High-speed photodetector

Details High-speed photodetector High-speed photodetector

2001-03-28

2002-12-24

Porta, David (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Photocell controlled circuit

C385S131000

Reexamination Certificate

active

06498337

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATION
The present application is based on Japanese priority application No.2000-205282 filed on Jul. 6, 2000, the entire contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to semiconductor devices and more particularly to a photodetector having an integral optical waveguide.
2. Description of the Related Art
With widespread use of information technology in human society, there is a demand for optical-fiber telecommunication systems that are capable of handling sharply increasing traffic of information.
In order to meet for this demand, there is a proposal to increase the transmission rate of the optical signals through the optical fibers from a conventional rate of 10 Gbps to a faster rate of 40 Gbps or more. When this approach is adopted, it is necessary to increase the response speed of the photodetector used in such an optical-fiber telecommunication system for detecting the optical signals.
Conventionally, PIN photodiodes have been used successfully in optical-fiber telecommunication systems as a high-speed photodetector. A PIN photodiode achieves a high response speed by providing a thin undoped semiconductor layer in a p-n junction such that the capacitance of the p-n junction is reduced.
Further, there is a proposal to increase the response speed and photosensitivity of a PIN photodiode further, by providing an integral optical waveguide adjacent to the PIN photodiode on a common semiconductor substrate.
FIG. 1
shows the construction of a conventional PIN photodetector
10
having such an integral optical waveguide.
Referring to
FIG. 1
, the PIN photodetector
10
is constructed on a compound semiconductor substrate
11
typically of InP and includes a PIN photodiode
12
, wherein the PIN photodiode
12
includes therein an optical absorption layer
12
A formed on the compound semiconductor substrate
11
, and an optical waveguide
13
is provided on the compound semiconductor substrate
11
adjacent to the PIN photodiode
12
in optical coupling therewith.
The optical waveguide
13
includes an optical waveguide layer
13
A having a first end surface to which an optical beam from an external optical waveguide, such as an optical fiber
14
, comes in and a second end adjacent to the PIN photodiode
12
for injecting the optical beam guided through the optical waveguide layer
13
A into the PIN photodiode
12
. Further, there is provided a cladding layer
13
B on the optical waveguide layer
13
A for confining the optical beam in the optical waveguide layer
13
A.
Thus, the optical waveguide layer
13
A is provided in optical coupling with the optical absorption layer
12
A of the PIN photodiode
12
, and there occurs an efficient injection of the optical beam guided through the optical waveguide layer
13
A into the optical absorption layer
12
A of the PIN photodiode
12
. The construction of
FIG. 1
is effective for reducing the thickness of the optical absorption layer
12
A below about 1 &mgr;m without causing degradation of optical coupling between the optical waveguide layer
13
A and the optical absorption layer
12
A. As a result of decreased thickness of the optical absorption layer
12
A, the PIN photodiode
12
shows a high response speed.
In such a PIN photodiode, it is possible to improve the response speed by providing the undoped optical absorption layer
12
A between a p-type layer and an n-type layer constituting a PN junction so as to reduce the junction capacitance. On the other hand, such a construction has to be designed such that the optical absorption layer
12
A has a sufficiently small thickness so as to avoid increase of the transit time of the optically excited carriers across the optical absorption layer
12
A and associated degradation of the response speed of the PIN photodiode
12
. From this viewpoint, it is desirable to reduce the thickness of the optical absorption layer
12
A as much as possible.
On the other hand, there arises a problem, in such a conventional photodetector
10
having the optical waveguide
13
integrally to the PIN photodiode
12
and injection of optical signal occurs from the external optical fiber
14
into the PIN photodiode
12
via the optical waveguide
13
, in that a large optical loss may occur at the first end surface of the optical waveguide
13
to which an optical signal in the optical fiber
14
is injected. It should be noted that the optical beam transmitted through the optical fiber
14
has a beam diameter of about 7.0 &mgr;m in terms of full-height width, while the optical waveguide layer
13
A in the optical waveguide
13
has a thickness of 1 &mgr;m or less, which is substantially identical with the thickness of the optical absorption layer
12
A in the PIN photodiode
12
. When the thickness of the optical absorption layer
12
A is increased for avoiding this problem, there arises a problem of degraded response speed of the photodiode
12
.
As explained previously, it is desirable to suppress the thickness of the optical absorption layer
12
A to be 1 &mgr;m or less in the PIN photodiode
12
for improving the response speed. Further, it is preferable to reduce the longitudinal length of the photodiode
12
A as much as possible so that the parasitic capacitance of the p-n junction is reduced. On the other hand, such a PIN photodiode having a short longitudinal length raises a problem in that the optical beam, entered into the PIN photodiode
12
at an incident end surface with offset from the optical absorption layer
12
A, tends to exit from the opposite end surface before the optical beam is effectively confined into the optical absorption layer
12
A by the optical confinement action.
In order to avoid these problems, there is a proposal to provide a spherical lens at an end of the optical fiber
14
such that the optical beam in the optical fiber
14
is injected efficiently into the thin optical absorption layer
12
A of the PIN photodiode
12
.
However, such a construction that uses a lens in the photodetector
10
is difficult to produce because of the stringent precision required for the optical system including the lens. As a result, such a construction inevitably increases the cost of the photodetector.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of the present invention to provide a novel and useful photodetector wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide a high-speed photodetector capable of minimizing optical loss with regard to an incoming optical beam having a large beam diameter.
Another object of the present invention is to provide a photodetector, comprising:
a substrate having a principal surface;
a photodetection part provided on a part of said principal surface of said substrate, said photodetection part comprising: an optical absorption layer of a semiconductor material extending parallel to said principal surface, said optical absorption layer causing excitation of carriers therein in response to an optical radiation supplied thereto; and
an optical waveguide provided on said principal surface of said substrate, said optical waveguide guiding an optical beam in a direction parallel to said principal surface from a first end surface to a second end surface adjacent to said photodetection part, such that said optical beam guided through said optical waveguide is injected into said optical absorption layer of said photodetection part, said optical waveguide comprising: a first tapered optical waveguide layer extending from said first end surface to said second end surface of said optical waveguide, said first tapered optical waveguide layer decreasing a thickness thereof continuously from said first end surface to said second end surface; a second tapered optical waveguide layer provided on said first tapered optical waveguide layer with a separation therefrom, said second tapered optical waveguide layer decreasing a thickness thereof continuously fr

Affiliated with

Also associated with

Rating

High-speed photodetector does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with High-speed photodetector, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and High-speed photodetector will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2917749