U.S. patent application number 13/910729 was filed with the patent office on 2013-12-19 for optically controlled power devices.
This patent application is currently assigned to The Curators of the University of Missouri. The applicant listed for this patent is The Curators of the University of Missouri, Helava Systems, Inc.. Invention is credited to Randy D. Curry, Heikki I. Helava.
Application Number | 20130334537 13/910729 |
Document ID | / |
Family ID | 49755069 |
Filed Date | 2013-12-19 |
United States Patent
Application |
20130334537 |
Kind Code |
A1 |
Helava; Heikki I. ; et
al. |
December 19, 2013 |
Optically Controlled Power Devices
Abstract
An electro-optically triggered power switch is disclosed
utilizing a wide bandgap, high purity III-nitride semiconductor
material such as BN, AN, GaN, InN and their compounds. The device
is electro-optically triggered using a laser diode operating at a
wavelength of 10 to 50 nanometers off the material's bandgap, and
at a power level of 10 to 100 times less than that required in a
conventionally triggered device. The disclosed device may be
configured as a high power RF MOSFET, IGBT, FET, or HEMT that can
be electro-optically controlled using photons rather than an
electrical signal. Electro-optic control lowers the power losses in
the semiconductor device, decreases the turn-on time, and
simplifies the drive signal requirements. It also allows the power
devices to be operated from the millisecond to the sub-picosecond
timeframe, thus allowing the power device to be operated at RF
frequencies (i.e., kilohertz to terahertz range) and at high
temperatures where the bandgap changes with temperature.
Inventors: |
Helava; Heikki I.; (Dix
Hills, NY) ; Curry; Randy D.; (Columbia, MO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Curators of the University of Missouri
Helava Systems, Inc. |
Columbia
Dix Hills |
MO
NY |
US
US |
|
|
Assignee: |
The Curators of the University of
Missouri
Columbia
MO
Helava Systems, Inc.
Dix Hills
NY
|
Family ID: |
49755069 |
Appl. No.: |
13/910729 |
Filed: |
June 5, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61659830 |
Jun 14, 2012 |
|
|
|
Current U.S.
Class: |
257/76 ; 257/431;
257/449 |
Current CPC
Class: |
H01L 31/03044 20130101;
H01L 31/112 20130101; H01L 31/162 20130101; H01L 31/1105 20130101;
Y02E 10/544 20130101 |
Class at
Publication: |
257/76 ; 257/431;
257/449 |
International
Class: |
H01L 31/16 20060101
H01L031/16; H01L 31/0304 20060101 H01L031/0304 |
Claims
1. An electro-optically triggered power device, comprising: a
semiconductor comprised of a III-nitride material; and a source of
photons, wherein an output beam of said source is coupled to said
semiconductor and generates carriers within said semiconductor.
2. The electro-optically triggered power device of claim 1, wherein
said source is comprised of a laser diode.
3. The electro-optically triggered power device of claim 2, wherein
said output beam is coupled to said semiconductor via an optical
fiber.
4. The electro-optically triggered power device of claim 1, wherein
said III-nitride material is selected from the group of III-nitride
materials consisting of BN, AlN, GaN, InN.
5. The electro-optically triggered power device of claim 1, wherein
said semiconductor is undoped and a Franz-Keldysh effect is applied
to said semiconductor to alter a photon absorption depth
corresponding to said semiconductor, and wherein a wavelength
corresponding to said source is between 2 and 150 nanometers below
a bandgap corresponding to said semiconductor.
6. The electro-optically triggered power device of claim 1, wherein
said semiconductor is undoped and a Franz-Keldysh effect is applied
to said semiconductor to alter a photon absorption depth
corresponding to said semiconductor, and wherein a wavelength
corresponding to said source is between 2 and 150 nanometers above
a bandgap corresponding to said semiconductor.
7. The electro-optically triggered power device of claim 1, wherein
said semiconductor is doped and a Franz-Keldysh effect is applied
to said semiconductor to alter a photon absorption depth
corresponding to said semiconductor, and wherein a wavelength
corresponding to said source is between 2 and 150 nanometers below
a bandgap corresponding to said semiconductor.
