Optical waveguide type photodiode and a process of producing the same

An optical waveguide type photodiode has a plurality of semiconductor layers formed one upon another on a semiconductor substrate and including an optical absorption layer sandwiched between a pair of optical confinement layers for guiding incident light in parallel with the semiconductor layers, wherein a light absorption quantity per unit length of an optical waveguide area constituted by the optical absorption layer is substantially constant throughout the entire area thereof. Specifically, the optical confinement factor .GAMMA.(x) of the optical waveguide area is set so as to increase with guided distance x of light. Preferably, a device structure is employed in which the thickness d(x) of the optical absorption layer increases with the guided distance x of light. Also, the optical absorption layer is formed by selective area growth with the use of a pair of selective area growth masks, and these masks have a mask pattern such that the mask width thereof gradually decreases/increases in the light guiding direction, whereby a photodiode with the above device structure can be fabricated with ease.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical waveguide type photodiode 
having a plurality of semiconductor layers which are formed one upon 
another on a semiconductor substrate and which include an optical 
absorption layer sandwiched between a pair of optical confinement layers 
for guiding light in parallel with the semiconductor layers. More 
particularly, the present invention relates to a photodiode which is quick 
in responsivity, ensures low modulation distortion with respect to 
incident light and thus is suited for large-capacity optical 
communications, and to a process of producing such a photodiode. 
2. Description of the Related Art 
In the field of optical communications using a long wavelength region, pin 
photodiodes of surface-incident type are widely used. The surface-incident 
photodiode absorbs light at the surface of a semiconductor layer (optical 
absorption layer), so that many photo carriers are locally generated in 
the vicinity of the surface of the optical absorption layer. With the 
surface-incident photodiode, therefore, modulation distortion with respect 
to incident light is liable to occur. Also, in surface-incident 
photodiodes in general, GaInAs semiconductor is used for the optical 
absorption layer. Since GaInAs semiconductor has a large absorption 
coefficient with respect to light with a wavelength of 1.3 .mu.m or 1.55 
.mu.m, the modulation distortion mentioned above poses a serious problem. 
Optical waveguide type photodiodes, on the other hand, have a basic device 
structure shown in FIG. 1, wherein a plurality of semiconductor layers 
including an n-type cladding layer 2, an n-type optical confinement layer 
3, an optical absorption layer 4, a p-type optical confinement layer 5, 
and p-type cladding layer 6 are successively formed on a emiconductor 
substrate 1. The optical confinement layers 3 and 5 are each made of a 
conductive semiconductor having a refractive index lower than that of the 
optical absorption layer 4. 
In the optical waveguide type photodiode with the above device structure, 
as light incident on the front facet of the semiconductor layers is guided 
in parallel with these layers, its light energy is absorbed by the optical 
absorption layer 4, and carriers excited by the thus-absorbed energy are 
detected as a photocurrent. Compared with the surface-incident type 
photodiode, the optical waveguide type photodiode has a thinner optical 
absorption layer 4, and thus the transit time of photo carriers is short, 
permitting high-speed operation. 
Generally, the thickness (film thickness) of the optical absorption layer 4 
of the optical waveguide type photodiode is set to 2 .mu.m or more in 
order to enhance the light sensitivity. Accordingly, light introduced into 
the optical absorption layer 4 is absorbed in large part in a region near 
the front facet while being guided for only a relatively short distance. 
As a result, a large number of photo carriers are generated locally in the 
vicinity of the front facet of the optical absorption layer 4, and such 
local generation of photo carriers causes modulation distortion. 
SUMMARY OF THE INVENTION 
A photodiode according to the present invention is based on the technical 
concept explained below. The aforementioned surface-incident type 
photodiode is produced using bulk crystalline materials, and thus its 
optical absorption coefficient is determined by the crystalline material 
(semiconductor material) forming the optical absorption layer. In the 
surface-incident photodiode, therefore, the optical absorption coefficient 
cannot be changed even if the device structure is elaborated. By contrast, 
the optical waveguide type photodiode has a device structure in which an 
optical waveguide is formed by an optical absorption layer sandwiched 
between a pair of optical confinement layers. The optical absorption 
coefficient of the optical waveguide is determined as a value 
.GAMMA..alpha. which is the product of a value a intrinsic to the 
semiconductor material forming the optical absorption layer and an optical 
confinement factor .GAMMA. determined by the structure of the optical 
waveguide. Thus, by elaborating the optical waveguide structure, it is 
possible to change the optical confinement factor .GAMMA. of the optical 
waveguide and thereby lower the optical absorption coefficient 
.GAMMA..alpha.. 
