JP3593264B2 - Phototransistor - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a phototransistor, and more particularly to a phototransistor with a built-in Schottky barrier diode useful as a photoexcited switching element.
[0002]
[Prior art]
Phototransistors are often used as switching elements. That is, a photocurrent is generated in the photodiode built in between the base and the collector by the light excitation, and the transistor performs a switching operation by this current.
[0003]
In the case of a normal switching transistor, the accumulation time is shortened in the switching time of the transistor by short-circuiting or reverse-biasing between the base and the emitter at the time of OFF. In comparison, the accumulation time was significantly longer.
[0004]
Further, since the base electrode of the phototransistor is normally open, it is affected by electromagnetic noise between the phototransistor and an LED chip (hereinafter, simply referred to as “LED”) when mounted on a photocoupler, for example. It was easier.
[0005]
Among them, in order to reduce the storage time, a technique of connecting the base and the collector of the phototransistor by a Schottky barrier diode 21 as shown in an equivalent circuit of FIG. No. 7-79010. This is to form a Schottky barrier diode by directly contacting the base electrode and the collector region of the semiconductor substrate, prevent carrier injection from the base to the collector, and reduce the accumulation time. In the figure, 22 is a photodiode and 23 is a transistor.
[0006]
However, when a Schottky barrier diode is built in a phototransistor and operates effectively, the Schottky barrier diode requires a certain area or more, regardless of the chip pattern.
[0007]
That is, since the Schottky barrier diode is formed at the contact portion between the base electrode and the collector region of the semiconductor substrate, the area of the base electrode is inevitably considerably larger than that of a normal phototransistor. It is more susceptible to electromagnetic noise than a phototransistor without a built-in diode, and for example, the common mode rejection ratio: CMR (Common Mode Rejection) characteristics when a photocoupler is mounted deteriorates.
[0008]
Therefore, as shown in FIG. 7, in a phototransistor with a built-in Schottky barrier diode, a metal guard ring 37 is formed around the base diffusion region (base diffusion layer 32), and the metal guard ring 37 is electrically connected to the emitter electrode 35. In some cases, the emitter potential is connected to shield the base electrode 36 when the phototransistor is mounted, for example, on a photocoupler, thereby removing noise entering the base electrode 36 and improving the CMR characteristic. In the drawings, (a) is a plan view, and (b) is a cross-sectional view taken along the line CC ′ of (a).
[0009]
In FIG. 7, reference numeral 31 denotes an N-type semiconductor substrate constituting a collector layer, 32 denotes a P-type base diffusion layer formed on the upper surface of the N-type semiconductor substrate 31, and 33 denotes an upper surface of the base diffusion layer 32. The N ⁺ type emitter diffusion layer is formed, 34 is an oxide film provided so as to insulate the upper surfaces of the respective layers 31, 32 and 33, and 35 is an oxide film of a part of the upper surface of the emitter diffusion layer 33. An emitter electrode 31a is provided on the emitter diffusion layer 33 by removing the film 34, and 31a is an exposed region (Schottky contact portion) in which a part of the semiconductor substrate 31 is exposed on the upper surface of the base diffusion layer 32 in an island shape. Reference numeral 36 denotes a portion of the exposed region 31a of the semiconductor substrate 31 and a portion of the oxide film 34 on the upper surface of the base diffusion layer 32 near the periphery thereof, which is removed to remove the exposed region 31a and the upper surface of the base diffusion layer 32 near the periphery thereof. Department Reference numeral 37 denotes a metal guard ring provided on the oxide film 34, surrounding the base diffusion layer 32 and having an emitter potential by electrical connection with the emitter electrode 35, and 38 denotes the semiconductor substrate. Reference numeral 39 denotes a collector electrode provided on the lower surface of the semiconductor substrate 31, and reference numeral 39 denotes a channel stopper provided on the outer peripheral portion of the upper surface of the semiconductor substrate 31.
[0010]
In the phototransistor, a photodiode is formed by using the collector layer 31 as a cathode, the base diffusion layer 32 as an anode, the PN junction of both layers 31 and 32 as a light-sensitive junction, and the collector layer 31, the base diffusion layer 32, and the emitter diffusion layer. An NPN transistor is formed using the layer 33 as a collector C, a base B, and an emitter E, respectively. The light incident on the photodiode is converted into a photocurrent by a PN junction, and the NPN transistor is switched by the photocurrent obtained by the conversion.
