|United States Patent
,   et al.
November 3, 1987
Negative transconductance device
A resonant tunneling device having a one-dimensional quantum well comprises
a semiconductor region capable of exhibiting one-dimensional quantization.
The device comprises source and drain contact regions adjoining such
semiconductor region as well as a gate contact for applying a field to
such region; the device can be implemented, e.g., by methods of III-V
deposition and etching technology. Under suitable source-drain bias
conditions the device can function as a transistor having negative
Capasso; Federico (Westfield, NJ);
Luryi; Sergey (Millington, NJ)
American Telephone and Telegraph Company, AT&T Bell Laboratories (Murray Hill, NJ)
November 27, 1985|
|Current U.S. Class:
||H01L 027/12; H01L 029/161|
|Field of Search:
357/4 S,4 L,16
References Cited [Referenced By]
U.S. Patent Documents
Applied Physics Letters, 24, Jun. 15, 1974, pp. 593-595.
Journal of Applied Physics, 58, Aug. 1, 1985, pp. 1366-1368.
Primary Examiner: Edlow; Martin H.
Attorney, Agent or Firm: Laumann; Richard D.
What is claimed is:
1. A semiconductor device comprising first and second semiconductor
a third semiconductor region which forms a quantum well wire layer and is
capable of exhibiting carrier motion quantized in two-dimensions (x and y)
and free in a third-dimension (z) said third region being between said
first and second semiconductor regions;
first and second electrical contacts to said first and second regions,
means for applying an electric field to said third region.
2. A device as recited in claim 1 in which said first and second regions
are capable of exhibiting one-dimensional (y) quantization, carriers being
free in two dimensions (x and z) thus forming first and second quantum
3. A device as recited in claim 2 in which said third region further
comprises two barrier layers on opposed sides of said quantum well wire
4. A device as recited in claim 3, said means for applying an electric
field comprising a third barrier layer contacting said first, second and
third regions, and said third barrier layer having a bandgap greater than
the bandgaps of said first, second and third regions.
5. A device as recited in claim 4, said means for applying an electric
field comprising a conducting gate layer on said third barrier layer and
an electric contact to said conducting gate layer.
This invention relates generally to the field of devices exhibiting
negative differential resistance and particularly to such devices based on
BACKGROUND OF THE INVENTION
For a long period of time, there has been interest in fabricating devices
exhibiting negative differential resistance. In such devices, the current
initially increases with increasing voltage but a point is reached after
which the current decreases as the voltage increases.
Several physical mechanisms, including resonant tunneling, can be exploited
in fabricating devices exhibiting such a characteristic. Perhaps the first
device based on resonant tunneling was the double-barrier heterostructure
proposed by Chang, Esaki and Tsu; see, for example, Applied Physics
Letters, 24, pp. 593-595, June 15, 1974. The original Tsu device was a
two-terminal device as were most of the other early negative differential
resistance devices. More recently, a three-terminal bipolar device has
been described which also exhibits negative differential resistance; see,
for example, Journal of Applied Physics, 58, pp. 1366-1368, Aug. 1, 1985.
This device utilizes resonant tunneling of minority carriers through a
quantum well in the base region.
Though obviously the details of all these devices differ, they do have one
element in common: they all utilize bulk carrier tunneling into a
two-dimensional density of states, typically of electrons, in a quantum
SUMMARY OF THE INVENTION
We have found that a resonant tunneling device in which the quantum well is
a linear, that is, a one-dimensional quantum well rather than
two-dimensional, has useful device properties. The tunneling in our
structure is of, for example, two-dimensional carriers into a
one-dimensional density of states. The latter will, for reasons of
convenience, be referred to as a quantum well wire.
The device comprises first and second semiconductor regions and a third
semiconductor region between said first and second regions which is
capable of exhibiting two-dimensional quantization. There are also first
and second electrical contacts to said first and second regions,
respectively; such contacts may be to highly doped semiconductor regions
having the same conductivity type. In a preferred embodiment, the first
and second regions are capable of exhibiting one-dimensional quantization.
The device further comprises means for applying an electric field such as,
e.g., a gate electrode for controlling a resonant tunneling current
through the quantum well wire.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a cross section of an exemplary embodiment of a device according
to our invention;
FIGS. 2 and 3 are energy band diagrams useful in explaining the operation
of the device depicted in FIG. 1; and
FIGS. 4 and 5 show the electrostatic potential distribution in the surface
resonant tunneling structure.
