U.S. patent application number 13/156525 was filed with the patent office on 2012-12-13 for unipolar diode with low turn-on voltage.
Invention is credited to Kwok K. Loi, Vesna Radisic, Donald J. Sawdai.
Application Number | 20120313105 13/156525 |
Document ID | / |
Family ID | 46124781 |
Filed Date | 2012-12-13 |
United States Patent
Application |
20120313105 |
Kind Code |
A1 |
Sawdai; Donald J. ; et
al. |
December 13, 2012 |
UNIPOLAR DIODE WITH LOW TURN-ON VOLTAGE
Abstract
A unipolar diode with low turn-on voltage includes a subcathode
semiconductor layer, a low-doped, wide bandgap cathode
semiconductor layer, and a high-doped, narrow bandgap anode
semiconductor layer. A junction between the cathode layer and the
anode layer creates an electron barrier in the conduction band,
with the barrier configured to produce a low turn-on voltage for
the diode. A unipolar diode with low turn-on voltage includes an
n.sup.+ subcathode semiconductor layer, a low-doped, wide bandgap
cathode semiconductor layer, and an n.sup.+ narrow bandgap anode
semiconductor layer. Again, a junction between the cathode layer
and the anode layer creates an electron barrier in the conduction
band, with the barrier configured to produce a low turn-on voltage
for the diode.
Inventors: |
Sawdai; Donald J.; (Redondo
Beach, CA) ; Loi; Kwok K.; (Cerritos, CA) ;
Radisic; Vesna; (Manhattan Beach, CA) |
Family ID: |
46124781 |
Appl. No.: |
13/156525 |
Filed: |
June 9, 2011 |
Current U.S.
Class: |
257/76 ;
257/E21.09; 257/E29.327; 438/478 |
Current CPC
Class: |
H01L 29/205 20130101;
H01L 29/861 20130101; H01L 29/66219 20130101 |
Class at
Publication: |
257/76 ; 438/478;
257/E29.327; 257/E21.09 |
International
Class: |
H01L 29/861 20060101
H01L029/861; H01L 21/20 20060101 H01L021/20 |
Claims
1. A unipolar diode with low turn-on voltage, comprising: a
subcathode semiconductor layer; a low-doped, wide bandgap cathode
semiconductor layer; and a high-doped, narrow bandgap anode
semiconductor layer, wherein a junction between the cathode layer
and the anode layer creates an electron barrier in the conduction
band, with the barrier configured to produce a low turn-on voltage
for the diode.
2. The diode of claim 1 wherein: the electron barrier can be
tuned.
3. The diode of claim 1 wherein: the electron barrier can be tuned
by appropriately choosing the composition of at least one of the
materials comprised in at least one of the anode layer and the
cathode layer.
4. The diode of claim 3, wherein the materials comprise at least
one of gallium and aluminum.
5. The diode of claim 1, wherein the cathode layer is an intrinsic
wide bandgap cathode semiconductor layer.
6. The diode of claim 1, wherein the cathode layer is a low-doped,
non-intrinsic wide bandgap cathode semiconductor layer.
7. The diode of claim 1, wherein the diode has a surface area of
approximately 9 .mu.m.sup.2 and has a turn-on voltage of less than
approximately 0.2 volts.
8. The diode of claim 1, wherein the diode has a surface area of
approximately 9 .mu.m.sup.2 and has a turn-on voltage of less than
approximately 0.1 volts.
9. The diode of claim 1, wherein the diode has a small junction
capacitance.
10. The diode of claim 7, wherein the junction capacitance of the
diode is less than approximately ten femtofarads.
11. The diode of claim 1, wherein: the subcathode layer is an
n.sup.+ subcathode semiconductor layer.
12. The diode of claim 1, wherein: the anode layer is a n.sup.+
narrow bandgap anode semiconductor layer.
13. The diode of claim 1, wherein: the subcathode layer is the
bottom layer.
14. The diode of claim 1, wherein: the anode layer is the bottom
layer.
15. The diode of claim 1, wherein: the diode is fabricated from an
epitaxial stack.