8. The electro-optically triggered power device of claim 1, wherein
said semiconductor is doped and a Franz-Keldysh effect is applied
to said semiconductor to alter a photon absorption depth
corresponding to said semiconductor, and wherein a wavelength
corresponding to said source is between 2 and 150 nanometers above
a bandgap corresponding to said semiconductor.
9. The electro-optically triggered power device of claim 1, said
electro-optically triggered power device comprising a
transistor.
10. The electro-optically triggered power device of claim 1, said
electro-optically triggered power device comprising a SCR
structure.
11. The electro-optically triggered power device of claim 1, said
electro-optically triggered power device comprising a GTO
structure.
12. The electro-optically triggered power device of claim 1,
wherein conductivity of said semiconductor varies in direct
proportion to an intensity corresponding to said output beam of
said source.
13. The electro-optically triggered power device of claim 1,
wherein a gate corresponding to said electro-optically triggered
power device operates in an avalanche mode.
14. The electro-optically triggered power device of claim 1,
wherein said III-nitride material is grown via HVPE.
15. The electro-optically triggered power device of claim 1,
further comprising a pair of ohmic contacts coupled to said
semiconductor.
16. The electro-optically triggered power device of claim 1,
further comprising a pair of Schottky contacts coupled to said
semiconductor.
17. The electro-optically triggered power device of claim 1,
further comprising a pair of metal contacts coupled to said
semiconductor.
18. The electro-optically triggered power device of claim 1, said
electro-optically triggered power device having a switching time in
the range of a millisecond to less than a picosecond.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of the filing date of
U.S. Provisional Patent application Ser. No. 61/659,830, filed 14
Jun. 2012, the disclosure of which is incorporated herein by
reference for any and all purposes.
FIELD OF THE INVENTION
[0002] The present invention relates generally to semiconductor
devices and, more particularly, to an electro-optically modulated
semiconductor switch.
BACKGROUND OF THE INVENTION
[0003] In a typical semiconductor switch, an electric field is used
to control the negative resistance characteristics of the device.
For example, U.S. Pat. No. 3,742,318 discloses a semiconductor
switch based on a field-effect type thyristor and a Hall-effect
element. Alternately, a semiconductor switch may be designed, such
as that disclosed in U.S. Pat. No. 4,107,721, to utilize an
injected light signal, rather than an electric field, to control
the conductivity of the device. By electro-optically controlling
the device, the gate or base or trigger electrode capacitance can
be eliminated, thereby lowering power losses, decreasing the
turn-on time, and simplifying the drive signal requirements. It
also allows the power devices to be operated at much higher
frequencies, which in turn extends the useful range of the
device.
[0004] In U.S. Pat. Nos. 4,782,222 and 4,864,119 a method of
optically-triggering avalanche breakdown in a semiconductor is
disclosed, the method using a laser diode as the optical trigger.
The wavelength of the optical trigger, i.e., the laser diode, and
the absorption coefficient of the semiconductor material are
selected so that the absorption depth of the majority of the
incident radiation is less than the distance between the contacts
applied to the semiconductor. As a result, most of the high voltage
is shifted beyond where the radiation is absorbed, causing
significant field compression. A stated advantage of this approach
is that the device is able to switch using a lower flux level
trigger, and at a lower applied bias voltage, than a device
designed to operate solely in an avalanche mode.
[0005] U.S. Pat. No. 8,183,512 discloses optically triggered power
systems and devices. The disclosed power device includes a pair of
terminals and a P-body region with an optical window. When
optically triggered, the P-body region causes current to be
conducted within an N.sup.- drift region provided between the
device's terminals. The disclosed power system includes, in
addition to the power device, a controller for generating control
signals and a converter for converting the electrical control
signals to optical control signals. U.S. Pat. No. 8,294,078, a
continuation-in-part of U.S. Pat. No. 8,183,512, further discloses
a multi-stage power system that includes at least one optically
triggered stage and at least one power device that is responsive to
an output signal generated by the at least one optically triggered
stage.
