U.S. patent application number 11/282730 was filed with the patent office on 2006-09-14 for semiconductor photodetector device.
This patent application is currently assigned to Mitsubishi Denki Kabushiki Kaisha. Invention is credited to Eitaro Ishimura.
Application Number | 20060202297 11/282730 |
Document ID | / |
Family ID | 36969948 |
Filed Date | 2006-09-14 |
United States Patent
Application |
20060202297 |
Kind Code |
A1 |
Ishimura; Eitaro |
September 14, 2006 |
Semiconductor photodetector device
Abstract
A transmitted light absorption/recombination layer, a barrier
layer, a wavelength selection/absorption layer, and an InP window
layer having a p-type region are supported by an n-type substrate
and arranged in that order. Light with a wavelength of 1.3 .mu.m
reaches the wavelength selection/absorption layer through the InP
window layer. Then, the light is absorbed by the wavelength
selection/absorption layer and drawn from the device as an electric
current signal. Light with a wavelength of 1.55 .mu.m reaches the
transmitted light absorption/recombination layer through the
barrier layer. Then, the light is absorbed by the transmitted light
absorption/recombination layer, generating electrons and holes.
These electrons and holes recombine with each other and hence
disappear.
Inventors: |
Ishimura; Eitaro; (Tokyo,
JP) |
Correspondence
Address: |
LEYDIG VOIT & MAYER, LTD
700 THIRTEENTH ST. NW
SUITE 300
WASHINGTON
DC
20005-3960
US
|
Assignee: |
Mitsubishi Denki Kabushiki
Kaisha
Tokyo
JP
|
Family ID: |
36969948 |
Appl. No.: |
11/282730 |
Filed: |
November 21, 2005 |
Current U.S.
Class: |
257/436 ;
257/E31.054; 257/E31.064; 257/E31.12; 257/E31.128 |
Current CPC
Class: |
H01L 31/101 20130101;
H01L 31/1075 20130101; H01L 31/02161 20130101; H01L 31/02327
20130101 |
Class at
Publication: |
257/436 |
International
Class: |
H01L 31/0232 20060101
H01L031/0232 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 14, 2005 |
JP |
2005-070646 |
Claims
1. A semiconductor photodetector device comprising: a first
semiconductor layer of a first conductivity type; a first
absorption layer; a second semiconductor layer having a bandgap
larger than the bandgap of said first absorption layer; and a
second absorption layer between said first semiconductor layer and
said first absorption layer, said second absorption layer having a
bandgap smaller than the bandgap of said first absorption layer,
wherein said first semiconductor layer, said first absorption
layer, and said second semiconductor layer are arranged from bottom
to top, in that order in a laminated structural said semiconductor
photodetector device detects light incident on a second
semiconductor layer side, and said second semiconductor layer
includes a doped region of a second conductivity type.
2. The semiconductor photodetector device according to claim 1,
further comprising a barrier layer between said first absorption
layer and said second absorption layer, said barrier layer having a
larger bandgap than said second absorption layer.
3. The semiconductor photodetector device according to claim 1,
further comprising a multilayer reflective layer on said second
semiconductor layer, reflecting light at wavelengths longer than
the bandgap wavelength of said first absorption layer.
4. The semiconductor photodetector device according to claim 1,
wherein said second semiconductor layer further includes a
multilayer reflective layer reflecting light at wavelengths longer
than the bandgap wavelength of said first absorption layer.
5. The semiconductor photodetector device according to claim 1,
further comprising a multiplication layer between said first
absorption layer and said barrier layer, said multiplication layer
including an AlInAs layer.
6. A semiconductor photodetector device comprising: a first
semiconductor layer of a first conductivity type; a first
absorption layer; a second semiconductor layer having a bandgap
larger than the bandgap of said first absorption layer; and a
second absorption layer between said second semiconductor layer and
said first absorption layer or on said second semiconductor layer,
said second absorption layer having a bandgap smaller than the
bandgap of said first absorption layer, wherein said first
semiconductor layer, said first absorption layer, said second
absorption layer, and said second semiconductor layer are arranged
from bottom to top, in that order, in a laminated structure, said
semiconductor photodetector device detects light incident on a
first semiconductor layer side, and said second semiconductor layer
includes a doped region of a second conductivity type.
7. The semiconductor photodetector device according to claim 6,
further comprising a multiplication layer between said first
absorption layer and said first semiconductor layer, said
multiplication layer including an AlInAs layer.
8. The semiconductor photodetector device according to claim 6,
further comprising a multilayer reflective layer between said first
semiconductor layer and said first absorption layer, reflecting
light at wavelengths longer than the bandgap wavelength of said
first absorption layer.
9. The semiconductor photodetector device according to claim 8,
further comprising a multiplication layer between said first
absorption layer and said multilayer reflective layer, said
multiplication layer including an AlInAs layer.
10. A semiconductor photodetector device comprising: a first
semiconductor layer of a first conductivity type; a first
absorption layer; a second semiconductor layer having a bandgap
larger than the bandgap of said first absorption layer; a second
absorption layer on said second semiconductor layer, said second
absorption layer having a bandgap larger than the bandgap of said
first absorption layer but smaller than the bandgap of said second
semiconductor layer; and a third absorption layer between said
first semiconductor layer and said first absorption layer, said
third absorption layer having a bandgap smaller than the bandgap of
the bandgap of said first absorption layer, wherein said first
semiconductor layer, said third absorption layer, said first
absorption layer, said second semiconductor layer, and said second
semiconductor absorption layer are arranged from bottom to top, in
that order, in a laminated structure, said semiconductor
photodetector device detects light incident on a second
semiconductor layer side, and said second semiconductor layer
includes a doped region of a second conductivity type.
11. The semiconductor photodetector device according to claim 10,
further comprising a barrier layer between said first absorption
layer and said third absorption layer, said barrier layer having a
larger bandgap than said third absorption layer.
12. The semiconductor photodetector device according to claim 10,
further comprising a multilayer reflective layer between said first
absorption layer and said second semiconductor layer or between
said second semiconductor layer and said second absorption layer or
on said second absorption layer, reflecting light at wavelengths
longer than the bandgap wavelength of said first absorption
layer.
13. The semiconductor photodetector device according to claim 10,
further comprising a multiplication layer between said first
absorption layer and said barrier layer, said multiplication layer
including an AlInAs layer.
14. A semiconductor photodetector device comprising: a first
semiconductor layer of a first conductivity type; a first
absorption layer; a second semiconductor layer having a bandgap
larger than the bandgap of said first absorption layer; a second
absorption layer on said second semiconductor layer, said second
absorption layer having a bandgap smaller than bandgap of said
first absorption layer; and a third absorption layer between said
first semiconductor layer and said first absorption layer, said
third absorption layer having a bandgap larger than the bandgap of
said first absorption layer, wherein said first semiconductor
layer, said third absorption layer, said first absorption layer,
said second semiconductor layer, and said second absorption layer
are arranged from bottom to top, in that order, in a laminated
structure, and said semiconductor photodetector device detects
light incident on a first semiconductor layer side, said second
semiconductor layer includes a doped region of a second
conductivity type.
15. The semiconductor photodetector device according to claim 14,
further comprising a barrier layer between said first absorption
layer and said third absorption layer, said barrier layer having a
larger bandgap than said third absorption layer.
16. The semiconductor photodetector device according to claim 14,
further comprising a multiplication layer between said first
absorption layer and said barrier layer, said multiplication layer
including an AlInAs layer.
17. The semiconductor photodetector device according to claim 14,
further comprising a multilayer reflective layer between said first
semiconductor layer and said third absorption layer and/or between
said third absorption layer and said barrier layer and/or between
said barrier layer and said first absorption layer, reflecting
light at wavelengths longer than the bandgap wavelength of said
first absorption layer.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention-relates to semiconductor photodetector
devices such as photodiodes and avalanche photodiodes.