16. The diode of claim 1, wherein: the diode is fabricated from an
epitaxial stack comprising: the anode layer; the cathode layer; and
the subcathode layer.
17. A unipolar diode with low turn-on voltage, comprising: An
n.sup.+ subcathode semiconductor layer; a low-doped, wide bandgap
cathode semiconductor layer fabricated on the subcathode layer; and
an n.sup.+ narrow bandgap anode semiconductor layer fabricated on
the cathode layer, wherein a junction between the cathode layer and
the anode layer creates a barrier in the conduction band, with the
barrier configured to produce a low turn-on voltage for the
diode.
18. The diode of claim 17 wherein: the electron barrier can be
tuned.
19. The diode of claim 17 wherein: the electron barrier can be
tuned by appropriately choosing the composition of at least one of
the materials comprised in at least one of the anode layer and the
cathode layer.
20. The diode of claim 17, wherein the materials comprise at least
one of gallium and aluminum.
21. A method for fabricating a unipolar diode with low turn-on
voltage, comprising: creating a subcathode semiconductor layer on a
substrate; creating a low-doped, wide bandgap cathode semiconductor
layer on the subcathode layer; creating a high-doped, narrow
bandgap anode semiconductor layer on the cathode layer; removing a
portion of the anode layer to expose the cathode layer; removing a
second portion of the cathode layer to expose the subcathode layer;
placing a metal cathode contact on the subcathode layer; depositing
a dielectric layer; removing portions of the dielectric layer;
placing a metal anode contact on the anode layer; forming a first
metal interconnect on the cathode contact; and forming a second
metal interconnect on the anode contact, wherein a junction between
the cathode layer and the anode layer creates an electron barrier
in the conduction band, with the barrier configured to produce a
low turn-on voltage for the diode.
22. The method of claim 21, wherein: at least one of the step of
creating the subcathode layer, the step of creating the cathode
layer, and the step of creating the anode layer is performed
epitaxially.
Description
BACKGROUND
[0001] The invention relates generally to a unipolar diode and more
particularly to a unipolar diode with a low turn-on voltage.
[0002] Diodes are a fundamental electronic building block. Their
ability to restrict current flow to substantially one direction is
a critical property relied upon in virtually every electronic
circuit manufactured, from the smallest power supply to the largest
industrial process controller.
[0003] Mixer and detector circuits use non-linearity in the diode
turn-on characteristics to up-convert or down-convert RF signals to
either intermediate signals or baseband. To operate, the voltage on
the diode must be in the range of its turn-on voltage. This can be
accomplished either by biasing the diode with a DC power supply or
by providing sufficient radio frequency (RF) or local oscillator
(LO) power to self-bias the diode. Introducing bias circuitry to
bias the diode increases noise, increases circuit size, increases
circuit DC power consumption, increases the conversion loss of the
circuit, and decreases the frequency performance of the circuit, so
self-biasing is preferred. In order to minimize the required RF or
LO power to self-bias the diode, minimizing diode turn-on voltage
is desirable.
[0004] The capacitance associated with the charge variation in a
diode's depletion layer is the junction capacitance. It is
generally desirable to minimize a diode's junction capacitance. The
barrier height of a diode refers to the potential barrier that must
be overcome to turn on the diode.
SUMMARY
[0005] In one set of embodiments, there is provided a unipolar
diode with low turn-on voltage comprising a subcathode
semiconductor layer, a low-doped, wide bandgap cathode
semiconductor layer, and a high-doped, narrow bandgap anode
semiconductor layer, wherein a junction between the cathode layer
and the anode layer creates an electron barrier in the conduction
band, with the barrier configured to produce a low turn-on voltage
for the diode.
[0006] In another set of embodiments, there is provided a unipolar
diode with low turn-on voltage comprising an n.sup.+ subcathode
semiconductor layer, a low-doped, wide bandgap cathode
semiconductor layer fabricated on the subcathode layer, and an
n.sup.+ narrow bandgap anode semiconductor layer fabricated on the
cathode layer, wherein a junction between the cathode layer and the
anode layer creates an electron barrier in the conduction band,
with the barrier configured to produce a low turn-on voltage for
the diode.
