Optoelectronic device

An avalanche photodiode including a substrate, a first semiconductor region of a first conductivity type having a relatively large band gap, a second semiconductor region of a second conductivity type having a relatively large band gap, and a third semiconductor region of the first conductivity type having a band gap smaller than the band gap of the first and second semiconductor regions, is disclosed in which, in order to suppress an increase in dark current and to cause the avalanche photodiode to operate on a low voltage, a fourth semiconductor region equal in conductivity type to and larger in impurity concentration than the first semiconductor region is formed in the first semiconductor region at a position below a central portion of a pn junction formed between the first semiconductor region and the second semiconductor region. The avalanche photodiode formed with this structure has low-noise and low operation voltage characteristics.

BACKGROUND OF THE INVENTION 
The present invention relates to optoelectronic devices for detecting or 
emitting light, such as an avalanche photodiode, other photodiodes which 
are used as photodetectors in fiber-optical communication, and a laser 
diode. 
In order to lower the noise level of an avalanche photodiode type, and to 
increase the operation speed of the avalanche photodiode, an avalanche 
photodiode made of compound semiconductor materials has been developed. In 
this avalanche photodiode, incident light is absorbed by a semiconductor 
region having a relatively small band gap, and a photocurrent thus 
obtained is amplified on the basis of the avalanche multiplication 
phenomenon in another semiconductor region having a band gap larger than 
the band gap of the light absorbing region. Semiconductor materials such 
as an InP compound and a GaSb compound are used for making the above 
avalanche photodiode. Further, in order to improve the performance of the 
above avalanche photodiode, a guard ring region for preventing the 
avalanche breakdown on the periphery of a pn junction is formed in the 
avalanche photodiode, and means for suppressing the carrier accumulcation 
effect based upon the difference in energy gap between the light absorbing 
region and the avalanche multiplication region to increase the operation 
speed of the avalanche photodiode is provided therein, that is, the region 
4 of FIG. 6 having a band gap which is intermediate between the band gap 
of the light absorbing region and that of the avalanche multiplication 
region, is interposed between these regions. 
However, in a compound semiconductor which has a small band gap and a small 
effective electron mass, there arises the following problem. That is, when 
a strong electric field is applied in the compound semiconductor, 
breakdown due to the tunneling effect is apt to occur before the avalanche 
breakdown is generated. Accordingly, when a reverse bias voltage applied 
across an avalanche photodiode with the SAM structure (namely, the 
separated absorption and multiplication structure) is increased so that a 
strong electric field extending from a pn junction which is formed in a 
semiconductor region having a large band gap (namely, the region 5 of FIG. 
6), is applied to a middle region (namely, the region 4 of FIG. 6) or 
light absorbing region (namely, the region 3 of FIG. 6) having a small 
band gap, a dark current is increased by the field emission due to the 
tunnel effect. In order to prevent such an increase in dark current, the 
impurity concentration of that portion of the semiconductor region having 
the large band gap (namely, the region 5 of FIG. 6) which exists at a 
portion between the pn junction and the middle region or light absorbing 
region, has been made high, or the thickness of the above portion has been 
made large. However, when the impurity concentration of the above portion 
is made high, a maximum electric field intensity at the pn junction 
becomes large. Then the ratio k of the ionization coefficient of hole 
.beta. to that of electron .alpha., k.ident..alpha./.beta., is decreased. 
Here, the ionization coefficient means the number of electron-hole pairs 
when one hole (or electron) transits a unit length. As a result, the 
avalanche magnification noise is increased (the noise of the avalanche 
photodiode is low when the ratio k is high). When the thickness of the 
above portion is made large, the operating voltage applied across the 
avalanche photodiode becomes high. Such a high operating voltage is 
undesirable from a practical point of view. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an avalanche photodiode 
which can prevent an increase in dark current caused by a tunneling 
current as well as one which, is low in noise, and which has a low 
operating voltage. 
According to the present invention, there is provided an optoelectronic 
device including a first semiconductor region of a first conductivity type 
having a relatively large band gap, a second region of a second 
conductivity type having a relatively large band gap and a third 
semiconductor region of the first conductivity type having a band gap 
smaller than the band gap of the first and second semiconductor regions, 
in which device a pn junction is formed between the first semiconductor 
region and the second semiconductor region, and a portion of the first 
semiconductor region is made larger in impurity concentration than the 
remaining portion of the first semiconductor region, to form a fourth 
semiconductor region. For example, in an avalanche photodiode shown in 
FIG. 1, a pn junction 26 is formed between a second semiconductor region 
16 and a first semiconductor region 15 which has a large band gap and a 
small impurity concentration to serve as an avalanche multiplication 
region, and a fourth semiconductor region 25 which is larger in impurity 
concentration than the first semiconductor region and has the large band 
gap, is formed in the first semiconductor region so that the fourth 
semiconductor region exists only under a central portion of the pn 
junction and lies in proximity to or comes in contact with a middle 
semiconductor region 14. 