According to the present invention, the optical waveguide structure is 
gradually varied in the light guiding direction to make the optical 
confinement factor .GAMMA. different at different portions of the optical 
waveguide and thereby cause the optical absorption coefficient 
.GAMMA..alpha. to vary depending on a guided distance x of light, so that 
light may be absorbed uniformly over the entire area of the optical 
waveguide. Namely, an optical waveguide structure is constructed in which 
light is not locally absorbed in a region in the vicinity of the light 
incident front facet. More specifically, the optical waveguide type 
photodiode according to the present invention has a waveguide structure 
such that the optical confinement factor .GAMMA. is small at the light 
incident front facet and gradually increases with distance x from the 
light incident front facet, whereby the light absorption quantity 
(absorbed energy) per unit length is made substantially constant 
throughout the entire area of the optical waveguide. 
In the example shown in FIG. 1, the photodiode has an optical waveguide 
structure which is uniform throughout the entire area, and thus the 
optical confinement factor .GAMMA. remains the same at any portion of the 
optical waveguide. Accordingly, the optical absorption coefficient 
.GAMMA..alpha. of the optical waveguide (optical absorption layer) also 
remains constant throughout the entire area thereof. Consequently, the 
optical waveguide has a light absorption characteristic such that the 
light absorption quantity is large in the vicinity of the front facet and 
gradually decreases with guided distance x, as indicated by characteristic 
A in FIG. 2. In this regard, if the optical waveguide structure is 
modified in the aforementioned manner such that the optical confinement 
factor .GAMMA. varies depending on the guided distance x, then it is 
possible to make the light absorption quantity per unit length constant, 
as indicated by characteristic B in FIG. 2. 
In order to make the light absorption quantity per unit length constant 
throughout the entire area of the optical waveguide, an optical 
confinement factor .GAMMA.(x) of the optical waveguide as a function of 
the guided distance x will be considered. Provided that the intensity of 
light introduced into a region with a unit length dx of the optical 
absorption layer having an absorption coefficient of .alpha. is P(x), the 
quantity dP(x) of light absorbed in this region is given by 
EQU -dP(x)=.alpha.P(x)dx (1) 
Given that the intensity of light incident on the front facet is Po, since 
the light is absorbed at a rate of absorption coefficient 
.alpha..GAMMA.(x) which is a function of the guided distance x, the 
intensity P(x) of light introduced into the above region is 
EQU P(x)=Po exp(-.alpha..intg..sub.0.sup.x .GAMMA.(x)dx) (2) 
Accordingly, to make the quantity dP(x) of light absorption in the above 
region constant regardless of the guided distance x, the optical 
confinement factor .GAMMA.(x) may be set so that the value obtained by 
differentiating equation (2) for the guided distance x, that is, the rate 
of light absorption indicated by 
##EQU1## 
may become constant. 
As one solution to this, according to the present invention, the optical 
confinement factor .GAMMA.(x) indicated, for example, by 
##EQU2## 
as shown in FIG. 3, is set as a function of the distance (guided distance) 
x from the light incident front facet, and in this case the light 
absorption quantity per unit length can be made constant, as indicated by 
characteristic B in FIG. 2. 