[0007] In yet another set of embodiments, there is provided a
method for fabricating a unipolar diode with low turn-on voltage
comprising: creating a subcathode semiconductor layer on a
substrate; creating a low-doped, wide bandgap cathode semiconductor
layer on the subcathode layer; creating a high-doped, narrow
bandgap anode semiconductor layer on the cathode layer; removing a
portion of the anode layer to expose the cathode layer; removing a
second portion of the cathode layer to expose the subcathode layer;
placing a metal cathode contact on the subcathode layer; depositing
a dielectric layer; removing portions of the dielectric layer to
expose the anode layer and the metal cathode contact; placing a
metal anode contact on the anode layer; forming a first metal
interconnect on the cathode contact; forming a second metal
interconnect on the anode contact, wherein a junction between the
cathode layer and the anode layer creates an electron barrier in
the conduction band, with the barrier configured to produce a low
turn-on voltage for the diode.
DESCRIPTION OF THE DRAWINGS
[0008] The accompanying drawings provide visual representations
which will be used to more fully describe various representative
embodiments and can be used by those skilled in the art to better
understand the representative embodiments disclosed herein and
their advantages. In these drawings, like reference numerals
identify corresponding elements.
[0009] FIG. 1 is a drawing of a cross section of a unipolar diode
with low turn-on voltage.
[0010] FIGS. 2A-2D are a set of drawings showing a method for
fabricating a unipolar diode with low turn-on voltage.
[0011] FIGS. 3A-3C are a set of energy band diagrams illustrating
the operation of a method for fabricating a unipolar diode with low
turn-on voltage in the cases of zero bias, positive bias, and
negative bias.
[0012] FIG. 4A is a graph showing a plot of current as a function
of voltage for a unipolar diode with low turn-on voltage compared
to prior-art Schottky diodes.
[0013] FIG. 4B is a graph showing a plot of conduction band barrier
height as a function of diode composition for a unipolar diode with
low turn-on voltage.
[0014] FIG. 5 is a flowchart of a method for fabricating a unipolar
diode with low turn-on voltage.
DETAILED DESCRIPTION
[0015] While the present invention is susceptible of embodiment in
many different forms, there is shown in the drawings and will
herein be described in detail one or more specific embodiments,
with the understanding that the present disclosure is to be
considered as exemplary of the principles of the invention and not
intended to limit the invention to the specific embodiments shown
and described. In the following description and in the several
figures of the drawings, like reference numerals are used to
describe the same, similar or corresponding parts in the several
views of the drawings.
[0016] Embodiments of the disclosed invention include a unipolar
diode with a low turn-on voltage. According to embodiments of the
invention, it may be fabricated from an epitaxial stack comprising
a subcathode semiconductor layer, a low-doped, wide bandgap cathode
semiconductor layer, and a high-doped, narrow bandgap anode
semiconductor layer. The anode layer may be placed on the cathode
layer, which may in turn be placed on the subcathode layer.
Alternatively, the subcathode layer may be placed on the cathode
layer, which may in turn be placed on the anode layer.
[0017] According to embodiments of the invention, the diode may be
fabricated by etching down to the subcathode around an
anode/cathode mesa. Following this step, a metal anode contact may
be placed on the anode layer and a metal cathode contact may be
placed on the subcathode layer. A small, tunable, extremely high
quality electron barrier is thereby created in the conduction band,
resulting in a low turn-on voltage. This barrier has blocking
characteristics that resemble those of the barrier between metal
and semiconductor in a Schottky diode.
[0018] According to embodiments of the invention, diodes with low
turn-on voltages may be created sequentially via epitaxial growth
and much simpler device fabrication processes than Schottky diodes.
For example, the layers may be created using molecular beam epitaxy
(MBE). For example, the layers may be created using metal-organic
chemical vapor deposition (MOCVD). Layers may be created in a
single semiconductor epitaxial growth reactor, substantially
reducing junction contamination, facilitating manufacture, and
greatly reducing junction capacitance. Junction capacitance
according to embodiments of the invention will typically be less
than approximately ten femtofarads (10 fF), compared to a typical
junction capacitance for a prior art backward tunneling diode of
approximately 70 fF or more. Embodiments of the invention provide
diodes that may operate faster relative to prior art backward
tunneling diodes.