SUMMARY OF THE INVENTION
[0006] An electro-optically triggered power device (e.g.,
transistor, SCR, GTO) is provided that is comprised of a
III-nitride semiconductor (e.g., BN, AN, GaN, InN and their
compounds) and a source of photons, where the output beam of the
photon source is coupled to the III-nitride semiconductor and
generates carriers within the semiconductor. The photon source may
be a laser diode and the output beam may be coupled to the
semiconductor using an optical fiber. The semiconductor may be
doped and the Franz-Keldysh effect may be applied to the
semiconductor to alter its photon absorption depth. The
semiconductor may be undoped and the Franz-Keldysh effect may be
applied to the semiconductor to alter its photon absorption depth.
The wavelength of the photon source may be between 2 and 150
nanometers above or below the bandgap of the semiconductor. The
conductivity of the semiconductor may vary in direct proportion to
the photon source output intensity. The gate of the
electro-optically triggered device may operate in an avalanche
mode. The III-nitride semiconductor material may be grown via HVPE.
The power device may further comprise a pair of ohmic contacts, or
a pair of Schottky contacts, or a pair of metal contacts, coupled
to the semiconductor. The electro-optically triggered power device
may have a switching time in the range of a millisecond to less
than a picosecond.
[0007] A further understanding of the nature and advantages of the
present invention may be realized by reference to the remaining
portions of the specification and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 provides a top view of an optically controlled
Schottky diode-based transistor in accordance with the
invention;
[0009] FIG. 2 provides a cross-sectional view taken along plane A-A
of the optically controlled Schottky diode-based transistor shown
in FIG. 1;
[0010] FIG. 3 illustrates the voltage measured between the contacts
of the optically controlled Schottky diode-based transistor shown
in FIGS. 1 and 2;
[0011] FIG. 4 provides a top view of a high speed optical switch in
accordance with the invention;
[0012] FIG. 5 provides a cross-sectional view taken along plane B-B
of the high speed optical switch shown in FIG. 4;
[0013] FIG. 6 illustrates the pulse characteristics of the device
shown in FIGS. 4 and 5 in a standard pulse-forming
configuration;
[0014] FIG. 7 illustrates the pulse characteristics of the device
shown in FIGS. 4 and 5 when the duration of the laser trigger is
less than 1 nanosecond;
[0015] FIG. 8 illustrates an electro-optically controlled MOSFET
using horizontal fabrication techniques in accordance with the
invention;
[0016] FIG. 9 illustrates a vertical MOSFET using a photonically
controlled gate where the gate may be symmetrical or
single-sided;
[0017] FIGS. 10A and 10B illustrate the effects of the Franz
Keldysh effect on penetration depth; and
[0018] FIG. 11 illustrates an electro-optically controlled bipolar
junction transistor in accordance with the invention.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
[0019] In accordance with the present invention, an
electro-optically triggered power switch is disclosed utilizing a
wide bandgap, high purity III-nitride semiconductor material.
Although the preferred embodiment is based on GaN, the invention is
equally applicable to other III-nitride materials such as BN, AN,
GaN, InN and their compounds. The device is electro-optically
triggered using a laser diode operating at a wavelength of 2 to 150
nanometers from the material's bandgap, and at a power level of 10
to 100 times less than that required in a conventionally triggered
device. The electro-optically triggered region of the semiconductor
may be either biased or unbiased. The disclosed device may be
configured as a high power RF MOSFET, IGBT, FET, HEMT, SCR or GTO
that can be electro-optically controlled using photons rather than
an electrical signal. Electro-optic control lowers the power losses
in the semiconductor device, decreases the turn-on time, and
simplifies the drive signal requirements. It also allows the power
devices to be operated from the millisecond to the sub-picosecond
timeframe, thus allowing the power device to be operated at RF
frequencies (i.e., kilohertz to terahertz range) and at high
temperatures where the bandgap changes with temperature.
[0020] In a conventional electro-optically triggered device, it is
often necessary to tailor the stoichiometry of the material, for
example using molecular beam epitaxy, CVD or even crystal growth
processes, in order to match the bandgap of the material to the
energy of the photons of the light source. Unfortunately, even
using these approaches it is often not possible to match the
bandgap to the photon energy available from a
commercially-available laser diode. Furthermore, while the
introduction of defect sites or impurities into the semiconductor
material may allow the use of lower photon energies to generate
carriers in the semiconductor, these impurities have the undesired
effect of decreasing the material's electric field breakdown,
resulting in significantly higher power losses than would otherwise
be achieved with a high purity material. In contrast, the present
invention utilizes extremely high purity material, thus allowing
compact, low inductance, fast rise time, high frequency power
devices to be fabricated. Additionally, by allowing conductive
regions in the semiconductor material to be generated locally
through the application of light to selected regions of the
semiconductor, the present invention can be used to generate
traveling wave devices.