Also, the present invention is based on the knowledge that, in the optical 
waveguide type photodiode having the device structure shown in FIG. 1, the 
thickness d of the optical absorption layer 4 and the optical confinement 
factor .GAMMA. have a nearly linear relationship, as shown in FIG. 4, 
especially in a region of the optical absorption layer 4 having a 
thickness d of 0.1 .mu.m or less. As a consequence, it was found that in 
order to fulfill the relationship shown in equation (4) above, the 
thickness d(x) of the optical absorption layer 4 as a function of the 
guided distance x may be set as follows: 
##EQU3## 
The optical waveguide type photodiode according to the present invention is 
obtained based on the technical knowledge described above, and has a 
device structure in which a plurality of semiconductor layers formed one 
upon another on a semiconductor substrate include an optical absorption 
layer and a pair of optical confinement layers sandwiching the optical 
absorption layer therebetween, the optical absorption layer and the pair 
of optical confinement layers forming an optical waveguide. The present 
invention is characterized especially in that the light absorption 
quantity per unit length of the optical waveguide is substantially 
constant throughout the entire area of the optical waveguide. Namely, the 
object of the present invention is to provide an optical waveguide type 
photodiode having a waveguide structure in which the light absorption 
quantity per unit length of the optical waveguide is made substantially 
constant so that light incident on and introduced into the optical 
waveguide may be absorbed at a constant rate over the entire area of the 
optical waveguide. 
To achieve the object, the present invention provides a photodiode having a 
waveguide structure in which the optical confinement factor .GAMMA.(x) of 
the optical waveguide is set so as to increase with distance x from the 
light incident front facet of the optical waveguide, thereby making the 
light absorption quantity per unit length substantially constant. 
Also, to achieve the above object, the present invention provides a 
photodiode having a waveguide structure in which the thickness d(x) of the 
optical absorption layer is set so as to increase with the distance x from 
the light incident front facet of the optical waveguide, whereby the 
optical absorption coefficient .GAMMA..alpha. varies in the light guiding 
direction so that the light absorption quantity per unit length may be 
substantially constant. 
With the optical waveguide type photodiode having the aforementioned 
waveguide structure, light energy is absorbed substantially uniformly over 
the entire area of the optical waveguide, whereby inconveniences such as 
local generation of photo carriers in large quantities are prevented and 
also the modulation distortion is suppressed. Further, since light energy 
is absorbed in the entire area of the optical waveguide, it is possible to 
provide an optical waveguide type photodiode having a thin optical 
absorption layer and yet having sufficiently high photo sensitivity. 
A photodiode production process according to the present invention 
comprises the steps mentioned below. After a first optical confinement 
layer made of a semiconductor having a first conductivity is formed on a 
semiconductor substrate [first step], a pair of selective area growth 
masks are formed on the first optical confinement layer in such a manner 
that a photodiode forming area on which a photodiode is to be formed is 
situated between the masks [second step]. The masks have a mask pattern 
whose mask width decreases/increases in a direction perpendicular to the 
direction of interposition of the photodiode forming area between the 
masks. Then, using the masks, an optical absorption layer is selectively 
grown on the first optical confinement layer [third step]. The optical 
absorption layer is made of a semiconductor having a smaller energy gap 
than that of the first optical confinement layer. Then, after the masks 
are removed, a second optical confinement layer made of a semiconductor 
having a second conductivity is formed on the optical absorption layer 
[fourth step]. The semiconductor forming the second optical confinement 
layer is opposite in conductivity to the first optical confinement layer 
and has a greater energy gap than that of the optical absorption layer. 
Thus, in the photodiode production process according to the present 
invention, the optical absorption layer is selectively grown using a pair 
of selective area growth masks whose mask width gradually 
increases/decreases in a longitudinal direction thereof, whereby the 
thickness and composition of the optical absorption layer are made to 
gradually vary in the light guiding direction which is the longitudinal 
direction of the optical absorption layer. This process permits easy and 
high-accuracy production of photodiodes having a waveguide structure such 
that the light energy absorption quantity per unit length of the optical 
waveguide is substantially constant over the entire area of the waveguide.

DETAILED DESCRIPTION OF THE INVENTION 
An optical waveguide type photodiode according to one embodiment of the 
present invention has a device structure shown in FIG. 5, for example, 
wherein a plurality of semiconductor layers including a pair of optical 
confinement layers, an optical absorption layer, etc. are successively 
formed by crystal growth on an n-InP substrate 11 having a carrier density 
of 4.times.10.sup.18 cm.sup.-3. 
