Note: Descriptions are shown in the official language in which they were submitted.
~(`A 6~,4()4A
.
1091361
I
The present invention relates to semiconductor
devices and more particularly to photovoltaic devices
and current rectifying devices whose active region is of
an amorphous silicon fabricated by a glow discharge in
silane.
Photovoltaic devices such as solar cells
and photodetectors are capable of converting solar
radiation into usable electrical energy. A problem
``! encountered in the field of solar cells is that the cost
, !
of producing electrical energy from solar cells is often
not competitive with other means of electrical energy
generation. One of the largest expenses involved in
solar cell manufacture is the cost of the semiconductor
material of the solar cell's active region. Often a
solar cell will require a thick, single crystal, active
layer, i.e. about ~0 microns or more, to ensure sufficient
absorption of solar radiation. Naturally, the more semi-
conductor material needed the higher the cost of a solar
cell. The lowering of the amount of semiconductor
material needed for photodetector devices would also lower
~¦ the~r cost. If this same semiconductor material ~ -
demonstrates current rectification properties in the dark,
it could also be utilized as the active region of semi-
conductor devices such as diodes. Thus, it would be most
desirable in the semiconductor field to have a material
of a semiconductor device's active region which demonstrates
3 either photovoltaic or current rectification properties,
-2-
I
~GA 69,404A
1091361
1 and reduces the cost of solar cell, photodetector and
current rectification devices.
:'.''
" A semiconductor device havin~ a semiconductor
junction and an active region Q~ amorphous silicon
fabricated by glow discharge in silane.
. .
FIGURE l is a cross-sectional view of a first
embodiment of the semiconductor device of the present
, 10 invention.
FIGURE 2 is a graph comparing the absorption
-~ coefficient of single crystal silicon to glow discharge
amorphous silicon in the visible light range.
FIGURE 3 is a schematic view of an apparatus
. 15 for carrying out the fabrication of amorphous silicon
. by a glow discharge in silane.
,:, FIGURE 4 is a cross-sectional view of a
second embodiment of the semiconductor device of the
present invention.
FIGURE 5 is a cross-sectional view of a third
embodiment of the semiconductor device of the present
invention.
Referring to FIGURE l, a first embodiment
of a seniconductor device of the present invention is
designated as lO. For the purpose of describing the
present invention the first embodiment of the semiconductor
device lO is a photovoltaic devicej specifically a ~chottky
barrier solar cell. The photovoltaic device lO includes
- 30 a substrate 12 of a material having both good electrical
--3-
~ A ~'3,404A
1091361
I conductivity properties and the ability of making an
ohmic contact with amorphous silicon deposited from a
glow discharge. Typically, the substrate 12 will be of
a metal such as aluminum, antimony, stainless steel or
highly doped N-type single crystalline or polycrystalline
silicon. On a surface of the substrate 12 is an active
. region 14 of amorphous silicon. By active region it is
~ meant that portion of the device in which electron-hole
- pairs can be generated for collection as current from a
photovoltaic device.
An amorphous material is one which has no long
range order in the periodicity of the matrix. Amorphous
silicon fabricated by a glow discharge in silane, SiH4,
possesses a short range order of no more than 20A. The
amorphous silicon of the active region 14 is formed
by a glow discharge in silane, SiH4, and has the kinetic
characteristics of a carrier lifetime greater than about
10 7 seconds and an average density of localized states
in the energy gap on the order of 1017/cm3 or less,
and a mobility for electrons and holes greater than
10 3cm2/V-sec. The active layer 14 is about 1 to 3
microns in thickness or less.
; ~n a surface of the active region 14 opposite
the substrate 12 is a metallic region 16, with an inter-
face 18 therebetween. The metallic region 16 is semi-
transparent to solar radiation and is of a metallic material
with good electrical conductivity, such as gold, platinum,
palladium, or chromium. The metallic region 16 may be
a single layer of a metal or it may be mul~i-layered. If
the metallic region 16 is multi-layered a first layer could
-4-
.. . . . .
~CA 69,404A
1091361
. .
I be of platinum on the active region 14 to assure a large
. .