[0011]
[Problems to be solved by the invention]
However, although the phototransistor with a built-in Schottky barrier diode shown in FIG. 7 surrounds the base electrode 36 with a metal layer having an emitter potential, most of the phototransistor is not arranged near the periphery of the base electrode 36 and is remote. In such a case, a sufficient shielding effect could not be obtained, and the CMR characteristics when, for example, a photocoupler was mounted still deteriorated.
[0012]
In view of the above problems, it is an object of the present invention to provide a photodiode capable of improving CMR characteristics when a photocoupler is mounted, for example, by surrounding substantially the entire periphery of a base electrode with a metal layer having an emitter potential.
[0013]
[Means for Solving the Problems]
In order to achieve the above object, a phototransistor according to claim 1 of the present invention is a phototransistor comprising a Schottky barrier diode formed between a base and a collector by contact between a collector region exposed on the surface and a base electrode. And a metal layer having an emitter potential surrounding substantially the entire periphery of the base electrode.
[0014]
The phototransistor according to claim 2 of the present invention is the phototransistor according to claim 1, wherein the collector region exposed on the surface is divided into a plurality of collector region portions, and the base electrode is connected to the plurality of collector regions. The base electrode portion is divided into a plurality of base electrode portions corresponding to the portions, and the vicinity of substantially the entire periphery of each base electrode portion is surrounded by the metal layer.
[0015]
Furthermore, a phototransistor according to a third aspect of the present invention is the phototransistor according to the first or second aspect, wherein a distance between the base electrode and the metal layer is 30 to 100 μm. .
[0016]
In addition, a phototransistor according to a fourth aspect of the present invention is the phototransistor according to the first, second or third aspect, wherein the base electrode and the metal layer are made of the same metal material. .
[0017]
According to the above configuration, the phototransistor according to the first aspect of the present invention surrounds substantially the entire periphery of the base electrode with the metal layer having the emitter potential. When a steep voltage is applied, lines of electric force are generated between the LED and the metal layer of the emitter potential of the phototransistor, and the lines of electric force shield the base electrode, causing extra noise on the base electrode. It becomes difficult to enter.
[0018]
The phototransistor according to claim 2 of the present invention is the phototransistor according to claim 1, wherein the collector region exposed on the surface is divided into a plurality of collector region portions, and the base electrode is connected to the plurality of collector regions. The base electrode portion is divided into a plurality of base electrode portions corresponding to the base portion, and substantially the entire periphery of each base electrode portion is surrounded by the metal layer, so that the shielding effect of the metal layer can be further improved, and the base electrode And extra noise is less likely to enter.
[0019]
Furthermore, in the phototransistor according to claim 3 of the present invention, since the distance between the base electrode and the metal layer is set to 30 to 100 μm in the phototransistor according to claim 1 or 2, an appropriate electrostatic withstand voltage value and base An electrode shielding effect is obtained.
[0020]
In addition, in the phototransistor according to claim 5 of the present invention, since the base electrode and the metal layer are the same metal in the phototransistor according to claim 1, 2, or 3, they can be formed simultaneously.
[0021]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, a phototransistor according to an embodiment of the present invention will be described with reference to the drawings.
[0022]
1A and 1B show a phototransistor according to an embodiment of the present invention. FIG. 1A is a plan view, and FIG. 1B is a cross-sectional view taken along line AA ′ of FIG.
[0023]
In FIG. 1, reference numeral 1 denotes an N-type semiconductor substrate constituting a collector layer (Schottky barrier diode cathode layer). A P-type impurity such as boron is selectively implanted into the upper surface of the semiconductor substrate 1 by an ion implantation method or the like to form a P-type base diffusion layer 2. At this time, an exposed region (Schottky contact portion) 11 in which a part of the collector layer 1 is surrounded by the base diffusion layer 2 and is exposed in an island shape is formed.
[0024]
Subsequently, by selectively thermally diffusing an N-type impurity such as phosphorus, the emitter diffusion layer 3 is formed on the upper surface of the base diffusion layer 2 and the channel stopper 4 is formed on the outer peripheral portion of the semiconductor substrate 1 at the same time.
[0025]
Next, an oxide film (specifically, a thermal oxide film) 5 for insulation is formed on the upper surface of each of the layers 1, 2, and 3. After that, a part of the oxide film 5 on the upper surface of the emitter diffusion layer 3 is removed. 1 and the part of the oxide film 5 on the upper surface of the base diffusion layer 2 near the entire periphery thereof is removed, and a metal material such as Al is coated on substantially the entire surface of the oxide film 5 together with the removed portion of the oxide film 5. Evaporate.