[0021] In the present devices, the high electric field arising from
the voltage applied across the positive and negative contacts
varies the photon absorption depth by up to four orders of
magnitude in accordance with the Franz-Keldysh effect, thereby
allowing the current density and carrier concentration to be
selected as required to electro-optically trigger the device using
below bandgap or above bandgap photons from a laser diode. Most
notably, this approach allows the absorption depth to be varied
without the inclusion of impurities or traps in the material, thus
not affecting the material's electric field breakdown. By operating
either below or above the bandgap threshold, the absorption depth
and absorption coefficient can be designed such that the depth of
the photons can be used to select the on-resistance of the
semiconductor device, and thus the current density. These
qualities, which allow either vertical or horizontal devices to be
fabricated, are important in both RF and power devices in order to
limit fault mode currents. Additionally, as the number of photons
required to generate the carriers in the semiconductor material
using this approach is one to seven orders of magnitude less than
that required using a conventional approach, the output power of
the optical trigger (i.e., diode laser or other photon source) can
be reduced, significantly enhancing the commercial viability of a
power switch designed in accordance with the invention.
[0022] It is well known that direct bandgap semiconductors, such as
GaAs, have significantly more attractive optical modulation
characteristics than indirect bandgap materials such as Si or SiC.
It is also known that wide bandgap (WBG) materials, with their
order-of-magnitude higher intrinsic electrical breakdown fields,
permit a much wider range of power devices than narrow bandgap
(NBG) materials such as GaAs and Si. For example, GaN has an
intrinsic breakdown field that is 10 times higher than that of
GaAs. Accordingly, the present invention is focused on the direct
bandgap family of III-nitride semiconductors including BN, AN, GaN,
InN and their compounds.
[0023] The intrinsic properties of GaN, an exemplary III-nitride
material used for much of the present testing, allow a wide range
of device possibilities. For example, undoped high-purity GaN has a
high resistivity (e.g., residual carrier concentration typically
less than 10.sup.15 cm .sup.-3) and can therefore be used as a bulk
device. GaN can also be easily doped as either n-type or p-type
material, allowing the material to be used in pn junction diodes,
pin diodes and Schottky diodes. These diodes can be converted to
optical transistors and other high power switches by injecting
light into the material to modulate its conductivity. Additionally,
at high enough field strengths the GaN devices are expected to be
usable in avalanche mode, in analogy with GaAs.
[0024] As noted above, undoped (i.e., intrinsic) high-purity GaN
can be used as a bulk device (i.e., a device that does not include
a junction) due to its low background carrier concentration,
assuming that care is taken to use high purity source materials and
that the material is accurately stoichiometric. The desired low
leakage current for such a bulk device can also be achieved by
compensating the usual residual negative carriers with positive
carriers via impurity doping, for example doping with an impurity
such as iron, thereby making the material highly resistive.
Impurity doping, however, leads to a more complex optical behavior
than that of pure GaN.
[0025] The inventors have found that uncompensated, high purity,
high resistivity III-nitride material shows conductivity that
follows and is directly proportional to the intensity of incident
light. In contrast, compensated, high resistivity III-nitride
material requires much greater optical intensity to generate equal
conductivity, the required optical intensity tracking the level of
compensation. Moreover, compensated material forms long-lived
optically excited states that continue long after the optical
signal has ended, as shown by a persistent conductivity with a
decay time on the order of seconds to minutes. Accordingly,
although highly compensated III-nitride material may be adequate
for some optically activated switches (e.g., MOSFET and IGBT
devices), assuming sufficient laser power is applied, they are not
as ideally suited as uncompensated material for many device types
(e.g., optically controlled high power transistor) given that the
conductivity of the uncompensated material accurately tracks the
optical modulation signal. The inventors have found, however, that
a hybrid approach whereby the gate operates in avalanche mode and
compensated GaN is utilized may be adequate for a power transistor
whereby slower switching speeds and recovery mitigate commutation
transients. It should be understood that the Franz-Keldysh
mechanism utilized in the present invention does not require
implantation or introduction of defects, although it can be used
with compensated material. Accordingly, this mechanism can be used
in either linear or avalanche mode.