Schottky barrier height and a second layer on the
first platinum layer could be gold or silver, for good
electrical conductivity. Since the metallic region 16
i-- 5 is a metal such as gold, platinum,palladium or chromium,
it should only be about lOOA in thickness in order to be
`- semi-transparent to solar radiation.
~ n a surface of the metallic layer 16 opposite
the interface 18 is a grid electrode 24. Typically, the
,.~
grid electrode 24 is of a metal having good electrical
conductivity. The grid electrode for purposes of dis-
.; closing the present invention has two sets of grid lines,
.-:
with the grid lines of each set substantially parallel
.~ . .
. to each other and the grid lines of each set are inter-
' 15 secting those of the other set. For purposes of discussion,
the grid lines are intersecting perpendicularly. The
grid electrode 24 occupies only a small area on the
surface of the metallic layer 16 since solar radiation
impinging the grid electrode 24 may be reflected away
from the active region 14. The function of the grid
', electrode 24 is for the uniform collection of current
from the metallic layer 16. The grid electrode 24 also
assures a low series resistance from the device 10 when
. .
in operation as part of a circuit. However, it is
anticipated that only a ~ingle set of grid lines may be
necessary for uniform current collection.
An antireflection layer 20 is on the grid
electrode 24 and on the surface of the metallic layer
16 opposite the interface 18 not occupied by the grid
electrode 24. The antireflection layer 20 has an incident
"
RCA 69,404A
' ``
109~361
.
I surface 22 on which impinges solar radiation 26. As
is well known in the art, there is an increase in
the solar radiation 26 traversing the metallic layer
16 and entering the active region 14, by having the
antireflection layer 20 of a thickness approximately equal
to A/4n, where ~ is the wavelength of the radiation
impinging the incident surface 22, and n is the index
of refraction of the antireflection layer 20 having an
appropriate value to increase the amount of solar
radiation 26 impinging the metallic layer 16. In essence
- the antireflection layer 20 reduces the amount of light
that would be reflected from the device 10. Usually, -;
the antireflection layer 20 will be of a dielectric
~ material such as zinc sulfide.
In the field of semiconductor devices it is
- well known that a surface barrier junction, generally
known as a Schottky barrier, is formed as a result of the
contacting of certain metals to certain semiconductor
materials. In the present invention a Schottky barrier
~ 20 is ~ormed at the interface 18 by contacting the metallic
; region 16 to the active region 14. A Schottky barrier
generates a space charge field in the semiconductor
material which penetrates into the active region 14 from
the interface 18 and is referred to as the depletion
region. It is preferable in the photovoltaic device 10
of the present invention that the depletion region extend
the entire width of active region 14 between the interface
18 and the substrate 12. With the depletion region
extending the entire width of the active region 14,
carriers created anywhere in the active region 14, as a
. ' . ' :
.
RCA 69,4û4A
.: ``
109~361
, I result of the absorption of solar radiation 26, are
swept by the electric field in the depletion region to
either the substrate 12 or the metallic region 16. The
substrate 12 functions as one of the electrodes to the
` 5 active region 14. If the depletion region did not
extend into a portion of the active region 14, any carriers
~, generated in this non-depleted portion of the active
region 14 would not be swept to an electrode by means
of an electric field. Carriers generated in a non-
depleted portion of the active region 14 must rely on
diffusion to either an electrode or the depleted region
in order to be collected. Also, any non-depleted
region would contribute to the series resistance when
drawing current from the device, and this series
resistance would lower device efficiency.
The amorphous silicon of the active region
14, fabricated by a glow discharge in silane, possesses
characteristics ideally suited for the active region of
a photovoltaic device. Carrier lifetime in amorphous
silicon fabricated by a glow discharge in silane is
greater than about 10 7 seconds, while carrier lifetime
in amorphous silicon formed by sputtering or evaporation
is in the order of 10 11 seconds. Since the mobility of
electrons and holes in glow discharge amorphous silicon
is greater than 10 3cm2/~-sec., large current collection
efficiencies can be obtained.
The optical absorption of glow discharge
amorphous silicon is superior to that of single crystalline
silicon over the visible light range, i.e., 4,000A to
- 30 O
7,000A. Referring to FIGURE 2, it is shown that the
-7-
RCA 69,4r~4A
_
109136:1
l amorphous silicon has a larger absorption coefficient over
the visible range than single crystalline silicon.