[0026]
Then, by selectively etching the metal material and performing a heat treatment, the exposed region 11 from which the oxide film 5 has been removed and a part of the base diffusion layer 2 near the entire periphery thereof and the oxide film 5 around the entire periphery thereof. A base electrode (Schottky barrier diode anode layer) 6 is formed on the upper surface of the emitter diffusion layer 3 from which the oxide film 5 has been removed, and an emitter electrode 7 is formed on the upper surface of the oxide film 5 around the entire surface. A metal guard ring 8 is formed on the entire upper surface of the film 5, and a metal shield wiring 9 is formed on the upper surface of the oxide film 5 close to the base electrode 6. Here, the emitter electrode 7 is electrically connected to the metal guard ring 8 and the metal shield wiring 9, and the emitter electrode 7, a part of the metal guard ring 8 and the metal shield wiring 9 form the base electrode 6. Constitutes a metal layer having an emitter potential that surrounds substantially the entire periphery. Therefore, the base electrode 6, the emitter electrode 7, the metal guard ring 8, and the metal shield wiring 9 are made of the same metal material such as Al and are formed in the same process.
Reference numeral 10 denotes a collector electrode provided on the lower surface of the semiconductor substrate 1.
[0027]
In the phototransistor, a photodiode is formed by using the collector layer 1 as a cathode, the base diffusion layer 2 as an anode, the PN junction of both layers 1 and 2 as a light-sensitive junction, and the collector layer 1, the base diffusion layer 2, and the emitter diffusion. An NPN transistor is formed using the layer 3 as a collector C, a base B, and an emitter E, respectively. The light incident on the photodiode is converted into a photocurrent by a PN junction, and the NPN transistor is switched by the photocurrent obtained by the conversion.
[0028]
According to the phototransistor having the above configuration, for example, when a steep voltage is applied between the input and output when the phototransistor is mounted on the photocoupler, as shown in FIG. The lines of electric force 13 are generated from the metal guard ring 8 and the metal shield wiring 9) to the LED 12 side. Since the metal layers 7, 8, and 9 having the emitter potential are disposed in the vicinity of the entire periphery of the base electrode 6, the electric flux lines 12 serve as a shield for the base electrode 6, and the base electrode 6 is shielded from electromagnetic noise. Protect. In FIG. 1, reference numeral 14 denotes a lead frame on which the LED 12 is mounted.
[0029]
FIG. 3 shows a circuit for measuring the CMR characteristic of a photocoupler having the phototransistor mounted thereon.
[0030]
FIG. 4 shows the measurement results of the CMR characteristics of the photocoupler equipped with the phototransistor. In the measurement circuit shown in FIG. 3, for example, _Vcc = 9 V, R _L = 470Ω, V _CM = 1.5 kV, and dv / FIG. 11 is a diagram illustrating a measurement result of V _np after dv / dt application when dt = 10 kV / μsec.
[0031]
As described above, according to the photocoupler equipped with the phototransistor having the above configuration, it is possible to obtain a characteristic value of V _np <20 mV, which is generally at a level without any problem.
[0032]
5A and 5B show a phototransistor according to another embodiment of the present invention, wherein FIG. 5A is a plan view, and FIG. 5B is a cross-sectional view taken along the line BB 'of FIG. In the present embodiment, only points different from the above-described embodiment will be described.
[0033]
In the phototransistor according to this embodiment, in the above-described embodiment, the exposed region (11) is divided into a plurality of exposed region portions 11a, and the base electrode (6) corresponds to the plurality of exposed region portions 11a. It is divided into a plurality of base electrode portions 11a, and the vicinity of substantially the entire periphery of each base electrode portion 6a is surrounded by the metal layers 7, 8, 9 having an emitter potential.
[0034]
More specifically, the exposed region (11) is divided into a plurality of exposed region portions 11a by interposing the base diffusion layer 2 therebetween, whereby the surface of each exposed region portion 11a is formed by the base diffusion layer 2. It is exposed as a surrounded island. The base diffusion layer 2 between the exposed regions 11a, 11a is covered with the oxide film 5 except for the vicinity of the exposed regions 11a, 11a.
[0035]
Further, the base electrode (6) is divided into a plurality of base electrode portions 6a by removing a part of a portion located on the base diffusion layer 2 interposed between the plurality of exposed region portions 11a. The Schottky barrier diode is formed by bringing the portion 6a into direct contact with the upper surfaces of the exposed region 11a and the base diffusion layer 2 near the entire periphery thereof.
[0036]
Further, on the upper surface of the oxide film 5 between the base electrode portions 6a, 6a, a metal shield wiring 9 is arranged close to the base electrode portions 6a, 6a.