As is apparent from FIG. 2 which shows the electric field intensity 
distribution in the avalanche photodiode of FIG. 1, a strong electric 
field required for the avalanche magnification is limited to that portion 
of the first semiconductor region 15 which exists between the fourth 
semiconductor region 25 and the pn junction 26. The above portion will 
hereinafter be referred to as a main junction or avalanche portion. 
As shown in FIG. 2, the intensity of electric field is greatly decreased at 
the fourth semiconductor region 25. Thus, a weak electric field is applied 
in the third semiconductor region 13 (serving as a light absorbing 
region), in the middle semiconductor region 14 and in that portion of the 
first semiconductor region which exists between the fourth semiconductor 
region 25 and the middle semiconductor region 14. That is, the electric 
field intensity in the semiconductor regions 13 and 14 which are smaller 
in band gap than the first semiconductor region and thus are apt to 
produce the tunneling effect, is kept at a level necessary for carriers to 
obtain a saturated drift velocity, and hence an increase in dark current 
caused by a tunneling current will not occur. It should be noted that the 
fourth semiconductor region 25 is absent under the peripheral portion of 
the pn junction 26. That is, only the first semiconductor region 15 having 
a low impurity concentration exists between the peripheral portion of the 
pn junction and the middle semiconductor region. Accordingly, a maximum 
electric field in that portion of the first semiconductor region which 
exists below the peripheral portion of the pn junction, will be smaller 
than a maximum electric field in the avalanche portion, and thus the above 
portion of the first semiconductor region can have a guard ring effect. 
Hence, this portion will hereinafter be referred to as a guard ring 
portion. The above guard ring effect can be explained as follows. 
Referring to FIG. 2, the area bounded by the intensity distribution curve 
along the line A--A' and the X-axis (namely, abscissa) will be equal to 
the area bounded by the intensity distribution curve along the line B--B' 
and the X-axis, for a reverse bias voltage applied across the avalanche 
photodiode. Thus, as shown in FIG. 2, the electric field is weak 
throughout the guard ring portion. Therefore, the guard ring portion can 
act effectively as a guard ring, and the avalanche photodiode can exhibit 
stable avalanche magnification characteristics. 
The avalanche multiplication phenomenon at the avalanche portion is 
governed mainly by the distance between the pn junction 26 and the fourth 
semiconductor region 25, since the first semiconductor region has a low 
impurity concentration. Accordingly, the operating voltage of the 
avalanche photodiode of FIG. 1 is stable without being affected by 
variations in impurity concentration at the first semiconductor region. 
Further, in the avalanche photodiode of FIG. 1, the middle semiconductor 
region 14 having an intermediate band gap is interposed between the first 
semiconductor region 15 and the light absorbing region 13 to lower the 
barrier for photo-excited carriers caused by the difference in band gap 
between the regions 15 and 13. Accordingly, the photo-excited carriers 
generated in the light absorbing region 13 are rapidly injected into the 
first semiconductor region 15 serving as the avalanche multiplication 
region, and hence the photoresponse time of the avalanche photodiode is 
fast. 
Another avalanche photodiode according to the present invention is shown in 
FIG. 3. The avalanche photodiode of FIG. 3 is basically identical with 
that of FIG. 1, except that a semiconductor region 47 is additionally 
provided. The semiconductor region 47 and a first semiconductor region 35 
which serves as the avalanche multiplication region, are made of the same 
semiconductor material having a relatively large band gap, but the region 
47 is larger in impurity concentration than the first region 35. 
Accordingly, the electric field extending from the avalanche portion to a 
light absorbing region 33 is weakened by a fourth semiconductor region 45, 
and is further weakened by the region 47. Thus, the electric field formed 
in the light absorbing region (namely, the third semiconductor region) 33 
having a relatively small band gap and a middle semiconductor region 34 
having an intermediate band gap, is weakened to a level necessary for 
carriers to obtain a saturated drift velocity. While, the electric field 
extending from the guard ring portion to the light absorbing region is 
weakened at the semiconductor region 47, and thus the electric field 
formed in the light absorbing region 33 and the middle semiconductor 
region 34 has a small intensity as in the case of the avalanche portion. 
