Quantum wire CCD charge pump

The very accurate quantization of charge transport or electron current is achieved within a heterostructure substrate by defining a quantum wire within the substrate and propagating a a.c. potential characterized by a travelling wave envelope along the length of the quantum wire. The travelling wave a.c. potential is applied to the length of the quantum wire by two opposing lateral gate arrays defined within the substrate. For each gate on the array is provided a corresponding gate on the opposing array which is offset in the direction of the current transport by a predetermined distance. A succeeding space is then provided in both arrays where there is no gate. An offset a.c. potential is then applied to the gate of one array, the offset gate of the opposing array and to the array as a whole through an overlying gate running the longitudinal length of the quantum, wire which gate applies a spatially independent a.c. potential along the length of the quantum wire.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to semiconductor devices exhibiting quantum 
mechanical performance characteristics, and in particular it relates to 
such semiconductor devices used as current sources. 
2. Description of the Prior Art 
Conventional semiconductor circuitry, even ultra miniaturized circuits, 
usually operate in a mode that can be described by classical or nonquantum 
mechanical, electrical theories of solids. However, in some cases the 
operation of the device cannot be explained by such classical theories, 
but only through a quantum mechanical theory. One such device is a 
Josephson Junction which operates to produce a quantized voltage in terms 
of e/h, where e is the charge of an electron and h Planck's constant. 
Additionally quantum behavior in quantum Hall effect devices is also 
observed and can be utilized to provide a quantized resistance which is a 
multiple of h/e.sup.2. 
Quantized adiabatic particle transport has been predicted and described by 
D. J. Thouless in Physical Review B 27,6083 (1983). According to Thouless 
an integral number of electrons is transported through any cross section 
as long as the Fermi gap in the conducting material is open at all times. 
Thouless has theoretically demonstrated that if the potential in a 
semiconductor is changed slowly in such a way that it returns to its 
starting value, the current across a boundary will be quantized for filled 
bands in an infinitely periodic system, but not in a finite torus. 
Thouless, however, did not propose any particular device in which 
quantization of current could be utilized or applied for any practical 
purpose. 
Therefore, what is needed is an apparatus and method for providing 
quantized current as predicted by theory which would allow control of the 
quantized current to be known and made usefully available. 
BRIEF SUMMARY OF THE INVENTION 
The invention is an apparatus, hereafter called a quantum charge pump, for 
transporting a selected number of electron charges. The quantum charge 
pump comprises primarily a quantum wire, three sets of gates and a pair of 
source and drains. 
The quantum wire serves as a channel for the charge flow. The source and 
drain are connected at the ends of the quantum wire, and they serve either 
as the storage places of the transported charges, or as the contacts to an 
external circuit. The three sets of gates are arranged along the top and 
the sides of the quantum wire. The charge flow is controlled by a moving 
potential wave along the quantum wire. The potential wave can be imposed 
by applying AC voltages on the lateral gates. 
In one embodiment of the apparatus as described in detail below, the 
quantum wire lies on an interface of semiconductor hetrostructure whereon 
a two-dimensional electron gas usually resides. The quantum wire is a 
narrow conducting channel, in which only one or a few lateral modes are 
occupied by the electrons. 
On one side of the quantum wire on the interface plane lies a first 
periodic array of gates with a predetermined spatial periodicity, a. Each 
gate substantially occupies one third of the unit distance, namely a/3. 
The essential function of the gates is to modify the effective thickness 
or the electronic potential of the quantum wire in the vicinity of the 
gates by applying voltages to the gates. A spatially periodic modulation 
of the quantum wire can be generated by applying a common voltage on this 
set of gates. In practice, the first array of gates of the wire are to be 
conductively connected together, so that they always have the same 
voltage. 
A second periodic array of gates lies on the opposing side of the quantum 
wire. This second set of gates is identical to the first set, but its 
position along the quantum wire is offset from the first set by one third 
of the unit distance a. A common voltage on the second set of gates will 
produce a spatially periodic modulation of the quantum wire at positions 
offset from those of the first set by a/3. 
The quantum wire is thus partitioned into succeeding segments as l.sub.1, 
l.sub.2, l.sub.3, l.sub.1, l.sub.2, l.sub.3, . . . each of length a/3. 
Here, l.sub.1 is a segment in the vicinity of a gate of the first set, 
l.sub.2 is in the vicinity of a gate of the second set, and l.sub.3 is a 
region without a gate of either of the sets. 
