Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure
Reexamination Certificate
2002-12-23
2004-06-29
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
C257S013000, C257S102000, C257S103000, C372S045013, C372S050121, C372S096000
Reexamination Certificate
active
06756609
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a semiconductor light receiving element and a method of manufacturing the same, and in particular, to a semiconductor light receiving element having high-speed and highly efficient light-receiving characteristics and a method for manufacturing the same.
BACKGROUND ART
Conventionally, a semiconductor light receiving element formed from a semiconductor element converting light signals into electric signals has been known.
FIG. 7
is a perspective view showing a structure of an end face refracting type semiconductor light receiving element as such a general semiconductor light receiving element.
FIG. 8
is a cross-sectional view showing a structure of the end face refracting type semiconductor light receiving element as such a general semiconductor light receiving element.
Namely, as shown in FIG.
7
and
FIG. 8
, this end face refracting type semiconductor light receiving element is structured such that a running layer
2
formed from i-InP is formed on a substrate
1
formed from n
+
-InP.
On this running layer
2
, a light absorbing layer
3
formed from p-InGaAs, a block layer
4
formed from p
+
-InGaAsP, and a contact layer
5
formed from p
+
-InGaAs are formed.
A p electrode
6
is mounted on the top surface of the contact layer
5
.
Further, an n electrode
7
is mounted on the bottom surface of the above-described substrate
1
.
Moreover, polyimide
8
is formed at one portion of the side surfaces of the running layer
2
, the light absorbing layer
3
, the block layer
4
, and the contact layer
5
, and at the bottom surface of the p electrode
6
, in order to reduce the capacitance.
As shown in
FIG. 8
, in the end face refracting type semiconductor light receiving element, light is made incident on an inclined end face
1
a
of the substrate
1
.
This incident light is refracted at the end face
1
a
, and thereafter, is made incident on the light absorbing layer
3
via the running layer
2
.
Then, the incident light is absorbed at the light absorbing layer
3
, and thereafter, is photoelectrically converted into electrons and positive holes.
Here, a predetermined reverse bias voltage is applied between the p electrode
6
and the n electrode
7
.
FIG. 9
is a diagram showing a band-diagram of the end face refracting type semiconductor light receiving element at the time of applying the reverse bias voltage.
Next, by using this band diagram, the principles of operation of the end face refracting type semiconductor light receiving element will be considered in detail.
As described above, the incident light is absorbed at the light absorbing layer
3
, and thereafter, the incident light is photoelectrically converted into electrons and positive holes. As a result, electrons
9
are generated at a conduction band, and positive holes
10
are generated at a valence band.
Generally, when the light absorbing layer
3
is not doped, because the mass of the positive holes
10
at the valence band is large, it is difficult for the positive holes
10
to move if a large bias voltage is not applied. Therefore, it is difficult for the positive holes
10
to be taken as electric current.
As a result, in the semiconductor light receiving element, a large bias voltage must be applied, and it is generally known that it is easy for heat destruction, which is due to the Joule heat provided by the product of the applied voltage and the flowing electric current being large, to arise.
In order to overcome such a problem, in the semiconductor light receiving element, a structure such as the following in which the light absorbing layer
3
is doped to p-type has been reported.
Namely, although an internal electric field does not exist in the light absorbing layer
3
, because the positive holes
10
are the majority carrier, the movement thereof is fast regardless of the fact that the mass thereof is large. The positive holes
10
move to the contact layer
5
formed from p
+
-InGaAs, and thereafter, are output to the exterior via the p electrode
6
.
On the other hand, the electrons
9
, which are the minority carrier at the conduction band, move to the running layer
2
by diffusing in the light absorbing layer
3
in which no internal electric field exists.
Because an internal electric field exists in the running layer
2
, the electrons
9
which have reached this region reach the substrate
1
formed from n
+
-InP at a high speed by drifting due to the internal electric field, and are taken out to the exterior via the n electrode
7
.
Here, the block layer
4
formed from p
+
-InGaAsP blocks the electrons
9
such that the electrons
9
generated at the light absorbing layer
3
do not flow toward the contact layer
5
formed from p
+
-InGaAs.
In this way, because the carriers running at the running layer
2
which is a non-doped layer are only the electrons
9
, the semiconductor light receiving element in which the light absorbing layer
3
is doped to p-type is called a unitraveling carrier photodiode (hereinafter, called UTC-PD) (refer to Jpn. Pat. Appln. KOKAI Publication No. 9-275224).
However, in the above-described UTC-PD, there are still problems to be solved as follows.
Namely, because the incident light is absorbed at the light absorbing layer
3
, there is the need to make the thickness of the light absorbing layer
3
thick in order to efficiently convert the light incident on the unitraveling carrier type semiconductor photodiode from the exterior into the electrons
9
and the positive holes
10
.
However, as described above, in the UTC-PD, an internal electric field does not exist in the light absorbing layer
3
doped to p-type, and the electrons
9
which are the minority carrier move only by diffusion in the light absorbing layer
3
.
In this case, although the mass of the electrons
9
is light, the diffusion length of the electrons
9
generally is not long.
Therefore, in such a unitraveling carrier type semiconductor photodiode, if the light absorbing layer
3
is made thicker than the diffusion length of the electrons
9
in order to increase the efficiency of the light incident from the exterior being converted into electric current, the electrons
9
cannot reach the running layer
2
, and the desired high-speed response cannot be obtained.
Moreover, in such a unitraveling carrier type semiconductor photodiode, light cannot be newly absorbed because the electrons
9
are not able to move and accumulate at the conduction band. As a result, the efficiency of the light incident from the exterior being converted into the electrons
9
and the positive holes
10
also deteriorates.
Namely, in the UTC-PD, due to the limit of the thickness of the light absorbing layer
3
doped to p-type, if the thickness is made thin in order to obtain a high-speed response, the efficiency of the light being converted into the electrons
9
and the positive holes
10
deteriorates. Conversely, if the thickness is made thick in order to obtain a high conversion efficiency, the high-speed response cannot be obtained. Therefore, there is the problem that it is difficult to realize a semiconductor light receiving element having both of two characteristics which are high speed and high efficiency.
Incidentally, in order to realize a response speed of 50 GHz or more with excellent reproducibility, the limit of the thickness of the light absorbing layer
3
is about 0.3 &mgr;m from the standpoint of the diffusion length of the electrons
9
.
Namely, this is because the problem arises that, if the thickness of the light absorbing layer
3
is made to be thick to, for example, about 0.6 &mgr;m in order to increase the efficiency of converting the incident light into the electrons
9
and the positive holes
10
, the operation frequency markedly deteriorates since the thickness becomes much greater than about 0.3 &mgr;m which is the diffusion length of the electrons
9
described above.
Moreover, in the UTC-PD, also when the thickness of the light absorbing layer
3
is thinner than the diffusion length of t
Hiraoka Jun
Kawano Kenji
Sasaki Yuichi
Yoshidaya Hiroaki
Anritsu Corporation
Frishauf Holtz Goodman & Chick P.C.
Tran Long
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