In other words, the voltage applied across the light absorbing region 33 
and the middle semiconductor region 34 is reduced by an amount 
corresponding to a voltage drop across the semiconductor region 47, and 
thus the electric field in the light absorbing region 33 and the middle 
semiconductor region 34 which are smaller in band gap than the first 
semiconductor region 35, is weakened by the region 47. Further, a strong 
electric field is formed in the avalanche portion which exists between a 
central portion of a pn junction 46 and the fourth semiconductor region 
45. While, the guard ring portion lying under a peripheral portion of the 
pn junction 46 does not include the fourth semiconductor region 45. 
Accordingly, the guard ring portion is larger in thickness of the first 
semiconductor region than the avalanche portion, and hence the electric 
field in the guard ring portion is weaker than that in the avalanche 
portion, as in the avalanche photodiode of FIG. 1. 
As mentioned above, according to the present invention, there is provided 
an avalanche photodiode including a first semiconductor region of a first 
conductivity type having a relatively large band gap, a second 
semiconductor region of a second conductivity type having a relatively 
large band gap and a third semiconductor region of the first conductivity 
type having a band gap smaller than the band gap of the first and second 
semiconductor regions, in which avalanche photodiode a pn junction is 
formed between the first semiconductor region and the second semiconductor 
region, and a fourth semiconductor region equal in conductivity type to 
and larger in impurity concentration than the first semiconductor region 
is formed in the first semiconductor region. In the avalanche photodiode 
having the above structure, the electric field at the pn junction is 
weakened due to the existence of the fourth region having a high impurity 
concentration, and thus the intensity of the electric field formed in the 
third semiconductor region which has a small band gap and serves as a 
light absorbing region, is small. Accordingly, an increase in dark current 
caused by a tunneling current can be prevented. Further, the guard ring 
portion is larger in thickness of the first semiconductor region than the 
avalanche portion, and the first semiconductor region has a low impurity 
concentration. Accordingly, the electric field formed in the guard ring 
portion is weaker than that formed in the avalanche portion, and hence the 
breakdown on the periphery of the pn junction can be prevented. Thus, 
stable avalanche multiplication is carried out. Further, the avalanche 
portion has a low impurity concentration and a large band gap, and hence 
the avalanche multiplication phenomenon is governed mainly by the 
thickness of the avalanche portion. Accordingly, uniform multiplication 
can be made without being affected by variations in impurity concentration 
at the avalanche portion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
EMBODIMENT I 
FIG. 1 shows a first embodiment of an optoelectronic device according to 
the present invention, and FIG. 2 shows the electric field intensity 
distribution in the first embodiment. Now, explanation will be made for a 
case where the first embodiment is made of compound semiconductor 
materials including InP. Referring to FIG. 1, an n-InP layer 12 having an 
impurity concentration of 2.times.10.sup.15 cm.sup.-3 and a thickness of 
2.5 .mu.m is formed on a highly-doped, n.sup.+ -InP substrate 11 through 
vapor phase epitaxial growth techniques. Then, an n-InGaAs layer 13 having 
an impurity concentration of 2.times.10.sup.15 cm.sup.-3 and a thickness 
of 3 .mu.m is grown on the InP layer 12, to be used as a light absorbing 
region, and an n-InGaAsP layer 14 having an impurity concentration of 
2.times.10.sup.15 cm.sup.-3 and a thickness of 0.3 .mu.m is grown on the 
InGaAs layer 13, to be used as a middle semiconductor region. The band gap 
of InGaAsP is intermediate between the band gap of InP and that of InGaAs, 
and corresponds to an optical wavelength of 1.3 .mu.m. Further, an n-InP 
layer 15 having an impurity concentration of 2.times.10.sup.15 cm.sup.-3 
and a thickness of 4 .mu.m is grown on the InGaAsP layer 14, to be used as 
a first semiconductor region. The substrate 11 and the layers 12 to 15 
make up the pellet of the first embodiment. Then, high-energy silicon ions 
are implanted into the InP layer 15 through selective ion implantation 
techniques so that the silicon concentration becomes maximum at a position 
which has a depth of 3 .mu.m from the surface of the layer 15, and a 
maximum silicon concentration becomes equal to 3.times.10.sup.16 
cm.sup.-3, to form a fourth semiconductor region 25. The n-type fourth 
semiconductor region 25 may be formed through techniques other than ion 
implantation techniques. That is, in the course of the growth of the first 
semiconductor region 15, this crystal growth is interrupted, and silicon 
is diffused in the grown layer through selective diffusion techniques. 