This means that an active region 14 of glow discharge
` amorphous silicon can be a factor of 10 thinner than
single crystal silicon and provide comparable light
absorption in the visible range. This is the reason
for the active region 14 being as thin or thinner than
one micron, and provide good device efficiency.
Furthermore, the average density of localized
states in the energy gap of glow discharge amorphous
silicon is on the order of 1017/cm3 or less. The
average density of localized states of glow discharge
amorphous silicon decreases with increasing deposition
temperatures and increasing purity of the silane in the
fabrication of the amorphous silicon. This average
density of localized states of the glow discharge amorphous
-I silicon is much lower than that of amorphous silicon
fabricated by other means, i.e., for sputtered or evaporated
amorphous silicon the average density of localized states
is 1019/cm3eV or greater. Significant about the average
density of localized states in the energy gap is that it
is inversely proportional to the square of the width
of the depletion region. Since glow discharge amorphous
silicon's density of states is relatively low a depletion
width on the order of one micron can be obtained. Also,
significant about the average density of localized
; states is the fact that carrier lifetime is inversely
proportional to the average density of states. This
point reaffirms that the carrier lifetime of glow dis-
charge amorphous silicon is larger than that of amorphous
~ RCA 69,404A
,
1091361
'
;: 1 silicon fabricatcd by the other processes mentioned.
Referring to FIGURE 3, a glow discharge
apparatus suitable for carrying out the fabrication of the
photovoltaic device 10 of the present invention is
generally designated as 30. The glow discharge apparatus
- 30 includes a vacuum chamber 32 defined by a vacuum
bell 34, typically of a glass material. In the vacuum
. chamber 32 is an anode 36, and a heating plate 38
, spaced from and opposite the anode 36. The anode 36,
is of a metallic material having good electrical
~ conductivity such as platinum and is in the form of a
:~ screen or coil. The heating plate 38 is a ceramic
frame which encloses heating coils which are energized
from a current source 40, external to the vacuum chamber
32.
A first outlet 44 into the vacuum chamber 32
is connected to a diffusion pump, a second outlet 46
is connected to a mechanical pump, and a third outlet
48 is connected to a gas bleed in system which is the
source of the various gases utilized in the glow discharge
process. While the second outlet 46 is described as
being connected to a diffusion pump, it is anticipated
that a diffusion pump mav not be necessary for evacuatin~
: the system.
~ In the fabrication of the photovoltaic device
-~ 25
10, the substrate 12, e.g., aluminum, is placed on the
heating plate 38 and is connected to the negative
terminal of a power source 42. Anode 36 is connected
to the positive termlnal of the power source 42. The
power source 42 may be DC or AC. Thus, there will-
g
.
.
~.
R~A 69,404A
.
` 1091361
l be a voltage potential between the anode 36 and the
substrate 12, which is essentially functioning as a
. .
cathode for DC operation, when the power source 42 is
energized.
The vacuum chamber 32 is then evacuated to
a pressure of about 0.5 to 1.0 x 10 6 torr, and the
substrate 12 is heated to a temperature in the range
of 150 to 400C. by energizing the heating coil of
the heating plate 38.
Next, silane, SiH4, is bled into the vacuum
chamber 32 to a pressure of 0.1 to 3.0 torr and as
a result, the substrate temperature is raised to a value
in the range of 200C. to 500C. To assure an ohmic
contact between the substrate 12 and active region 14,
the active region 14 should be deposited on substrate
12 at a temperature greater than 350C. so as to assure
the forming of a eutectic between the aluminum substrate
12 and amorphous silicon active region 14.
To initiate the glow discharge between the anode
20 36 and the substrate 12, resulting in the deposition of --
`~ the amorphous silicon of active region 14 onto a surface
of the substrate 12, the power source 42 is energized.
For deposition of the active region 14 the voltage
potential between the anode 36 and substrate 12 should be ~-
in the range of 0.3 to 3.0 ma/cm2 at the surface of the
substrate 12. The deposition rate of the amorphous
silicon increases with the vapor pressure of the silane
and the current density. Under the conditions described
deposition of one micron of amorphous silicon occurs in
less than 5 minutes.
