Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
2000-09-06
2002-06-11
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S090000, C257S461000
Reexamination Certificate
active
06404029
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photosensitive device, as well as a photosensitive device having internal circuitry, for use in an optical pickup or the like which supports write operations.
2. Description of the Related Art
Optical pickups are employed for optical disk apparatuses, e.g., CD-ROM or DVD (digital video disk) apparatuses. In recent years, optical disk apparatuses have been increasing in operation speed, and there has been a trend for processing a large amount of data, e.g., moving image data, at higher speeds. Against such a background, there has been an intense demand for increasing the operation speed of optical pickups.
Over the past years, optical disk apparatuses which are capable of writing data on optical disks, e.g., CD-R/RW and DVD-R/RAM, have been developed. Such an optical disk apparatus which is capable of write operations writes information on an optical disk by causing a phase change in a dye which is provided on the disk based on laser-induced heat. High power laser light is radiated onto an optical disk, and light reflected therefrom is incident on a photodiode. Therefore, a much greater amount of laser light is radiated onto the photodiode at the time of writing than at the time of reading. Such writable optical disk media have also been subjected to intense demands for faster operation.
FIGS. 1A and 1B
illustrate the structure of a conventional photodiode
1000
which is disclosed in Japanese Laid-Open Publication No 9-153605. As shown in
FIG. 1A
, this photodiode
1000
includes an epitaxial layer
85
of a second conductivity type provided on a semiconductor substrate
84
of a first conductivity type. The epitaxial layer
85
of the second conductivity type is subdivided into a plurality of regions by diffusion layers
87
and
88
of the first conductivity type. A junction between each subdivided region and the underlying portion of the semiconductor substrate
84
of the first conductivity type provides the photodiode
1000
.
The response speed of the conventional photodiode
1000
of the aforementioned structure is a function of a CR time constant, which in turn is a function of the capacitance (C) and the resistance (R) of the photodiode, and the migration distance of carriers, which are generated on the side of a depletion layer
86
closer to the substrate
84
, migrating via diffusion.
Therefore, according to this conventional technique, the impurity concentration within the semiconductor substrate
84
of the first conductivity type is prescribed at a low level as shown in
FIG. 1B
, which illustrates the impurity concentration profile of a cross-section along line I-I′ in
FIG. 1A
, so as to obtain a large expanse of the depletion layer
86
within the semiconductor substrate
84
of the first conductivity type. As a result, the junction capacitance of the photodiode
1000
is reduced, thereby decreasing the CR time constant and hence increasing the response speed of the photodiode
1000
. Furthermore, since the depletion layer
86
extends deeply into the substrate
84
, carriers which are generated at relatively deep portions within the substrate
84
will not have to travel over a large distance via diffusion, thereby also increasing the response speed of the photodiode
1000
.
The C component in the CR time constant, which determines the response speed of the photodiode, can also be reduced by increasing the resistivity of the substrate
84
up to a certain value. Thus, as shown in
FIG. 2
, the response speed (i.e., cut off frequency) of the photodiode can be improved until the resistivity of the substrate
84
reaches that value. However, increasing the resistivity of the substrate
84
further above that value will result in an increase in the serial resistance of the anode side (which contributes to an increase in the R component), so that the response speed of the photodiode, as a function of the CR time constant, is decreased rather than increased, as shown in FIG.
2
.
Accordingly, in order to further enhance the response speed of a photodiode, Japanese Laid-Open Publication No. 61-154063, for example, proposes a photosensitive device
2000
having a structure as shown in
FIG. 3
, where a photodiode is constructed on a laminate substrate obtained by forming a P-type high resistance crystal growth layer
142
on a P-type low resistance substrate
141
.
The photosensitive device
2000
shown in
FIG. 3
includes an N-type epitaxial layer
143
, a P-type separation diffusion layer
144
, an N-type contact region
145
, an N-type embedded region
146
, a P-type base region
147
, an N-type emitter region
148
, a silicon oxide film
149
, electrode wiring layers
150
a,
150
b,
and
150
c,
a photodiode structural portion
180
for detecting signal light, and a circuit structural portion
190
for processing a detected signal.
A high resistance crystal growth layer
142
includes an autodope layer
142
a,
which has a gradually decreasing impurity concentration beginning from the low resistance substrate
141
, and a layer
142
b,
which has a constant impurity concentration. According to this conventional technique, the high resistance crystal growth layer
142
makes it easy for the depletion layer
160
to expand into the substrate
141
, thereby reducing the junction capacitance. Furthermore, the serial resistance on the anode side is reduced by the P-type low resistance substrate
141
, which lies much below the expanse of the depletion layer
160
. As a result, both the C component and the R component (which determine the response speed) of the photodiode are reduced, thereby enhancing the response speed of the photosensitive device
2000
.
In order to improve the response speed of a photodiode by employing the aforementioned laminate substrate, it is necessary to reduce the junction capacitance by allowing the depletion layer
160
to adequately expand into the high resistance crystal growth layer
142
. Therefore, it is desirable to increase the resistivity of the high resistance crystal growth layer
142
up to 1000 &OHgr;cm, which corresponds to the maximum controllable resistivity under epitaxial growth, and to prescribe the thickness of the high resistance crystal growth layer
142
at about 20 &mgr;m (where the constant impurity concentration portion
142
b
of the high resistance layer would be about 13 &mgr;m thick) so that the depletion layer
160
fully expands in the constant impurity concentration portion
142
b
of the high resistance layer. Any increase in the region into which the high resistance crystal growth layer
142
does not expand would cause an increase in the serial resistance on the anode side, which in turn prevents the improvement of response speed.
In the case of an optical pickup which supports write operations, the amount of light which is radiated by a laser onto an optical disk increases in proportion with the writing speed, whereby the amount of laser light which is reflected from the optical disk and enters the photodiode also increases. If the amount of light entering the photodiode exceeds a certain level, the response speed of the photodiode may deteriorate.
FIG. 4
shows the dependency of the response speed (i.e., cut off frequency) of the photodiode having the structure shown in
FIG. 1
on the incident light amount. As seen from
FIG. 4
, the response speed (i.e., cut off frequency) of the photodiode decreases as the amount of light entering the photodiode exceeds a certain level. It can also be seen that such a decrease in the response speed becomes more prominent as the resistivity of the substrate increases.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a photosensitive device including: a semiconductor substrate of a first conductivity type; a semiconductor layer of the first conductivity type formed on the semiconductor substrate and having a lower impurity concentration than that of the semiconductor substrate; a semiconductor layer of a second conductivity type formed on th
Fukunaga Naoki
Fukushima Toshihiko
Hosokawa Makoto
Kubo Masaru
Ohkubo Isamu
Huynh Andy
Nelms David
Nixon & Vanderhye
Sharp Kabushiki Kaisha
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