Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
1997-11-24
2001-01-16
Saadat, Mahshid (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S458000, C257S461000
Reexamination Certificate
active
06175142
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a PIN photodiode used as a receiving element for converting a light wave to an electric signal in an optical communication, more particularly relates to improvements in characteristics such as a frequency characteristic or a wavelength sensitivity characteristic.
2. Background Art
Examples of conventional PIN photodiodes are shown in
FIGS. 4 and 7
. A high concentration N-type layer (an N
30
layer)
91
in a PIN photodiode
90
shown in FIG.
4
(A) is formed by a diffusion from one surface of a high specific resistance silicon wafer
92
A. The thickness of an I-type layer
92
as a remaining part is determined by the control of the diffusion. Further, a high concentration P-type layer (a P
30
layer)
93
is formed by a diffusion from the opposite surface to the N-type layer
91
, thereby forming the basic constitution of a PIN photodiode
90
.
Since the high concentration N-type layer (an N
+
layer)
91
is formed by the diffusion as described, some N
+
carriers (hereinafter referred to as “N carriers”) are diffused into the I-type layer
92
. The concentration distribution B
1
of the N carriers in the N-type layer
91
and the I-type layer
92
will have a gentle slope as shown in FIG.
4
(B). This means a depletion layer
94
is a shallow layer which does not reach a junction surface
95
between the N-type layer
91
and the I-type layer
92
.
An I-type layer
82
in a PIN photodiode
80
shown in FIG.
7
(A) is formed by an epitaxial growth on one surface of a high concentration N-type wafer
81
A as shown in FIG.
8
. In this conventional art, the thickness of the I-type layer
82
is determined by, for example, a time control of the epitaxial growth. Further, a P-type layer
83
is formed as well as the above-described conventional art.
Since the I-type layer
82
is formed by the epitaxial growth on the preformed N-type layer (the N
+
layer)
81
, it is possible to form the I-type layer
82
which does not contain so many N carriers. The concentration distribution B
2
of the N carriers in the N-type layer
81
and the I-type layer
82
will have a step-like shape as shown in FIG.
7
(B). This means a depletion layer
84
substantially reaches a junction surface
85
between the N-type layer
81
and the I-type layer
82
.
However, there are some problems with regard to the PIN photodiodes
80
and
90
as following. As for the photodiode
90
, although there is an advantage that the wavelength sensitivity of light rays depending on the thickness of the I-type layer
92
can be easily defined due to the easiness for controlling the thickness of the I-type layer
92
, there is a problem that it can not deal with optical communication requiring a high-speed characteristic due to the shallowness of the depletion layer
94
which leads a response frequency characteristic F
1
to be 10 MHz at most as shown in FIG.
6
.
As for the photodiode
80
, the depletion layer
84
substantially reaches the junction surface
85
of the N-type layer
81
and the I-type layer
82
, and therefore about 200 MHz of a response frequency characteristic F
2
can be obtained as shown in FIG.
9
. However, it is substantially impossible to form a thick I-type layer with a thickness of, for example, at least 30 &mgr;m (preferably at least 40 &mgr;m) by the epitaxial growth, which causes the peak value of wavelength sensitivity to be shifted to a short wavelength side. This makes a problem that the peak value can not be conformed to the oscillating wavelength of a semiconductor laser which is practically used.
In addition, since the epitaxial growth process by which the I-type layer
82
in the photodiode
80
is formed is a difficult process, it takes too much time for forming the I-type layer
82
even if the thickness of the I-type layer
82
is restricted to, for example, 20 &mgr;m, and leads the PIN photodiode
80
to a low yield, which in turn raises the cost of the PIN diode
80
.
SUMMARY OF THE INVENTION
Accordingly, a general object of the present invention is to provide a PIN photodiode having a high response frequency, by which it is possible to freely define the wavelength sensitivity.
In order to achieve the foregoing object and others, the present invention provides a PIN photodiode comprising a P-type layer, an N-type layer, and an I-type layer provided between said P-type layer and said N-type layer, wherein a junction surface between said I-type layer and said N-type layer is formed by joining a high specific resistance wafer preformed as said I-type layer to a high concentration N-type wafer preformed as said N-type layer.
In the PIN photodiode of the present invention, the thickness of said high specific resistance wafer has been preferably adjusted in accordance with the peak value of the wavelength of light rays to be received.
Other features and advantages of the present invention will be apparent from the following description taken in connection with the accompanying drawing.
REFERENCES:
patent: 5061977 (1991-10-01), Funaba
patent: 5391910 (1995-02-01), Fujimura et al.
patent: 5596186 (1997-01-01), Kobayashi
patent: 5682037 (1997-10-01), De Cesare et al.
Eckert II George C.
Saadat Mahshid
Stanley Electric Co. Ltd.
Weingarten, Schurgin Gagnebin & Hayes LLP
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