,
Quantum Cellular Automata Nanotechnology
, Vol. 4, pp. 49-57.)
FIGS. 18A and 18B
schematically show an inverter circuit incorporating no conventional transistors, which is called a “cell-connected inverter circuit.” As
FIGS. 18A and 18B
show, the cell-connected inverter circuit comprises six 5-quantum dot cells C
1
to C
6
. A 5-quantum dot cell has five quantum dots D
1
to D
5
, two of which contain one electron each, as illustrated in
FIGS. 19A and 19B
. It can assume two recognizable, stable ground states &PSgr;
1
and &PSgr;
2
since the two electrons undergo Coulomb interaction and repel each other, trying to acquire a lower energy level.
How the cell-connected inverter circuit operates will be described, with reference to
FIGS. 18A and 18B
. An input “1” is supplied to the 5-quantum dot cell C
1
, setting the cell C
1
into ground state &PSgr;
1
. Then, the 5-quantum dot cell C
2
takes ground state &PSgr;
1
which is electrically stable with respect to the cell C
1
. Further, the 5-quantum dot cell C
3
assumes ground state &PSgr;
1
which is electrically stable with respect to the cell C
2
. Next, the 5-quantum dot cell C
4
assumes ground state &PSgr;
2
which is electrically stable with respect to the cell C
3
, and the 5-quantum dot cell C
5
assumes ground state &PSgr;
2
which is electrically stable with respect to the cell C
4
. Finally, the 5-quantum dot cell C
6
takes ground state &PSgr;
2
which is electrically stable with respect to the cell C
5
. As a result, the cells C
1
and C
6
, which are the input and output cells, respectively, assume different ground states. The cell-connected inverter circuit therefore outputs “0” when it receives “1.”
The cell-connected inverter circuit is, however, disadvantageous in one respect. When the input binary value is changed from “1” to “0, ” the 5-quantum dot cells C
1
to C
3
assume ground state &PSgr;
2
, but the 5-quantum dot cells C
4
to C
6
remain in ground state &PSgr;
2
, as is illustrated in FIG.
18
B. In this case, the output binary value is identical to the input binary value. In other words, the circuit fails to function as an inverter.
A plurality of 5-quantum dot cells can transfer a signal if they are arranged in a row or a column. When a bias is applied on the first 5-quantum dot cell, thereby setting this cell into, for example, ground state &PSgr;
1
the second 5-quantum dot cell assumes the same ground state (&PSgr;
1
) due to its Coulomb interaction with the first cell. Similarly, the third to sixth 5-quantum dot cells assume ground state &PSgr;
1
. Therefore, the ground state &PSgr;
1
, is transferred as a signal from the first cell to the last via the remaining cells. When the first 5-quantum dot cell is set into ground state &PSgr;
2
, this ground state is transferred as a signal from the first cell to the last via the remaining cells.
If 5-quantum dot cells are arranged in a row or a column, they can transfer a signal (either ground state) along a straight path only. In an integrated circuit, 5-quantum dot cells must be arranged, forming bent paths and branched paths. Otherwise they could not change the direction of transferring signals or branch a signal transfer path in order to supply the signals to an arithmetic logic unit or a memory located at a given position in the integrated circuit.
To change the direction of transferring signals, 5-quantum dot cells may be arranged as shown in FIG.
48
. To branch a signal transfer path, they may be arranged as shown in FIG.
49
. Both arrangements are those proposed by Lent et al.
The cell arrangement shown in
FIG. 48
is a so-called “bent-back wire” consisting of five 5-quantum dot cells C
1
to C
5
. A signal S is first transferred through the cells C
1
to C
3
in the direction of 3 o'clock and then through the cells C
3
to C
5
in the direction of 6 o'clock. The bent-back wire has a problem, however. The 5-quantum dot cells C
2
and C
4
are unstable. They do not always assume a ground state since one of the electrons in the cell C
2
and one of the electrons in the cell C
4
are so close to have Coulomb interaction and repel each other, inevitably rendering the states of the cells C
2
and C
4
electrically unstable. It is difficult for the bent-back wire to change the direction of transferring the signal S, while maintaining the information which the signal S conveys.
The cell arrangement shown in
FIG. 49
is a so-called “multi fan-out branched wire” consisting of ten 5-quantum dot cells C
1
to C
10
. In the multi fan-out branched wire, the input signal S is branched twice—first at the cell C
3
, and then at the cell C
5
. As a result, the signal S is split into two signals S
1
and S
2
which are transferred in the direction of 3 o'clock along two parallel paths. The multi fan-out branched wire has a problem, too. Since the cells C
4
, C
6
, C
7
and C
9
are not electrically stable, it is difficult for the multi fan-out branched wire to split the input signal S into two signals S
1
and S
2
, while maintaining the information which the signal S conveys.
C. S. Lent et al. have proposed that a plurality of 5-quantum dot cells be combined to constitute a quantum effect device named “cellular automaton.”(See Appl. Phys. Lett. 62 (1993), p. 714. ) As diagrammatically shown in
FIG. 42
, a 5-quantum dot cell is rectangular and comprised of five spherical quantum boxes. Each quantum box is small enough to confine electrons in 0-dimensional fashion. Of these five quantum boxes, four are located in the corners of the cell, and the remaining one is located at the center of the cell. Two of the five quantum boxes, i.e., boxes 1017, contain one electron each.
The five quantum boxes are arranged so close to one another that electrons can move back and forth between any two adjacent quantum boxes by virtue of tunnel effect. In contrast, the 5-quantum dot cells constituting a quantum effect device are spaced apart by so long a distance that electrons cannot move among the cells despite tunnel effect.
The two electrons in one 5-quantum dot cell repel each other due to Coulomb interaction, and assume the lowest energy level when they are at the ends of a diagonal. Therefore, the 5-quantum dot cell can take two electrically stable states (“0” and “1”). Namely, the cell is bistable.
Since 5-quantum dot cells are bistable (0,1), they can form a memory when arranged close to one another, each for storing one bit of information. For example, four 5-quantum dot cells may be arranged in a row as shown in FIG.
43
. If this is the case, when state “0” is input to the left cell, it will be transferred to the right cell via the intermediate two cells—by virtue of the Coulomb repulsion acting between the two electrons contained in each 5-quantum dot cell.
Obviously, 5-quantum dot cells can form a wire for transferring electric signals only if they are arranged close to one another. Moreover, 5-quantum dot cells can constitute a logic circuit if they are arranged in an appropriate pattern.
However, 5-quantum dot cells of the type described above have the following problem.
As indicated above, the conventional 5-quantum dot cell is rectangular, and the five quantum boxes in the cell are spherical. It follows that any two adjacent quantum boxes arranged in a diagonal are less spaced apart than are the two adjacent quantum boxes arranged in a line parallel to any side of the cell. The tunneling probability is therefore high, and electrons can move between any adjacent quantum boxes due to tunnel effect.
Certainly, the cell indeed remains in a specific state while a bias is being applied to it. When the application of the bias to it is stopped, however, its state becomes less stable
Katoh Riichi
Mashita Masao
Naruse Yujiro
Saba Francis Minoru
Takahashi Shigeki
Chaudhuri Olik
Ha Nathan W.
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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