Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1 46,931
LIGHT-ACTIVATED P-I-N SWITCH
BACKGROUND OF THE INVENTION
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Field_of the Invention:
This invention relates to the field of light-
activated or light-sensitive semiconductor devices and
relates particularly to semiconductor switches gated by
light radiation.
Description of the Prior Art:
Light-activated-silicon-switches (LASS) promise
to be very useful in power electronic systems. The major
accepted advantages of LASS are: electrical isolation of
the control circuit from the power circuit; very fast
turn-on; simultaneous turn-on of related units; reduction
of cusioning components when units are series connected;
and insensitivity to electrical noise. In addition, there
are cer~ain other advantages such as lower cabling weight
for fiber optic cable as compared to copper cable.
Light-activated thyristors will be particularly
useful in electronic power circuits. They can be turned on
when optically excited by means of a laser or other light
source. However, like conventional thyristors, they can
only be turned off by reducing the conduction current to
zero. Transistors, on the other hand, can be turned on and
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turned off by appropriate electrical inputs to the base
electrode. Power transistors, however, are typically
slower than thyristors.
SUMMARY OF THE INVENTION
.
A light-activated semiconductor switch comprises
a first coupling means for coupling the switch in a circuit
and for facilitating current flow between the switch and
the circuit, which first coupling means includes means for
admitting electromagnetic radiation therethrough. A first
region in a semiconductor body having a first type of con-
ductivity is responsive to radiation admitted through the
first coupling means for absorbing photons therefrom and
thereby creating electron-hole pairs in the first region.
A lightly doped second region in the semiconductor body
having a second type of conductivity is adjacent to the
first region and forms a p-n junction therewith. A heavily
doped third region in the semiconductor body having the
same second-type conductivity as the second region is
adjacent thereto and facilitates ohmic contact between the
second region and a second coupling means disposed on the
third semiconductor region.
More particularly, in a preferred embodiment, a
metal grid emitter electrode ohmically contacts the surface
of the p region in a p-i-n (p-n -n ) rectifier. A collec-
tor electrode which may be a metal grid structure is dis-
posed on the surface of the n region (n+).
When the p-n junction in the switch is electri-
cally reverse biased, a depletion region is formed and an
electric field is created in the semiconductor body When
light passes through the emitter grid and impinges upon the
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p region, electron-hole pairs are created in the depleted
region thereby causing a drift current to flow in the
external circuit. Even though the strength of the electric
field in the semiconductor body decreases in direct pro-
portion to the intensity of the light impinging upon the p
region when the device is in series with a load resistance,
the low doping or impurity concentration of the i(n) region
" ss rcs
~sY~e~ that an electric field continues to exist over most
of the absorption volume (depletion region) in order to0 provide electron current flow in the external circuit.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a sectional view of a light-activated
semiconductor switch according to the present invention;
Figure 2 shows a top view of the emitter elec-
trode grid referred to in Figure l;
Figure 3 is a graph of the magnitude of the elec-
tric field in the semiconductor crystal shown in Figure l;
Figure 4 is an alternative embodiment of the
present invention wherein the collector electrode has a
grid structure;
Figure 5 is an alternative embodiment of the
present invention having semiconductor regions of conduc-
tivity type opposite that shown in Figure 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Figure 1 shows a sectional view of a p-i-n sili-
con body 10 in accordance with the teachings of the present
invention. The body 10 is comprised of layers or regions
14, 15, and 16. Region 14 is a p-type region approximately
25~m thick which is doped with any suitable substance which
provides acceptor atoms, for example, boron, aluminum, or
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gallium. The dopant or impurity concentration is prefer-
ably such that the region 14 is a p region, for example
greater than 1016 acceptor atoms/cm3 and preferably 1018 to
1021 atoms/cm3. The p doping is preferred since it pro-
vides for a steeper slope in the electric field distribu-
tion as later discussed herein with reference to Figure 3.
Since the slope of the electric field distribution is
steeper in such a p region, a thinner region can be used.
Region 15 is an i or n type region approximately 450 ~m
thick lightly doped with any suitable substance which
provides donor atoms, for example, phosphorus, antimony, or
arsenic, such that the impurity or dopant concentration is
effective to provide an n region, for example, less than
1016 donor atoms/cm3 and preferably about 1014 atoms/cm3.
The slope of the electric field distribution when the p-n
junction is reverse biased, is directly proportional to the
level of doping or impurity concentration therein. It is
desirable, according to the teaching of the present inven-
tion, to provide an n region 15 having a low impurity
concentration in order to effect an electric field distribu-
tion across the region 15 having a slow rate of decrease
(shallow slope) as later discussed herein with reference to
Figure 3. The p region 14 and the n region 16 from p-n
junction 18. Region 16 is an n-type region approximately
~m thick heavily doped with, for example, any substance
hereinbefore mentioned as suitable for doping the n region
15, such that the impurity or dopant concentration is
effective to provide an n+ region, for example, greater
than 1016 donor atoms/cm3. It is desirable according to
the teachings of the present invention to provide an n
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region 16 having a heavy doping or impurity concentration
in order to facilitate ohmic contact between the n region
15 and a metal collector electrode 22 disposed on a surface
of the region or layer 16. A gridded emitter electrode 24
is disposed on a surface of the p region or layer 14 so
that electromagnetic radiation or light is allowed to
impinge upon the p region 14 through the openings in the
emitter electrode 24. The emitter electrode 24 can be
formed in any conventional manner such as depositing a
layer of metal over the entire surface of the p region 14
and using a photo resist method to etch away the desired
portions.