[0003] 2. Background Art
[0004] In a multiple-wavelength optical communications system, the
optical receivers must have a function to selectively receive light
having a desired wavelength.
[0005] FIG. 11 is a diagram showing the configuration of a
conventional optical receiver. In the figure, two wavelengths of
light, 1.3 .mu.m and 1.55 .mu.m, are incident on the optical
receiver, but the avalanche photodiode 161 receives only the 1.3
.mu.m wavelength light. Specifically, a wavelength filter 163 for
reflecting 1.55 .mu.m wavelength light 162 is provided in front of
the avalanche photodiode 161, acting as a photodetector device, to
selectively receive 1.3 .mu.m wavelength light 164.
[0006] FIG. 12 is a cross-sectional view of a conventional
avalanche photodiode (hereinafter referred to as a "conventional
APD") for optical communications. Referring to the figure,
reference numeral 171 denotes an anode electrode; 172 denotes a
p-type diffusion layer region; 173, a nonreflective film; 174, an
undoped InP window layer; 175, an n-type InP-electric field
reduction layer; 176, an undoped InGaAsP graded layer; 177, an
undoped InGaAs light absorption layer; 178, an n-type InP
substrate; 179, a cathode electrode; 180, an anode electrode; 181,
a multiplication region; and 182, a guard ring region.
[0007] The nonreflective film 173 and the InP window layer 174 also
act as a surface protective film and a multiplication layer,
respectively. It should be noted that the InP window layer 174 has
a large bandgap and hence does not absorb the wavelengths used in
typical optical communications; such as 1.3 .mu.m and 1.55 .mu.m,
allowing these wavelengths to pass without change. The guard ring
region 181 is provided to prevent edge multiplication and is a
p-type region having a low carrier concentration.
[0008] Light entering the nonreflective film 173, as shown at the
top of the FIG. 12, is passed through the InP window layer 174 and
then absorbed by the InGaAs light absorption layer 177, generating
electrons and holes. It should be noted that the avalanche
photodiode (APD) is reverse-biased with a high voltage
(approximately 25 V), which depletes the InGaAs light absorption
layer 177, the InGaAsP graded layer 176, the n-type InP electric
field reduction layer 175, and the multiplication region 181.
Therefore, the generated electrons flow toward the n-type InP
substrate 178 through the depleted layers. On the other hand, the
holes flow toward the multiplication region 181 having a high
electric field applied thereto. The holes that have reached the
multiplication region 181 causes avalanche multiplication,
generating a large number of new electrons and holes. As a result,
the light signal that has entered the APD is drawn from it as a
multiplied electric current signal. The magnitude of the obtained
electric current signal is ten-odd times larger than when no
multiplication occurs.
[0009] Further, there is a conventional semiconductor photodetector
device which, upon reception of two different wavelengths of light,
photoelectrically converts only the longer wavelength light and
outputs the resultant signal (see, e.g., Japanese Patent Laid-Open
No. 2000-77702). That is, this semiconductor photodetector device
has sensitivity to only the longer wavelength light.
[0010] FIG. 13 is a cross-sectional view of this semiconductor
photodetector device. Referring to the figure, reference numeral
191 denotes an n.sup.--type InGaAs second absorption layer; 192, an
n-type InP buffer layer; 193, an InGaAsP first absorption layer;
193a, a p-type InGaAsP region; 193b, an n.sup.--type InGaAsP
region; 194, an n-type InP substrate; 195, an antireflective film;
196, a p-type diffusion layer region; and 197, a nonreflective
film.
[0011] The following description assumes that 1.3 .mu.m wavelength
light and 1.55 .mu.m wavelength light are incident on the
photodetector device shown in FIG. 13. In the photodetector device,
the 1.55 .mu.m wavelength light, whose wavelength is longer than
the bandgap wavelength of the InGaAsP first absorption layer 193,
reaches the n.sup.--type InGaAsP second absorption layer 191 and
then is drawn from the device as a photocurrent. On the other hand,
the 1.3 .mu.m wavelength light, whose wavelength is shorter than
the bandgap wavelength of the InGaAsP first absorption layer 193,
is absorbed by the InGaAsP first absorption layer 193. In this
case, since no electric field is applied to the InGaAsP first
absorption layer 193, the generated carriers recombine with each
other. Therefore, this shorter wavelength light is not drawn from
the device as a photocurrent.
[0012] Further, there is a conventional technique in which a
reflective film for reflecting the shorter wavelength light is
formed to receive only the longer wavelength light and convert it
into a photocurrent (see, e.g., Japanese Patent Laid-Open No.
2002-33503). This photodetector device also has sensitivity to only
the longer wavelength light.
[0013] Incidentally, recent multiple-wavelength optical
communications systems require optical receivers having a very high
wavelength selectivity ratio as much as 1000:1, or 30 dB, for 1.3
.mu.m and 1.55 .mu.m wavelengths. This means that these optical
receivers must have high sensitivity to 1.3 .mu.m wavelength light
but substantially no sensitivity to 1.55 .mu.m wavelength
light.
[0014] However, to achieve such a high selectivity ratio,
conventional APDs must be provided with a wavelength filter, as
described below.
[0015] Referring to FIG. 12, the bandgap wavelength of the InGaAs
light absorption layer 177 is 1.67 .mu.m, and that of the InP
window layer 174 is 0.92 .mu.m. Therefore, this APD has high
sensitivity to a wide range of wavelengths, from 0.92 .mu.m to 1.67
.mu.m, which means that the APD has approximately the same
sensitivity to 1.3 .mu.m and 1.55 .mu.m wavelengths. As a result,
the APD cannot receive the shorter wavelength 1.3 .mu.m without
receiving the longer wavelength 1.55 .mu.m unless it is provided
with a wavelength filter.
[0016] Further, as described above, although photodetector devices
for selectively receiving the longer wavelength light have been
available, there is no known photodetector device capable of
selectively receiving the shorter wavelength light.
SUMMARY OF THE INVENTION
[0017] The present invention has been devised in view of the above
problems. It is, therefore, an object of the present invention to
provide a semiconductor photodetector device having a high
wavelength selectivity ratio without using a wavelength filter.
[0018] Another object of the present invention is provide a
semiconductor photodetector device capable of selectively receiving
the shorter wavelength light.
[0019] According to one aspect of the present invention, a
semiconductor photodetector device comprises a first semiconductor
layer of a first conductive type, a first absorption layer, and a
second semiconductor layer having a larger bandgap than the first
absorption layer. The first semiconductor layer, the first
absorption layer, and the second semiconductor layer are formed
from bottom to top in that order so as to form a laminated
structure. The semiconductor photodetector device is adapted to
receive light incident on the second semiconductor layer side. The
second semiconductor layer includes an impurity region of a second
conductive type. The semiconductor photodetector device further
comprises a second absorption layer formed between the first
semiconductor layer and the first absorption layer. The second
absorption layer has a smaller bandgap than the first absorption
layer.
[0020] According to another aspect of the present invention, a
semiconductor photodetector device comprises a first semiconductor
layer of a first conductive type, a first absorption-layer, and a
second semiconductor layer having a larger bandgap than the first
absorption layer. The first semiconductor layer, the first
absorption layer, and the second semiconductor layer are formed
from bottom to top in that order so as to form a laminated
structure. The semiconductor photodetector device is adapted to
receive light incident on the first semiconductor layer side. The
second semiconductor layer includes an impurity region of a second
conductive type. The semiconductor photodetector device further
comprises a second absorption layer formed between the second
semiconductor layer and the first absorption layer or formed on the
second semiconductor layer. The second absorption layer has a
smaller bandgap than the first absorption layer.
