JPH06260674A - Phototransistor - Google Patents

Abstract

PURPOSE:To accelerate response speed in a phototransistor in which an input transistor is Darlington-connected to an output transistor. CONSTITUTION:A phototransistor has an input transistor 11, an output stage transistor 12 in such a manner that the transistor 11 is Darlington-connected to the transistor 12, and comprises a life time killer diffused layer 24 formed only in the transistor 12.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Darlington-connected phototransistor.

[0002]

2. Description of the Related Art FIG. 2 is a diagram showing an equivalent circuit of a general NPN phototransistor chip connected in Darlington. As shown in the figure, the first-stage transistor 1 and the output-stage transistor 2 are Darlington-connected. In the figure, 3 is a light receiving part, 4 is a collector terminal, and 5 is an emitter terminal.

In this phototransistor chip,
FIG. 4 shows input / output waveforms when the response speed is measured by the measuring method shown in FIG. In FIG. 3, 6 is a light emitting diode, Vcc is a DC power source, R _L is a collector load resistance, I _C is an output current (collector current), I _B is a photoexcitation current (base current), and I _F is the light emitting diode 6 which emits light. This is the input current to be applied. Further, in FIG. 4, tr is the rise time of the output current I _C of the phototransistor, and t _off is the output current I _{C of the} phototransistor after the input current I _F of the light emitting diode 6 is cut off and is 10% or less of the steady value. It shows the time until it becomes.

The response speed t _off can be expressed by the following relational expression.

[0005] _{t off = t op + t stg} + t f ······ (1) t op: time until the minority carriers generated by photoexcitation to disappear f _stg: storage time _{t stg = τ s ln (h} FE I _B / I _C ) ・ ・ ・ ・ ・ (2) τ _s ≈0.6 / 2π _{f b} + τ _nc / 2 ・ ・ ・ (3) h _FE : DC current amplification factor f _b : Base cutoff frequency τ _nc : minority carrier lifetime of the collector layer t _f: ShitaNoboru time _{t f = τ F h FE ···············} (4) τ F ≒ 1 / 2πf T + 1.7R L C _TC (5) f _T : transition frequency _RL : collector load resistance C _TC : collector transition capacitance Therefore, the response speed t _off becomes longer as the DC current amplification factor h _FE of the transistor increases, and is required. Response speed t
_It is necessary to select the DC current gain h _{FE according} to _off .

[0006]

In particular, a Darlington-connected phototransistor is originally intended to achieve high output, and a high response speed is a major issue.

Therefore, in order to increase the response speed t _off , the direct current amplification factor h _FE must be reduced, which means that the output of the phototransistor is reduced. Therefore, in order to secure the output, the photoexcitation current I _B must be improved by increasing the output of the light emitting diode 6 or increasing the light receiving area of the light receiving unit 3 in the phototransistor. However, this involves an increase in cost and an increase in size of the device.

Further, the above relational expressions (1), (2), (3)
_Therefore , the storage time t _stg depends on the life τ _nc of the minority carriers in the collector layer, and the response speed t _off can be shortened by shortening the life τ _nc of the minority carriers in the collector layer. As a method therefor, there is a method of diffusing a lifetime killer such as Au or Pt in the phototransistor. However, when the lifetime killer is diffused, the following problems occur.

(1) Generally, when the lifetime killer is diffused in the entire phototransistor, the collector-emitter cutoff current (leakage current) increases, but the leakage current of the first-stage transistor 1 is the base of the output-stage transistor 2. Since it flows into the, it may malfunction at high temperature.

(2) When the lifetime killer is diffused in the entire phototransistor, the direct current amplification factor h _FE is lowered.

(3) When the lifetime killer is diffused in the light receiving portion 3 of the phototransistor, the photoexcitation current I
_B decreases.

In view of the above problems, it is an object of the present invention to provide a semiconductor device capable of achieving a high response speed.

[0013]

A phototransistor of the present invention comprises a first stage transistor and an output stage transistor, and in the phototransistor in which the first stage transistor and the output stage transistor are Darlington connected, only the output stage transistor is a lifetime killer. It is characterized in that a diffusion layer is formed.