- 1 0 -
.
` RC~ 69,404A
., . ~
``` 1091361
. `
I Once the glow discharge is initiated electrons
from the substrate 12 are emitted from the substrate
and strike silane molecules, SiH4, both ionizing and
disassociating the molecules. The silicon ions and
silicon hydrides, such as SiH , are of a positive charge
, and are thus attracted to the substrate 12, which is the
~`` cathode, and silicon is thereby deposited on the substrate
..
12. The substrate temperature is greater than 350C.
;.
and promotes pyrolytic decomposition of deposited silicon
: lO hydrides.
After deposition of the amorphous silicon, the
wafer of substrate 12 and active region 14 is placed in
~ a state of the art evaporation system and the metallic
- region 16 is evaporated onto the active region 14. Like-
15 wise, the grid electrode 24 and antireflection layer 20 ~-
are deposited on the metallic region 16 by state of the
~` art evaporation and masking techniques. The entire
processing may be accomplished in a single system accommodat-
ing both glow discharge and evaporation.
Fabrication of the photovoltaic device 10 is
completed by the connecting of wire electrodes (not shown)
to the substrate 12 and grid electrode 24 for connection
to external circuitry.
Referring to FIGURE 4, a second embodiment of
the semiconductor device of the present invention is
designated as 110. For the purpose of describing the
' present invention, the semiconductor device 110 is a
photovoltaic device and more particularly a PIN solar cell.
The photovoltaic device 110 includes an active region 114
of amorphous silicon fabricated by a glow discharge in
- 11 -
':
P~A 69,404A
`` 1091361
;
1 silane, SiH4. Active layer 114 includes a first doped
layer 113~ a second doped layer 115 spaced from and
opposite the first doped layer 113, and an intrinsic
layer 117 in contact with and between the first and
second doped layers 113 and 115. The intrinsic layer
117 is undoped. The first and second doped layers 113
and 115 are of opposite conductivity type. For purposes
of discussion the second doped layer 115 is of N-type
conductivity while the first doped layer 113 is of P-type
conductivity. Both the first and second layers 113
and 115 are of a high doping concentration, i.e., greater
than lnl9/cm3 of electrically active dopants. Typically
the N-type second doped layer 115 is doped with phosphorous
- and the P-type first doped layer 113 is doped with boron.
A solar radiation transmissive electrode 128
is on a surface of the first doped layer 113 opposite the
second doped layer 115. The transmissive electrode 128
has an incident surface 129 opposite the first doped
layer 113. The function of transmissive electrode 128
is to be either transparent or semi-transparent to solar
radia~ion and be able to collect current generated in the
active region 114. Solar radiation 126 enters the device ~ -
110 at the incident surface 129. The solar radiation
transmissive electrode 128 may be a single layer of a
material such as indium tin oxide or tin oxide which are
both transparent to solar radiation and have good electrical
- conductivity. Also the transmissive electrode 128 can be
a thin film metal, i.e., about lO~A in thickness, such
as gold, antimony, or platinum, which will be semi-transparent
to solar radiation. If the transmissive electrode 128 is of
-12-
: ,
.
~CA 69,404A
.
~ 1091361
:~"
` ,. .
~- l a thin film metal it is preferable that an antireflection
;~ layer as described in the first embodiment, be on the
` incident surface 129 of the electrode 128 to decrease
reflection of the solar radiation 126. Furthermore,
- 5 the electrode 128 may be multi-layered such as a layer
of indium tin oxide commercially available on a layer
of a glass material. In such an instance the indium
- tin oxide is in intimate contact with the first doped
layer 113.
If the surface resistivity of the electrode
128 at the first doped layer 113 is on the order of
'r` about lOQ/~ or more it is preferable to also have a grid
;',.'.
contact like that of the first embodiment of the present
invention on the first doped layer 115 for collection
-~ 15 of the current generated in the active region 114.
An electrical contact 127 is on a surface
l of the second doped layer 115 opposite the tranmissive
:,. j
~ electrode 128. The electrical contact 127 is of a
.
material having reasonable electrical conductivity, such
~;';; 20 as aluminum, chromium, or antimony.
