U.S. patent application number 13/948084 was filed with the patent office on 2014-02-06 for light emitting and lasing semiconductor methods and devices.
This patent application is currently assigned to The Board of Trustees of The University Of IIIinois. The applicant listed for this patent is The Board of Trustees of The University Of IIIinois, Quantum Electro Opto Systems Sdn. Bhd.. Invention is credited to Milton Feng, Nick Holonyak, JR., Gabriel Walter, Chao-Hsin Wu.
Application Number | 20140036949 13/948084 |
Document ID | / |
Family ID | 43067956 |
Filed Date | 2014-02-06 |
United States Patent
Application |
20140036949 |
Kind Code |
A1 |
Walter; Gabriel ; et
al. |
February 6, 2014 |
Light Emitting And Lasing Semiconductor Methods And Devices
Abstract
A method for producing light emission from a two terminal
semiconductor device with improved efficiency, includes the
following steps: providing a layered semiconductor structure
including a semiconductor drain region comprising at least one
drain layer, a semiconductor base region disposed on the drain
region and including at least one base layer, and a semiconductor
emitter region disposed on a portion of the base region and
comprising an emitter mesa that includes at least one emitter
layer; providing, in the base region, at least one region
exhibiting quantum size effects; providing a base/drain electrode
having a first portion on an exposed surface of the base region and
a further portion coupled with the drain region, and providing an
emitter electrode on the surface of the emitter region; applying
signals with respect to the base/drain and emitter electrodes to
obtain light emission from the base region; and configuring the
base/drain and emitter electrodes for substantial uniformity of
voltage distribution in the region therebetween.
Inventors: |
Walter; Gabriel; (Champaign,
IL) ; Holonyak, JR.; Nick; (Urbana, IL) ;
Feng; Milton; (Champaign, IL) ; Wu; Chao-Hsin;
(Champaign, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Board of Trustees of The University Of IIIinois
Quantum Electro Opto Systems Sdn. Bhd. |
Urbana
Melaka |
IL |
US
MY |
|
|
Assignee: |
The Board of Trustees of The
University Of IIIinois
Urbana
IL
Quantum Electro Opto Systems Sdn. Bhd.
Melaka
|
Family ID: |
43067956 |
Appl. No.: |
13/948084 |
Filed: |
July 22, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12799080 |
Apr 16, 2010 |
8509274 |
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13948084 |
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12655806 |
Jan 7, 2010 |
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12799080 |
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61204560 |
Jan 8, 2009 |
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61204602 |
Jan 8, 2009 |
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61208422 |
Feb 24, 2009 |
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61212951 |
Apr 17, 2009 |
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61268119 |
Jun 9, 2009 |
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Current U.S.
Class: |
372/45.01 ;
257/13 |
Current CPC
Class: |
H01S 5/34 20130101; H01S
5/04257 20190801; H01L 33/30 20130101; B82Y 20/00 20130101; H01L
33/06 20130101; H01S 5/34313 20130101; H01L 33/0016 20130101; H01S
5/1835 20130101; H01S 5/06213 20130101; H01S 5/06203 20130101; H01S
5/18311 20130101 |
Class at
Publication: |
372/45.01 ;
257/13 |
International
Class: |
H01S 5/34 20060101
H01S005/34; H01L 33/06 20060101 H01L033/06 |
Goverment Interests
GOVERNMENT RIGHTS
[0003] This invention was made with Government support, and the
Government has certain rights in the invention.
Claims
1. A method for producing light emission from a three terminal
semiconductor device with improved efficiency, comprising the steps
of: providing a layered semiconductor structure including a
semiconductor collector region comprising at least one collector
layer, a semiconductor base region disposed on said collector
region and including at least one base layer, and a semiconductor
emitter region disposed on a portion of said base region and
comprising an emitter mesa that includes at least one emitter
layer; providing, in said base region, at least one region
exhibiting quantum size effects; providing a collector electrode on
said collector region, providing a base electrode on an exposed
surface of said base region, and providing an emitter electrode on
the surface of said emitter region; applying signals with respect
to said collector, base, and emitter electrodes to obtain light
emission from said base region; and configuring said base and
emitter electrodes for substantial uniformity of voltage
distribution in the region therebetween.
2. The method as defined by claim 1, further comprising configuring
the geometry of said emitter mesa between said base and emitter
electrodes for substantial uniformity of voltage distribution in
said region between said base and emitter electrodes.
3. The method as defined by claim 1, further comprising providing
an optical cavity for said light emission in the region between
said base electrode and said emitter electrode
4. The method as defined by claim 1, wherein said emitter mesa has
a substantially rectilinear surface portion, and wherein said step
of providing said electrodes comprises providing said emitter
electrode along one side of said surface portion of the emitter
mesa and providing said base electrode on a portion of the base
region surface adjacent the opposite side of said emitter mesa
surface portion.