Specifically, a buffer layer 12 formed on the n-InP substrate 11 and having 
a thickness of 1 .mu.m is made of n-InP having a carrier density of 
2.times.10.sup.18 cm.sup.-3. A first optical confinement layer 13, which 
is formed on the buffer layer 12 and is 2 .mu.m thick, is made of, for 
example, n-GaInAsP having a carrier density of 1.times.10.sup.18 cm.sup.3 
and having a band gap wavelength of 1.25 .mu.m. An optical absorption 
layer 14 formed on the first optical confinement layer 13 is made of 
undoped GaInAs having a band gap wavelength of 1.65 .mu.m. The thickness 
d(x) of the optical absorption layer 14 varies with distance x from an 
front facet thereof in accordance with equation (5) mentioned above. The 
optical absorption layer 14 of which the thickness d(x) varies with the 
distance x from its front facet is formed by epitaxial growth on a 
selective area with the use of a pair of dielectric masks, as described 
later. 
A second optical confinement layer 15, which is formed on the GaInAs 
optical absorption layer 14 and is 2 .mu.m thick, is made of p-GaInAsP 
having a carrier density of 1.times.10.sup.18 cm.sup.-3 and having a band 
gap wavelength of 1.25 .mu.m. A cladding layer 16 formed on the second 
optical confinement layer 15 and having a thickness of 2 .mu.m is made of 
p-InP having a carrier density of 1.times.10.sup.18 cm.sup.-3. A contact 
layer 17, which is formed on the cladding layer 16 and is 0.4 .mu.m in 
thickness, is made of p.sup.+ -GaInAsP having a carrier density of 
2.times.10.sup.19 cm.sup.-3. The band gap wavelength of the p.sup.+ 
-GaInAsP contact layer 17 is 1.55 .mu.m. 
In FIG. 5, reference numeral 18 denotes a P electrode of Ti/Pt/Au ohmically 
deposited on the p.sup.+ -GaInAsP contact layer 17, and 19 denotes an N 
electrode of AuGeNi/Au deposited on the reverse side of the n-InP 
substrate 11. Further, reference numeral 20 represents a non-reflective 
film of SiN.sub.x deposited over on an front facets of the aforementioned 
semiconductor layers. Although not illustrated, a 0.4 .mu.m-thick region 
having a carrier density of 1.times.10.sup.16 cm.sup.-3 is formed on and 
beneath the GaInAs optical absorption layer 14 as a junction interface 
with the optical confinement layers 13 and 15. 
The individual semiconductor layers 12, 13, 15, 16 and 17 have their 
lattices matched with the n-InP substrate 11. The optical absorption layer 
14 includes a lattice unmatched portion because it is formed by selective 
area growth, but the amount of lattice mismatching of the optical 
absorption layer 14 is as small as 1% or less. The thickness of the 
optical absorption layer 14 is restricted to a small value of 0.2 .mu.m at 
the maximum, and accordingly, there is substantially no possibility of the 
device characteristics being deteriorated due to lattice mismatching. 
The optical absorption layer 14, of which the thickness gradually varies 
with the distance x from its front facet as mentioned above, is formed by 
selective area growth of GaInAs with the use of a pair of dielectric masks 
whose width varies depending on the distance x. The selective area growth 
is achieved by a vapor-phase crystal growing process such as MOCVD (Metal 
Organic Chemical Vapor Deposition), MBE (Molecular Beam Epitaxy), gas 
source MBE, or CBE (Chemical Beam Epitaxy), utilizing the phenomenon that 
the presence of an area covered with a dielectric mask causes the rate of 
growth of a semiconductor layer on an exposed area, which is not covered 
with the dielectric mask, to vary depending on the width of the 
surrounding dielectric mask, with the result that the thickness and 
composition vary within the resultant semiconductor layer. Specifically, 
since no semiconductor layer is grown on the dielectric mask, the 
semiconductor material flying onto the dielectric mask flows into the 
exposed area with no dielectric mask thereon. Thus, the selective area 
growth of the optical absorption layer 14 is carried out while 
accelerating the rate of growth of the semiconductor layer on the exposed 
area by the inflow of the semiconductor material from the dielectric mask 
side. 