[0037]
According to this configuration, by dividing the exposed region and the base electrode, a desired Schottky barrier diode area can be obtained in the base electrode portion 6a having a small area. Further, since the metal layers 7, 8, and 9 surround substantially the entire periphery of each divided base electrode portion 6a, the area of the base electrode (base electrode portion 6a) that is easily affected by noise is small. In addition, noise is less likely to occur, and the CMR characteristics can be further improved as compared with the phototransistor of the above embodiment.
[0038]
That is, in order to obtain a desired Schottky barrier diode area, when the exposed region and the base electrode are divided, the total area of the base electrode (the total area of each base electrode portion) is increased as compared with the above embodiment. However, since the base electrode portion 6a having a small area is surrounded by the metal layers 7, 8, and 9, respectively, the shielding effect by the metal layers 7, 8, and 9 can be further improved, and as a result, CMR characteristics can be further improved as compared with the embodiment.
[0039]
In the above-described embodiment, in order to enhance the shielding effect, the distance between the base electrode 6 or the base electrode portion 6a and the metal layers 7, 8, 9 provided near substantially the entire periphery thereof is preferably 100 μm or less. In order to maintain the electrostatic withstand voltage characteristics at 3 kV or more, which has no problem in practical use, the above interval is preferably 30 μm or more. Therefore, it is preferable that the interval 12 between the base electrode 6 or the base electrode portion 6a and the metal layers 7, 8, 9 provided in the vicinity of substantially the entire periphery is 30 μm or more and 100 μm or less.
[0040]
Further, by using the same material (for example, Al) as the electrodes 6 (6a) and 7 and the guard ring 8 for the metal shield wiring 9, all of them can be formed in one process, which is economically advantageous. .
[0041]
【The invention's effect】
As described above, according to the phototransistor of the first aspect of the present invention, the metal layer of the emitter potential surrounds substantially the entire periphery of the base electrode, so that, for example, the CMR characteristic when the photocoupler is mounted is improved. it can.
[0042]
According to the phototransistor of claim 2 of the present invention, in the phototransistor of claim 1, the collector region exposed on the surface is divided into a plurality of collector region portions, and the base electrode is connected to the plurality of collector regions. Since the base electrode portion is divided into a plurality of base electrode portions corresponding to the collector region portion and substantially the entire periphery of each base electrode portion is surrounded by the metal layer, the CMR characteristics can be further improved.
[0043]
Furthermore, according to the phototransistor of the third aspect of the present invention, in the phototransistor of the first or second aspect, since the distance 12 between the base electrode and the metal layer is set to 30 to 100 μm, an appropriate electrostatic capacity can be obtained. The withstand voltage value and the shielding effect of the base electrode can be obtained.
[0044]
In addition, according to the phototransistor according to claim 4 of the present invention, in the phototransistor according to claim 1, 2, or 3, since the base electrode and the metal layer are made of the same metal material, they can be formed simultaneously. , Easy to form.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a phototransistor according to an embodiment of the present invention.
FIG. 2 is a cross-sectional view showing a state where a steep voltage is applied between input and output in a photocoupler equipped with the phototransistor shown in FIG.
FIG. 3 is a measurement circuit diagram of a CMR characteristic.
FIG. 4 is a diagram showing measurement results of CMR characteristics.
FIG. 5 is a configuration diagram of a phototransistor according to another embodiment of the present invention.
FIG. 6 is a circuit diagram of a phototransistor.
FIG. 7 is a configuration diagram of a conventional phototransistor.
[Explanation of symbols]
1 Semiconductor substrate (collector layer)
2 Base diffusion layer 3 Emitter diffusion layer 4 Channel stopper 5 Oxide film 6 Base electrode 6a Base electrode part 7 Emitter electrode 8 Metal guard ring 9 Metal shield wiring 10 Collector electrode 11 Exposed area 11a Exposed area part 12 Base electrode 6 or base electrode part 6a and the metal layers 7, 8, 9 provided in the vicinity of substantially the entire periphery.

Claims (4)

In a phototransistor formed by forming a Schottky barrier diode between the base and the collector by contact between the collector region exposed on the surface and the base electrode,
A phototransistor, wherein substantially the entire periphery of the base electrode is surrounded by a metal layer having an emitter potential. The collector region exposed on the surface is divided into a plurality of collector region portions, and the base electrode is divided into a plurality of base electrode portions corresponding to the plurality of collector region portions. 2. The phototransistor according to claim 1, wherein the phototransistor is surrounded by the metal layer. The phototransistor according to claim 1, wherein a distance between the base electrode and the metal layer is 30 to 100 μm. 4. The phototransistor according to claim 1, wherein said base electrode and said metal layer are made of the same metal material.

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