Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Light responsive structure
Reexamination Certificate
2000-07-06
2002-06-04
Tran, Minh Loan (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Light responsive structure
C257S185000, C257S186000, C257S187000, C257S432000, C257S436000
Reexamination Certificate
active
06399967
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light receiving device which selectively receives light having a specific wavelength range. More particularly, the present invention relates to a light receiving device which selectively receives a signal light beam having a longer wavelength among a plurality of signal light beams having different wavelengths.
2. Description of the Related Art
Currently, pin photodiodes made of a compound semiconductor are widely used as light receiving devices for the optical fiber communication. The pin photodiodes include a window structure for the purpose of improving the light reception sensitivity. In the pin photodiodes, a light absorbing layer having a narrow forbidden bandwidth (long absorption edge wavelength) is provided near a semiconductor substrate. A window layer having a wide forbidden bandwidth (short absorption edge wavelength) is provided on the light absorbing layer so that light having a wavelength between the absorption edge wavelengths is efficiently absorbed in the light absorbing layer. The absorption edge wavelength of a layer herein means the maximum wavelength of light absorbed by the layer. A most typical pin photodiode made of InGaAs/InP includes a light absorbing layer made of InGaAs and a window layer made of InP. In this case, assuming that this structure receives light from the window layer, the light absorbing layer can receive light in the absorption edge range between 0.9 &mgr;m of InP and 1.65 &mgr;m of InGaAs.
Pass-band pin photodiodes having sensitivity only to light in a narrower wavelength range have been developed. For example, when a wavelength multiplex communication is performed using a signal light beam having a wavelength of 1.3 &mgr;m and a signal light beam having a wavelength of 1.55 &mgr;m, a pass-band photodiode having sensitivity only to each wavelength is used. One of the pass-band photodiodes needs to have a pass-band characteristic in which a sensitivity to a signal light beam having a wavelength of 1.3 &mgr;m is sufficient, but a sensitivity to the signal light beam having a wavelength of 1.55 &mgr;m is substantially zero. Such a characteristic can be achieved if the light absorbing layer is made of InGaAsP having an absorption edge wavelength of 1.4 &mgr;m, instead of InGaAs. With this configuration, the sensitivity to the wavelength 1.3 &mgr;m can be separated 30 dB or more away from the sensitivity to the wavelength 1.55 &mgr;m. This is because an electron-hole pair is not generated in the light absorbing layer by the light beam having a wavelength of 1.55 &mgr;m. Although some absorption of light having a wavelength of 1.55 &mgr;m occurs due to impurity levels in the forbidden band, such absorption has an extremely low efficiency. Therefore, substantially no photoelectric current occurs.
The other pass-band photodiode needs to have a pass-band characteristic in which there is a sufficient sensitivity to the signal light beam having a wavelength of 1.55 &mgr;m but substantially no sensitivity to the signal light beam having a wavelength of 1.3 &mgr;m. An example of a structure achieving such a characteristic is disclosed in Japanese Publication for Opposition No. 1-48663 (1989). In this publication, a heterojunction phototransistor, but not a pin photodiode, is provided as a light receiving device. Referring to
FIG. 6
, a heterojunction phototransistor
500
includes a collector layer
502
, a base layer
503
, an emitter layer
504
, and a wavelength filter
505
, which are provided on an upper side of a semiconductor substrate
501
. A collector electrode
506
is provided on a lower side of the semiconductor substrate
501
. An emitter electrode
507
is provided on an upper side of the wavelength filter
505
. The collector layer
502
, the emitter layer
504
, and the wavelength filter
505
are of the same conductivity type as that of the semiconductor substrate
501
. The base layer
503
has the opposite conductivity type to that of the layers
502
,
504
, and
505
. The emitter layer
504
has a forbidden bandwidth larger than that of the base layer
503
. The forbidden bandwidth of the wavelength filter
505
is intermediate between the forbidden bandwidths of the base layer
503
and the emitter layer
504
.
The heterojunction phototransistor
500
has a current amplifying function of a transistor as well as a light receiving function. In terms of the light receiving function, the base layer
503
functions as a light absorbing layer of a pin photodiode, and the emitter layer
504
functions as a window layer thereof. Unless the wavelength filter
505
is provided, the base layer
503
in the heterojunction phototransistor
500
has a high sensitivity to light in the absorption edge range from the absorption edge of the emitter layer
504
to the absorption edge of the base layer
503
. Unfortunately, the wavelength filter
505
absorbs light having a wavelength corresponding to the absorption edge of the wavelength filter or less. Therefore, the heterojunction phototransistor
500
has a pass-band characteristic in which only light having a wavelength longer than the absorption edge of the wavelength filter
505
. In order to achieve the selective light reception in which the wavelength of 1.3 &mgr;m is rejected and the wavelength of 1.5 &mgr;m is selected, for example, the absorption edge wavelength of the emitter layer
504
is set to 0.9 &mgr;m, the absorption edge wavelength of the base layer
503
is set to 1.65 &mgr;m, and the absorption edge wavelength of the wavelength filter
505
is set to 1.4 &mgr;m. Such settings allow achievement of a long wavelength pass-band characteristic in which a sensitivity to a signal light beam having a wavelength of 1.55 &mgr;m is high, but a sensitivity to the signal light beam having a wavelength of 1.3 &mgr;m is low.
Japanese Laid-Open Publication No. 9-83010 discloses a pin photodiode which achieves a selective-wavelength capability using the above-described heterojunction phototransistor structure. This example has a complicated structure which includes two pin photodiodes so as to receive light having two wavelengths and further includes other electronic devices. Only the selective-wavelength capability will be described in the following example. Referring to
FIG. 7
, the heterojunction phototransistor
600
includes a wavelength filter
602
, a buffer layer
603
, a light absorbing layer
604
, and a window layer
605
, which are successively provided on a semiconductor substrate
601
. An island-like diffusion region
606
in which p-type impurities are diffused is provided in the window layer
605
. The light absorbing layer
604
under the diffusion region
606
functions as a light receiving region. A negative electrode
607
is provided on the diffusion region
606
. A positive electrode
608
is deposited over a portion of the semiconductor substrate
601
which has been exposed by etching the window layer
605
and the light absorbing layer
604
(on the buffer layer
603
). In the example, a signal light beam enters from below the semiconductor substrate
601
. The absorption edge wavelengths of the light absorbing layer
604
and the wavelength filter
602
are set to 1.65 &mgr;m and 1.4 &mgr;m, respectively. Such settings allow achievement of a long wavelength pass-band characteristic in which a sensitivity to a signal light beam having a wavelength of 1.55 &mgr;m is high, but a sensitivity to the signal light beam having a wavelength of 1.3 &mgr;m is low.
Among the above-described conventional techniques, the heterojunction phototransistor
500
shown in
FIG. 6
receives a signal light beam from the upper side thereof. The phototransistor
500
is in the shape of mesa which is created by etching a region which has been doped during crystal growth. Such a mesa-type light receiving device has a drawback in that a leakage current is likely to occur.
The heterojunction phototransistor
600
shown in
FIG. 7
is of a planer type, having a window layer which is caused to be of
Dickey Thomas
Matsushita Electric - Industrial Co., Ltd.
Snell & Wilmer L.L.P.
Tran Minh Loan
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