5. The method as defined by claim 4, wherein said step of providing
said electrodes further comprises providing said emitter electrode
and said base electrode as opposing linear conductive strips.
6. The method as defined by claim 5, wherein said step of providing
said emitter electrode and said base electrode as opposing linear
conductive strips further comprises providing said conductive
strips as having substantially the same length.
7. The method as defined by claim 5, wherein said step of providing
said emitter electrode and said base electrode as opposing linear
conductive strips further comprises providing said conductive
strips as being of different lengths, so that the surface of
emitter mesa between said strips is trapazoidal.
8. The method as defined by claim 1, wherein said step of
providing, in said base region, a region exhibiting quantum size
effects comprises providing at least one quantum well.
9. A three terminal light-emitting semiconductor device for
producing light emission in response to electrical signals,
comprising: a layered semiconductor structure including a
semiconductor collector region comprising at least one collector
layer, a semiconductor base region disposed on said collector
region and including at least one base layer, and a semiconductor
emitter region disposed on a portion of said base region and
comprising an emitter mesa that includes at least one emitter
layer; said base region containing at least one region exhibiting
quantum size effects; and a collector electrode on said collector
region, a base electrode on an exposed surface of said base region,
and an emitter electrode on the surface of said emitter region,
said electrical signals being applied with respect to said
collector, base, and emitter electrodes to cause light emission
from said base region; said base and emitter electrodes being
configured to obtain substantial uniformity of voltage distribution
in the region between said electrodes.
10. The device as defined by claim 9, wherein the geometry of said
emitter mesa between said base and emitter electrodes is configured
to obtain substantial uniformity of voltage distribution in said
region.
11. The device as defined by claim 9, further comprising an optical
cavity for said light emission in the region between said base
electrode and said emitter electrode.
12. The device as defined by claim 9, wherein said emitter mesa has
a substantially rectilinear surface portion, and wherein said
emitter electrode is disposed along one side of said surface
portion of the emitter mesa and said base electrode is disposed on
a portion of the base region surface adjacent the opposite side of
said emitter mesa surface portion.
13. The device as defined by claim 12, wherein said emitter
electrode and said base electrode comprise opposing linear
conductive strips.
14. The device as defined by claim 13, wherein said opposing linear
conductive have substantially the same length.
15. The device as defined by claim 13, wherein said opposing linear
conductive strips have different lengths, so that the surface of
emitter mesa between said strips is trapazoidal.
16. The device as defined by claim 9, wherein said region
exhibiting quantum size effects comprises at least one quantum
well.
Description
PRIORITY CLAIMS
[0001] This is a divisional of U.S. patent application Ser. No.
12/799,080, filed Apr. 16, 2010, which is incorporated herein by
reference. Said U.S. patent application Ser. No. 12/799,080 is a
continuation-in-part of U.S. patent application Ser. No.
12/655,806, filed Jan. 7, 2010, which, in turn, claimed priority
from three U.S. Provisional Patent Applications; namely, U.S.
Provisional Application Ser. No. 61/204,560, filed Jan. 8, 2009,
U.S. Provisional Application Ser. No. 61/204,602, filed Jan. 8,
2009, and U.S. Provisional Application Ser. No. 61/208,422, filed
Feb. 24, 2009. Said U.S. patent application Ser. No. 12/799,080
also claims priority from U.S. Provisional Patent Application Ser.
No. 61/212,951, filed Apr. 17, 2009, and from U.S. Provisional
Patent Application Ser. No. 61/268,119, filed Jun. 9, 2009, and all
of the listed Patent Applications are incorporated herein by
reference.
RELATED APPLICATION
[0002] The subject matter of this Application relates to subject
matter disclosed in copending U.S. patent application Ser. No.
______, filed of even date with the parent Application hereof and
assigned to the same assignees as the present Application.
FIELD OF THE INVENTION
[0004] This invention relates to methods and devices for producing
light emission and laser emission in response to electrical
signals. The invention also relates to methods for producing light
emission and laser emission from semiconductor devices with
improved efficiency, and to increasing light output from
semiconductor light-emitting devices.
BACKGROUND OF THE INVENTION
[0005] A part of the background hereof lies in the development of
heterojunction bipolar transistors which operate as light-emitting
transistors and transistor lasers. Reference can be made for
example, to U.S. Pat. Nos. 7,091,082, 7,286,583, 7,354,780,
7,535,034 and 7,693,195; U.S. Patent Application Publication
Numbers US2005/0040432, US2005/0054172, US2008/0240173,
US2009/0134939, and US2010/0034228; and to PCT International Patent
Publication Numbers WO/2005/020287 and WO/2006/093883. Reference
can also be made to the following publications: Light-Emitting
Transistor: Light Emission From InGaP/GaAs Heterojunction Bipolar
Transistors, M. Feng, N. Holonyak, Jr., and W. Hafez, Appl. Phys.