For reasons of clarity, the elements of the devices depicted are not drawn
An exemplary embodiment of a resonant tunneling device according to our
invention is depicted in FIG. 1. The device comprises substrate 1 and
disposed thereon: first and second highly doped n-type GaAs layers 3 and
5. Interleaved between said n-type layers 3 and 5 is an undoped region
which comprises third, fourth and fifth GaAs layers 11, 13 and 15 and,
between these GaAs layers, first and second AlGaAs layers 21 and 23 which
are between the third and fourth, and fourth and fifth GaAs layers,
respectively. The structure depicted is a mesa structure, and commonly
contacting the layers 11, 21, 13, 23, and 15 is an undoped AlGaAs layer 41
on a side of the mesa.
More specifically, the AlGaAs layers may be represented as Al.sub.x
Ga.sub.1-x As. Layer 41 has an aluminum content which preferably
corresponds to values of x in a range of from 0.3 to 1.0 (and typically
near 0.75); the AlGaAs layers 21 and 23 have an aluminum content
corresponding to preferred values of x in a range of from 0.2 to 1.0 (and
typically near 0.35). The undoped layer 41 is covered by conducting layer
42 (typically made of metal or a heavily doped semiconductor material) and
contacted electrically by gate contact 310. Also, there are electrical
contacts 320 and 330 to layers 3 and 5, respectively. A two-dimensional
electron gas is formed in region 151 of layer 15 and region 111 of layer
11 when an appropriate positive voltage is applied to contact 310; the
tunneling probability of electrons through region 131 of layer 13 depends
on the voltage at contact 310 as well as on source-drain bias voltage
As will be appreciated by the skilled artisan, the structure depicted
comprises an undoped planar quantum well, i.e., layer 13, which is
surrounded by the double barrier layers 21 and 23 of AlGaAs. These in turn
are sandwiched between two undoped GaAs layers 11 and 15 which are in
contact with the heavily doped GaAs layers 3 and 5, respectively.
The particular embodiment depicted uses the AlGaAs materials system. Other
embodiments using this materials system will be readily thought of by
those skilled in the art as well as similar embodiments using other
materials systems. For example, there may be a two-dimensional electron
gas in layers 11 and 15 even in the absence of a gate voltage as, e.g., in
a normally-on device.
The structure is conveniently grown, for example, by molecular beam
epitaxy. Details of an expedient growth technique will be readily known to
those skilled in the art and need not be given in detail. The working
surface may be further defined by selective etching to form the V-groove
and then subsequently overgrown epitaxially with the thin AlGaAs layer.
Details of the etching and regrowth will be readily known to the skilled
artisan. Electrical contacts may then be formed in well-known manner.
The overgrown layer 41 will be termed the gate barrier layer and both its
thickness and the aluminum content in the layer are selected to minimize
gate leakage. In particular, the gate barrier layer should have a bandgap
greater than the bandgaps of the mesa layers. The thickness of the gate
barrier layer should be greater than approximately 50 Angstroms to avoid
excessive gate leakage by tunneling.
The quantum well barrier layers 21 and 23 preferably are made to have
thicknesses, as measured along the slanted mesa surface, which are less
than approximately 50 Angstroms each, larger values being undesirable
because they could lead to a significantly reduced tunneling probability.
The thicknesses ot the two undoped GaAs layers 11 and 15 outside the
double-barrier region should be sufficiently large, typically greater than
approximately 1000 Angstroms, to inhibit the creation of a parallel
conduction path by conventional (bulk) resonant tunneling.
The aluminum content in the quantum well barrier layers typically
corresponds to a value of x which is less than 0.45 to ensure optimum
electron tunneling probability through the GlGaAs barrier; however,
generally suitable are aluminum contents corresponding to values of x in a
range of from 0.2 to 1.0.
The application of a positive gate voltage induces the formation of
two-dimensional electron gases in the interface regions 151 and 111 of the
edges of the respective undoped GaAs layers 1 and 5. These gases
effectively act as the source and drain electrodes. The region 131 of
layer 13 will be termed, as previously explained, a quantum well wire;
this region exists because of the additional dimensional quantization in
the direction parallel to the interface.
This may be better understood by considering the energy band diagram in the
absence of a source-to-drain voltage as depicted in FIG. 2. The energy
band diagram is taken along the x-axis as indicated in FIG. 1. For reasons
of simplicity, only the conduction band is shown. As shown, this direction
is parallel to the surface channel. The normal direction, i.e., the
y-axis, is the direction normal to gate barrier layer 42, and the
z-direction is along the quantum well wire, i.e., perpendicular to the
plane of FIG. 1.