[0021] According to other aspect of the present invention, a
semiconductor photodetector device comprises a first semiconductor
layer of a first conductive type, a first absorption layer, and a
second semiconductor layer having a larger bandgap than the first
absorption layer. The first semiconductor layer, the first
absorption layer, and the second semiconductor layer are formed
from bottom to top in that order so as to form a laminated
structure. The semiconductor photodetector device is adapted to
receive light incident on the second semiconductor layer side. The
second semiconductor layer includes an impurity region of a second
conductive type. The semiconductor photodetector device further
comprises a second absorption layer formed on the second
semiconductor layer. The second absorption layer has a bandgap that
is larger than the bandgap of the first absorption layer but
smaller than the bandgap of the second semiconductor layer. A third
absorption layer is formed between the first semiconductor layer
and the first absorption layer. The third absorption layer has a
smaller bandgap than the first absorption layer.
[0022] According to other aspect of the present invention, a
semiconductor photodetector device comprises a first semiconductor
layer of a first conductive type, a first absorption layer, and a
second semiconductor layer having a larger bandgap than the first
absorption layer. The first semiconductor layer, the first
absorption layer, and the second semiconductor layer are formed
from bottom to top in that order so as to form a laminated
structure. The semiconductor photodetector device is adapted to
receive light incident on the first semiconductor layer side. The
second semiconductor layer includes an impurity region of a
second-conductive type. The semiconductor photodetector device
further comprises a second absorption layer formed on the second
semiconductor layer, the second absorption layer having a smaller
bandgap than the first absorption layer, and a third absorption
layer formed between the first semiconductor layer and the first
absorption layer. The third absorption layer has a larger bandgap
than the first absorption layer.
[0023] Other objects and advantages of the present invention will
become apparent from the following description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a cross-sectional view of an APD according to a
first embodiment.
[0025] FIG. 2(a), in a first embodiment, shows wavelength
dependence of sensitivity of an APD and of an amount of light
returned due to reflection within the APD.
[0026] FIG. 2(b) shows the corresponding characteristics of a
comparative example of the APD showing in FIG. 2(a).
[0027] FIG. 3 is a dross-sectional view of an APD according to a
second embodiment.
[0028] FIG. 4 is a cross-sectional view of an APD according to a
third embodiment.
[0029] FIG. 5 is a cross-sectional view of an APD according to a
fourth embodiment.
[0030] FIG. 6 is a cross-sectional view of an APD according to a
fifth embodiment.
[0031] FIG. 7 is a cross-sectional view of an APD according to a
sixth embodiment.
[0032] FIG. 8, in a sixth embodiment, shows wavelength dependence
of sensitivity of an APD and of an amount of light returned due to
reflection within the APD.
[0033] FIG. 9 is a cross-sectional view of an APD according to a
seventh embodiment.
[0034] FIG. 10 is a cross-sectional view of an PD according to a
eighth embodiment.
[0035] FIG. 11 is a diagram showing the configuration of a
conventional optical receiver.
[0036] FIG. 12 is a cross-sectional view of one conventional
APD.
[0037] FIG. 13 is a cross-sectional view of another conventional
APD.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] Preferred embodiments of the present invention will be
described with reference to the accompanying drawings.
First Embodiment
[0039] A semiconductor photodetector device of a first embodiment
of the present invention includes a laminated structure made up of
a semiconductor layer of a first conductive type, a first
absorption layer, and a window layer formed from bottom to top in
that order, and light enters the device from the window layer side.
The window layer includes an impurity region of a second conductive
type, and a second absorption layer having a smaller bandgap than
the first absorption layer is provided on the "semiconductor layer
of the first conductive type" side of the first absorption layer.
Specifically, the second absorption layer may be provided between
the semiconductor layer of the first conductive type and the first
absorption layer.
[0040] According to the present embodiment, the semiconductor layer
of the first conductive type corresponds to an n-type InP
substrate; the first absorption layer corresponds to a wavelength
selection/absorption layer; and the second absorption layer
corresponds to a transmitted light absorption/recombination
layer.
[0041] FIG. 1 is a cross-sectional view of an APD according to the
present embodiment. Referring to the figure, over an n-type InP
substrate 1 are formed a transmitted light absorption/recombination
layer 2, a barrier layer 3, a wavelength selection/absorption layer
4, an undoped InGaAsP graded layer 5, an n-type InP electric field
reduction layer 6, and an undoped InP window layer 7 also acting as
a multiplication layer. A p-type diffusion layer region 8,
corresponding to the impurity region of the second conductive type,
is formed in the InP window layer 7. Further, a nonreflective film
9, also acting as a surface-protective film, is formed on the InP
window layer 7. The nonreflective film 9 may be formed of, for
example, an SiN film. It should be noted that according to the
present embodiment, the barrier layer 3 may be omitted.
[0042] Further according to the present embodiment, the wavelength
selection/absorption layer 4 may be an InGaAsP layer or AlGaInAs
layer having a bandgap wavelength of 1.4 .mu.m.
[0043] The transmitted light absorption/recombination layer 2
absorbs the 1.55 .mu.m wavelength light transmitted through the
wavelength selection/absorption layer 4 and recombines the
electrons and holes generated as a result of the absorption.
According to the present embodiment, the transmitted light
absorption/recombination layer 2 may be, for example, an InGaAs
layer, AlGaInAs layer, InGaAsP layer, or the like which has a
bandgap wavelength longer than 1.55 .mu.m.
[0044] The barrier layer 3 prevents the holes generated in the
transmitted light absorption/recombination layer 2 from diffusing
to the wavelength selection/absorption layer 4. Therefore, the
barrier layer 3 is formed of a material having a larger bandgap
than the transmitted light absorption/recombination layer 2.
According to the present embodiment, the barrier layer 3 may be,
for example, an InP layer, AlInAs layer, AlGaInAs layer, InGaAsP
layer, or the like.
[0045] The wavelength selection/absorption layer 4 is an undoped or
low carrier concentration, p-type or n-type semiconductor layer.
The transmitted light absorption/recombination-layer 2 and the
barrier layer 3 are high carrier concentration n-type semiconductor
layers. However, according to the present embodiment, only one of
the transmitted light absorption/recombination layer 2 and the
barrier layer 3 needs to be n-type.
[0046] It should be noted that according to the present embodiment,
an n-type InP layer, corresponding to the semiconductor layer of
the first conductive type, may be formed on an insulative
substrate, and then the above transmitted light
absorption/recombination layer 2, barrier layer 3, wavelength
selection/absorption layer 4, undoped InGaAsP graded layer 5,
n-type InP electric field reduction layer 6, and undoped InP window
layer 7 may be formed over the n-type InP layer.
[0047] Still referring to FIG. 1, a cathode electrode 10 is a first
electrode for energizing the n-type InP substrate 1, while an anode
electrode 11 is a second electrode for energizing the p-type
diffusion layer region 8. Further, reference numeral 12 denotes a
guard ring region formed around the p-type diffusion layer region
8. The guard ring region is a p-type region having a low carrier
concentration. Reference numeral 13 denotes a multiplication
region.
[0048] Light with a wavelength of 1.3 .mu.m entering the portion of
the nonreflective film 9 not covered by the anode electrode 11, as
shown at the top of the figure, goes through the InP window layer 7
since the bandgap wavelength of the InP window layer 7 is 0.92
.mu.m, and reaches the wavelength selection/absorption layer 4. The
wavelength selection/absorption layer 4 has a bandgap wavelength of
1.4 .mu.m, which is longer than the wavelength of the incident
light (1.3 .mu.m). Therefore, the 1.3 .mu.m wavelength light is
absorbed by the wavelength selection/absorption layer 4, generating
electrons and holes. Further, since the carrier concentration of
the wavelength selection/absorption layer 4 is low, this layer is
depleted when a bias voltage is applied to the APD. Therefore, the
holes move to the multiplication region 13 and are multiplied
therein. Then, they are drawn from the device as an electric
current signal.