Lett. 84, 151 (2004); Quantum-Well-Base Heterojunction Bipolar
Light-Emitting Transistor, M. Feng, N. Holonyak, Jr., and R. Chan,
Appl. Phys. Lett. 84, 1952 (2004); Type-II GaAsSb/InP
Heterojunction Bipolar Light-Emitting Transistor, M. Feng, N.
Holonyak, Jr., B. Chu-Kung, G. Walter, and R. Chan, Appl. Phys.
Lett. 84, 4792 (2004); Laser Operation Of A Heterojunction Bipolar
Light-Emitting Transistor, G. Walter, N. Holonyak, Jr., M. Feng,
and R. Chan, Appl. Phys. Lett. 85, 4768 (2004); Microwave Operation
And Modulation Of A Transistor Laser, R. Chan, M. Feng, N.
Holonyak, Jr., and G. Walter, Appl. Phys. Lett. 86, 131114 (2005);
Room Temperature Continuous Wave Operation Of A Heterojunction
Bipolar Transistor Laser, M. Feng, N. Holonyak, Jr., G. Walter, and
R. Chan, Appl. Phys. Lett. 87, 131103 (2005); Visible Spectrum
Light-Emitting Transistors, F. Dixon, R. Chan, G. Walter, N.
Holonyak, Jr., M. Feng, X. B. Zhang, J. H. Ryou, and R. D. Dupuis,
Appl. Phys. Lett. 88, 012108 (2006); The Transistor Laser, N.
Holonyak and M Feng, Spectrum, IEEE Volume 43, Issue 2, February
2006; Signal Mixing In A Multiple Input Transistor Laser Near
Threshold, M. Feng, N. Holonyak, Jr., R. Chan, A. James, and G.
Walter, Appl. Phys. Lett. 88, 063509 (2006); and Collector Current
Map Of Gain And Stimulated Recombination On The Base Quantum Well
Transitions Of A Transistor Laser, R. Chan, N. Holonyak, Jr., A.
James, and G. Walter, Appl. Phys. Lett. 88, 14508 (2006); Collector
Breakdown In The Heterojunction Bipolar Transistor Laser, G.
Walter, A. James, N. Holonyak, Jr., M. Feng, and R. Chan, Appl.
Phys. Lett. 88, 232105 (2006); High-Speed (/spl ges/1 GHz)
Electrical And Optical Adding, Mixing, And Processing Of
Square-Wave Signals With A Transistor Laser, M. Feng, N. Holonyak,
Jr., R. Chan, A. James, and G. Walter, Photonics Technology
Letters, IEEE Volume: 18 Issue: 11 (2006); Graded-Base InGaN/GaN
Heterojunction Bipolar Light-Emitting Transistors, B. F. Chu-Kung
et al., Appl. Phys. Lett. 89, 082108 (2006); Carrier Lifetime And
Modulation Bandwidth Of A Quantum Well AlGaAs/InGaP/GaAs/InGaAs
Transistor Laser, M. Feng, N. Holonyak, Jr., A. James, K. Cimino,
G. Walter, and R. Chan, Appl. Phys. Lett. 89, 113504 (2006); Chirp
In A Transistor Laser, Franz-Keldysh Reduction of The Linewidth
Enhancement, G. Walter, A. James, N. Holonyak, Jr., and M. Feng,
Appl. Phys. Lett. 90, 091109 (2007); Photon-Assisted Breakdown,
Negative Resistance, And Switching In A Quantum-Well Transistor
Laser, A. James, G. Walter, M. Feng, and N. Holonyak, Jr., Appl.
Phys. Lett. 90, 152109 (2007); Franz-Keldysh Photon-Assisted
Voltage-Operated Switching of a Transistor Laser, A. James, N.
Holonyak, M. Feng, and G. Walter, Photonics Technology Letters,
IEEE Volume: 19 Issue: 9 (2007); Experimental Determination Of The
Effective Minority Carrier Lifetime In The Operation Of A
Quantum-Well n-p-n Heterojunction Bipolar Light-Emitting Transistor
Of Varying Base Quantum-Well Design And Doping; H. W. Then, M.
Feng, N. Holonyak, Jr., and C. H. Wu, Appl. Phys. Lett. 91, 033505
(2007); Charge Control Analysis Of Transistor Laser Operation, M.
Feng, N. Holonyak, Jr., H. W. Then, and G. Walter, Appl. Phys.
Lett. 91, 053501 (2007); Optical Bandwidth Enhancement By Operation
And Modulation Of The First Excited State Of A Transistor Laser, H.
W. Then, M. Feng, and N. Holonyak, Jr., Appl. Phys. Lett. 91,
183505 (2007); Modulation Of High Current Gain (.beta.>49)
Light-Emitting InGaN/GaN Heterojunction Bipolar Transistors, B. F.