As previously recited in the discussion of
~`~ the first embodiment of the present invention, the
; absorption coefficient of glow discharge amorphous silicon
- is better than that of single crystal silicon in the
~ l 25 visible range. For this reason only a thin layer of
., I
amorphous silicon is needed for sufficient solar radiation
absorption. Typically, the intrinsic region of amorphous
silicon is about one to three microns or less in thickness,
while the first and second doped layers 113 and 115 are
each a few hundred angstroms in thickness.
-13-
:-
R~A 69,404A
` 1091361
I Well known to those in the PIN solar cell
art is that as a result of the equalization in Fermi
levels between layers 113, 115 and 117 there is a
negative space charge in the irst doped layer 113
and a positive space charge in the second doped layer115, and the formation of a depletion region in the
intrinsic layer 117. How far the electric field of the
depletion region extends into the intrinsic layer 117
is a function of the average density of localized
10 states in the energy gap, as explained in the discussion --
of the first embodiment of the present invention. Also,
from the earlier discussion of semiconductor device 10,
it is forseen that the depletion region will extend
across the entire thickness of the intrinsic layer 117,
i.e., about one to three microns or less in thickness.
Therefore, any carriers generated into the intrinsic
layer 117 by the absorption of solar radiation, will
be swept up in the electric field of the depletion region
and be collected as an electrical current.
In the fabrication of the photovoltaic device
- 110 the transmissive electrode 128 is assumed to be a
layer of indium tin oxide commercially available on a
layer of glass material. The electrode 128 is placed
on the heating plate 38 of the apparatus 30 shown in
FIGURE 3. The glass layer of electrode 128 is in intimate ~-
contact with the heating plate 38.
The apparatus 30 is then prepared for deposition
of the first doped layer 113, of P-type conductivity
- onto the indium tin oxide layer of electrode 128. The
vacuum chamber 32 is evacuated to a pressure of about
. ,
' ' .
P~CA 69,404A
-
1091361
` I 10 6 torrs and then silane with about one-half to five
percent diborane, i.e., the diborane constitutes one-
half to five percent of the silane-diborane atmosphere,
at a pressure of 0.1 to 1.0 torrs is bled into the
vacuum chamber 32, while the electrode 128 is brought
to a temperature in the range of 200 to 500C.
A glow discharge is initiated in the vacuum
chamber 32 for about one to two seconds with a current
density of about 0.5 ma/cm2 at the electrode 128 for
deposition of the first doped layer 113, on the order of
a few hundred angstroms in thickness.
The atmosphere in the vacuum chamber 32 is then
pumped out by the mechanical pump 46.
With the vacuum chamber 32 at a pressure of
,~ 15 10 6 torrs, silane is bled into the vacuum chamber 32 at
a pressure of 0.1 to 3 torrs. Again a glow discharge is
initiated for 1 to 5 minutes with a current density of
from 0.3 ma/cm2 to 3.0 ma/cm2 at the first doped layer
113 for the deposition of the intrinsic layer 117
of about one micron in thickness.
Next, about 0.1 to 1.0 percent phosphine, as
the doping gas, is bled into the vacuum chamber 32, so
that the phosphine constitutes 0.1 to 1.0 percent of the
silane-phosphine atmosphere. A glow discharge is initiated
with a current density of from 0.3 ma/cm2 to 3.0 ma/cm2
at the intrinsic layer 117 and the N-type second doped
layer 115 on the order of a few hundred angstroms thick
is deposited on a surface of the intrinsic layer 117.
While phosphine and diborane were mentioned
as the doping gases for the first and second doped layers
-15-
RCA 69J4n4A
.
1091361
1 113 and 115, it is anticipated that other appropriate dop-
- ing gases well known in the art can also be used. ~-
The electrical contact 127 is then deposited on
a surface of the second doped layer 115 by state of the art
evaporation techniques. Final fabrication of the photovoltaic
device 110 includes connecting contacting wires (not shown)
: to the contact 127 and electrode 128 for electrical connec-
` tion to external circuitry.