[0019] Embodiments of the invention reduce the turn-on voltage of
the diode, thereby reducing the RF or LO power required to turn on
a mixer or detector circuit that uses diodes in a self-biased
configuration. A reduction in RF power improves the circuit's
sensitivity, and a reduction in LO power improves the circuit's
efficiency and/or conversion loss.
[0020] According to embodiments of the invention, a unipolar diode
may be fabricated from an epitaxial stack comprising an n.sup.+
narrow bandgap anode semiconductor layer on a low-doped, wide
bandgap cathode semiconductor layer on an n.sup.+ subcathode
semiconductor layer. The diode may be fabricated by etching down to
the subcathode layer around an anode/cathode mesa. Then a metal
anode contact may be placed on the anode layer and a metal cathode
contact may be placed on the subcathode layer.
[0021] According to embodiments of the invention, the junction
between the anode layer and the cathode layer creates a small, high
quality electron barrier in the conduction band. This barrier
creates current rectification with a low turn-on voltage. The
barrier height can be tuned, according to embodiments of the
invention, to any level appropriate for a particular application by
varying the composition of the anode and cathode layers. For
example, according to embodiments of the invention, the barrier
height can be tuned by varying a composition ratio describing the
relative presence of gallium (Ga) and aluminum (Al) fractions used
in the cathode and anode layers. By varying the composition of the
cathode and anode layers according to embodiments of the invention,
the alignment of the conduction bands of the cathode and anode
layers can be varied, and thereby the height of the barrier can be
tuned.
[0022] According to embodiments of the invention, an n.sup.+ narrow
bandgap anode semiconductor layer is used in conjunction with a
low-doped, wide bandgap cathode semiconductor layer. This may
create an electron barrier in the conduction band that is similar
to the barrier between the metal and the semiconductor in a
Schottky diode. The cathode layer may be an intrinsic wide bandgap
cathode semiconductor layer. The cathode layer may be a low-doped,
non-intrinsic wide bandgap cathode semiconductor layer.
[0023] The diode in this invention is unipolar since its electrical
characteristics are dominated by only one polarity of free
carriers. In the case of the embodiment of this invention with an
n.sup.+ subcathode semiconductor layer, a low-doped, wide bandgap
cathode semiconductor layer fabricated on the subcathode layer, and
an n.sup.+ narrow bandgap anode semiconductor layer fabricated on
the cathode layer, the electrical characteristics of the diode are
dominated by electrons. Electrons are the majority carriers in all
layers of the diode and provide almost all of the electrical
current, while holes are negligibly present. Therefore, this diode
has negligible minority carriers and hence negligible minority
carrier charge storage. Unipolar diodes are faster compared to
comparable bipolar diodes such as PN junction diodes, which have a
capacitive delay when switched from the on-state to the off-state
due to minority carrier charge storage.
[0024] FIG. 1 is a drawing of a cross section of a unipolar diode
100 with a low turn-on voltage. The unipolar diode 100 comprises a
high-doped, narrow bandgap anode semiconductor layer 110, a
low-doped, wide bandgap cathode semiconductor layer 120, and a
subcathode semiconductor layer 130. The diode is fabricated on a
semi-insulating substrate 140.
[0025] The diode 100 is fully sealed in that the semiconductor
materials forming the diode 100 are covered or fully surrounded by
metal or dielectric layers.
[0026] Depending upon a particular application and need, the size
and shape of the diode 100 can be selected from a range of options.