Chu-Kung, C. H. Wu, G. Walter, M. Feng, N. Holonyak, Jr., T. Chung,
J.-H. Ryou, and R. D. Dupuis, Appl. Phys. Lett. 91, 232114 (2007);
Collector Characteristics And The Differential Optical Gain Of A
Quantum-Well Transistor Laser, H. W. Then, G. Walter, M. Feng, and
N. Holonyak, Jr., Appl. Phys. Lett. 91, 243508 (2007); Transistor
Laser With Emission Wavelength at 1544 nm, F. Dixon, M. Feng, N.
Holonyak, Jr., Yong Huang, X. B. Zhang, J. H. Ryou, and R. D.
Dupuis, Appl. Phys. Lett. 93, 021111 (2008); and Optical Bandwidth
Enhancement Of Heterojunction Bipolar Transistor Laser Operation
With An Auxiliary Base Signal, H. W. Then, G. Walter, M. Feng, and
N. Holonyak, Jr. Appl. Phys. Lett. 93, 163504 (2008).
[0006] FIGS. 1 and 2 illustrate an example of an existing tilted
charge light emitter; that is, a light-emitting transistor ("LET")
as described in the above referenced patent documents and
publications. An n+ GaAs subcollector region 105 has an n-type GaAs
collector region 110 deposited thereon, followed by a p+
AlGaAs/GaAs base region 120, having an n-type InGaAs quantum well
(QW) 126. An emitter mesa is deposited over the base, and includes
n-type InGaP emitter layer 130, and n-type AlGaAs aperture layer
140, and an n+ GaAs cladding layer 150. Lateral oxidation can be
conventionally used to obtain annular oxide 141 and form the
central aperture. The collector electrode or contact metallization
is shown at 107, the base contact metallization is shown at 122,
and the emitter contact metallization is shown at 152. FIG. 2 shows
a plan view of the FIG. 1 metallizations; that is, opposing
collector contacts (common connection not shown), the base contact
122 including an outer annular ring, and the emitter contact 152
including an inner annular ring.
[0007] FIG. 1 also has arrows that illustrate the flow of electron
current and hole current in typical light-emitting transistor
operation. As described, for example, in the above referenced
documents, light-emitting transistors, transistor lasers, and
certain two terminal light emitters are sometimes referred to as
"tilted charge" devices, owing to the "tilted" base charge
distribution (as could be illustrated on the device band diagram)
which locks the base electron-hole recombination in "competition"
with the charge "collection" at the reverse-biased collector
junction, thus selecting ("filtering") and allowing only "fast"
recombination in the base (assisted by the quantum well(s)) at an
effective lifetime of the order of picoseconds. [Reference can be
made, for example, to the above-listed documents, including
reference to a two-terminal tilted charge light emitter disclosed
in U.S. Patent Application Publication No. US2010/0034228, and
reference can also be made to U.S. patent application Ser. No.
12/655,806, filed Jan. 7, 2010, which is a parent application
hereof.]
[0008] In existing tilted charge devices, the optical cavity or
window, defined in part by an aperture formed with an oxide, is
placed after the base and emitter contact. Due to the high base
sheet resistant and large current gain (emitter current) of the
tilted charge device, the voltage difference across the base
emitter junction is the greatest along the edge defined by the
oxide aperture. This forces the recombination events (which result
in the desired optical output) to localize along the perimeter of
the oxide aperture, as current injection is largest in the region
where voltage difference is largest. The junction voltage decreases
towards the center of the optical cavity. This phenomenon is
represented in FIGS. 1 and 2, and can be further understood from
the modeling of device operation as shown in the simplified circuit
model of FIG. 3. In FIG. 3, the regions and contacts correspond
with those of like reference numerals in FIG. 1. In the model, 307,
320, and 330 respectively represent the collector, base, and
emitter resistances, 308 represents collector current components,
and 340 represents the spatial components of base/emitter voltage.
As was first seen in FIG. 1, the path of least resistance for
electron conduction is along the edge defined by the oxide
aperture. In the model of FIG. 3, this results in V4 being
substantially greater than V3, and V1 being substantially greater
than V2. This causes most of the recombination events to localize
nearer the edge of the base layer, and less recombination at and
near the center of the base layer (see sketch of light output
representation of FIG. 2).
[0009] FIG. 4 is a graph showing detected optical output of the
device (as detector photo current in pA) as a function of device
base current (in mA). The optical output for larger emitter
diameter devices saturates at larger base current input. Saturation
of light is attributed to quantum well saturation.
[0010] In FIG. 5, the optical output density and emitter current
density for different emitter sizes (and hence, aperture sizes) is
conveniently normalized to the aperture perimeter "area" (shaded
area of inset in FIG. 5.) The area is determined by assuming a
constant shallow penetration into the optical cavity. The result
indicates that recombination is localized along the edge of the
device. Maximum light output is therefore determined by the active
perimeter defined by the oxide aperture rather than the area of the
total optical cavity.