The quantum wire in the l.sub.3 regions may be modulated by a top gate on 
the top of the quantum wire. The top gate extends uniformly along the 
quantum wire. The top gate voltage V.sub.3 is to be applied with respect 
to the source which connects one end of the quantum wire. The voltages 
V.sub.1 and V.sub.2 on the first and second sets of lateral gates are to 
be applied with respect to the top gate. 
The time dependencies of V.sub.1, V.sub.2 and V.sub.3 are chosen such that 
a potential wave of the following form is imposed along the quantum wire: 
##EQU1## 
where u(x) is a local potential centered at x=0 and having an effective 
range of a/3. The amplitudes A.sub.j (t) are phase-shifted sinusoidal 
waves, i.e., 
EQU A.sub.j (t)=C+C.sub.1 cos [2.pi.(j/3-t/T)] 
where j=1, 2, 3, and C and C.sub.1 are predetermined constants. 
The potential U(x,t) simulates a periodic array of moving potential wells. 
The Fermi energy can be controlled by timing the value of C on the top 
gate, such that n levels of each potential well is occupied, where n is a 
predetermined integer. The substrate is cooled to a predetermined 
temperature to limit high energy electron states the occupation. In a 
cycle, two 2n electrons (with spins up and down) get transported from one 
end of the quantum wire to the other. As a result, a selected number of 
electrons may be pumped from the source to the drain by controlling the 
voltages on the lateral gates and the top gate. When Coulomb effect is 
taken into account, a level is split into two with a Coulomb gap between 
them. If the Fermi energy is placed in a Coulomb gap, an odd number of 
electrons may be pumped. 
The quantum wire and the lateral gates can be constructed by tailoring a 
two-dimensional electron gas in the interface of a semiconductor 
heterostructure, using focused ion beam technology. 
The quantum charge pump, as invented, can precisely control the number of 
electrons pumped in a cycle; the possible error can be made exponentially 
small as the temperature and driving frequency get smaller than the energy 
gap at the Fermi energy. The device is also robust against small 
imperfections in the structure, in the sense that small amount of disorder 
does not influence the precise nature of the device at all. The 
temperature and frequency for precision operation can be greatly increased 
by raising the energy gap in the quantum wire through minimization of the 
feature sizes of the device. 
The invention comprises the following major applications: 
1) Memory Device 
The quantum charge pump is coupled to a capacitor which provides and 
receives the pumped charges. Information is to be stored on the capacitor 
in terms of the number of charges that the capacitor carries. A selected 
number of electron charges can be deposited on or retrieved from the 
capacitor, corresponding to writing or reading the information, 
respectively. A separate circuit checks whether the capacitor has cleared 
out of charges. 
2) Capacitive Standard 
A selected number of charges is pumped onto a capacitor. The capacitance 
can then be determined by a precise measurement of the voltages on the 
capacitor. 
3) A Precise DC Current Source 
A selected strength of DC current can be produced by tuning the frequency 
(number of cycles per second) at which the pump is driven. Such a current 
source can also be used as a current standard. 
More formally, the invention is an apparatus for transporting a selected 
number of electron charges. The apparatus comprises a quantum wire for 
providing a channel for flow of the electrons, and an element for applying 
a moving a.c. potential wave along the quantum wire so that a 
predetermined number of electrons are disposed within each well of the 
moving a.c. potential. 
As a result, the number of electrons flowing through the quantum wire is 
selectively determined and controlled. 
The element comprises a first and second plurality of opposing conductive 
gates defined along the sides of the quantum wire. Each gate of the first 
plurality of gates corresponds to a gate of the second plurality of gates 
and vice versa. Each gate of the first plurality of gates is spatially 
offset along the quantum wire from the corresponding one of the gates of 
the second plurality of gates. A control element applies a separate a.c. 
potential to the first and second plurality of gates. The a.c. potential 
applied to the first plurality of gates is phase shifted from the a.c. 
potential applied to the second plurality of gates. 
The pairs of corresponding gates of the first and second plurality of gates 
are spaced apart from a succeeding pair of corresponding gates from the 
first and second plurality of gates by a gap along the quantum wire. 
The corresponding gates of each pair of gates of the first and second 
plurality of gates and the corresponding gap succeeding the pair of gates 
along the quantum wire is equally spatially set off one from the other so 
that the pair of gates and gap extend a predetermined unit of distance 
along the quantium wire. Each gate substantially occupies one third of the 
predetermined unit distance. 
The control means applies an a.c. potential to each gate of the pair of 
corresponding gates of the first and second plurality of gates and to the 
gap. The potential is phase shifted by one third cycle between each 
opposing gate and gap in an order corresponding to the spatial order of 
the gates and gap along the quantum wire. 