Referring to FIGURE 5, a third embodiment of the -
` lO semiconductor device of the present invention is designated ~-
- as 210. Again the semiconductor device 210 is a photovoltaic
device and more particularly a P-N iunction solar cell. The
photovoltaic device 210 includes a body 211 of amorphous
silicon fabricated by a glow discharge in silane, SiH4,
, 15 with the appropriate doping gases. The body 211 comprises
;l a first doped layer 252 of one conductivity type in contact
with a second doped layer 254 of an opposite conductivity
type with a P-N junction 256 therebetween. For purposes of
discussion it is assumed the first doped layer 252 is of
P-type conductivity and the second doped layer 254 is of
N-type conductivity. Both the first and second doped
layers 252 and 254 are the active region 214 of the photo-
voltaic device 210. The body 211 includes a third doped
layer 258 on a surface of the second doped layer 254 opposite
the P-N junction 256. The third doped layer 258 is of the
- same conductivity type as the second doped layer 254 but has
a higher doping concentration than the second doped layer
~; 254. Thus, the third doped layer 258 is of N+ type
conductivity. The third doped layer 258 assists in making
ohmic contact to the active region 214.
-16-
.
.. .
. .
RCA 69,404A
1091361
I ~n a surface of the third doped layer 258
;` opposite the P-N junction 256 there is an electrical
contact 227 the same as the electrical contact 127 of
the second embodiment of the present invention. A solar
radiation transmissive electrode 228 having a solar
- radiati~n incident surface 229 is on a surface of the
first doped layer 252 opposite the P-N junction 256.
Solar radiation 226 enters the device 210 at the incident
surface 229. The solar radiation transmissive electrode
228 is the same as the solar radiation transmissive
l electrode 128 of the second embodiment of the present
.. ,' lnvention.
In the operation of the photovoltaic device
210 solar radiation 226 enters the device 210 at the
incident surface 229 and some of the solar radiation
; 226 is absorbed in the active region 214 forming electron-
hole pairs. These carriers then di~use to the R-N
junction 256 and i~ the~ arr~ve at t~e space charge f~eld
of the P-N junction 256 before recombining they are
collected and contribute to the current of the device
210.
In the fabrication of the device 210, as
in device 110, the transmissive electrode 228 is assumed
as being a layer of indium tin oxide on a layer of glass
material. The electrode 228 is placed on the heating
plate 38 of apparatus 30 so that the glass layer is
in intimate contact with the heating plate 38.
Next, the apparatus is prepared for the
deposition of the first doped layer 252 onto the indium
-17-
'
~ A 6~,404A
1091361
`'
1 tin oxide layer of the transmissive electrode 228.
` The vacuum chamber is evacuated to a pressure of about
10 6 torrs and then silane with about 1 to 5 percent
diborane at a pressure of 0.1 to 1.0 torrs is bled
into chamber 32, while the electrode 228 is brought to
a temperature in the range of 200C. to 500C.
A glow discharge is initiated in the vacuum
.. ~ - .
- chamber 32 for about one to two seconds with a current
density of about 0.5 ma/cm2 at the surface of electrode
' 10 228 for deposition of the first doped layer 252 on the
order of a few hundred angstroms in thickness.
The atmosphere in the vacuum chamber 32 is
;~ then pumped out by the mechanical pump 46. The chamber
32 is brought to a pressure of about 10 6 torrs and
silane with about 0.01 percent phosphine is then bled
into the chamber 32 at a pressure of 0.1 to 3 torrs. The
glow discharge is initiated for about 1 to 30 minutes
with a current density of from 0.3 ma/cm2 to 3.0 ma/cm2
at the surface of the first doped layer 252, thus the
second doped layer 254 is deposited of a thickness in
the range of 1 to 20 microns.
Next, phosphine is bled into the vacuum
chamber 32 so that there is a 0.5 percent mixture of
- phosphine with the silane. Again a glow discharge is
- 25 initated with a current density of from 0.3 ma/cm to
3.0 ma/cm2 at the second doped layer 254 for the
deposition of the third doped layer 258 a thickness of a
few hundred angstroms.
` The electrical contact 227 is deposited on
the third doped layer 258 by state of the art evaporation
-18-
' ~`
R(`A 69,404A
` ~09136~
:,.
l techniques. The fabrication of device 210 is completed
: by connecting contacting wires (not shown) to contact
227 and electrode 228.