[0011] FIG. 6 illustrates pulsed current measurement for various
emitter sizes showing light output for both 10% and 50% pulsed
current measurements to be substantially the same. Results indicate
that light saturation for the device was not caused by heating but
by localized quantum well saturation.
[0012] FIG. 7 is a top view photograph of the type of existing
device of FIG. 1, wherein the collector (C), base (B), and emitter
(E) metallizations are denoted, and the optical cavity or window is
indicated by an arrow. The light-emitting transistor of the Figure
has a 10 .mu.m emitter mesa and aperture defined optical cavity of
6 .mu.m. The optical cavity is located after the base and emitter
contacts (i.e., above them, as in FIG. 1). The active perimeter of
this device is therefore about 18 .mu.m. Similarly, FIG. 8 shows an
existing tilted charge light-emitting diode (see, for example,
copending U.S. patent application Ser. No. 12/655,806, filed Jan.
7, 2010, assigned to the same assignees as the present
Application), wherein the emitter (E) and base/drain (BD)
metallizations are denoted and, again, the device has a 10 um
emitter mesa and aperture defined optical cavity of 6 um. The
optical cavity is again located after the base and emitter
contacts. Again, the active perimeter of this device is about 18
.mu.m.
[0013] In the described types of devices, as above noted, the
optical window or cavity is placed after the base and emitter
contact. Due to the high base sheet resistant and large current
gain (emitter current) of the tilted charge device, the voltage
difference across the base emitter junction is greatest along the
edge defined by the oxide aperture. As explained above, this forces
the recombination events (which result in the desired optical
output) to localize along the perimeter of the oxide aperture, as
current injection is largest in the region where voltage difference
is largest. The junction voltage decreases towards the center of
the optical cavity, with attendant disadvantages.
[0014] Among the objects of the present invention are to overcome
these and other limitations of existing light-emitting devices,
such as the described tilted-charge light emitters, and to improve
light emission of light-emitting and lasing semiconductor
devices.
SUMMARY OF THE INVENTION
[0015] In a form of the present invention, the light-emitting
semiconductor devices are configured to obtain uniformity of
carrier injection into the base region, and the optical cavity
between base and emitter electrodes does not cause a deleterious
non-uniformity of voltage distribution between the emitter and base
(or base/drain) electrodes of the device, as in the prior art.
[0016] In accordance with a form of the invention, a method is set
forth for producing light emission from a two terminal
semiconductor device with improved efficiency, including the
following steps: providing a layered semiconductor structure
including a semiconductor drain region comprising at least one
drain layer, a semiconductor base region disposed on said drain
region and including at least one base layer, and a semiconductor
emitter region disposed on a portion of said base region and
comprising an emitter mesa that includes at least one emitter
layer; providing, in said base region, at least one region
exhibiting quantum size effects; providing a base/drain electrode
having a first portion on an exposed surface of said base region
and a further portion coupled with said drain region, and providing
an emitter electrode on the surface of said emitter region;
applying signals with respect to said base/drain and emitter
electrodes to obtain light emission from said base region; and
configuring said base/drain and emitter electrodes for substantial
uniformity of voltage distribution in the region therebetween.
[0017] In an embodiment of this form of the invention, the geometry
of said emitter mesa between said electrodes is configured to
promote substantial uniformity of voltage distribution in the
region between the electrodes. In a form of this embodiment, the
emitter mesa has a substantially rectilinear surface portion, and
the step of providing said electrodes comprises providing said
emitter electrode along one side of said surface portion of the
emitter mesa and providing the first portion of said base/drain
electrode on a portion of the base region surface adjacent the
opposite side of said emitter mesa surface portion. The emitter
electrode and said first portion of the base/drain electrode can be
opposing linear conductive strips.
[0018] In accordance with another form of the invention, a method
is provided for producing light emission from a three terminal
semiconductor device with improved efficiency, including the
following steps: providing a layered semiconductor structure
including a semiconductor collector region comprising at least one
collector layer, a semiconductor base region disposed on said
collector region and including at least one base layer, and a
semiconductor emitter region disposed on a portion of said base
region and comprising an emitter mesa that includes at least one
emitter layer; providing, in said base region, at least one region
exhibiting quantum size effects; providing a collector electrode on
said collector region, providing a base electrode on an exposed
surface of said base region, and providing an emitter electrode on
the surface of said emitter region; applying signals with respect
to said collector, base, and emitter electrodes to obtain light
emission from said base region; and configuring said base and
emitter electrodes for substantial uniformity of voltage
distribution in the region therebetween.
[0019] Further features and advantages of the invention will become
more readily apparent from the following detailed description when
taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a cross-sectional representation of an example of
an existing tilted charge light-emitting transistor device.
[0021] FIG. 2 is a plan view of the contacts or electrodes of the
FIG. 1 device.
[0022] FIG. 3 is a circuit model representing the relevant
operation of the FIG. 1 device.
[0023] FIG. 4 is a graph showing optical output (as detector
photocurrent) as a function of base current, for devices of
different emitter diameters D.sub.E.