[0014]

As described above, since the phototransistor of the present invention has a structure in which the lifetime killer diffusion layer is formed only in the output stage transistor, the relational expression (1),
In (2) and (3), the minority carrier lifetime τ _nc of the collector layer in the output stage transistor becomes short,
The accumulation time t _stg becomes short. Therefore, the response speed t _off is shortened and the speed can be increased.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to FIG.

FIG. 1 is a sectional view of an NPN phototransistor in which a first stage transistor and an output stage transistor are Darlington connected.

As shown in FIG. 1, NPN phototransistor of the present invention includes a first-stage transistor 11 and an output stage transistor 12 and the light receiving portion 13 the N ^- is formed on the epitaxial layer 14, the N ^- epitaxial layer 14 Is the collector layer 15 of the first-stage transistor section 11 and the collector layer 16 of the output-stage transistor section 12, and the N ⁺ substrate 1
It is connected to the collector electrode 18 via 7. The emitter layer 19 of the first-stage transistor section 11 and the base layer 20 of the output-stage transistor section 12 are connected by the emitter electrode (base electrode of the output-stage transistor section 12) 21 of the first-stage transistor section 11. Further, the emitter layer 22 of the output stage transistor section 12 is connected to the emitter electrode 23 of the output stage transistor section 12. In addition, the output stage transistor section 12 has a structure in which the lifetime killer diffusion layer 24 is formed of a heavy metal film such as Au or Pt. In the figure, 25 is an anode layer of the light receiving portion 13 (base layer of the first-stage transistor portion 11), 26 is an N ⁺ channel stopper, and 27 is S.
This is an iO ₂ film.

The lifetime killer diffusion layer 24 can be formed, for example, as follows. Before forming the emitter electrode 23 of the output stage transistor section 12, the SiO ₂ film 27 formed on the output stage transistor section 12 is formed.
Of the Au, P, etc.
A heavy metal film such as t is deposited.

Next, for example, a heat treatment is performed at a temperature of about 900 ° C. for about 1 hour, and then an aqua regia treatment is performed to remove the heavy metal film in unnecessary portions.

As described above, the lifetime killer is selectively diffused only in the output stage transistor section 12 to form the lifetime killer diffusion layer 24.

As described above, the phototransistor of the present invention has a structure in which the lifetime killer diffusion layer 24 is formed only in the output stage transistor section 12. Therefore, in the above relational expressions (1), (2) and (3), The minority carrier lifetime τ _nc of the collector layer 16 of the output stage transistor unit 12 becomes short, and the accumulation time t _stg becomes short. Therefore, the response speed t _off is shortened and the speed can be increased.

Here, the phototransistor of the present invention is
Since the lifetime killer diffusion layer 24 is formed only in the output stage transistor section 12, the direct current amplification factor h _FE of the output stage transistor is reduced. However, in order to prevent a decrease in DC current gain h _FE of a phototransistor which are Darlington connected, an decrement of the DC current amplification factor h _FE of the output stage transistor by increasing the DC current amplification factor h _FE of the first stage transistor I will make up for it.

[0023]

As described above, according to the present invention, in the phototransistor, since the lifetime killer diffusion layer is formed only in the output stage transistor, the minority carrier life of the collector layer in the output stage transistor becomes short. , The accumulation time becomes shorter. Therefore, the response speed becomes short and the speed is increased.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a phototransistor showing an embodiment of the present invention.

FIG. 2 is a diagram showing an equivalent circuit of a general NPN phototransistor chip connected in Darlington.

FIG. 3 is a diagram showing a method of measuring a response speed of the phototransistor shown in FIG.

4 is a diagram showing input / output waveforms when measured by the response speed measuring method shown in FIG.

[Explanation of symbols]

 11 First Stage Transistor Section 12 Output Stage Transistor Section 24 Lifetime Killer Diffusion Layer

Claims (1)

[Claims] 1. A phototransistor comprising a first stage transistor and an output stage transistor, wherein the first stage transistor and the output stage transistor are Darlington connected, wherein a lifetime killer diffusion layer is formed only in the output stage transistor. Transistor.