The element for applying a moving a.c. potential applies an a.c. potential 
of the form 
##EQU2## 
where U(x,t) is a local a.c. potential having an effective nonzero value 
across an interval a/3 and where 
EQU A.sub.j (t)=C+C.sub.1 cos [2.pi.(j/3-t/T)] 
where j is an integer and T is the time periodicity of the amplitude 
A.sub.j (t). 
The quantum wire and the lateral gates are defined on the interface of a 
semiconductor heterostructure. The heterostructure comprises an aluminum 
gallium arsenide layer and a gallium arsenide layer. The quantum wire is 
cooled to a predetermined temperature to ensure that in the quantum wire 
those states below the Fermi energy are almost fully occupied, and those 
above it are almost totally empty. 
The element for applying the moving a.c. potential generates a moving a.c. 
potential comprised of a plurality of localized a.c. potentials simulating 
localized square wave a.c. potentials. 
The control element further comprises a top gate for applying a spatially 
independent a.c. potential along the quantum wire. 
The apparatus further comprises a capacitor. The capacitor is coupled to 
the quantum charge pump and receives and provides electron current to and 
from the pump respectively as controlled by the element for applying the 
moving a.c. potential. 
The apparatus is used in further combination with a memory circuit wherein 
a plurality of distinguishable charge states is stored on the capacitor by 
control of the element of applying a moving a.c. potential transporting to 
the capacitor a predetermined number of electron charges corresponding to 
a selected one of the predetermined plurality of charge states on the 
capacitor. 
The element for applying a moving a.c. potential applies a predetermined 
number of the moving a.c. potentials along a predetermined length of the 
substrate to transport a corresponding predetermined number of electric 
charges across a substrate so that the apparatus serves as a precise 
direct current standard. 
The apparatus is used in further combination with a capacitor so that the 
apparatus is adaptable as a precise measuring standard of capacitance. 
In summary and alternatively the invention is an apparatus for selectively 
transporting a predetermined number of electronic charges. The invention 
comprises a quantum wire for providing a defined direction of current flow 
of the electrons. A plurality of gates define a quantum wire. The gates 
are arranged in periodic arrays along the sides of the quantum wire. An 
element applies a plurality of periodically varying a.c. potentials to the 
plurality of gates to form a travelling wave envelope along the quantum 
wire. 
As a result, a selectively controlled number of electrons is transported 
along the quantum wire. 
The invention is also a method for transporting a selected discrete number 
of electron charges from a first to a second terminal. The method 
comprises the steps of defining a gated quantum wire within a 
heterostructure interface to provide channeled flow of the electrons 
through the quantum wire. The quantum change pump is coupled between the 
first and second terminals of an external circuit. An a.c. potential is 
imposed on the quantum wire characterized by a travelling wave envelope 
moving in the direction of the quantum wire to transport electrons from 
the first terminal to the second terminal. 
As a result, a discrete number of electrons may be transported between the 
first and second terminals. 
The invention can be better visualized by now turning to the following 
drawings wherein like elements are referenced by like numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The very accurate quantization of charge transport or electron current is 
achieved within a heterostructure interface by defining a quantum wire 
within the interface and propagating an a.c. potential characterized by a 
travelling wave envelope along the length of the quantum wire. The 
travelling wave a.c. potential is applied to the length of the quantum 
wire by two opposing lateral gate arrays and a uniform top gate defined 
along the sides and top of the quantum wire. For each gate on one lateral 
array is provided a corresponding gate on the opposing array which is 
offset in the direction of the current transport by a predetermined 
distance. A succeeding space is then provided in both arrays where there 
is no gate. An offset a.c. potential is then applied to the gate of one 
array, the offset gate of the opposing array and to the array as a whole 
through an overlying gate running the longitudinal length of the quantum 
wire, which gate applies a spatially independent a.c. potential along the 
length of the quantum wire. 
The invention is an apparatus for transporting a selected number of 
electron charges comprising a quantum wire for providing a channel for 
flow of the electrons. A circuit element applies a moving potential along 
the flow of electrons in the quantum wire so that a predetermined number 
of electrons are disposed within each cycle of the moving potential along 
the quantum wire. 
As a result, the number of electrons flowing through the quantum wire is 
selectively determined and controlled. The circuit element defines a gated 
quantum wire within the interface. 