Provided that the width of the area on which the semiconductor layer is to 
be grown is Wg and the width of a pair of selective growth dielectric 
masks formed on opposite sides of this area is Wm, the ratio in thickness 
between semiconductor layers grown on the area (selected area) situated 
between the pair of masks and on an area (non-selected area) which is not 
situated between the masks varies depending on the mask width Wm, as shown 
in FIG. 6. Namely, in the case where the width Wm of the pair of masks is 
small, there occurs almost no difference in thickness between the 
semiconductor layers grown respectively on the above two areas, as shown 
in FIG. 6. However, if the mask width Wm is large and thus the width Wg of 
the semiconductor layer-growing area (selected area) situated between the 
pair of masks is correspondingly small, then the thickness of the 
semiconductor layer grown on the selected area becomes large compared with 
that grown on the non-selected area. 
In the case where GaInAs, for example, is grown by means of the selective 
area growth technique, since In has a greater diffusion length than Ga, 
the effect of accelerating the growth of the semiconductor layer is 
conspicuous. Accordingly, if the mask width Wm is large and GaInAs is 
selectively grown on the exposed area (selected area) with a narrow width 
Wg between the pair of masks, the proportion of In to the grown GaInAs 
layer becomes large, and the increased proportion of In induces a 
corresponding reduction in the energy gap of this area. 
The thickness of the optical absorption layer 14 is controlled by actively 
utilizing the aforementioned feature of the selective area growth. 
Specifically, the thickness of the optical absorption layer 14 is 
gradually increased with the distance x from its front facet so that the 
absorption coefficient of the layer 14 may gradually increase with the 
distance x. Thus, a pseudo-absorption coefficient .GAMMA..alpha. of the 
optical waveguide is increased with the distance x, thereby obtaining an 
optical waveguide structure in which the light absorption quantity per 
unit length is almost constant. 
A photodiode production process according to the present invention, which 
actively utilizes the selective area growth technique for the optical 
absorption layer 14, will be now explained. To produce the photodiode, 
first, the n-InP buffer layer 12 and the n-GaInAsP optical confinement 
layer 13 are successively formed on the n-InP substrate 11 by an MOCVD 
process, for example [first step]. Subsequently, an SiN.sub.x film of 0.12 
.mu.m thick is deposited on the n-GaInAsP optical confinement layer 13 by 
a PCVD process. This SiN.sub.x film is then subjected to photolithography 
to be shaped into a pattern as shown in FIG. 7a, for example, thereby 
obtaining a pair of selective area growth masks 21 [second step]. 
The pair of masks 21 are formed in such a manner that an area (photodiode 
forming area) of 20 .mu.m wide and 50 .mu.m long, for example, which is 
part of the optical absorption layer to be formed on the optical 
confinement layer 13, is situated between the pair of masks 21. Also, the 
pattern of the masks 21 has a shape such that the mask width Wm gradually 
increases from 5 .mu.m at a light incident end up to 50 .mu.m at the other 
terminal end. Basically, the pattern of the masks 21 may be such that the 
mask width Wm varies linearly; in this embodiment, however, the edges of 
the masks 21 are convexly curved, as shown in FIG. 7a, thus providing a 
gradual broadening of the mask width Wm. 
The pair of masks 21 having the pattern shown in FIG. 7 correspond to one 
photodiode (segment) to be formed on the semiconductor substrate 11. 
Accordingly, in cases where a plurality of photodiodes (segments) are to 
be collectively formed on the semiconductor substrate 11, a mask pattern 
may be used of which the overall shape consists of a plurality of basic 
patterns (pairs of masks 21) of FIG. 7a arranged at a predetermined pitch, 
though the illustration thereof is omitted. 
Then, using the pair of masks 21, the optical absorption layer 14 of 
undoped GaInAs is selectively grown on the n-GaInAsP optical confinement 
layer 13 by an MOCVD process [third step] The selective area growth of the 
GaInAs optical absorption layer 14 is performed in such a manner that the 
thickness of the optical absorption layer 14 at the light incident end 
where the width Wm of the masks 21 is 5 .mu.m is controlled to 0.05 .mu.m. 
In this case, the composition of the GaInAs grown on an area not affected 
by the masks 21 at all (i.e., the area not situated between the masks 21) 
was set such that the resultant lattice was unmatched by approximately 
-0.2% from the latticematching condition. As a result, the GaInAs optical 
absorption layer 14 had a film thickness distribution characteristic C 
shown in FIG. 7b, and it was confirmed that the film thickness at the end 
terminal (rear facet) was approximately 0.1 .mu.m. 