The additional dimensional quantization, in the x-direction results in a
zero-point energy, E.sub.0 ', which is greater than the level indicated by
E.sub.0. The latter level corresponds to free motion in both the x- and
z-directions. It is readily appreciated by those skilled in the art that,
when the thicknesses of the undoped source and drain layers are
sufficiently large, then the carrier motion in the x-direction in these
layers can be considered as free. And it is similarly appreciated that in
the quantum-well-wire region of the surface channel there is an additional
dimensional quantization along the x-direction. As a result, energy levels
are quantized in two directions and carriers move freely in only the
The extra zero-point energy is given by E.sub.0 '-E.sub.0, as illustrated
in FIG. 2. If, in the following, t denotes the thickness of the quantum
well layer as measured along the face of the mesa in the x-direction, then
the extra energy is approximately equal to .pi..sup.2 n.sup.2 /2mt.sup.2,
where n is Planck's constant and m is the effective mass. Application of a
gate voltage can move the two-dimensional sub-band E.sub.O with respect to
the bottom of the conduction band, E.sub.C, and the Fermi level, E.sub.F.
The contemplated operating regime of our device corresponds to the case in
which the Fermi level lies in the interval between E.sub.0 'and E.sub.O.
The energy band diagram for the resonant tunneling condition is depicted in
FIG. 3. The range of energy of the carriers which can participate in
resonant tunneling through the base is represented as .DELTA.. This
condition is brought about by application of a drain voltage positive with
respect to the source. In the resonant tunneling situation, some electrons
in the source will have energy levels which match those of unoccupied
levels in the quantum well wire. Some of these electrons, when
conservation of lateral momentum is considered, can participate in
resonant tunneling. However, not all electrons in this band of matched
energy levels can tunnel because of the requirement of momentum
conservation. As the drain voltage increases, more carriers can undergo
resonant tunneling. At a sufficiently high drain voltage, however, there
will be no electrons in the source which can tunnel into the quantum well
wire and also conserve lateral momentum. Thus, a negative differential
resistance occurs in the drain circuit and the current will decrease as
the voltage increases.
In addition to controlling the resonant tunneling by the source-drain
voltage, the gate voltage may also be utilized to control the tunneling.
This is better understood by considering the electric fields depicted in
FIGS. 4 and 5. The projections of equipotential surfaces are shown as
broken lines. The electric field configuration for the situation in which
the source voltage V.sub.S is equal to the drain voltage V.sub.D and in
which the gate voltage V.sub.G is positive is depicted in FIG. 4. The
structure is equivalent to a double parallel-plate capacitor with a common
electrode, namely the gate electrode. The separation d between the
parallel plates is equal to the thickness of the AlGaAs gate barrier
layer, and the slit width 2 l is equal to the thicknesses of the tunneling
barrier layers and the quantum well layer as measured along the face of
the mesa in the x-direction.
The electric field configuration for equal source and gate voltages
together with a positive drain voltage is depicted in FIG. 5. A detailed
analysis, using a conformal mapping, shows that when the value of d is
near l, the gate potential is nearly as effective in lowering the level in
the quantum wire with respect to E.sub.0 in the source as is the drain
potential. The details of this transformation and analysis will be readily
apparent to those skilled in the art and need not be given in detail.
However, it can be shown that the gate potential is nearly as effective in
lowering the value of E.sub.0 ' in the quantum well wire relative to
E.sub.0 in the source (and thus in affecting the resonant tunneling
condition), as is the source-drain voltage.
Of course, the typical operating regime involves the situation in which
both V.sub.G is positive as well as V.sub.D is greater than V.sub.S, in
which case no suitable conformal mapping onto a simple-connected domain is
available. It is clear in this respect that the effects described above in
connection with FIG. 4 and 5 can be treated as additive, at least
qualitatively. As a result, the gate potential can be used to control the
resonant-tunneling condition set up by the source-to-drain voltage. In
particular, if .DELTA.is initially greater than or equal to zero for a
fixed drain voltage, .DELTA.can be made to go negative by further
increasing the gate voltage. The result is that the tunneling current
decreases and a range of negative transconductance has been achieved.
Such a device, namely a transistor having negative transconductance can
perform the functions of a complementary device analogous to a p-channel
transistor in silicon CMOS technology. And, a circuit formed by combining
a conventional n-channel field effect transistor with a negative
transconductance transistor can act as a low-power inverter in which a
significant current flows only during switching.
While the invention has been described primarily as making use of a single
GaAs quantum well, alternate structures are not precluded. For example,
such quantum well may be supplanted by a superlattice of alternating
layers of GaAs and AlGaAs. Also, there are embodiments of the invention
based on the movement of holes instead of electrons, for example,
utilizing silicon barriers instead of AlGaAs barriers and a
silicon-germanium alloy instead of GaAs for the low-bandgap material.
Furthermore, such structure may be be replaced by a superlattice of
alternating layers of silicon and silicon-germanium.
* * * * *