[0049] Light with a wavelength of 1.55 .mu.m entering the APD shown
in FIG. 1 from an optical fiber (not shown) also goes through the
InP window layer 7 and reaches the wavelength selection/absorption
layer 4. However, since the bandgap wavelength of the wavelength
selection/absorption layer 4 is shorter than 1.55 .mu.m, the light
transmits through the wavelength selection/absorption layer 4 and
reaches the transmitted light absorption/recombination layer 2
through the barrier layer 3. Since the bandgap wavelength of the
transmitted light absorption/recombination layer 2 is longer than
1.55 .mu.m, the 1.55 .mu.m wavelength light is absorbed by the
transmitted light absorption/recombination layer 2, generating
electrons and holes. These generated electrons and holes recombine
with each other within this layer, producing heat.
[0050] Thus, the APD of the present embodiment includes the
wavelength selection/absorption layer 4 and the transmitted light
absorption/recombination layer 2. This arrangement allows the 1.3
.mu.m wavelength light to be selectively drawn from the device, as
an electric current, while preventing the 1.55 .mu.m wavelength
light from being drawn.
[0051] The present embodiment also has the following effect due to
the transmitted light absorption/recombination layer 2.
[0052] If the transmitted light absorption/recombination layer 2 is
not provided, the 1.55 .mu.m wavelength light that has been
transmitted through the wavelength selection/absorption layer 4 is
reflected from the cathode electrode 10 and returned to the
wavelength selection/absorption layer 4. A portion of this returned
light is absorbed by the wavelength selection/absorption layer 4,
and some of the remaining portion is transmitted through the
wavelength selection/absorption layer 4 and then emitted from the
top of the APD. The emitted light returns to the optical fiber.
These phenomena are undesirable since they reduce the wavelength
selectivity ratio of the APD and increase the amount of light
returned due to reflection.
[0053] On the other hand, with the transmitted light
absorption/recombination layer 2 provided in the device, the light
that has been transmitted through the wavelength
selection/absorption layer 4 is prevented from returning to the
wavelength selection/absorption layer 4. As a result, the APD has a
high wavelength selectivity ratio, and in the APD the amount of
light returned due to reflection is small.
[0054] FIG. 2(a) shows the wavelength dependence of the sensitivity
of the APD of the present embodiment and the wavelength dependence
of the amount of light returned due to reflection within the APD.
FIG. 2(b) shows the corresponding characteristics of a comparative
example in which the transmitted light absorption/recombination
layer is not provided.
[0055] As can be seen by comparison between FIGS. 2(a) and 2(b),
the APD of the present embodiment has reduced sensitivity to 1.4
.mu.m and longer wavelengths, as compared to the comparative
example. This indicates that the wavelength selectivity ratio can
be increased by forming the transmitted light
absorption/recombination layer. Further, in the comparative example
the amount of light returned due to reflection drastically
increases at a wavelength of around 1.4 .mu.m and remains large at
longer wavelengths, whereas in the APD of the present embodiment
substantially no amount of returned light is observed regardless of
the wavelength. Therefore, the transmitted light
absorption/recombination layer is also effective in reducing the
amount of light returned due to reflection.
[0056] The present embodiment does not necessarily require the
barrier layer. However, to increase the wavelength selectivity
ratio, it is preferable to form the barrier layer, as described
below.
[0057] If the electrons and holes generated in the transmitted
light absorption/recombination layer 2 have diffused to the
wavelength selection/absorption layer 4, they are undesirably drawn
from the device as an electric current signal since an electric
field is applied to the wavelength selection/absorption layer 4.
However, this can be prevented by forming the barrier layer 3,
having a larger bandgap than the transmitted light
absorption/recombination layer 2, between the transmitted light
absorption/recombination layer 2 and the wavelength
selection/absorption layer 4. Specifically, with this arrangement,
the electrons and holes generated in the transmitted light
absorption/recombination layer 2 can be prevented from diffusing to
the wavelength selection/absorption layer 4 before they recombine
with each other. Further, since the transmitted light
absorption/recombination layer 2 and the barrier layer 3 have a
high carrier concentration and hence are hardly depleted, the
electrons and holes generated in the transmitted light
absorption/recombination layer 2 do not pass through the barrier
layer 3.
[0058] As described above, the semiconductor photodetector device
of the present embodiment can selectively receive light having the
shorter wavelength 1.3 .mu.m without using a wavelength filter.
Further, a wavelength filter having a simpler configuration than
conventional wavelength filters may be used with the semiconductor
photodetector device. Still further, a conventional wavelength
filter may be used with the semiconductor photodetector device to
further increase the selectivity ratio for the wavelength of light
to be received.
Second Embodiment
[0059] FIG. 3 is a cross-sectional view of an APD according to a
second embodiment of the present invention. Referring to the
figure, over an n-type InP substrate 21, also acting as a
semiconductor layer of a first conductive type, are formed a
transmitted light absorption/recombination layer 22 corresponding
to a second absorption layer, a barrier layer 23, a wavelength
selection/absorption layer 24 corresponding to a first absorption
layer, an undoped InGaAsP graded layer 25, an n-type-InP electric
field reduction layer 26, and an undoped-InP window layer 27 also
acting as a multiplication layer. A p-type diffusion layer region
28, corresponding to an impurity region of the second conductive
type, is formed in the InP window layer 27. Further, a
nonreflective film 29, also acting as a surface protective film, is
formed on the InP window layer 27. The nonreflective film 29 may be
formed of, for example, an SiN film. It should be noted that
according to the present embodiment, the barrier layer 23 may be
omitted.
[0060] The present embodiment is different from the first
embodiment in that a multilayered reflective layer 34 is formed on
the p-type diffusion layer region 28.
[0061] The multilayered reflective layer 34 is formed by repeatedly
laminating layers having different refractive indices. The
multilayered reflective layer 34 reflects light at wavelengths
longer than the bandgap wavelength of the wavelength
selection/absorption layer 24.
[0062] According to the present embodiment, the multilayered
reflective layer 34 reflects light at a wavelength of 1.55 .mu.m,
and is formed by, for example, repeatedly laminating an InP layer
and an InGaAs layer (that is, InP/InGaAs/InP/InGaAs/ . . . ). In
this case, the InGaAs layer may be replaced by an InGaAsP layer. Or
alternatively, the multilayered reflective layer 34 may be formed
by repeatedly laminating an AlInAs layer and a GaInAs layer (that
is, AlInAs/GaInAs/AlInAs/GaInAs/ . . . ). In this case, the GaInAs
layer may be replaced by an AlGaInAs layer. It should be noted that
the multilayered reflective layer 34 may be a p-type semiconductor
layer.
[0063] Further, each layer in the multilayered reflective layer 34
is set to an optical thickness of one quarter of the wavelength of
the incident light to be reflected. For example, the optical
thickness of each layer may be set to a quarter of 1.55 .mu.m to
reflect only 1.55 .mu.m wavelength incident light. It should be
noted that the multilayered reflective layer 34 may be undoped,
p-type, or n-type.