Depending upon a particular application and need, the size and
shape of the interface of the anode layer 110 with the cathode
layer 120 is formed according to one of numerous exemplary
configurations including the following:
[0027] A=10 .mu.m.sup.2: 0.5.times.20, 1.times.10, 2.times.5,
3.times.3.33
[0028] A=4 .mu.m.sup.2: 0.5.times.8, 0.8.times.5, 1.times.4,
2.times.2
[0029] A=2 .mu.m.sup.2: 0.5.times.4, 1.times.2
[0030] A=1 .mu.m.sup.2: 0.5.times.2, 0.8.times.1.2, 1.times.1
[0031] As an example, the anode layer 110 may comprise a grown
epitaxial Indium Gallium Arsenide (InGaAs) layer. As an example,
the cathode layer 120 may comprise a grown epitaxial Indium
Aluminum Arsenide (InAlAs) layer. As an example, the subcathode
layer 130 may comprise a grown epitaxial Indium Aluminum Arsenide
(InAlAs) layer, and etched to form the desired shape for supporting
the cathode layer 120. Alternatively, the subcathode layer 130 may
comprise a grown epitaxial Indium Aluminum Arsenide (InAlAs)
sub-layer (not shown) and a grown epitaxial Indium Gallium Arsenide
(InGaAs) sub-layer (not shown). As an example, the substrate 140
may comprise a semi-insulating Indium Phosphide (InP) wafer.
[0032] A dielectric layer 145 provides isolation when required
between the structure of the diode 100 and, for example, a first
metal interconnect 150 and a second metal interconnect 155. The
anode connection is, in this example, provided by first metal
interconnect 150 through metal ohmic anode contact 160, which
contacts and is supported by anode layer 110. A cathode connection
is provided by the second metal interconnect 155 through metal
ohmic cathode contact 170, which contacts and is supported by
subcathode layer 130.
[0033] FIGS. 2A-2D are a set of drawings showing a method for
fabricating a unipolar diode 100 with low turn-on voltage. In this
example, the diode 100 is fabricated by epitaxially growing a
subcathode layer 130, then growing a cathode layer 120 on the
subcathode layer 130, then growing an anode layer 110 on the
cathode layer 120. Next the fabrication entails etching down to the
subcathode layer 130 around a mesa comprising cathode layer 120 and
anode layer 110. Following this step, a metal anode contact 160 is
placed on the anode layer and a metal cathode contact 170 is placed
on the subcathode layer.
[0034] In FIG. 2A, a starting cross section 210 results from
epitaxially growing on a substrate, for example, a semi-insulating
indium phosphide (InP) wafer 140, a subcathode semiconductor layer
130, then epitaxially growing a low-doped, wide bandgap cathode
semiconductor layer 120, then epitaxially growing a high-doped,
narrow bandgap anode semiconductor layer 110. For example, the
layers may be created using molecular beam epitaxy (MBE). For
example, the layers may be created using metal-organic chemical
vapor deposition (MOCVD). The semiconductor material may comprise,
for example, various compositions of Indium Aluminum Gallium
Arsenide (InAlGaAs) or, for another example, Aluminum Gallium
Arsenide (AlGaAs).
[0035] In FIG. 2B, the cross section 220 shows the result of the
next fabrication steps. Etching to the desired shape is carried out
for subcathode layer 130, for cathode layer 120, and for anode
layer 110. Ohmic metal cathode contact 170 is then deposited so
that it is supported by subcathode semiconductor layer 130.
[0036] In FIG. 2C, the cross section 230 shows the result of the
next fabrication step, in which dielectric layer 145 is deposited.
For example, the dielectric layer 145 may comprise silicon nitride
(SiN). Portions of the dielectric layer 145 are then removed to
reveal contact locations to the underlying layers.
[0037] In FIG. 2D, the cross section 240 shows the result of the
final fabrication steps. Ohmic metal anode contact 160 is
deposited, for example, by electron beam evaporation. Then first
metal interconnect 150 is formed on metal ohmic anode contact 160,
for example, by electron beam evaporation. A second metal
interconnect 155 is formed on metal ohmic cathode contact 170, for
example, by electron beam evaporation. Second metal interconnect
155 thereby connects to ohmic cathode contact 170 and seals
dielectric layer 145. The cross section 240 illustrates the
deposited first metal interconnect 150 and the deposited second
metal interconnect 155.