In the photovoltaic operation of the first,
second and third embodiments of the present invention,
the substrate 12 and electrical contacts 127 and 227
may reflect unabsorbed solar radiation back into the
active regions 14, 114 and 214 respectively, thereby
improving the possibility for solar radiation absorption.
It should be mentioned tha~ in the first
embodiment of the present invention, the substrate 12
was described as a support for the device while in
the second and third embodiments the light transmissive
electrodes 128 and 228 are supports for their respective
device
Although the three embodiments of the semi-
conductor device of the present invention have been
described as solar cells, it is anticipated by the present
invention that these three embodiments can also be
utilized as high frequency photodetectors, i.e. devices
; which respond to radiant energy. It has been discovered
that these photodetectors having an active region of
amorphous silicon prepared by a glow dishcarge in silane
have a high frequency response, on the order of ln
megahertz or more. In utilizing the first three embodiments
of the present invention as a photodetector it is
known by those in the semiconductor art that the amount
of radiant energy entering the active region may not be
as critical as if the three embodiments were utilized
as solar cells. Therefore, modifications well known to
~ I - 19-
~ A 69,404A
1091361
:. :
1 those in the art can be made to the first three embodiments
, of the present invention if they are used as photodetectors,
e.g. removal of antireflection layers and grid electrodes.
The second embodiment of the present invention,
semiconductor device 110, is a PIN structure, and if
~; utilized as a photodetector its spectral response can be ~ -
tailored to the relative sensitivity of the human eye.
Tailoring the spectral response of semiconductor device
110 is accomplished through the thickness and dopant
~, 10 concentration of the layer which is of P type conductivity,
i.e. either first doped layer 113 or second doped layer
115, and by the thickness of the intrinsic layer 117.
As an example, the spectral response of device 110
approximates that of the human eye if the P type layer
has an acceptor dopant concentration on the order of 5
.: O
atomic percent boron and is of a thickness of about 500A,
while the intrinsic region is approximately 0.3 micro-
meters in thickness.
The utilization of glow discharge amorphous -
silicon in the active region of photovoltaic and
photodetector devices provides a device with a thinner
active region than devices of the same basic structure
but of single cr~stalline silicon. Also devices utilizing
glow discharge amorphous silicon are capable of solar
radiation absorption comparable to that of single crystal
silicon photovoltaic and photodetector devices having
active regions of a factor of 10 times thicker. Thus,
the specific advantage of the present invention as a
, - photovoltaic or photodetector device is the cost reduction realized by the utilization of a thinner active region.
-20-
RC~ ~9,404A
109136~
~`.
`` 1 Moreover, the present invention as a photovoltaic device
also provides a cost reduction in generation of electrical
; power from solar radiation because there is less energy
expended in making devices of the present invention since
fabrication is at temperatures lower than single crystal
device fabrication; and larger area solar cells can be
fabricated as compared to single crystalline solar cell
fabrication.
It has also been discovered that the semi-
conductor device of the present invention having an active
region of amorphous silicon fabricated by a glow dis-
charge in silane, is capable of current rectification in
- the dark. As an example the Schottky barrier semiconductor
- device 10 of FIGURE 1, with a substrate 12 of N type
single crystalline silicon, a metallic region 16 of gold
and without the grid electrode 24 and antireflection
: layer 20, demonstrates current rectification at +.4 volts,
i.e. forward biased, on the order of 104 times greater
. .
than the current at -.4 volts, i.e. reverse biased. While
the three embodiments of the present invention are
described as solar cells they can function as described
as current rectifiers but it would bé obvious to those
skilled in the semiconductor art that their utility
as rectifiers would be more desirable with some minor
modification, such as the removal of grid electrodes
and antireflection layers. The semiconductor devices
of the present invention possess a potential barrier as
a result of having a semiconductor junction, i.e.,
either a P-N junction, a PIN junction or a Schottky
barrier junction-
-21-
.
RCA 69,404A
1091361
..
1 In the semiconductor device of the present
invention, the active region is of amorphous silicon
' fabricated by a glow discharge in silane and these
devices function either as solar cells, photodetectors
or current rectifiers.
:
: ~
' ~
,, .
-22-
` . . ~ . :