[0064] According to the present embodiment, the wavelength
selection/absorption layer 24, the transmitted light
absorption/recombination layer 22, and the barrier layer 23 may be
formed of the same materials as the corresponding layers of the
first embodiment. Specifically, the wavelength selection/absorption
layer 24 may be an InGaAsP layer or AlGaInAs layer having a bandgap
wavelength of 1.4 .mu.m. The transmitted light
absorption/recombination layer 22 may be an InGaAs layer, AlGaInAs
layer, InGaAsP layer, or the like which has a bandgap wavelength
longer than 1.55 .mu.m. The barrier layer 23 is formed of a
material having a larger bandgap than the transmitted light
absorption/recombination layer 22. Specifically, the barrier layer
23 may be an InP layer, AlInAs layer, AlGaInAs layer, InGaAsP
layer, or the like.
[0065] The wavelength selection/absorption layer 24 is an undoped
or low carrier concentration, p-type or n-type semiconductor layer.
The transmitted light absorption/recombination layer 22 and the
barrier layer 23 are high carrier concentration n-type layers.
[0066] It should be noted that according to the present embodiment,
an n-type InP layer, corresponding to the semiconductor layer of
the first conductive type, may be formed on an insulative
substrate, and then the above transmitted light
absorption/recombination layer 22, barrier layer 23, wavelength
selection/absorption layer 24, undoped InGaAsP graded layer 25,
n-type InP electric field reduction layer 26, and undoped InP
window layer 27 may be formed over the n-type InP layer.
[0067] Still referring to FIG. 3, a cathode electrode 30 is; a
first electrode for energizing the n-type InP substrate 21, while
an anode electrode 31 is a second electrode for energizing the
p-type diffusion layer region 28. Further, reference numeral 32
denotes a guard ring region formed around the p-type diffusion
layer region 28. The guard ring region is a p-type region having a
low carrier concentration. Reference numeral 33 denotes a
multiplication region.
[0068] Light with a wavelength of 1.3 .mu.m entering the device
from the top, as shown in FIG. 3, goes through the multilayered
reflective layer 34 and reaches the InP window layer 27. Since the
InP-window layer 27 has a bandgap wavelength of 0.92 .mu.m, which
is shorter than 1.3 .mu.m, the 1.3 .mu.m wavelength light also goes
through the InP window layer 27 and reaches the wavelength
selection/absorption layer 24. The wavelength selection/absorption
layer 24 has a bandgap wavelength of 1.4 .mu.m, which is longer
than the wavelength of the incident light (1.3 .mu.m). Therefore,
the 1.3 .mu.m wavelength light is absorbed by the wavelength
selection/absorption layer 24, generating electrons and holes.
Further, since the carrier concentration of the wavelength
selection/absorption layer 24 is low, this layer is depleted when a
bias voltage is applied to the APD. Therefore, the holes move to
the multiplication region 33 and are multiplied therein. Then, they
are drawn from the device as an electric current signal.
[0069] On the other hand, light with a wavelength of 1.55 .mu.m is
reflected by the multilayered reflective layer 34. It should be
noted that the multilayered reflective layer 34 typically has a
reflectance of approximately 90% to 1.55 .mu.m wavelength light.
Therefore, 10% of the incident light goes through the multilayered
reflective layer 34 and reaches the wavelength selection/absorption
layer 24 through the InP window layer 27. Since the bandgap
wavelength of the wavelength selection/absorption layer 24 is
shorter than 1.55 .mu.m, the light passes through the wavelength
selection/absorption layer 24 and reaches the transmitted light
absorption/recombination layer 22 through the barrier layer 23.
Then, since the bandgap wavelength of the transmitted light
absorption/recombination layer 22 is longer than 1.55 .mu.m, the
1.55 .mu.m wavelength light that has gone through the multilayered
reflective layer 34, etc. is absorbed by the transmitted light
absorption/recombination layer 22, generating electrons and holes.
These generated electrons and holes recombine with each other
within this layer, producing heat.
[0070] The present embodiment has the following effect in addition
to the effects described with regard to the first embodiment. Since
the multilayered reflective layer reflects a major portion of the
1.55 .mu.m wavelength light, it is possible to reduce the amount of
1.55 .mu.m wavelength light absorbed by the wavelength
selection/absorption layer. Specifically, by providing the
multilayered reflective layer having a reflectance of 90%, the
selection ratio of 1.3 .mu.m wavelength light to 1.55 .mu.m
wavelength light can be increased by a factor of approximately 10,
as compared to the first embodiment.
Third Embodiment
[0071] FIG. 4 is a cross-sectional view of an APD according to a
third embodiment of the present invention. Referring to the figure,
over an n-type InP substrate 41, also acting as a semiconductor
layer of a first conductive type, are formed a transmitted light
absorption/recombination layer 42, a barrier layer 43, a wavelength
selection/absorption layer 44, an undoped InGaAsP graded layer 45,
an n-type InP electric field reduction layer 46, and an undoped InP
window layer 47 also acting as a multiplication layer. The
wavelength selection/absorption layer 44, the transmitted light
absorption/recombination layer 42, and the barrier layer 43 may be
formed of the same materials as the corresponding layers of the
first embodiment. It should be noted that according to the present
embodiment, the barrier layer 43 may be omitted.
[0072] The present embodiment is characterized in that a
multilayered reflective layer 54, also acting as a window layer, is
formed on the InP window layer 47. The multilayered reflective
layer 54 is formed of the same material as the corresponding layer
of the second embodiment.
[0073] A p-type diffusion layer region 48, corresponding to an
impurity region of a second conductive type, is formed in the InP
window layer 47 and in the multilayered reflective layer 54.
Further, a nonreflective film 49, also acting as a surface
protective film, is formed on the multilayered reflective layer 54.
The nonreflective film 49 may be formed of, for example, an SiN
film.
[0074] It should be noted that according to the present embodiment,
an n-type InP layer, corresponding to the semiconductor layer of
the first conductive type, may be formed on an insulative
substrate, and then the above transmitted light
absorption/recombination layer 42, barrier layer 43, wavelength
selection/absorption layer 44, undoped InGaAsP graded layer 45,
n-type InP electric field reduction layer 46, and undoped InP
window layer 47 may be formed over the n-type InP layer.
[0075] Still referring to FIG. 4, a cathode electrode 50 is a first
electrode for energizing the n-type InP substrate 41, while an
anode electrode 51 is a second electrode for energizing the p-type
diffusion layer region 48. Further, reference numeral 52 denotes a
guard ring region formed around the p-type diffusion layer region
48. The guard ring region 52 is a p-type region having a low
carrier concentration. Reference numeral 53 denotes a
multiplication region.
[0076] The present embodiment has the following effects in addition
to the effects described with regard to the second embodiment. By
forming a multilayered reflective film also acting as a window
layer, it is possible to reduce the total crystal growth-thickness
of the layers making up the APD. Further, the anode electrode side
of the APD can be formed such that its surface is flat, as compared
to the second embodiment.
Fourth Embodiment
[0077] FIG. 5 is a cross-sectional view of an APD according to a
fourth embodiment of the present invention. Referring to the
figure, over an n-type InP substrate 71, also acting as a
semiconductor layer of a fist conductive type, are formed a
transmitted light absorption/recombination layer 72, a barrier
layer 73, an AlInAs multiplication layer 74, a p-type InP electric
field reduction layer 75, a wavelength selection/absorption layer
76, an undoped InGaAsP graded layer 77, and an undoped InP window
layer 78. It should be noted that according to the present
embodiment, the barrier layer 78 may be omitted.
[0078] Thus, the present embodiment is characterized in that a
multiplication layer of AlInAs and an electric field reduction
layer are formed between the barrier layer and the wavelength
selection/absorption layer in that order. This structure has the
same effect as that described with regard to the first
embodiment.
[0079] The wavelength selection/absorption layer 76, the
transmitted light absorption/recombination layer 72, and the
barrier layer 73 may be formed of the same materials as the
corresponding layers of the first embodiment.
[0080] A nonreflective film 79, also acting as a surface protective
film, is formed on the InP window layer 78. The nonreflective film
79 may be formed of, for example, an SiN film.