[0026] While the invention may be applied to any semiconductor
material, preferably the selected material is a direct bandgap
semiconductor, more preferably a direct bandgap semiconductor with
a wide bandgap (i.e., greater than 2 eV), and still more preferably
a direct bandgap, highly resistive semiconductor with a wide
bandgap that matches the photon energy of an available laser, for
example a diode laser made from the same material system as the
direct bandgap material used in the optical switch. As noted above,
the present invention is focused on the direct bandgap family of
III-nitride semiconductors including BN, AN, GaN, InN and their
compounds, although at least one preferred embodiment is based on
GaN. Additionally, the material can be grown using any of a variety
of techniques. For example, GaN can be bulk grown or pseudo-bulk
grown as a thick epitaxy using a process such as HVPE, and
epitaxial GaN films grown can be grown by any process (e.g., HVPE,
LPE, MOVD, ammonothermal, and sodium flux) on any substrate (e.g.,
sapphire, silicon carbide, GaAs, and silicon). The inventors have
found that HVPE is preferred due to the high intrinsic purity and
the high growth rate of the process as well as the capability to
include regions of high p- and n-type doping which are of primary
interest for pin structures, contact formation and compensation
doping.
[0027] In accordance with the invention, when a voltage is applied
between the positive and negative contacts a leakage current is
formed in the device, where the device is formed from a III-nitride
material, preferably GaN. The leakage current is a function of the
material resistivity as well as the thickness and type of metal
used for the contacts. If the contact metal forms an ohmic contact
with the device, e.g., Ni or Ti with the GaN in the preferred
embodiment, the leakage current is determined only by the residual
carrier concentration and is independent of the polarity of the
applied voltage. If the contacts are Schottky metal, e.g., Au or Hg
with the GaN in the preferred embodiment, the leakage current is
determined by the metal/GaN Schottky and the polarity of the
applied voltage. These switch characteristics define the background
leakage current of the device, but not the operation of the switch.
When a photon of energy greater than or near the bandgap energy is
absorbed in the GaN an electron hole pair is formed. The electron
is attracted to the positive contact and the hole is attracted to
the negative contact. The drift velocity and mean free path, i.e.,
the speed of movement and the distance the hole or electron will
drift in the electric field, in high purity GaN is longer than in
low purity GaN, and therefore the hole-electron pair will drift
farther in pure GaN before being recombined. As a result, less
optical energy (e.g., photons from a diode laser) is required in
order to generate sufficient hole-electron pairs to reduce the
resistance of the GaN to a low value, i.e., turn the device on.
Note that if the photons from the optical source are injected
rapidly enough, the electric field is compressed by the generated
conductivity and a carrier avalanche occurs.
[0028] It should be understood that the invention may be applied to
a variety of switching device configurations including (i) pn
junctions where the depletion layer of the pn junction forms the
highly resistive layer, which is limited to relatively low voltage
due to the junction breakdown; and (ii) pin structures where the
intrinsic material forms the highly resistive layer (i.e., pure,
highly resistive layer in the ideal case and compensated layer in
the lowest leakage, highest voltage case). In general, due to the
intrinsic breakdown strength of the selected material, devices in
accordance with the invention may be designed to operate in the
range of 100 V to 20 MV. The transistor and thyristor structures
described in this patent have high gain and the electro-optical
control of the devices reduces the light source to levels that are
commercially feasible. Although the inherent breakdown strength of
the material allows very high voltage switches to be built, lower
voltage switches can be fabricated and operated either in series or
parallel. Furthermore, the lower capacitance of the integrated or
even distributed semiconductor device decreases the power losses
and the relative high current gate or trigger requirements in
traditional semiconductors.