[0024] FIG. 5 shows a graph of normalized optical output density as
a function of emitter current over edge density, for devices
different emitter diameters, D.sub.E. The inset shows a
representation of the light emitting area, as a normalized aperture
perimeter area. The area is determined by assuming a constant
shallow penetration into the optical cavity.
[0025] FIG. 6 shows photocurrent measurements as a function of
emitter current for devices of various emitter sizes (in .mu.m),
showing 10% and 50% pulsed current points on each curve.
[0026] FIG. 7 is a top view photograph of the type of existing
device of FIG. 1, wherein the collector (C), base (B), and emitter
(E) metallizations are denoted, and the optical cavity is indicated
by an arrow.
[0027] FIG. 8 is a top view photograph of a tilted charge
light-emitting diode of the type described in copending U.S. patent
application Ser. No. 12/655,806, filed Jan. 7, 2010, and assigned
to the same assignees as the present application.
[0028] FIG. 9 is a cross-sectional diagram of an example of an
improved tilted charge light-emitting transistor in accordance with
an embodiment of the invention and which can be used in practicing
an embodiment of the method of the invention.
[0029] FIG. 10 shows a circuit model of device operation of the
FIG. 9 embodiment.
[0030] FIGS. 11(a) and 11(b) show opposing base and emitter contact
or electrode strips as employed in embodiments of the
invention.
[0031] FIG. 12 is a top view photograph of a titled charge light
emitting transistor with a 10 .mu.m.times.10 .mu.m Type 2 optical
cavity design.
[0032] FIG. 13 shows the light-emitting transistor optical output
(detector photo current) vs emitter current for the devices shown
in FIG. 7 (solid line) and FIG. 12 (dashed line).
[0033] FIG. 14 is a simplified cross-sectional diagram of a
two-junction tilted-charge light emitting diode in accordance with
an embodiment of the invention.
[0034] FIG. 15 is top view photograph of the device of FIG. 14,
wherein the emitter (E) and base/drain (BD) metallizations are
denoted, and the optical cavity is indicated by an arrow.
[0035] FIG. 16 is a table showing the semiconductor layers of an
example of the FIG. 15 device.
[0036] FIG. 17 shows the I-V characteristic of the device of FIGS.
15 and 16.
[0037] FIG. 18 shows the optical light output L-I characteristic of
the FIG. 15, 16 device, measured from the device substrate bottom,
and, in the inset, the output optical spectrum in arbitrary
units.
[0038] FIG. 19 shows the optical output response of the FIGS. 15,
16 device at bias currents I.sub.E=40, 50, and 60 mA showing the -3
dB frequency f.sub.3db of 3.2, 5, and 7 GHz, respectively.
[0039] FIG. 20 is a simplified cross-sectional diagram of an
embodiment of the invention that utilizes a tunnel junction as the
device's drain region.
DETAILED DESCRIPTION
[0040] FIG. 9 is a diagram of an improved tilted charge
light-emitting transistor device in accordance with an embodiment
of the invention. The devices hereof can be fabricated using, for
example, conventional semiconductor deposition techniques for
depositing III-V semiconductor layers and device fabricating and
finishing techniques as described, for example, in the patents and
publications listed in the Background portion hereof. From the
bottom up, the device includes n+ subcollector region 905, n-type
collector region 910, and p+ base region 920 containing quantum
well 926. The emitter mesa includes n-type emitter layer 930 and n+
emitter cladding 950. In the present example, the device is an npn
tilted charge light emitting transistor, it being understood that
the principles hereof also apply to a pnp device. The collector
electrode or contact metallization is represented at 907. The base
contact is represented at 922, and the emitter contact is
represented at 952.
[0041] In the embodiment of FIG. 9, the optical cavity is
advantageously placed in between the emitter and base electrodes.
The emitter resistance (R.sub.E) is tuned relative to the emitter
current to base current ratio (.beta.+1) so that the voltage drop
due to electron conduction equals the voltage drop due to base
current as holes conduct laterally from the opposite direction.
This results in a more uniform voltage drop across the base-emitter
junction. Emitter resistance can be tuned by changing sheet
resistances and by changing the geometry of the emitter mesa (FIG.
11, below).
[0042] FIG. 10 shows a circuit model of device operation of the
FIG. 9 embodiment. In FIG. 10, the regions and contacts correspond
with those of like reference numerals in FIG. 9. In the model,
1007, 1020, and 1030 respectively represent the collector, base,
and emitter resistances, 1008 represents collector current
components, and 1040 represents the spatial components of
base/emitter voltage. As seen in the Figure, the voltage drops
across the base-emitter junction are made substantially uniform so
that V1, V2, V3 and V4 will be approximately the same. This means
that the recombination events will be approximately uniform in the
optical cavity.