[0081] It should be noted that according to the present embodiment,
an n-type-InP layer, corresponding to the semiconductor layer of
the first conductive type, may be formed on an insulative
substrate, and then the above transmitted light
absorption/recombination layer 72, barrier layer 73, AlInAs
multiplication layer 74, p-type InP electric field reduction layer
75, wavelength selection/absorption layer 76, undoped InGaAsP
graded layer 77, and undoped InP window layer 78 may be formed over
the n-type InP layer.
[0082] Still referring to FIG. 5, a p-type diffusion layer region
80, corresponding to an impurity region of a second conductive
type, is formed in the InP window layer 78. Further, a cathode
electrode 81 is a first electrode for energizing the n-type InP
substrate 71, while an anode electrode 82 is a second electrode for
energizing the p-type diffusion layer region 80.
[0083] It should be noted that in the APD shown in FIG. 5, the
AlInAs multiplication layer 74 and the p-type InP electric field
reduction layer 75 are provided under the wavelength
selection/absorption layer 76 to inject electrons into the AlInAs
multiplication layer 74. Therefore, according to the present
embodiment, since the AlInAs multiplication layer 74, at which
electric field concentration occurs, is not in contact with the
p-type diffusion layer region 80, a guard ring need not be provided
around the p-type diffusion layer region 80.
Fifth Embodiment
[0084] A semiconductor photodetector device of a fifth embodiment
of the present invention includes a laminated structure made up of
a semiconductor layer of a first conductive type, a first
absorption layer, and a window layer formed from bottom to top in
that order, and light enters the device from the "semiconductor
layer of the first conductive type" side. The window layer includes
an impurity region of a second conductive type, and a second
absorption layer having a smaller bandgap than the first absorption
layer is provided on the window layer. Further, a multilayered
reflective layer for reflecting light at wavelengths loner than the
bandgap wavelength of the first absorption layer is provided
between the semiconductor layer of the first conductive type and
the first absorption layer.
[0085] According to the present embodiment, the semiconductor layer
of the first conductive type corresponds to an n-type InP
substrate; the first absorption layer corresponds to a wavelength
selection/absorption layer; and the second absorption layer
corresponds to a transmitted light absorption/recombination
layer.
[0086] FIG. 6 is a cross-sectional view of an APD according to the
present embodiment. Referring to the figure, over an n-type InP
substrate 91 are formed a multilayered reflective layer 92, a
barrier layer 93, an AlInAs multiplication layer 94, a p-type InP
electric field reduction layer 95, a wavelength
selection/absorption layer 96, an undoped InGaAsP graded layer 97,
and an undoped InP window layer 98.
[0087] The wavelength selection/absorption layer 96 may be formed
of the same material as the corresponding layer of the first
embodiment.
[0088] The multilayered reflective layer 92 reflects light at
wavelengths longer than the bandgap wavelength of the wavelength
selection/absorption layer 96. On the other hand, the barrier layer
93 has a larger bandgap than the multilayered reflective layer 92.
According to the present embodiment, at least one of the
multilayered reflective layer 92 and the barrier layer 93 may be an
n-type semiconductor layer. It should be noted that according to
the present embodiment, the barrier layer 93 may be omitted.
[0089] A p-type diffusion layer region 99, corresponding to the
impurity region of the second conductive type, is formed in the InP
window layer 98, and a transmitted light absorption/recombination
layer 100 is formed on the p-type diffusion layer region 99. The
transmitted light absorption/recombination layer 100 may be a
p-type semiconductor layer.
[0090] A nonreflective film 101, also acting as a surface
protective film, is formed on the portions of the InP window layer
98 not covered by the transmitted light absorption/recombination
layer 100. The nonreflective film 101 may be formed of, for
example, an SiN film.
[0091] A cathode electrode 102, which is a first electrode for
energizing the n-type InP substrate 91, is formed on a
predetermined region of the back surface of the n-type InP
substrate 91. On the other hand, an anode electrode 103, which is a
second electrode for energizing the p-type diffusion layer region
99, is formed on the transmitted light absorption/recombination
layer 100. Light enters the portion of the back surface of the
n-type InP substrate 91 not covered by the cathode electrode
102.
[0092] It should be noted that according to the present embodiment,
an n-type InP layer, corresponding to the semiconductor layer of
the first conductive type, may be formed on an insulative
substrate, and then the above multilayered reflective layer 92,
barrier layer 93, AlInAs multiplication layer 94, p-type InP
electric field reduction layer 95, wavelength selection/absorption
layer 96, undoped InGaAsP graded layer 97, and undoped InP window
layer 98 may be formed over the n-type InP layer.
[0093] The operation of the APD of the present embodiment will now
be described.
[0094] Light with a wavelength of 1.3 .mu.m entering the n-type InP
substrate 91, as shown at the bottom of FIG. 6, goes through the
multilayered reflective film 92 and reaches the wavelength
selection/absorption layer 96. Since the wavelength
selection/absorption layer 96 has a bandgap wavelength of 1.4
.mu.m, which is longer than the wavelength of the incident light
(1.3 .mu.m), the 1.3 .mu.m wavelength light is absorbed by the
wavelength selection/absorption layer 96, generating electrons and
holes, which are drawn from the device as an electric current
signal.
[0095] On the other hand, light with a wavelength of 1.55 .mu.m is
reflected by the multilayered reflective layer 92. It should be
noted that the multilayered reflective layer 92 typically has a
reflectance of approximately 90% to 1.55 .mu.m wavelength light.
Therefore, 10% of the incident light goes through the multilayered
reflective layer 92 and reaches the transmitted light
absorption/recombination layer 100 through the InP window layer 98.
Then, since the bandgap wavelength of the transmitted light
absorption/recombination layer 100 is longer than 1.55 .mu.m, the
1.55 .mu.m wavelength light, that has gone through the multilayered
reflective layer 92, is absorbed by the transmitted light
absorption/recombination layer 100, generating electrons and holes.
These generated electrons and holes recombine with each other
within this layer, producing heat.
[0096] The present embodiment has the following effects in addition
to the effects described with regard to the first and second
embodiments. Since the multilayered reflective layer is provided
between the n-type InP substrate and the barrier layer, the
thickness of the multilayered reflective layer can be increased
without degrading the flatness of the APD surface. This allows the
reflectance of the multilayered reflective layer to be
increased.
Sixth Embodiment
[0097] A semiconductor photodetector device of a sixth embodiment
of the present invention includes a laminated structure made up of
a semiconductor layer of a first conductive type, a first
absorption layer, and a window layer formed from bottom to top in
that order, and light enters the device from the window layer side.
The window layer includes an impurity region of a second conductive
type, and a second absorption layer having a larger bandgap than
the first absorption layer is provided on the window layer side of
the first absorption layer. Particularly, according to the present
embodiment, a multilayered reflective layer for reflecting light at
wavelengths longer than the bandgap wavelength of the first
absorption layer may be laminated to the window layer, and then the
second absorption layer may be laminated to the multilayered
reflective layer. Further, a third absorption layer having a
smaller bandgap than the first absorption layer is provided on the
"semiconductor layer of the first conductive type" side of the
first absorption layer. Specifically, the third absorption layer
may be provided between the semiconductor layer of the first
conductive type and the first absorption layer.
[0098] According to the present embodiment, the semiconductor layer
of the first conductive type corresponds to an n-type InP
substrate; the first absorption layer corresponds to a wavelength
selection/absorption layer; the second absorption layer corresponds
to a wavelength selection/recombination layer; and the third
absorption layer corresponds to a transmitted light
absorption/recombination layer.