The circuit element comprises a first and second plurality of opposing 
conductive gates which, together with the quantum wire, define a gated 
quantum wire within the interface. Each gate of the first plurality of 
gates corresponds to a gate of the second plurality of gates and vice 
versa. Each gate of the first plurality of gates is spatially offset along 
the direction of current flow within the quantum wire from the 
corresponding one of the gates of the second plurality of gates. A control 
circuit applies a separate a.c. potential to the first and second 
plurality of gates. The a.c. potential applied to the first plurality of 
gates is phase shifted from the a.c. potential applied to the second 
plurality of gates. 
The pairs of corresponding gates of the first and second plurality of gates 
are spaced apart from a succeeding pair of corresponding gates from the 
first and second plurality of gates by a gap or no-gate space extending 
along the distance of the current flow within the quantum wire. 
The corresponding gates of each pair of gates of the first and second 
plurality of gates and the corresponding gap succeeding the pair of gates 
along the direction of current flow within the quantum wire is equally 
spatially set off one from the other so that the pair of gates and gap 
extend a predetermined unit of distance along the direction of the current 
flow within the quantum wire. Each of the gate substantially occupies one 
third of the predetermined unit distance. 
The control circuit applies an a.c. potential to each gate of the pair of 
corresponding gates of the first and second plurality of gates and to the 
gap with a phase differential of one third cycle offset each gate and gap 
from the other in an order corresponding to the spatial order of the gates 
and gap along the direction of current flow within the quantum wire. 
The corresponding gates of each pair of gates of the first and second 
plurality of gates in the corresponding gap succeeding the pair of gates 
along the direction of current flow within the quantum wire is equally 
divided so that the pair of gates and gap extend a predetermined unit of 
distance along the direction of the current flow within the quantum wire. 
and wherein each the gate substantially occupies one third of the 
predetermined unit distance. 
The circuit element for applying a moving a.c. potential applies a a.c. 
potential of the form: 
##EQU3## 
where U(x, t) is a local a.c. potential having an effective nonzero value 
across an interval a/n and where 
EQU A.sub.j (t)=C+C.sub.1 cos[2.pi.(j/n-t/T)] 
where j is an integer and T is the time periodicity of the amplitude 
A.sub.j (t) and where n is an integer. 
The smallest number of the local a.c. potentials combinable to form the 
moving a.c. potential along the current direction of the quantum wire is 
n=3. 
The quantum wire a channel of defines electron flow within a 
two-dimensional interface between a first and second semiconducting layer. 
In the illustrated embodiment the heterostructure comprises an aluminum 
gallium arsenide layer and a gallium arsenide layer. 
The quantum wire is cooled to a predetermined temperature to prevent higher 
energy state within the quantum wire being occupied. 
The circuit element for applying the moving a.c. potential generates a 
moving a.c. potential comprised of a plurality of moving localized a.c. 
potentials simulating localized square wave a.c. potentials. 
The circuit element further comprises a top gate for applying a spatially 
independent a.c. potential along the direction of the current flow within 
the quantum wire. 
The apparatus further comprises a capacitor. The capacitor is coupled to 
the quantum wire for receiving and providing electron current to and from 
the quantum wire respectively as controlled by the circuit element for 
applying the moving a.c. potential. 
The invention is the apparatus described above in further combination with 
a memory circuit wherein a plurality of distinguishable charge states is 
stored on the capacitor by control of the circuit element of applying a 
moving a.c. potential transporting to the capacitor a predetermined number 
of electron charges corresponding to a selected one of the predetermined 
plurality of charge states on the capacitor. 
The circuit element for applying a moving a.c. potential applies a 
predetermined number of the moving a.c. potentials along a predetermined 
length of the quantum wire to transport a corresponding predetermined 
number of electric charges across a quantum wire so that the apparatus 
serves as a precise direct current standard. 
The apparatus is provided in further combination with a capacitor so that 
the apparatus is adaptable as a precise measuring standard of capacitance. 
The invention is also characterized as an apparatus for selectively 
transporting a predetermined number of electronic charges comprising a 
quantum wire for providing a defined direction of current flow of the 
electrons. A plurality of gates, together with a quantum wire, define a 
gated quantum wire. The gates are arranged in a periodic array along the 
sides of the quantum wire. A circuit element applies a plurality of 
periodically varying a.c. potentials to the plurality of gates to form a 
travelling wave envelope along the quantum wire of the gated quantum wire. 
As a result, a selectively controlled number of electrons is transported 
along the gated quantum wire. 
