Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
2003-04-24
2004-08-31
Kang, Donghee (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S461000, C257S463000, C257S464000, C257S465000, C257S257000, C257S258000, C438S048000, C438S087000, C438S309000
Reexamination Certificate
active
06784513
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor light receiving device, such as a phototransistor and the like, and an electronic apparatus incorporating the same. More particularly, the present invention relates to a semiconductor light receiving device used for a photocoupler, and an electronic apparatus incorporating the same.
2. Description of the Related Art
Phototransistors are used as switching elements, such as an optical switch and the like. The phototransistor performs a switching operation which is triggered by a photocurrent generated in a photodiode provided between a base electrode and a collector electrode. Such a photocurrent is generated by carries excited by light incident on a light receiving region.
When a phototransistor is incorporated in, for example, a photocoupler having a light emitting element, a stray capacitance Cf is caused between the phototransistor and a light emitting diode chip (light emitting diode is hereinafter abbreviated as LED) of the photocoupler. The base region of the phototransistor typically serves as a light receiving region. The surface of the base region is uncovered and exposed. Therefore, when a steeply changing pulse voltage is applied between the phototransistor and the LED of the photocoupler, a displacement current (noise current) caused by electromagnetic noise occurs in the base region of the phototransistor via the stray capacitance Cf, which may cause malfunctions of the phototransistor.
In general, the phototransistor comprises an N type collector layer serving as a collector (C), a P type base layer serving as a base (B), and an N type emitter layer serving as an emitter (E). Typically, even when a minute value of base current Ib is input to the P type base layer, a large value of collector current (output current) Ic=hfe×Ib (where hfe represents an amplification factor and Ib represents a base current) is output. Therefore, in order to prevent malfunctions of the phototransistor due to electromagnetic noise or the like, it is most important to suppress a noise current occurring in the P type base layer and its surrounding region.
As described above, when the phototransistor is incorporated in the photocoupler, a sharply changing, pulse-like displacement voltage occurs between the LED and the phototransistor in the photocoupler. In this case, the displacement voltage causes electromagnetic noise via a stray capacitance Cf therebetween, resulting in a noise current in the P type base layer of the phototransistor. In the case of the occurrence of such a noise current in the P type base layer, the N type collector layer of the phototransistor serving as a collector (C) outputs a collector current Ic represented by:
Ic=hfe×Ib=hfe×Cfb×d
(
VCM
)/
dt
(1)
where hfe is the amplification factor of the transistor, Ib represents a base current, Cfb represents a stray capacitance between the LED of the photocoupler and the P type base layer of the phototransistor, VCM represents a voltage between the LED of the photocoupler and the phototransistor, and t represents a time.
Thus, when a malfunction is caused in the phototransistor, a large value of collector current Ic is output from the phototransistor in accordance with the above-described expression (1), which may have an adverse influence on external circuitry.
In order to address malfunctions due to such electromagnetic noise or the like, phototransistors may be equipped with a common mode rejection (CMR) characteristic which is used as an indicator representing the performance of the phototransistor.
The CMR characteristic represents an ability to absorb noise as follows. With the CMR characteristic, a noise current, which is input to the phototransistor via a stray capacitance Cf, is caused to flow into the N type emitter layer of the transistor, but not the P type base layer region, when a steeply changing voltage (e.g., a noise signal) is applied between the LED and the phototransistor of the photocoupler. Since the N type emitter layer is designed so that a large current flows, a minute noise current caused by a noise signal can be caused to flow out of the phototransistor quickly. The reason the minute noise current is caused to flow into the N type emitter layer through which a large value of current flows, is that the current passing through the N type emitter layer is not substantially affected by the minute noise current.
Accordingly, in order to improve the CMR characteristic when the phototransistor is incorporated in the photocoupler, the light receiving region of the phototransistor needs to be provided with a metal line which is coupled to the N type emitter layer and thus has the same potential as that of the N type emitter layer.
FIG. 9
is a cross-sectional view showing a structure of a photocoupler incorporating a phototransistor in which malfunctions due to electromagnetic noise are suppressed.
A phototransistor
200
shown in
FIG. 9
is configured so that a planer N type semiconductor substrate has an N+ type collector layer
22
and an N type collector layer
30
laminated on the N+ type collector layer
22
. An N+ type channel stopper layer
130
is embedded at an outer periphery (along a surrounding edge) of the N type collector layer
30
, where the N+ type channel stopper layer
130
is exposed from an outer peripheral surface of the N type collector layer
30
. A P type base layer
40
having a predetermined thickness is embedded in a middle portion of the N type collector layer
30
surrounded by the N+type channel stopper layer
130
, where there is a predetermined space between the P type base layer
40
and the N+type channel stopper layer
130
and the P type base layer
40
is exposed from a surface of the N type collector layer
30
. A surface of the P type base layer
40
receives incident light. An N+ type emitter layer
50
having a predetermined thickness is embedded in the vicinity of an edge of the P type base layer
40
, where the N+ type emitter layer
50
is exposed from a surface of the P type base layer
40
.
An emitter underlying electrode
70
is provided on a middle portion of the N+ type emitter layer
50
. An oxide insulating film
60
is provided around the emitter underlying electrode
70
, where the oxide insulating film
60
covers surfaces of the N type collector layer
30
, the P type base layer
40
, the N+ type emitter layer
50
, and the N+ type channel stopper layer
130
. An emitter electrode
120
is provided on the emitter underlying electrode
70
and the oxide insulating film
60
, where the emitter electrode
120
faces the N+ type emitter layer
50
.
A metal guard ring line
90
is provided on the oxide insulating film
60
along each inner edge of regions in which the N+ type channel stopper layer
130
is embedded, where the metal guard ring line
90
is electrically connected to the emitter electrode
120
. Further, a metal shield line
80
is provided on the oxide insulating film
60
covering the surface of the P type base layer
40
, where the metal shield line
80
is electrically connected to the emitter electrode
120
(in
FIG. 9
, the connections between the emitter electrode
120
, and the metal shield line
80
and the metal guard ring line
90
are not shown). A collector electrode
21
is provided on a surface of the N+ type collector layer
22
.
The phototransistor
200
is thus provided with an NPN transistor comprising the N+ type collector layer
22
and the N type collector layer
30
, the P type base layer
40
, and the N+ type emitter layer
50
, which serve as collectors (C), a base (B), and an emitter (E), respectively; and a photodiode having a PN junction at the interface between the P type base layer
40
and the N type collector layer
30
where the N+ type collector layer
22
and the N type collector layer
30
serve as cathodes and the P type base layer
40
serves as an anode.
Now it
Aki Motonari
Yasuda Yoshiki
Kang Donghee
Sharp Kabushiki Kaisha
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