[0038] FIG. 3A is an energy band diagram illustrating the operation
of a method for fabricating a unipolar diode with low turn-on
voltage in the case of zero bias, according to embodiments of the
invention comprising an n.sup.+ subcathode semiconductor layer, a
low-doped, wide bandgap cathode semiconductor layer fabricated on
the subcathode layer, and an n.sup.+ narrow bandgap anode
semiconductor layer fabricated on the cathode layer.
[0039] The junction between the low-doped, wide bandgap cathode
semiconductor layer and the high-doped, narrow bandgap anode
semiconductor layer creates a barrier in the conduction band, with
the barrier configured to produce a diode with a low turn-on
voltage.
[0040] FIG. 3A depicts an energy band diagram of the semiconductor
layers of the diode at zero bias according to this set of
embodiments. The energy band diagram schematically plots electron
energy as a function of depth in the diode 100, which comprises
n.sup.+ subcathode semiconductor layer 130, a low-doped, wide
bandgap cathode semiconductor layer 120 fabricated on the
subcathode layer 130, and n.sup.+ narrow bandgap anode
semiconductor layer 110 fabricated on the cathode layer 120.
[0041] A conduction band 310 is created in the diode 100. A
discontinuity in the conduction band 310 at the junction between
the n.sup.+ narrow bandgap anode semiconductor layer 110 and the
wide bandgap cathode semiconductor layer 120 creates a conduction
band barrier 320.
[0042] Line 330 indicates the quasi-Fermi energy level
E.sub.F-anode in the n.sup.+ anode semiconductor layer 110. Line
340 indicates the quasi-Fermi energy level E.sub.F-subcathode in
the n.sup.+ subcathode semiconductor layer 130. E.sub.F-anode is
approximately equal to the voltage at the metal ohmic anode contact
160 (not shown) of a fully assembled diode 100. Similarly,
E.sub.F-subcathode is approximately equal to the voltage at the
metal ohmic cathode contact 170 (not shown) of a fully assembled
diode 100. For the zero bias case, E.sub.F-anode is approximately
equal to E.sub.F-subcathode.
[0043] Since the quasi-Fermi energy line 330 is located above the
conduction band 310 in the n.sup.+ anode semiconductor layer 110,
and quasi-Fermi energy line 340 is located above the conduction
band 310 in the n.sup.+ subcathode semiconductor layer 130, both of
these semiconductor layers have a large supply of free
electrons.
[0044] FIG. 3B depicts an energy band diagram of the semiconductor
layers of the diode at positive bias according to this same set of
embodiments. The energy band diagram again schematically plots
electron energy as a function of depth in the diode 100.
[0045] A conduction band 310 is again created in the diode 100,
with a discontinuity in the conduction band 310 at the junction
between the n.sup.+ narrow bandgap anode semiconductor layer 110
and the wide bandgap cathode semiconductor layer 120 again creating
a conduction band barrier 320.
[0046] Line 330 again indicates the quasi-Fermi energy level
E.sub.F-anode in the n.sup.+ anode semiconductor layer 110, and
line 340 again indicates the quasi-Fermi energy level
E.sub.F-subcathode in the n.sup.+ subcathode semiconductor layer
130. For the positive bias case, E.sub.F-anode is less than
E.sub.F-subcathode. The positive bias allows electrons to overcome
the conduction band barrier 320 and flow from subcathode 130 to
anode 110, such that the diode 100 conducts current.
[0047] FIG. 3C depicts an energy band diagram of the semiconductor
layers of the diode at negative bias according to this same set of
embodiments. The energy band diagram again schematically plots
electron energy as a function of depth in the diode 100.
[0048] A conduction band 310 is again created in the diode 100,
with a discontinuity in the conduction band 310 at the junction
between the n.sup.+ narrow bandgap anode semiconductor layer 110
and the wide bandgap cathode semiconductor layer 120 again creating
a conduction band barrier 320.