It was also confirmed by calculation that the optical confinement factor 
.GAMMA.(x) had a distribution characteristic D shown in FIG. 7c. As for 
the composition distribution of the GaInAs optical absorption layer 14, it 
was found that the band gap wavelength became longer with increase in the 
film thickness. Further, lattice mismatching of the GaInAs optical 
absorption layer 14 at the terminal end with a maximum thickness was 
approximately 0.2%. 
Then, after the masks 21 composed of the SiN.sub.x film are removed by 
etching, the p-GaInAsP optical confinement layer 15 is grown on the GaInAs 
optical absorption layer 14 by an MOCVD process. Further, on the p-GaInAsP 
optical confinement layer 15 are successively formed the p-InP cladding 
layer 16 and the p.sup.+ -GaInAsP contact layer 17 [fourth step]. After 
these layers 15, 16 and 17 were grown, a level difference of about 0.05 
.mu.m attributable to the aforementioned film thickness distribution of 
the GaInAs optical absorption layer 14 was almost reduced and the surface 
of the uppermost layer was nearly flat. 
Subsequently, the multiple semiconductor layers formed in the above manner 
are subjected to wet etching to shape the aforementioned photodiode 
forming area of 20 .mu.m wide and 50 .mu.m long into ridge stripe form. 
This step is carried out by setting a stripe of 15 .mu.m wide so as to 
pass through the center of a light receiving area. Then, SiN is deposited 
on the semiconductor multilayer structure shaped into ridge stripe form 
(i.e., on the p.sup.+ -GaInAsP contact layer 17 which is the uppermost 
semiconductor layer), thereby performing a passivation process and an 
electrical insulation process. Further, polyimide is deposited on the SiN, 
and after the upper surface of the thus-deposited polyimide is flattened, 
only part of the polyimide in an area at which photodiodes are to be 
separated from each other is removed. 
Then, a window for electrode contact is formed in the SiN and the polyimide 
remaining on the stripe and the P electrode 18 of Ti/Pt/Au is ohmically 
deposited in the window. The P electrode 18 is extended from the upper 
part of the stripe and is connected, for example, to a 50 .mu.m-square 
bonding area. On the other hand, the reverse side of the n-InP substrate 
11 is polished for adjustment of the thickness thereof, and the N 
electrode 19 of AuGeNi/Au is ohmically deposited on the reverse side. 
Subsequently, the multiple semiconductor layers are cleaved perpendicularly 
to the stripe at a location corresponding to the end of the photodiode 
forming area, that is, at a location where the thickness of the GaInAs 
optical absorption layer 14 is the smallest, thereby obtaining a 
photodiode bar having a flat light incident surface. Thus, the stripe is 
restricted in length to 50 .mu.m. Then, the non-reflective film 20 of 
SiN.sub.x is deposited on the cleaved surface which later serves as the 
light incident front facet, and the photodiode bar is cut to obtain 
individual separate photodiodes (segments). 
Using the photodiode fabricated in this manner, a single-mode optical fiber 
was butt-jointed to the incident front facet of the photodiode, and light 
was introduced into the photodiode via the optical fiber to measure the 
light sensitivity. As a result, it was found that under a maximum coupling 
condition, the light sensitivity was 0.95 A/W for light with a wavelength 
of 1.55 .mu.m and was 0.9 A/W for light with a wavelength of 1.3 .mu.m. 
Also, the length of the photodiode was varied on purpose to restrict the 
stripe length, and the light sensitivity of the photodiode was measured to 
examine the light absorption quantity. It was confirmed that the light 
absorption quantity per unit length was constant. 
Further, the photodiode was examined as to second- and third-order 
intermodulation distortions by an optical heterodyne method. Under 
operating conditions of modulation frequencies of 244 MHz and 250 MHz, 
modulation factor of 70% and average input optical power of 0 dBm, the 
second- and third-order modulation distortions were -90 dBc and -110 dBc 
on the average. These values signify excellent performance as compared 
with conventional photodiodes generally employed, proving that with the 
photodiode of the present invention, the modulation distortion is 
remarkably suppressed. Also, the modulation distortion was examined with 
the degree of coupling between the photodiode and the optical fiber 
changed, and no substantial deterioration in the modulation distortion was 
observed. 