[0043] A substantially symmetrical voltage drop across the base and
emitter junction can be achieved by tuning the sheet resistance and
geometry of the emitter mesa; e.g. by employing a geometry of the
optical window or cavity (defined by in this case the exposed
emitter mesa) to obtain the desired resistances. For example, the
diagrams of FIGS. 11(a) and 11(b), show opposing base and emitter
contact or electrode strips and, as the shaded area, the exposed
emitter mesa from which generated light can be emitted. Compared to
the "Type 2" device of FIG. 11(a), the "Type 1" device of FIG.
11(b) will exhibit larger emitter resistance and smaller base
resistance.
[0044] FIG. 12 is a top view photograph of a tilted charge light
emitting transistor with a 10 .mu.m.times.10 .mu.m "Type 2" optical
cavity or window design. An approximate symmetric voltage
distribution to obtain uniform light emission is achieved by
designing R.sub.B=(.beta.+1)R.sub.E. The active perimeter of this
device is 10 .mu.m.
[0045] FIG. 13 shows the light-emitting transistor optical output
(detector photo current) vs emitter current for the devices shown
in FIG. 7 (solid line--existing device) and FIG. 12 (dashed
line--example of an embodiment hereof). The distributed design
structure hereof, despite having an active perimeter of 10 .mu.m
(FIG. 12), which is almost half of the 18 .mu.m perimeter of the
existing design (FIG. 7), is seen to be capable of about two times
larger emitter current injection before reaching optical
saturation. This indicates that a larger effective area of the
optical window or cavity is involved in recombination as a result
of the distributed design hereof.
[0046] FIGS. 14 and 15 show a two-terminal tilted charge
light-emitting diode of the type described in above-referenced
copending U.S. patent application Ser. No. 12/655,806, filed Jan.
7, 2010, but having the distributed design feature of an embodiment
hereof, with the optical cavity placed between the emitter and
base/drain and the tuned emitter resistance. In FIG. 14, a p-type
base region 1440 is disposed between unintentionally doped n-type
drain region 1433 and n-type emitter region 1450, so that there is
a first semiconductor junction between said emitter and base region
and second semiconductor junction between the base region and the
drain region. The base region 1440 includes quantum size region
1441, such as, for example, one or more quantum wells or one or
more regions of the quantum dots. Below the drain 1433 is n-type
sub-drain 1434. Above the emitter is an emitter cladding and
contact region 1460. The emitter region has emitter electrode
coupled thereto, in the form of emitter contact 1453. A base/drain
electrode is coupled with the base and drain regions. The
base/drain electrode is a metallic contact 1470 that is deposited,
in this embodiment, on the base region and sub-drain region. As
shown in FIG. 14, a positive bias voltage 1491 is applied to the
base/drain contact 1470 with respect to the emitter contact 1453,
and an AC voltage 1492 is also applied with respect to these
contacts. The flow of electrons and holes in the FIG. 14 device is
shown by the arrows in the Figure. Recombination in the base
region, aided by the quantum well, results in light emission.
Waveguide and cavity configurations can be added to this structure
in order to allow this device to function as a two junction laser
diode, two junction resonance cavity light emitting diode, or two
junction vertical cavity transistor laser. (For example, typical
upper and lower distributed Bragg reflectors (DBRs) can be provided
in the FIG. 14 device to obtain an optical resonant cavity.)
Radiative recombination is optimized in the active optical region,
as represented in FIG. 14 at 1485. From the top view photograph of
FIG. 15, the emitter and base/drain metallizations, and the optical
cavity or window region of the FIG. 14 device can be seen.
[0047] For an example of the embodiment of FIG. 14 (see also
Walter, Wu, Then, Feng, and Holonyak, Applied Physics Letters, 94,
231125 (June, 2009)), the epitaxial layers of the crystal used for
making a two-junction tilted-charge light emitting diode includes,
upward from the substrate, a 3000 .ANG. n-type doped GaAs buffer
layer, a 500 .ANG. graded Al.sub.0.30Ga.sub.0.70As confining layer,
a 213 .ANG. graded Al.sub.0.30Ga.sub.0.70As to
Al.sub.0.90Ga.sub.0.10As oxide buffer layer, a 595 .ANG. n-type
Al.sub.0.98Ga.sub.0.02As oxidizable aperture layer and another 213
.ANG. graded Al.sub.0.90Ga.sub.0.10As to Al.sub.0.30Ga.sub.0.70As
oxide buffer layer. A 557 .ANG. n-type GaAs contact layer, a 120
.ANG. InGaP etch stop layer, and a 2871 .ANG. undoped "drain" layer
are grown on top. The "drain" layer is just beneath the 1358 .ANG.
base layer, which includes two undoped 112 .ANG. InGaAs quantum
wells and an Al.sub.0.05Ga.sub.0.95As layer with average doping of
3.times.10.sup.19 cm.sup.-3. The heterostructure emitter includes
of a 511 .ANG. n-type In.sub.0.49Ga.sub.0.51P layer, a 213 .ANG.
graded Al.sub.0.30Ga.sub.0.70As to Al.sub.0.90Ga.sub.0.10As oxide
buffer layer, a 595 .ANG. n-type Al.sub.0.98Ga.sub.0.02As
oxidizable aperture layer, another 213 .ANG. graded
Al.sub.0.90Ga.sub.0.10As to Al.sub.0.30Ga.sub.0.70As oxide buffer
layer, and a 500 .ANG. graded Al.sub.0.30Ga.sub.0.70As confining
layer. The structure is completed with a 2000 .ANG. GaAs top
contact layer. The aperture is optional. Reference can be made to
the table of FIG. 16, the last column of which indicates the layer
description relative to the diagram of FIG. 15.