[0049] Line 330 again indicates the quasi-Fermi energy level
E.sub.F-anode in the n.sup.+ anode semiconductor layer 110, and
line 340 again indicates the quasi-Fermi energy level
E.sub.F-subcathode in the n.sup.+ subcathode semiconductor layer
130. For the negative bias case, E.sub.F-anode is greater than
E.sub.F-subcathode. The negative bias creates conduction band
barrier 320, which prevents electrons from flowing from the anode
110 to the subcathode 130, so that the diode 100 does not conduct
appreciable current.
[0050] Accordingly, the basic operation of the diode according to
embodiments of this invention is similar to the basic operation of
a typical Schottky diode. According to embodiments of the
invention, the metal-semiconductor barrier of a typical Schottky
diode is replaced by a semiconductor-semiconductor conduction band
barrier operating according to principles outlined in FIGS.
3A-3C.
[0051] FIG. 4A is a graph showing a plot of current through the
diode as a function of voltage across the diode for a unipolar
diode with low turn-on voltage. More specifically, FIG. 4A shows a
plot of diode current I.sub.d in amperes as a function of diode
voltage V.sub.d in volts, with the current plotted logarithmically
against a linear plot of voltage, for three different diodes with a
surface area of approximately 9 .mu.m.sup.2 (3 .mu.m.times.3
.mu.m): for a unipolar diode with low turn-on voltage according to
embodiments of the invention and for two classes of prior art
Schottky diodes, namely, for Indium Phosphide (InP) Schottky diodes
and for Gallium Arsenide (GaAs) Schottky diodes.
[0052] If we define turn-on voltage as the voltage at which the
current is at least 10.sup.-6 amperes, then embodiments of the
invention with a surface area of 9 .mu.m.sup.2 have a turn-on
voltage of less than approximately 0.2 volts. Other embodiments of
the invention with a surface area of approximately 9 .mu.m.sup.2
have a turn-on voltage of less than approximately 0.1 volts, by
comparison with a turn-on voltage for prior art InP Schottky diodes
of greater than approximately 0.25 volts and a turn-on voltage for
prior art GaAs Schottky diodes of more than approximately 0.5
volts.
[0053] The barrier height of the diode may be tuned by
appropriately changing the material and alloy composition of the
cathode and anode layers that make up the junction. In one set of
embodiments, the barrier height of the diode may be tuned by
appropriately changing the relative presence in the cathode and
anode layers of aluminum (Al) and of gallium (Ga). Prior art
backward tunneling diodes do not typically have a tunable barrier.
Prior art Schottky diodes typically are difficult to tune due to
surface states caused by contamination or material imperfections at
its metal-semiconductor junction.
[0054] FIG. 4B is a graph showing a plot of conduction band barrier
height as a function of diode composition for a unipolar diode with
low turn-on voltage. More specifically, FIG. 4B shows a plot of
conduction band barrier height in electron volts (eV) as a function
of composition ratio x, with the barrier height plotted linearly
against a linear plot of composition ratio x, for a unipolar diode
with low turn-on voltage with a surface area of approximately 4
.mu.m.sup.2 (2 .mu.m.times.2 .mu.m) and an anode composed of InGaAs
and a cathode composed of InAlGaAs, both materials which are
lattice-matched to InP. Composition ratio x is defined as the ratio
in the cathode layer of the number of aluminum (Al) atoms to the
total number of atoms of aluminum (Al) and gallium (Ga).
x=Al/(Al+Ga). When x=0, there results a cathode comprising InGaAs,
and when x=1, there results a cathode comprising InAlAs. The
greater the value of x, the greater the presence of aluminum, and
the higher the barrier height. Tunability is evident from the
inflection of the plot of barrier height against composition
ratio.
[0055] As shown by FIGS. 4A and 4B, embodiments of the invention
behave as if they were Schottky diodes with a very small tunable
Schottky barrier.
[0056] The semiconductor and metal layers that comprise a Schottky
diode may be fabricated in different steps, so that the interface
between them contains a significant density of contaminants and
defects. This contamination creates surface states that cause
significant difficulty in reproducing and controlling the diode
barrier height and diode leakage current, and hence its electrical
characteristics.
[0057] Embodiments of the invention use a fixed-composition barrier
that may offer greater design flexibility relative to prior art.