[0099] FIG. 7 is a cross-sectional view of an APD according to the
present embodiment. Referring to the figure, over an n-type InP
substrate 111, also acting as the semiconductor layer of the first
conductive type, are formed a transmitted light
absorption/recombination layer 112, a barrier layer 113, an, AlInAs
multiplication layer 114, a p-type InP electric field reduction
layer 115, a wavelength selection/absorption layer 116, an undoped
InGaAsP graded layer 117, and an undoped InP window layer 118. The
wavelength selection/absorption layer 116 may be formed of the same
material as the corresponding layer of the first embodiment.
[0100] The barrier layer 113 has a larger bandgap than the
transmitted light absorption/recombination layer 112 and may be an
n-type semiconductor layer. It should be noted that according to
the present embodiment, the barrier layer 113 may be omitted.
[0101] Further according to the present embodiment, a second
barrier layer may be provided between the wavelength
selection/absorption layer 116 and a wavelength
selection/recombination layer 121 in addition to or in place of the
barrier layer 113. The second barrier layer may be a p-type
semiconductor layer.
[0102] A p-type diffusion layer region 119, corresponding to the
impurity region of the second conductive type, is formed in the InP
window layer 118. Further, a multilayered reflective layer 120 and
the wavelength selection/recombination layer 121 are formed over
the p-type diffusion layer region 119. The multilayered reflective
layer 120 may be a p-type semiconductor layer.
[0103] A nonreflective film 122, also acting as a surface
protective film, is formed on the portions of the InP window layer
118 not covered by the multilayered reflective film 120. The
nonreflective film 122 may be formed of, for example, an SiN
film.
[0104] Still referring to FIG. 7, a cathode electrode 123 is a
first electrode for energizing the n-type InP substrate 111, while
an anode electrode 124 is a second electrode for energizing the
p-type diffusion layer region 119.
[0105] It should be noted that according to the present embodiment,
an n-type InP layer, corresponding to the semiconductor layer of
the first conductive type, may be formed on an insulative
substrate, and then the above transmitted light
absorption/recombination layer 112, barrier layer 113, AlInAs
multiplication layer 114, p-type InP electric field reduction layer
115, wavelength selection/absorption layer 116, undoped InGaAsP
graded layer 117, and undoped InP window layer 118 may be formed
over the n-type InP layer.
[0106] The present embodiment assumes that three wavelengths of
light (for example, 1.3 .mu.m, 1.49 .mu.m; and 1.55 .mu.m) enter
the device. In this case, the bandgap wavelength of the wavelength
selection/recombination layer 121 may be set to 1.4 .mu.m, and the
bandgap wavelength of the wavelength selection/absorption layer 116
may be set to 1.52 .mu.m.
[0107] The 1.3 .mu.m wavelength light entering the device from the
top, as shown in FIG. 7, is absorbed by the wavelength
selection/recombination layer 121, generating electrons and holes.
Since no electric field is applied to the wavelength
selection/recombination layer 121, the generated electrons and
holes recombine with each other within this layer and hence
disappear.
[0108] The 1.55 .mu.m wavelength light, on the other hand,
transmits through the wavelength selection/recombination layer 121
and then is reflected by the multilayered reflective layer 120. At
that time, however, a portion of the light goes through the
multilayered reflective layer 120 and reaches the wavelength
selection/absorption layer 116. Then, since the bandgap wavelength
of the wavelength selection/absorption layer 116 is shorter than
1.55 .mu.m, the light, that has gone through the multilayered
reflective layer 120, also goes through the wavelength
selection/absorption layer 116 and reaches the transmitted light
absorption/recombination layer 112 through the barrier layer 113.
The light that has reached the transmitted light
absorption/recombination layer 112 is absorbed therein, generating
electrons and holes. The generated electrons and holes recombine
with each other within this layer and hence disappear.
[0109] Therefore, the 1.3 .mu.m wavelength light and the 1.55 .mu.m
wavelength light are not output from the device as electric
current-signals.
[0110] On the other hand, the 1.49 .mu.m wavelength light passes
through the wavelength selection/recombination layer 121 and the
multilayered reflective layer 120 and reaches the wavelength
selection/absorption layer 116. Then, since the wavelength
selection/absorption layer 116 has a bandgap wavelength of 1.52
.mu.m, which is longer than the wavelength of the incident light
(1.49 .mu.m), the 1.49 .mu.m wavelength light is absorbed by the
wavelength selection/absorption layer 116, generating electrons and
holes, which are drawn from the device as an electric current
signal.
[0111] FIG. 8 shows the wavelength dependence of the sensitivity of
the APD of the present embodiment and the wavelength dependence of
the amount of light returned due to reflection within the APD. As
can be seen from the figure, the APD of the present embodiment has
sensitivity to the wavelength range from 1.4 .mu.m to 1.52
.mu.m.
[0112] As described above, the APD of the present embodiment
includes a wavelength selection/recombination layer to absorb light
at wavelengths shorter than the bandgap wavelength of the layer.
Therefore, this layer may be combined with a multilayered
reflective layer, a wavelength selection/absorption layer, a
barrier layer, and a transmitted light absorption/recombination
layer so as to selectively extract the middle one of three
wavelengths.
Seventh Embodiment
[0113] A semiconductor photodetector device of a seventh embodiment
of the present invention includes a laminated structure made up of
a semiconductor layer of a first conductive type, a first
absorption layer, and a window layer formed from bottom to top in
that order, and light enters the device from the "semiconductor
layer of the first conductive type" side. The window layer includes
an impurity region of a second conductive type, and a second
absorption layer having a smaller bandgap than the first absorption
layer is provided on the window layer side of the first absorption
layer. Specifically, the second absorption layer may be provided on
the window layer. Further, a third absorption layer having a larger
bandgap than the first absorption layer is provided on the
"semiconductor layer of the first conductive type" side of the
first absorption layer. Specifically, the third absorption layer
may be provided between the semiconductor layer of the first
conductive type and the first absorption layer.
[0114] According to the present embodiment, the semiconductor layer
of the first conductive type corresponds to an n-type InP
substrate; the first absorption layer corresponds to a wavelength
selection/absorption layer; the second absorption layer corresponds
to a transmitted light absorption/recombination layer; and the
third absorption layer corresponds to a wavelength
selection/recombination layer.
[0115] FIG. 9 is a cross-sectional view of an APD according to the
present embodiment. This APD is a variation of the APD of the sixth
embodiment, adapted to receive light incident on its back surface.
Referring to the figure, over an n-type InP substrate 131 are
formed a wavelength selection/recombination layer 132, a
multilayered reflective layer 133, a barrier layer 134, an AlInAs
multiplication layer 135, a p-type InP electric field reduction
layer 136, a wavelength selection/absorption layer 137, an undoped
InGaAsP graded layer 138, and an undoped InP window layer 139.
[0116] The multilayered reflective layer 133 reflects light at
wavelengths longer than the bandgap wavelength of the wavelength
selection/absorption layer 137. The multilayered reflective layer
133 may be an n-type semiconductor layer.
[0117] The barrier layer 134 has a larger bandgap than the
multilayered reflective layer 133 and may be an n-type
semiconductor layer. It should be noted that according to the
present embodiment, the barrier layer 134 may be omitted.
[0118] A p-type diffusion layer region 140, corresponding to the
impurity region of the second conductive type, is formed in the InP
window layer 139. Further, a transmitted light
absorption/recombination layer 141 is formed on the p-type
diffusion layer region 140. Further, a nonreflective film 142, also
acting as a surface protective film, is formed on the portions of
the InP window layer 139 not covered by the transmitted light
absorption/recombination layer 141. The nonreflective film 142 may
be formed of, for example, an SiN film.