[0048] The two-junction tilted-charge LED is fabricated by first
performing wet etching steps to form emitter and base-"drain"
mesas, followed by an isolation etch from the sub-"drain" layer to
the substrate. Metallization steps are then performed to provide
the required electrical contacts. The completed LED has only two
terminals: (a) a contact to the emitter layer, and (b) another
across the base and "drain" layers (see FIG. 15). The base-"drain"
forms a p-n junction with a reverse built-in field that is
maintained by a common potential (zero potential difference)
obtained via the common contact metallization extending to the
base. The zero base-"drain" potential difference ensures that there
is no base charge population density at the base-"drain" boundary,
hence establishing the dynamic "tilted" emitter-to-"drain"
population in the base, which was first described above. The
"drain" layer performs therefore a role similar to the collector in
a three-terminal HBLET. It allows excess minority carriers to be
removed from the base (I.sub.D), "swept" from base to "drain" by
the built-in field at the base-"drain" p-n junction. Base carriers
in transit from the emitter to the "drain" that do not recombine
within the base transit time are removed, "drained". This enables
fast modulation of the tilted-charge LED by preventing the build-up
of "slow" charge in the base. The tilted-charge LED possesses the
high speed optical modulation characteristics of an HBLET.
[0049] The tilted-charge LED can be biased as a usual two-terminal
device, simply operating faster. Externally the tilted-charge LED
displays an electrical I-V characteristic resembling that of a p-n
junction diode (see FIG. 17). The "turn-on" voltage is determined
by the emitter-base potential difference since the base and "drain"
are metallized and unified in potential. The L-I.sub.E optical
output characteristic, shown in FIG. 18, is obtained from the
bottom emission (through the substrate) of the device. The broad
radiative emission spectrum (FWHM.about.96 nm) of the inset shows
that the LED is operating in spontaneous recombination. The
spectral peak occurs at .lamda.=1000 nm, corresponding to the
ground state transition (1.24 eV) of the InGaAs quantum-wells. The
optical output saturates with I.sub.E beyond 10 mA as the internal
"transistor" gain, .beta.=I.sub.D/I.sub.B, increases resulting in
the base (recombination) current, I.sub.B=I.sub.E/(.beta.+1)
saturating. The optical output for this example is in the low
microwatt range because the light extraction efficiency, assuming a
single escape cone from the semiconductor GaAs-air interface, is
only about 1.4%. To obtain the optical response of the device, the
optical output is collected from the device top emission through a
fiber, and measured with a 12 GHz p-i-n photodetector connected to
an Agilent N5230A network analyzer. The optical response of the
tilted-charge LED for I.sub.E=40, 50, and 60 mA are shown in FIG.
19. The data show an excellent fit to a single-pole response of the
form, H(f)=A.sub.o/(1+jf/f.sub.3 dB) where f.sub.3
dB=1/(2.pi..tau..sub.B). For this example, a -3 dB bandwidth,
f.sub.3dB, of 7 GHz at I.sub.E=60 mA is obtained, corresponding to
an effective .tau..sub.B=23 ps.
[0050] FIG. 20 shows another embodiment hereof which utilizes a
tunnel junction as the drain region. Reference can be made, for
example, to Tunnel Junction Transistor Laser, M. Feng, N. Holonyak,
Jr., H. W. Then, C. H. Wu, and G. Walter, Appl. Phys. Lett. 94,
04118 (2009) and to U.S. Patent Application Publication No.
US2010/0034228. In FIG. 20, elements with like reference numerals
to those of FIG. 14 correspond to those elements of FIG. 14. In
FIG. 20, the p+ layer 1930 of the tunnel junction is adjacent the
base 1440 and the n+ layer 1931 of the tunnel junction is adjacent
an n-type sub drain layer 1434.
[0051] The invention has been described with reference to
particular preferred embodiments, but variations within the spirit
and scope of the invention will occur to those skilled in the art.
For example, appropriate reflectors can be employed to enhance
extraction of output spontaneous optical emission. Also, where
spontaneous emission LETs and diodes have been described, it will
be understood that by employing appropriate reflective resonators,
transistor lasers and diode lasers that benefit from the described
features can also be devised.
* * * * *