According to embodiments of the invention, layers of a unipolar
diode with low turn-on voltage may be created in a single
semiconductor epitaxial growth reactor. This fabrication method
significantly reduces junction contamination. The resulting diode
can be manufactured very repeatedly. Relative to prior art
conventional Schottky diodes, the epitaxial growth process and
device fabrication process according to embodiments of this
invention is much simpler.
[0058] Furthermore, junction capacitance according to embodiments
of the invention will typically be less than approximately ten
femtofarads (10 fF), compared to a typical junction capacitance for
a prior art backward tunneling diode of 70 fF or more.
[0059] FIG. 5 is a flowchart of a method 500 for fabricating a
unipolar diode with low turn-on voltage. The order of the steps in
the method 500 is not constrained to that shown in FIG. 5 or
described in the following discussion. Several of the steps could
occur in a different order without affecting the final result.
[0060] In block 510, a subcathode semiconductor layer is created on
a substrate. Block 510 then transfers control to block 520.
[0061] In block 520, a low-doped, wide bandgap cathode
semiconductor layer is created on the subcathode layer. Block 520
then transfers control to block 530.
[0062] In block 530, a high-doped, narrow bandgap anode
semiconductor layer is created on the cathode layer. Block 530 then
transfers control to block 540.
[0063] In block 540, a portion of the anode layer is removed to
expose the cathode layer. Block 540 then transfers control to block
550.
[0064] In block 550, a second portion of the cathode layer is
removed to expose the subcathode layer. Block 550 then transfers
control to block 560.
[0065] In block 560, a metal cathode contact is placed on top of
the subcathode layer. Block 560 then transfers control to block
570.
[0066] In block 570, a dielectric layer is deposited. Block 570
then transfers control to block 580.
[0067] In block 580, portions of the dielectric layer are removed
to expose the anode layer and the metal cathode contact. Block 580
then transfers control to block 590.
[0068] In block 590, a metal anode contact is placed on the anode
layer. Block 590 then transfers control to block 592.
[0069] In block 592, a first metal interconnect is formed on the
cathode contact to allow for connections to the diode. Block 592
then transfers control to block 595.
[0070] In block 595, a second metal interconnect is formed on the
anode contact to allow for connections to the diode. As a result, a
junction creates an electron barrier in the conduction band, with
the barrier configured to produce a low diode turn-on voltage.
Block 595 then terminates the process.
[0071] Alternatively, embodiments of the invention may comprise a
p.sup.+ narrow bandgap cathode semiconductor layer, a wide bandgap
anode semiconductor layer, and a p.sup.+ subanode semiconductor
layer. According to this set of embodiments, a hole barrier is
created in the valence band, instead of an electron barrier in the
conduction band. The operation and advantages of this alternative
set of embodiments are comparable to the operation and advantages
of the sets of embodiments described above.
[0072] While the above representative embodiments have been
described with certain components in exemplary configurations, it
will be understood by one of ordinary skill in the art that other
representative embodiments can be implemented using different
configurations and/or different components. For example, it will be
understood by one of ordinary skill in the art that the order of
certain fabrication steps and certain components can be altered
without substantially impairing the functioning of the
invention.
[0073] For example, the fabrication step of placing a metal anode
contact could instead entail providing one of a metal anode contact
self-aligned to the anode semiconductor layer. As another example,
the fabrication step of placing a metal anode contact could
additionally provide a cantilever beam to the metal interconnect.
As another example, the fabrication step of placing the metal
interconnect could additionally provide an airbridge over the
subcathode semiconductor region. As another example, the device
could comprise the anode layer as the bottom layer, with the
subcathode layer as the top layer, and could be fabricated
accordingly without substantially impairing the functioning of the
invention.
[0074] The representative embodiments and disclosed subject matter,
which have been described in detail herein, have been presented by
way of example and illustration and not by way of limitation. It
will be understood by those skilled in the art that various changes
may be made in the form and details of the described embodiments
resulting in equivalent embodiments that remain within the scope of
the appended claims. Moreover, fabrication details are merely
exemplary; the invention is defined by the following claims.
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