The invention is a method for transporting a selected discrete number of 
electron charges from a first to a second terminal comprising the steps of 
defining a gated quantum wire within an interface to provide channeled 
flow of the electrons through the quantum wire. The quantum wire is 
coupled between the first and second terminals. An a.c. potential is 
imposed on the quantum wire, which a.c. potential is characterized by a 
travelling wave envelope moving in the direction of the quantum wire to 
transport electrons from the first terminal to the second terminal. 
As a result, a discrete number of electrons may be transported between the 
first and second terminals. 
The invention now having been generally described without reference to the 
drawings, turn to the drawings for a better visualization of the 
invention. A quantum charge pump, generally denoted in the Figures by 
reference numeral 10, is devised to provide a selectively controlled 
current in multiples of electrons. A selectively controlled current as 
small as one electron is produced. Charge pump 10 in the illustrated 
embodiment is disposed in a gallium arsenide heterostructure substrate 
although silicon substrates or other semiconducting materials could be 
utilized with equal generality. Gallium arsenide is chosen in preference 
to silicon because the electrons within the interface of gallium arsenide 
heterostructure have a smaller effective mass and higher mobility than in 
a silicon structure. Charge pump 10 is formed of a heterostructure of 
aluminum gallium arsenide and gallium arsenide so that the quantum wire 22 
is formed out of a two-dimensional electron gas situated at the interface 
of the gallium arsenide and aluminum gallium arsenide. 
Again, in the illustrated embodiment charge pump 10 is operated at 
approximately 1-20 degrees K although it is expected that the charge pump 
of the invention will be able to operate at higher temperatures, possibly 
within the range of liquid nitrogen and higher. 
An end electrode, denoted by reference numeral 14, is formed at one end 
with a similar opposing end electrode 16 formed at the opposite end of the 
quantum wire. Utilizing a focused gallium ion beam gun, such as model 
Nanofab 150UHV manufactured by Micro Beam Inc. of California, a focused 
beam of 150 ke V gallium ions is implanted near the heterostructure 
interface 12 to form lattice distortions and disruptions. The dislocations 
are selectively disposed into interface 12 to form a first insulating 
pattern 18 and corresponding opposing insulating pattern 20 as shown in 
the plan view of FIG. 1. Regions 18 and 20 become substantially 
nonconductive in the lateral direction of substrate 12, namely in the 
plane of the illustration of FIG. 1. See A. Wieck et. al., "In-Plane Gated 
Quantum Wire Transistor Fabricated with Directly Written Focused Ion 
Beams", Applied Physical Letters 56(10), 5 Mar. 1990. 
The energy band structure in the vertical direction of the heterostructure 
maintains the electron gas within a two-dimensional flow of the plane of 
the illustration of FIG. 1. 
Therefore, electrons are able to flow from contact 14 to 16 only through a 
longitudinal channel 22 which is also termed a quantum wire 22. A quantum 
wire is defined as a path length of electron flow wherein the quantum 
state of the conducting electrons is selectively controlled. 
Nonconducting regions 18 and 20 are formed by the focused gallium beam in a 
periodic array offset relative to each other. The pattern is shown in 
highly idealized square form in FIG. 1 and in actuality will be much 
smoother due to the finite diameter of the beam, i.e. about 60-80 nm, 
impinging upon the hererostructure as well as practical limitations upon 
the fineness of its control. 
In the illustrated embodiment, quantum wire 22 is formed with a length of 
approximately 1.5 microns from boundary 24 to 26. The geometrical width 28 
of quantum wire 22 is of the order of 1 micron, but is not critical. The 
effective width of quantum wire 22 can always be effectively adjusted by 
application of appropriate voltage levels to side contacts 30 and 32. 
Nonconductive regions 18 and 20 are formed so that conducting gates 34 and 
36 of conductive regions 30 and 32, respectively, extend laterally into 
the nonconducting region 18 and 20 toward quantum wire 22 in a generally 
perpendicular direction to quantum wire 22. As shown in FIG. 1, gate 36 is 
the leftmost gate in the array of charge pump 10 as shown in FIG. 1, while 
gate 34 in the arrays is disposed in the next adjacent but opposing 
position one step to the right in FIG. 1. At a third step no gate extends 
into nonconductive regions 18 and 20. The three steps, two gates and one 
no-gate space, comprise a unit cell 38 of the device. The pattern as 
established with the first three longitudinal steps down the length of 
quantum wire 22 is then periodically repeated along its entire length. 