[0119] A cathode electrode 143, which is a first electrode for
energizing the n-type InP substrate 131, is formed on a
predetermined region of the back surface of the n-type InP
substrate 131. On the other hand, an anode electrode 144, which is
a second electrode for energizing the p-type diffusion layer region
140, is formed on the transmitted light absorption/recombination
layer 141. Light enters the portion of the back surface of the
n-type InP substrate 131 not covered by the cathode electrode
143.
[0120] It should be noted that according to the present embodiment,
an n-type InP layer, corresponding to the semiconductor layer of
the first conductive type, may be formed on an insulative
substrate, and then the above wavelength selection/recombination
layer 132, multilayered reflective layer 133, barrier layer 134,
AlInAs multiplication layer 135, p-type InP electric field
reduction layer 136, wavelength selection/absorption layer 137,
undoped InGaAsP graded layer 138, and undoped InP window layer 139
may be formed over the n-type InP layer.
[0121] The present embodiment assumes that three wavelengths of
light (for example, 1.3 .mu.m, 1.49 .mu.m, and 1.55 .mu.m) enter
the device. In this case, the bandgap wavelength of the wavelength
selection/recombination layer 132 may be set to 1.4 .mu.m, and the
bandgap wavelength of the wavelength selection/absorption layer 137
may be set to 1.52 .mu.m.
[0122] The 1.3 .mu.m wavelength light entering the device from the
bottom, as shown in FIG. 9, is absorbed by the wavelength
selection/recombination layer 132, generating electrons and holes.
Since no electric field is applied to the wavelength
selection/recombination layer 132, the generated electrons and
holes recombine with each other within this layer and hence
disappear.
[0123] The 1.55 .mu.m wavelength light, on the other hand,
transmits through the wavelength selection/recombination layer 132
and then is reflected by the multilayered reflective layer 133. At
that time, however, a portion of the light goes through the
multilayered reflective layer 133 and reaches the transmitted light
absorption/recombination layer 141 through the window layer 139.
Then, if the bandgap wavelength of the transmitted light
absorption/recombination layer 141 is longer than 1.55 .mu.m, the
1.55 .mu.m wavelength light that has reached the transmitted light
absorption/recombination layer 141 is absorbed therein, generating
electrons and holes. The generated electrons and holes recombine
with each other within this layer and hence disappear.
[0124] Therefore, the 1.3 .mu.m wavelength light and the 1.55 .mu.m
wavelength light are not output from the device as electric current
signals.
[0125] On the other hand, the 1.49 .mu.m wavelength light passes
through the wavelength selection/recombination layer 132 and the
multilayered reflective layer 133 and reaches the wavelength
selection/absorption layer 137. Then, since the wavelength
selection/absorption layer 137 has a bandgap wavelength of 1.52
.mu.m, which is longer than the wavelength of the incident light
(1.49 .mu.m), the 1.49 .mu.m wavelength light is absorbed by the
wavelength selection/absorption layer 137, generating electrons and
holes, which are then drawn from the device as an electric current
signal.
[0126] As described above, the APD of the present embodiment,
adapted to receive light incident on its back surface, includes a
wavelength absorption/recombination layer to absorb light at
wavelengths shorter than the bandgap wavelength of the layer.
Therefore, this layer may be combined with a multilayered
reflective layer, a wavelength selection/absorption layer, a
barrier layer, and a transmitted light absorption/recombination
layer so as to selectively extract the middle one of three
wavelengths.
Eighth Embodiment
[0127] The first to seventh embodiments have been described as
applied to APDs. However, the present invention can also be applied
to another type of semiconductor photodetector device, namely
photodiode (PD).
[0128] FIG. 10 is a cross-sectional view of a PD according to an
eighth embodiment of the present invention. Referring to the
figure, over an n-type InP substrate 151, also acting as a
semiconductor layer of a first conductive type, are formed a
transmitted light absorption/recombination layer 152, a barrier
layer 153, a wavelength selection/absorption layer 154, and an
undoped InP window layer 155. The wavelength selection/absorption
layer 154, the transmitted light absorption/recombination layer
152, and the barrier layer 153 may be formed of the same materials
as the corresponding layers of the first embodiment. It should be
noted that according to the present embodiment, the barrier layer
153 may be omitted.
[0129] A p-type diffusion layer region 156, corresponding to an
impurity region of a second conductive type, is formed in the InP
window layer 155 such that it reaches the wavelength
selection/absorption layer 154. Further, a nonreflective film 157,
also acting as a surface protective film, is formed on the InP
window layer 155. The nonreflective film 157 may be formed of, for
example, an SiN film.
[0130] It should be noted that according to the present embodiment,
an n-type InP layer, corresponding to the semiconductor layer of
the first conductive type, may be formed on an insulative
substrate, and then the above transmitted light
absorption/recombination layer 152, barrier layer 153, wavelength
selection/absorption layer 154, and undoped InP window layer 155
may be formed over the n-type InP layer.
[0131] Still referring to FIG. 10, a cathode electrode 158 is a
first electrode for energizing the n-type InP substrate 151, while
an anode electrode 159 is a second electrode for energizing the
p-type diffusion layer region 156.
[0132] Light with a wavelength of 1.3 .mu.m entering the portion of
the nonreflective film 157 not covered by the anode electrode 159,
as shown at the top of the figure, goes through the InP window
layer 155 since the bandgap wavelength of the InP window layer 155
is 0.92 .mu.m, and reaches the wavelength selection/absorption
layer 154. The wavelength selection/absorption layer 154 has a
bandgap wavelength of 1.4 .mu.m, which is longer than the
wavelength of the incident light (1.3 .mu.m). Therefore, the 1.3
.mu.m wavelength light is absorbed by the wavelength
selection/absorption layer 154 and then drawn from the device as an
electric current signal.
[0133] Light with a wavelength of 1.55 .mu.m also goes through the
InP window layer 155 and reaches the wavelength
selection/absorption layer 154. However, since the bandgap
wavelength of the wavelength selection/absorption layer 154 is
shorter than 1.55 .mu.m, the light transmits through the wavelength
selection/absorption layer 154 and reaches the transmitted light
absorption/recombination layer 152 through the barrier layer 153.
Since the bandgap wavelength of the transmitted light
absorption/recombination layer 152 is longer than 1.55 .mu.m, the
1.55 .mu.m wavelength light is absorbed by the transmitted light
absorption/recombination layer 152, generating electrons and holes.
These generated electrons and holes then recombine with each other
within this layer and hence disappear.
[0134] Thus, like the APD of the first embodiment, the PD of the
present embodiment includes a wavelength selection/absorption layer
and a transmitted light absorption/recombination layer, which
allows the 1.3 .mu.m wavelength light to be selectively drawn from
the device, as an electric current, while preventing the 1.55 .mu.m
wavelength light from being drawn.
[0135] It should be noted that the PD of the present embodiment may
also be configured such that a multilayered reflective layer for
reflecting light at wavelengths longer than the bandgap wavelength
of the wavelength selection/absorption layer is formed on or in the
window layer. In this case, the multilayered reflective layer may
be made of p-type semiconductor.
[0136] It should be further noted that the present invention is not
limited to the embodiments described above, and various alterations
may be made thereto without departing from the spirit and scope of
the invention.
[0137] The features and advantages of the present invention may be
summarized as follows.
[0138] The semiconductor photodetector devices of the present
invention can selectively receive the shorter wavelength light
without using a wavelength filter.
[0139] Obviously many modifications and variations of the present
invention are possible in the light of the above teachings. It is
therefore to be understood that within the scope of the appended
claims the invention may be practiced otherwise than as
specifically described.
[0140] The entire disclosure of a Japanese Patent Application No.
2005-070646, filed on Mar. 14, 2005 including specification,
claims, drawings and summary, on which the Convention priority of
the present application is based, are incorporated herein by
reference in its entirety.
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