Moreover, reduction in the light sensitivity of the photodiode caused by 
displacement of the connecting portions of the photodiode and the optical 
fiber was examined. Where the connecting portions were displaced from each 
other by .+-.3 .mu.m with respect to the maximum coupling position, the 
observed reduction in the light sensitivity was as low as 0.5 dB. In other 
words, an allowable displacement range of as large as .+-.3 .mu.m is 
ensured for the connection between the photodiode and an optical fiber, 
and this is extremely useful in constructing light-receiving modules 
including optical fibers. 
The dark current of the photodiode was as small as 100 pA at a reverse bias 
of 3 V. Also, it was confirmed that the Zener breakdown voltage of the 
photodiode with an increased reverse bias voltage applied thereto was as 
high as 15 V. Further, the capacitance of the photodiode was 0.2 pF, and 
it was found that the high-frequency response was as high as 20 GHz at a 
-3 dB point. 
As another embodiment of the present invention, a photodiode designed 
exclusively for receiving light with a wavelength of 1.55 .mu.m was 
fabricated in the following manner. In this case, GaInAsP having a band 
gap wavelength of 1.35 .mu.m was used for the optical confinement layers 
13 and 15, so that the relative index difference between the optical 
confinement layer 13, 15 and the optical absorption layer 14 was small. 
This is effective in reducing the optical confinement factor .GAMMA. of 
the optical waveguide. Also, the reduction in the optical confinement 
factor .GAMMA. of the optical waveguide induces a corresponding 
enlargement of the spot size of the guided light, whereby the loss of 
coupling with an optical fiber can effectively be reduced. 
Basically, the photodiode of this embodiment is fabricated following the 
same procedure as employed in the foregoing embodiment. However, in this 
embodiment, a pair of masks 22 used to selectively grow the optical 
absorption layer 14 had a mask pattern as shown in FIG. 8a. The masks 22 
also are formed in such a manner that a light receiving area of 20 .mu.m 
wide and 100 .mu.m long is situated between the pair of masks. The pattern 
of the masks 22 is in this case such that the mask width Wm gradually 
varies from 5 .mu.m at the light incident end to 40 .mu.m at the other 
terminal end, thus defining a generally trapezoidal area. 
Using the masks 22, the GaInAs optical absorption layer 14 was formed by 
crystal growth on the optical confinement layer 13 so that the film 
thickness thereof at the light incident end might be 0.04 .mu.m. The 
optical absorption layer 14 formed in this case had a film thickness 
distribution characteristic E shown in FIG. 8b, proving that the film 
thickness gradually varied up to about 0.08 .mu.m at the terminal end. 
Also, it was confirmed by calculation that the optical confinement factor 
.GAMMA.(x) of the optical waveguide had a characteristic F shown in FIG. 
8c. 
Then, following the same procedure as employed in the foregoing embodiment, 
the photodiode with a stripe length of 100 .mu.m was produced. The 
modulation distortion of the thus-obtained photodiode was measured in the 
same manner as in the foregoing embodiment, and it was found that the 
second- and third-order modulation distortions of the photodiode were -80 
dBc and -110 dBc on the average. Compared with the foregoing embodiment, 
this photodiode had a greater stripe length and the optical absorption 
layer 14 had a smaller thickness; therefore, the capacitance of the 
photodiode was slightly higher and was found to be 0.5 pF at a maximum. 
The photodiode had a slightly lower high-frequency response of 10 GHz at a 
-3 dB point, yet proving that the photodiode had sufficient practicality. 
The present invention is not limited to the embodiments described above. 
Although in each of the above embodiments GaInAsP is used for the optical 
confinement layers, other materials having large band gap energy as 
compared with the optical absorption layer, such as AlGaInAs, may also be 
used for the optical confinement layers. Further, the present invention is 
of course applicable to the case where a photodiode is formed on a GaAs 
substrate. Thus, various modifications can be made without departing from 
the scope or spirit of the invention.