The two-dimensional electron gas outside quantum wire 22 is used to 
influence the potential within quantum wire 22. Since the electron gas 
outside the quantum wire can be confined or controlled in a periodic 
manner along the wire, the influences along the wire will also be 
periodic. As shown in FIG. 1, there are two sets of lateral gates on 
either side of the quantum wire which are phase-shifted with respect to 
each other. In between each of the two gates on each side there is an area 
with no lateral gate with the result that as shown in FIG. 3 there are 
three alternating a.c. potentials along quantum wire 22, namely lateral 
gate 36, lateral gate 34 and then no gate. 
The lateral gates 30 and 32 are formed by a focused ion beam of 
approximately 600 Angstroms in diameter. Tunneling through lateral gates 
30 and 32 is very unlikely because of their thickness which is chosen for 
this reason. A previously developed lateral gating technique is used to 
change a.c. potential in a periodic manner along the length of quantum 
wire 22 as described by Wieck et. al., "In-Plane Gated Quantum Wire 
Transistor Fabricated with Directly Written Focused Ion Beams", Applied 
Physical Letters 56(10), Mar. 5, 1990. See also German Patent Application 
P 39 14 007.5 (1989). 
The physical width 28 of quantum wire 22 is approximately 100-1,000 
nanometers. However, in order to avoid multiple modes and therefore 
multiple numbers of electron states with within each cell 38 along the 
length of quantum wire 22, a.c. potentials of side contacts 30 and 32 are 
adjusted so that the effective electrical width available for electron 
states within quantum wire 22 is approximately 150 Angstroms. 
Quantum charge pump 10 is operated to precisely pump a charge of Ne per 
operating cycle where e is the charge of a single electron and N is an 
integer, typically between 1 and 10. Charge pump 10 is bidirectional so 
that the charges may enter electrode 12 or 16 depending upon the direction 
of cycling or pumping which is utilized as described below. The pumping 
action is effectuated by applying voltages to side electrodes 30 and 32 
and top gate 40 shown in perspective view of FIG. 2, which is a planar 
sheet overlying the entire periodic array of cells 38 shown in FIG. 1. The 
electron mechanics in the charge pump can be characterized in essence as a 
one-dimensional noninteracting electron gas in a sliding periodic a.c. 
potential, U(X-at/T), where a is the spatial periodicity of the a.c. 
potential and the propagation speed is a/T. 
If it can be assumed that the electrons are at zero temperature, they will 
initially occupy an integer number of Bloch bands within the gallium 
arsenide crystal. Assume, for example, that h/T is much smaller than the 
electron energy gap of the quantum wire at the Fermi energy so that 
initially all the filled bands remain filled. The electron density then 
shifts rigidly with the sliding a.c. potential. The number of electrons 
flowing across any point in time T is then equal to the number of 
electrons in a spatial period of the sliding a.c. potential. This number 
is an even integer, due to the spin degeneracy of the electron states. 
In practice, the a.c. potential may consist of a series of histograms as 
diagrammatically depicted in FIG. 3. The histographic potential can be 
viewed as spatially stationery with its height oscillating in time at each 
gate so that the minima propagate down the quantum wire. For example, the 
top panel in FIG. 3 is the histographic a.c. potential at time T0. The 
next panel below is the histogram a.c. potential at T1 followed in 
succeeding panels with the a.c. potential at times T2 through T4 which are 
each separated by one twelfth of a period. The horizontal axis is the 
spatial coordinate down the longitudinal length of quantum wire 22. 
The phases of the oscillating heights of the sliding a.c. potential are 
synchronized so that the envelope forms a travelling wave. The Fermi 
energy is fixed in the gap above the lowest Bloch band by application of 
an appropriate a.c. potential to the Fermi gate 40. The number of 
electrons transported across any point in time is then two. In other 
words, the electrons are locked at the minima of a a.c. potential wave and 
travel with it. 
As best depicted in perspective view in FIG. 2, on top of the entire 
quantum wire 22 is a metallic top gate which is vapor deposited and is 
used to change the overall a.c. potential of all the characters within 
wire 22. The a.c. potential applied to gate 40, the top gate, is the 
spatially independent term within the a.c. potential C(t), reference and 
equations 1 and 2 below. 
FIG. 4 is a diagrammatic side view of device 10 as seen through section 
4--4 of FIG. 1. The voltage on gate 40 (biased with respect to the source 
14) is A.sub.3 (t), the voltage on contact 32 (biased with respect to gate 
40) and hence gate 36 is A.sub.1 (t)-A.sub.3 (t) and the voltage on 
contact 30 (biased with respect to gate 40) and hence gate 34 is A.sub.2 
(t)-A.sub.3 (t). The voltages A.sub.1, A.sub.2 and A.sub.3 are varied 
sinusoidally with 120 degrees shifted phase to produce the sliding a.c. 
potential of FIG. 3. The periodic travelling wave therefore realized 
within quantum wire 22 has impressed upon it by contacts 30 and 32 and the 
gates corresponding with them and gate 40. The frequency of the periodic 
wave is typically 1-10 GHz and is limited only by the frequency generator 
of an atomic clock. 