Then, the crystal growth of the first semiconductor region 15 is again 
started, to complete the first semiconductor region 15. The impurity 
concentration N and the thickness d of the fourth semiconductor region 25 
are determined, depending upon how much the electric field intensity is to 
be reduced by the fourth semiconductor region 25, and a reduction in 
electric field intensity caused by the fourth semiconductor region 25 is 
given by qNd/E, where q indicates the electronic charge, and E a 
dielectric constant. What is material for the fourth region 25 is that it 
sufficiently raises the average electric field intensity in that portion 
of the region 15 sandwiched between the junction 26 and the fourth region 
25. In other words, the electric field intensity keeps a high value from 
the junction to the fourth region, but becomes sufficiently low in the 
region 13. Thus, it will be apparent that the fourth region 25 may be 
formed to touch the region 14 as shown in righthand part of FIG. 2. Next, 
a p-type layer 16 serving as a second semiconductor region is formed in 
the first semiconductor region 15 through ion implantation or diffusion 
techniques. Thereafter, a two-layer film 17 which includes an SiN.sub.x 
layer by plasma deposition and an SiO.sub.2 layer by chemical vapor 
deposition, is formed, as a passivation film, on the first and second 
semiconductor regions 15 and 16, and then an undesired portion of the 
two-layer film 17 is removed through the well-known photoetching 
techniques. An SiN.sub.x film 18 is formed, as an anti-reflection film, on 
a desired surface area, and electrodes 19 and 20 are formed on the second 
semiconductor region 16 and the substrate 11 through the well-known 
evaporation techniques so that the electrodes 19 and 20 are kept in ohmic 
contact with the second semiconductor region 16 and the substrate 11, 
respectively. 
Now, explanation will be made of the operation of the first embodiment 
which is fabricated in the above-mentioned manner. Incident light within a 
wavelength range from 1 to 1.6 .mu.m passes through the first 
semiconductor region 15 and the fourth semiconductor region 25, and is 
then absorbed by one or both of the middle semiconductor region 14 and the 
InGaAs layer 13. Since a reverse bias voltage is applied between the 
electrodes 19 and 20, the InGaAs layer 13 and the middle semiconductor 
region 14 become a depletion layer. Accordingly, photo-excited carriers 
are moved, at a drift velocity, to the avalanche portion which is applied 
with a strong electric field sufficient to generate avalanche 
multiplication. The carriers injected into the avalanche portion cause the 
avalanche multiplication, and thus an amplified photocurrent flows through 
an external circuit connected between the electrodes 19 and 20. 
EMBODIMENT II 
FIG. 3 shows a second embodiment of an optoelectronic device according to 
the present invention. Referring to FIG. 3, an n-InP layer 32, an n-InGaAs 
layer 33, an n-InGaAsP layer 34, an n-InP layer 47 and an n-InP layer 35 
are successively grown through vapor phase growth techniques, to form a 
desired crystal on a substrate 31. The layers 32, 33, 34 and 35 correspond 
to the layers 12, 13, 14 and 15 of the first embodiment, respectively. The 
n-InP layer 47 which is not included in the first embodiment, has an 
impurity concentration of 2.times.10.sup.16 cm.sup.-3 and a thickness of 
0.4 .mu.m. A fourth semiconductor region 45 and a second semiconductor 
region 36 are both formed through ion implantation techniques. The second 
embodiment having the above structure can operate in substantially the 
same manner as the first embodiment. 
In the above, explanation has been made on a case where each of the first 
and second embodiments is made of compound semiconductor materials 
including InP. However, the present invention is not limited to such 
semiconductor materials, but it is possible to make the light absorbing 
region 13 or 33 of GaSb and to make the first, second, fourth and middle 
semiconductor regions of composition controlled GaAlSbAs, since the band 
gap of GaSb is smaller than that of GaAlSbAs. Further, the first 
semiconductor region 13 or 33 may be made of InAlAs or silicon, instead of 
InP. As mentioned above, various combinations of semiconductor materials 
can be used for making each of the first and second embodiments. 
EMBODIMENT III 
In the first and second embodiments, the peripheral portion of the pn 
junction 26 or 46 can produce a guard ring effect by forming the fourth 
semiconductor region 25 or 45 only under the central portion of the pn 
junction. FIGS. 4 and 5 show a third embodiment of the optoelectronic 
device according to the present invention. In the embodiment of FIGS. 4 
and 5, a graded junction having a low impurity concentration is formed on 
the periphery of a second semiconductor region 56 or 76, to enhance the 
guard ring effect. It is needless to say that, like the first and second 
embodiments, the third embodiment of FIGS. 4 and 5 have the characteristic 
features of the present invention. 
The embodiments of FIGS. 1, 3, 4 and 5 are concerned with an avalanche 
photodiode. However, the technical thought of the present invention is not 
limited to the avalanche photodiode, but is applicable to other 
optoelectronic devices such as a photodiode and a laser diode.