Exemplary Embodiments
Embodiment 1
[0029] FIGS. 1 and 2 provide top and cross-sectional views,
respectively, of an optically controlled Schottky diode-based
transistor 100. Device 100 is comprised of a layer 101 of high
purity, highly resistive GaN grown on a sapphire substrate 201 by
HVPE. A pair of mercury contacts 103/104 forms a lateral Schottky
diode with the GaN 101. When reverse biased, diode 100 effectively
blocks any current flow between the contacts. Output beam 203 of a
405 nanometer laser diode is focused between the contacts 103/104,
or focused on contact 104, of the lateral diode 100 and is
modulated by a square wave signal generator. The current carried by
the diode is proportional to the intensity of the laser light. If
sufficient laser light is injected, the conductivity of the GaN can
be increased at will. FIG. 3 shows the voltage measured between
contacts 103/104 with the voltage decreasing as the current
increases. Residual voltage 301 is due to the Schottky contact
potential.
Embodiment 2
[0030] FIGS. 4 and 5 provide top and cross-sectional views,
respectively, of a high speed optical switch in accordance with the
invention. As shown, device 400 is comprised of GaN 501 grown on a
sapphire substrate 503 mounted between a pair of GaIn eutectic
Ohmic contacts 401/402 on a 50 Ohm transmission line 403. Device
400 is mounted to a sapphire substrate 505. Since device 400 does
not include a barrier such as pn junction or a Schottky contact to
limit the current, the pulse forming line is pulse charged to high
voltage. The typical charging time is 1 microsecond, albeit a
slower charge pulse or even a nanosecond pulse can be used to
charge the stripline configuration. The output beam 507 of a 405
nanometer laser diode is focused to fill the gap between the pulse
forming line and the output 50 Ohm line.
[0031] FIG. 6 illustrates the output pulse of device 400 when the
laser diode is driven by a 250 picosecond rise-time, 2 nanosecond
duration pulse generator. As shown, in a standard pulse-forming
configuration the amplitude of output pulse 601 is half of the
charging voltage 603 and the output pulse duration is 1 nanosecond,
which is approximately half the duration of the laser charging
pulse.
[0032] When the laser pulse duration for device 400 is shorter than
1 nanosecond, the switch resistivity recovers before the pulse is
over and the pulse duration is truncated by the now open switch.
For example, using device 400 and adjusting the pulse generator to
a pulse width of 500 picoseconds, output pulse 701 is also
approximately 500 picoseconds as illustrated in FIG. 7.
[0033] As previously noted, devices fabricated in accordance with
the present invention may be used for fast, medium and slow
switching applications. Basically any application that requires
mitigation of the gate or base capacitance and that utilizes a
transistor or switching geometry is an application that can be
significantly simplified by the present invention. Exemplary
applications include HEMTs, MOSFETs, FETs, transistors, SCRs, GTOs
and any device that is controlled by an external signal applied to
a gate or base structure. Additionally, high voltage applications
that require the switching device to operate serially and at very
high voltages, e.g., 1000 V to 4 MV, may utilize the present
invention since the inherent properties of many III-nitride
materials (e.g., GaN, AlGaN, etc.) have very high breakdown
voltages (e.g., 1-8 MV/cm). Furthermore, due to the inherent thin
structures used in the present devices, relatively fast switching
times can be achieved. For example, using uncompensated material
(e.g., GaN) switching times less than a picosecond up to several
nanoseconds are obtainable. Lastly, the optical signal applied to
the gate or base of the disclosed device can be provided by a laser
diode or other optical source, either directly or by an optical
transport mechanism such as an optical fiber. FIGS. 8-11 illustrate
a few of the configurations that may be used with the present
invention.
[0034] It should be understood that the accompanying figures are
only meant to illustrate, not limit, the scope of the invention and
should not be considered to be to scale.
[0035] Systems and methods have been described in general terms as
an aid to understanding details of the invention. In some
instances, well-known structures, materials, and/or operations have
not been specifically shown or described in detail to avoid
obscuring aspects of the invention. In other instances, specific
details have been given in order to provide a thorough
understanding of the invention. One skilled in the relevant art
will recognize that the invention may be embodied in other specific
forms, for example to adapt to a particular system or apparatus or
situation or material or component, without departing from the
spirit or essential characteristics thereof. Therefore the
disclosures and descriptions herein are intended to be
illustrative, but not limiting, of the scope of the invention.
* * * * *