In FIG. 3, three histographic blocks represent a single wave length. The 
minima are at the positions 42. The height of each a.c. potential A.sub.j 
(t) varies in time according to the equation: 
EQU A.sub.j (t)=C(t)+C.sub.1 cos [2.pi.(j/3a-t/T),] Equation #1 
so that the a.c. potential is: 
##EQU4## 
where u(x)=1 for absolute value of x less than a/6 and otherwise u(x)=0, 
where T is the time cycle period of the wave envelope. The additionally 
spatially independent term, C(t), is chosen so that the Fermi energy of 
the source 14 always lies in the energy gap. C(t) can only be varied 
through the voltage on the top gate 40. 
The local a.c. potentials in the illustrated embodiment, idealized in FIG. 
3 as square waves, are equally offset in phase one from the other by a 
third of a cycle. Therefore, at three points during each cycle one of the 
a.c. potentials will decrease to zero while the adjacent a.c. potentials 
are equal, nonzero and form a well. Thus, the minimum number of a.c. 
potentials which are required to be used in combination to perform a well 
are three in a one-dimensional channel since the well and the two opposing 
side walls of the well must be formed if the electron is to be localized 
within the well. The same three offset a.c. potentials are then applied to 
the next cell along the quantum wire as diagrammatically depicted in FIG. 
3. Every third localized a.c. potential is thus in phase and equal. 
Every one third of the cycle of the localized a.c. potentials will thus 
cause a well 42 to move down the direction of current flow in the quantum 
wire by a distance equal to one third of the wave length of the frequency 
of the moving wave envelope. Movement by one third of the wave length is 
shown in FIG. 3 between the times T0 and T4. It is entirely within the 
scope of the invention that other fractions of a cycle could be used as 
desired to shape or create moving a.c. potential wells. 
It must be understood that square wave a.c. potentials, as idealized in 
FIG. 3, are not necessarily applied to the quantum wire. Gates 34 and 36 
are not precisely rectangular and thus the a.c. potentials which are 
impressed upon quantum wire 22 by them are also not exact square waves as 
depicted in FIG. 3. Instead, it is enough that the a.c. potential be 
generally localized within the area of the gate or gap so that a moving 
a.c. potential well 42, which may be of arbitrary shape, propagates down 
the length of the quantum wire. Propagation along the length of the 
quantum wire is assured by virtue of the periodicity of gates 34 and 36 
and the two opposing arrays of gates integrally extending from contacts 
30, 32. 
Lateral gates 34 and 36 are formed with a periodicity of 150 nanometers 
with twenty periods resulting in an energy gap of about 1 meV. The 
temperature device 10 must there be kept at about 1 degree K. in order to 
prohibit higher line states from being propagated. When the a.c. potential 
is modulated with the frequency of 1 GHz, the direct current within 
quantum wire 22 is of the order of several nanoamps. The voltage frequency 
on the lateral gates and top gate of device 10 can be very accurately 
controlled by atomic clocks. 
It is predicted on the basis of quantum mechanical theory of adiabatic 
particle transport that for a finite Fermi energy gap within quantum wire 
22, the electron motion will be insensitive to many kinds of 
perturbations, such as thermal activation, non adiabatic excitation, 
static disorder, many-body interaction, finite size and the like. The 
correction due to thermal activation is of the order of exp(-Eg/(KT)) 
which can be very small if the thermal energy gets smaller than the energy 
of the energy gap, Eg. The correction due to nonadiabatic excitations is 
an exponential of exp(-Eg/(h.nu.)) and therefore could be easily 
controlled. It has been theoretically shown by Niu and Thouless in the 
Journal of Physics, A17, page 2453 (1984) that quantization of the charge 
transport is not affected by disorder of the crystal and many-body 
interaction, as long as the adiabatic ground state of the system is 
separated from the excitations by finite energy gap. 
Finally, although the contacts 14 and 16 at the end of device 10 will 
certainly close the Fermi gap in the device at those locations, the states 
in the gap are localized in the leads. In order for these states to have 
an effect, they must tunnel from one end of device 10 to the other. 
Correction to quantization is therefore of the order of exp(-L/1) where L 
is the length of the quantum wire 22 and 1 is the localization length of 
the edge states at contacts 14 and 16. 
