Active solid-state devices (e.g. – transistors – solid-state diode – Bipolar transistor structure
Reexamination Certificate
2003-07-08
2004-11-02
Pham, Hoai (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Bipolar transistor structure
C257S577000
Reexamination Certificate
active
06812546
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a current mirror circuit formed in an integrated circuit. Further, the present invention relates to an optical signal circuit, which includes the integrated circuit having the current mirror circuit, and which is provided in a vicinity of an electro-optic conversion element and a photo-electric conversion element such as a light-emitting diode and a photodiode.
BACKGROUND OF THE INVENTION
In an integrated circuit (an IC for infrared remote control reception, optical pickup signal reception, LED driving, etc.) provided in a vicinity of an electro-optic conversion element (such as a light-emitting diode) and an photo-electric conversion element (such as a photodiode), diffracted and scattered light due to signal light, and noise light such as fluorescent light generate a photocurrent at a parasitic photodiode, thereby causing circuit malfunction.
A p-type transistor, in particular, has a large area of an n-type epitaxial layer (base diffusion layer). Thus, the photocurrent at the parasitic photodiode increases a base current, thereby significantly affecting the circuit characteristics. This will be explained with reference to
FIGS. 7 through 10
.
FIG. 7
is a diagram schematically showing a structure of a p-type transistor
1
, and
FIG. 8
is its equivalent circuit diagram.
In this structure, an n-type epitaxial layer
3
is formed on a p-type substrate layer
2
. The n-type epitaxial layer
3
is separated by a trench
4
, and each separated n-type epitaxial layer
3
becomes an element region.
Here, because of the structure of the integrated circuit, a parasitic photodiode
5
is generated between the n-type epitaxial layer
3
and the substrate layer
2
. Further, the parasitic photodiode
5
is connected between the base terminal of the p-type transistor
1
and the substrate layer
2
(ground).
Thus, as shown in
FIG. 7
, when light incidence causes a photocurrent Ipd that flows from the n-type epitaxial layer
3
toward the substrate
2
, the photocurrent Ipd serves as a base current of the p-type transistor
1
, thereby significantly affecting the circuit characteristics.
Because the photocurrent Ipd increases in accordance with an amount of incident light, the photocurrent Ipd increases when the p-type transistor
1
is located in a vicinity of the photo-electric conversion element. Further, since the photocurrent Ipd increases in accordance with an area S of the n-type epitaxial layer
3
, the photocurrent Ipd increases in accordance with a current capacitance of the p-type transistor
1
.
Likewise,
FIG. 9
is a diagram schematically showing a structure of an n-type transistor
11
, and
FIG. 10
is its equivalent circuit diagram.
In this structure, an n-type epitaxial layer
13
is formed on a p-type substrate layer
12
. The n-type epitaxial layer
13
is separated by a trench
14
, and each separated n-type epitaxial layer
13
becomes an element region.
Here, because of the structure of the integrated circuit, a parasitic photodiode
15
is generated between the n-type epitaxial layer
13
and the substrate layer
12
. Further, the parasitic photodiode
15
is connected between the collector terminal of the n-type transistor
11
and the substrate layer
12
(ground).
Thus, as shown in
FIG. 9
, when light incidence causes a photocurrent Ipd that flows from the n-type epitaxial layer
13
toward the substrate
12
, the photocurrent Ipd bypasses a collector current of the n-type transistor
11
, thereby significantly affecting the circuit characteristics.
The photocurrent Ipd increases in accordance with an amount of incident light, and increases in accordance with an area S of the n-type epitaxial layer
13
. However, the n-type transistor
11
has larger current driving force compared with the p-type transistor
1
, and can reduce the area S of the n-type epitaxial layer
13
. Further, in the n-type transistor
11
, the generated photocurrent influences the collector current, so that the influence of the photocurrent seems to be smaller by an amount corresponding to a current amplification ratio.
As a method to reduce the influences of the photocurrent due to the parasitic photodiodes
5
and
15
, an element front face may be covered with wiring metal so as to shield light entering therefrom.
However, this method may not be able to sufficiently address light entering from a chip side face and a chip edge which cannot shield light. Further, in these years, because of the demand to cut costs by reducing a chip area and the number of masks, the wiring metal can no longer shield light sufficiently. Further, in accordance with the trend for low current consumption to save energy, the influence of the photocurrent due to the parasitic photodiode are relatively increasing.
Japanese Unexamined Patent Publication No. 262153/1991 (Tokukaihei 3-262153, published on Nov. 21, 1991; corresponding to Japanese Patent Publication No. 2634679) discloses a typical conventional technique that eliminates the influence of the photocurrent due to the parasitic photodiode in terms of circuit configuration.
FIG. 11
is an electric circuit diagram in which the conventional technique is applied to a current mirror circuit. This current mirror circuit
20
has a current mirror section
21
composed of a pair of p-type transistors q
1
and q
2
.
The emitters of the transistors q
1
and q
2
are both connected to a high-level power supply. Further, the input-side transistor q
1
has a diode structure in which the base and the collector are connected with each other. From the base and collector, a signal current iin is drawn out by a signal source
22
.
The base of the output-side transistor q
2
is connected to the base and collector of the transistor q
1
. Thus, the collector of the output-side transistor q
2
outputs an output current iout, which is the signal current iin that is mirrored by a current ratio i
2
/i
1
of the transistors q
1
and q
2
.
When areas of the n-type epitaxial layers of the transistors q
1
and q
2
are s
1
and s
2
, respectively, a photocurrent ipd flowing out from the bases of the transistors q
1
and q
2
is expressed as follows:
ipd
=(
s
1
+
s
2
)×
io,
(1)
where io is a value of photocurrent per unit area of the n-type epitaxial layer.
To compensate the photocurrent ipd, a current mirror section
23
composed of a pair of p-type transistors q
3
and q
4
is provided. The emitters of the transistors q
3
and q
4
are both connected to a high-level power supply. Further, the input-side transistor q
3
has a diode structure in which the base and the connector are connected with each other. The base of the output-side transistor q
4
is connected to the base and collector of the transistor q
3
.
Thus, the collector of the output-side transistor q
4
outputs a compensation current ic, which is obtained by amplifying a photocurrent ipdc that flows out from the bases of the transistors q
3
and q
4
. The compensation current ic is then supplied to the bases of the transistors q
1
and q
2
.
When areas of the n-type epitaxial layers of the transistors q
3
and q
4
are s
3
and s
4
, respectively, the photocurrent ipd is expressed as follows.
ipdc
=(
s
3
+
s
4
)×
io
(2)
Then, for simplicity, the base currents of the transistors q
3
and q
4
are ignored, namely, a current amplification ratio hfe is assumed to ∞ (infinity). Here, when areas of the n-type epitaxial layers of the transistors q
1
, q
2
, q
3
, and q
4
are s
1
, s
2
, s
3
, and s
4
, respectively, and i
2
/i
1
and i
4
/i
3
are current ratios of the current mirror sections
21
and
23
, respectively, Kirchhoff law gives the following equations.
ic
=(
i
4
/
i
3
)×(
s
3
+
s
4
)×
io
(3)
i
out=(
i
2
/
i
1
)×(
i
in+(
s
1
+
s
2
)×
io−ic
) (4)
These two equations further derive the following equation.
i
out=(
i
2
/
i
1
)×(
i
in+((
s
1
+
s
2
)−(
i
4
/
i
3
)×(
s
3
+
s
4
Inoue Takahiro
Yokogawa Naruichi
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