A top view of the emitter electrode 24 suitable
for use in accordance with the teachings of the present
invention is shown in Figure 2 wherein a layer of metal
includes a conducting area 40 approximately 2 cm long and
2.5 mm wide. Electrode fingers 411 through 41n extend from
the conducting area 40 and are each approximately 2 cm long
and 2.5 mm wide. The variable n represents the total
number of electrode fingers extending from the conducting
area 40. Gaps 431 through 43n 1 between the electrode
fingers are approximately .5 mm wide but in any case must
be no narrower than the wavelength of the light or electro-
magnetic radiation 55 to which it is desired that the
switch respond. The variable n in this case is 267.
Figure 1 will be used to describe the operation
of the switch of the present invention. The emitter elec-
trode 24 is coupled to a terminal 30 of a resistor 31 by an
electrical conductor 32. A terminal 33 of the resistor 31
is coupled to an electrical conductor 34 to a negative
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terminal 37 of the power supply 36 is coupled by an elec-
trical conductor 38 to the collector electrode 22. The
power supply 36 causes the p-n junction 18 to become re-
verse or back biased and a depletion region is formed which
extends partially into the p-region 14, throughout the n
region 15, and partially into the n+ region 16. An elec-
tric field E is created in the depletion region the magni-
tude distribution of which is shown in Figure 3 by the
curve portions 52, 53 and 54. In Figure 3, an axis 50 is a
measure of the magnitude of the electric field E and an
axis 51 is a locus of distance points through the regions
14, 15 and 16. The magnitude of the electric field E rises
rapidly in the depleted portion of the p region as depicted
by the curve portion 52. This rapid rise in magnitude
A (steep slope) is ~u~ to the heavy doping concentration in
the P region 14. The magnitude of the electric field
decreases through the n region 15 as shown by a curve
portion 53. However, the slope of curve portion 53 is
determined by the doping concentration of the region 15 as
hereinbefore discussed. The low doping concentration of
the region lS provides for a shallow slope of the curve
portion 53. The magnitude of the electric field decreases
rapidly (steep negative slope) in the depleted portion of
the n+ region 16 as shown by a curve portion 54 due to the
heavy doping concentration therein. When no light is
impinging upon the p region 14, the switch is said to be in
the blocking state and no current flows in the external
circuit through the resistor 31 since the semiconductor
body 10 is depleted of free electrons.
When light from a source denoted as 55 of suffi-
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ciently short wavelength or sufficient intensity impinges
upon the p region 14, preferably at the Brewster angle,
pl~ons
,q~ most of the photoc are created in the high electric field
region as shown in Figure 3, creating electron-hole pairs.
The phenomenon of photon or optical absorption is old in
the electrical arts. It has been determined that 11,500 A
is the maximum wavelength or energy of light at which
electron-hole pairs can be created in silicon (which has a
band gap of 1.1 eV) by photon absorption. The voltage
across the switch, i.e., the potential difference between
the conductors 32 and 38, decreases since free electrons
are being created thereby causing current to flow in the
resistor 31. The holes created will recombine at p-n
junction 18 while the electrons created (free electrons)
will be swept out through the n-type regions 15 and 16 and
flow around the circuit through the resistor 31 in order to
neutralize the hole current.
The magnitude of the electric field in the de-
pleted region decreases as a result of the decrease in
voltage across the switch hereinbefore discussed and is
shown in Figure 3 by the curve portions 56, 57 and 58. The
n region 15 is effective to ensure that though a low
voltage potential may exist across the switch, the slope of
the curve portion 57 is shallow so that a finite magnitude
of electric field is maintained in order to continue to
propel the free electron created by photon absorption
through the external circuit thereby maintaining current
flow in the external circuit.
When the light 55 is removed, the switch will
revert to the blocking state when the electrons and holes
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are swept out of the depletion region by the electric
field. There will be some residual current due to stored
charge in the non-depleted portions of the p region 14 and
the n region 16 which then diffuses into the depletion
region after the light 55 is removed. In order to minimize
this residual current, the p region 14 and the n region 16
should be as narrow as possible and of low lifetime.
Present technology can provide thicknesses as narrow as
25 ~m, but the regions 14 and 16 should be no thicker than
about 50 ~m in order to minimize the deleterious effects
of stored charge therein.
The power gain ~ of a switch according to the
present invention can be written as:
V
G = n Vs
where: n = the quantum efficiency
Vs = blocking (system) voltage
Vp = photon voltage
It will be appreciated by those skilled in the
art that the scope of the present invention is not limited
by the details of the foregoing description and that the
present invention may be carried out in various ways and
may take various forms and embodiments other than the
illustrative embodiment hereinbefore described. Figure 4,
for example, shows an embodiment of the present invention
si~ilar to that shown in Figure 1 and wherein like charac-
ters refer to similar elements. A switch according to the
present invention includes the semiconductor body 10 and a
collector electrode 122 having a grid structure for allow-
ing light or other electromagnetic energy to pass there-
9 46,931through and impinge upon the n+ region 16. An emitter
electrode 114 comprises a layer of metal having no holes or
grid for admitting light.
Other possible embodiments would include, for
example, a switch according to the present invention where-
in both the collector and the emitter electrodes include
means for allowing light to impinge upon the semiconductor
region upon which it is disposed. Regions of conductivity
type opposite that of regions 14, 15 and 16 as shown in
Figure 1 can be used. Such an embodiment is shown in
Figure 5 wherein like characters refer to similar elements.
In addition, the electrode 24 can be realized in any number
of various ways and forms so long as the desired wavelength
of electromagne~ic radiation can impinge upon the region 14
according to the teachings of the present invention. These
variations are not meant to be exhaustive nor are they
meant to limit the scope of the present invention in any
substantive way.