Once device 10 lies near an ideal configuration, every conceivable 
correction to the quantization is either identically zero or exponentially 
small when a perturbation analysis is made. Therefore, charge transport 
through the length of device 10 can be quantized as a practical matter to 
lead to an extremely accurate standard for electric charge and current 
measurements. 
FIG. 5 is a symbolic diagram which illustrates the relationship of the 
phenomena exploited by the methodology of the invention to other known 
observable quantum mechanical effects. For example, the Josephson effect, 
denoted by circle 44, is a phenomenon wherein the frequency of electrical 
excitation is quantized in units of e/h times the applied V. The quantum 
Hall effect, denoted by symbolic circle 46, similarly is a phenomenon 
wherein voltage is quantized in multiples of h/e.sup.2 times current I. 
The quantum charge pump of the invention, symbolically denoted by circle 
48, is the conceptual Ohms Law analog of the other two in that charge 
transport or current is quantized in multiples of e, the charge of a 
single electron. Therefore, it is within the intent and scope of the 
invention that a charge pump device such as shown in FIG. 1 can be 
controlled and its output coupled to a conventional capacitor which can 
then be controllably charged by a discrete number of electrons with 
precision equal to one electron charge. Such a controllably charged 
capacitor can then be used in measurements and measuring devices as a 
standard or reference with the highest degree of precision capable given 
the observed quantization of charge in nature. The amount of pumped 
charges is easily and accurately controlled by controlling the number of 
operating cycles or periodic wave transversals of quantum wire 22 executed 
within device 10. 
In addition, device 10 can be used as a high resistance current source. The 
resistance is considered as exponentially large along the length of 
quantum wire 22 until the voltage becomes comparable to Eg/e. 
Further applications of device 10 can be used in semiconductor memories. 
For example, a controlled number of electrons can be stored into a 
capacitor again by driving device 10 with a corresponding number of pulses 
or travelling wave transversals of the length of quantum wire 22. Insofar 
as the capabilities of the charge pump device 10 are concerned, it is 
possible to discriminate between states on the capacitor by an amount as 
small as a single electron charge. Device 10 may be used to charge up a 
capacitor as well as discharging a capacitor for read-out. 
FIG. 6 illustrates a diagrammatic depiction of such a memory cell. Charge 
pump 10 is coupled to capacitor 50 and is directionally driven by an 
oscillator 52 coupled to electrodes 30, 32 and 40 in the manner described 
above. In a practical device, distinguishable charge states may vary by 
more than a single electron charge by partitioning the a.c. potential 
charge on capacitor 50 into a predetermined number of discrete levels. For 
example, it is entirely possible to build a decimal state memory in which 
ten very accurately controlled charge levels can be stored and read from 
capacitor 30 by charge pump 10. The ten distinguishable charge levels on 
capacitor 50 may be arbitrarily chosen in order to provide easy and 
practical charge discriminations by a sense amplifier which would be 
coupled to the memory cell of FIG. 6. 
As a result, the information density storable on memory cells not limited 
to binary memory states is dramatically increased. For example, in order 
to store a megabyte of information in conventional binary memory, 8 
million memory cells are required. A memory cell of FIG. 6 which was 
configured to pump any one of ten charge levels into capacitor 50, 
including the zero charge state, could hold five times as much information 
as a binary cell. The number of such decimal cells which would be required 
to hold an equivalent amount of information to that in one megabyte of 
binary memory cells would then be five times fewer. If the same memory 
cell of FIG. 6 were operated so as to generate a hundred distinguishable 
states within capacitor 50, including the zero charge state, a fifty-fold 
increase over a binary cell would be realized. The increase in information 
density by using the charge pump memory cell of FIG. 6 as compared to 
conventional binary memory cells is then limited only by the ability of 
the sense amplifier reading the memory cells to distinguish between charge 
states of capacitor 50 and the maximum charge carrying capacity of 
capacitor 50. 
The speed and size of memory cells constructed according to FIG. 6 is 
easily comparable and is expected to be substantially several orders of 
magnitude better than that realizable by the best performing and highest 
density conventional binary memory cell now known. 
Many alterations and modifications may be made by those having ordinary 
skill in the art without departing from the spirit and scope of the 
invention. Therefore, the invention has been described above only for the 
purposes of example and clarification. The specification should therefore 
not be read as limiting the invention which is defined by the following 
claims, which include all equivalent means as well as means for performing 
substantially similar functions for obtaining substantially similar 
results even though performed in a substantially different way.