JPH0547990B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a photothyristor, and more particularly to a semiconductor device that can be used as an optically coupled semiconductor device in combination with a light emitting diode.

Conventionally, as shown in Figure 1, a photothyristor coupler has been put into practical use that uses a light emitting diode GL on the input side and a photothyristor PT on the output side to form a single package. Compared to electromagnetic relays, these photothyristor couplers have extremely good insulation between input and output, faster operation speed, longer life, less noise generation, no influence from external magnetic fields, and smaller size. It has many advantages, and as the electronic circuits of various devices progress, it is used in a wide range of fields such as isolation of signal transmission systems and AC control.

However, when a steep voltage is applied between the anode A and the cathode K, the photothyristor turns on at a voltage lower than the original breakover voltage of the photothyristor. This phenomenon occurs when a steep rising voltage (dv/dt) is applied, and as shown in the photothyristor equivalent circuit diagram in Figure 2, a displacement current i _D shown by the following equation flows through the capacitor C ₀ (junction capacitance, etc.). It depends.

i _D = dQ/dt=d(c ₀ v)/dt=C ₀ dv/dt+
Vdc ₀ /dt (1) Here, assuming that C0 is constant, equation (1) becomes further as follows.

i _D =C ₀ dv/dt (2) As a result, when the value of dv/dt is large, the photothyristor is turned on. The value of the maximum rising voltage (dv/dt) _M that does not cause this phenomenon is called the critical off-voltage rise rate.

When actually using a photothyristor coupler, as shown in Figure 3, a resistor R _G and a capacitor are connected between the photothyristor gate PG and cathode K.
_CG is connected to prevent malfunction when a steep voltage is applied. By the way, when designing an actual circuit, externally attaching resistors and capacitors is very inconvenient in terms of installation locations and increased costs.

Conventionally, the following methods have been reported for improving the malfunction caused by the steep rising voltage dv/dt of the photothyristor as described above.

(1) Reduce h _FE of PNP transistor.

(2) Reduce gate resistance R _G.

(3) Control the gate resistance with a transistor.

(4) Control gate resistance with MOSFET.

However, in both methods, even if it is possible to increase the (dv/dt) _M value, the minimum trigger current I _FT increases, and the manufacturing of the circuit requires a complicated or special process. However, there were problems in practical application.

The present invention eliminates the need for external parts and complicated processes unlike conventional devices (DV/DT).
The present invention provides a semiconductor device with improved _M.

To summarize the present invention, in a vertical photothyristor, the h _FE of the PNP transistor included in the photothyristor is made larger than 1.0, the resistance connected to the P gate is made smaller than 20 kΩ, and the PNP
This can be achieved by setting the base width of the transistor to 100μ to 300μ. Further, if the resistor connected to the gate is formed integrally with the photothyristor by simultaneous diffusion with the anode of the photothyristor, it is convenient for lowering the resistance and increasing the base width.

The present invention will be explained in detail below with reference to Examples.

FIG. 4 shows the structure of a vertical photothyristor according to the present invention.

In the figure, 1 is an N-type semiconductor substrate, usually 20
Silicon is used with a resistivity of ~50Ω·cm and a thickness of about 150–500μ. P-type impurities such as boron or gallium are diffused around the element region from both sides of the N-type semiconductor substrate 1, and after partially penetrating and separating the N-type substrate 1, P-type impurities are further diffused over the entire back surface. Anode 2 is created. Furthermore, board 1
If the silicon is thick, the silicon in the isolation region can be etched in advance and then separated and diffused, or the isolation can be performed using mesa etching. 3 is N from the main surface of the board
The P gate is formed by diffusing 5 to 60 μm of P type impurity boron into the mold substrate 1, and is generally created at the same time as the entire back surface diffusion of the anode (the diffusion depth varies depending on the breakdown voltage and h _FE) . obtain). A base width d is set between the anode 2 and the gate 3 in the substrate thickness direction. Reference numeral 4 denotes a cathode formed by diffusing N-type impurity phosphorus into the gate 3. The depth is generally about 2 to 20 microns. The main surface of the semiconductor substrate on which the above regions are formed by diffusion is covered with an insulating film 5, and SiO ₂ is generally used as the insulating film 5. Each of the above regions has an anode electrode 6, a gate electrode 7, and a cathode electrode 8.
is made of metal such as Al.

As mentioned above, the cross-sectional structure is the same as that of the conventional vertical photothyristor, but in order to increase (dv/dt) _M , the photothyristor according to this embodiment optimizes the base width of the PNP transistor included in the photothyristor. In addition, h _FE is increased and gate resistance is decreased. That is, the characteristics of h _FE and gate resistance described above are usually obtained by increasing the lifetime of the base region of the transistor, and are obtained by performing heat treatment.

As an example, after the completion of diffusion of cathode region 4, 900
When heat treated in ℃ _N2 , the PNP transistor
h _FE is improved by 1.5-3 times. In this case, the surface of the general photothyristor is protected with SiO ₂ ,
Heat treatment in an oxygen atmosphere significantly degrades the h _FE of PNP transistors. Therefore, it is necessary to add another process such as once removing the SiO ₂ film. Other methods such as dislocation-free diffusion technology and optimization of impurity concentration may also be used.

In the structure of the vertical photothyristor mentioned above, first, the base width of the PNP transistor was made to a constant value of, for example, 100μ, and then the h _FE of the PNP transistor was changed by heat treatment etc., and the measured values are shown in Figure 5. Shown below. The figure shows the relationship between the minimum forward current of the light emitting diode (minimum trigger current: I _FT ) required to transition the photothyristor from OFF to ON state and the critical OFF voltage increase rate dv/dt in a photothyristor coupler. show. () in the diagram
The value of h _FE shown in each element is
The peak value of h _FE is shown when the collector current is changed.

Straight line A in Figure 5 shows that when the gate resistance R _G is fixed at 20 kΩ (the h _FE of the NPN transistor is also fixed),
The relationship between I _FT and (dv/dt) _M is shown when h _FE of the PNP transistor is sequentially changed to 0.15, 0.5, 2.5, and 5. Straight line A has a relatively gentle slope, and I _FT
(dv/dt) indicates that the change in _M is small.

Next, line B shows the relationship between I _FT and (dv/dt) _M when the h _FE of the PNP transistor is kept constant (the h _FE of the NPN transistor is also constant) and the gate resistance R _G is varied. Straight line B represents h _FE of PNP transistor as 5
Therefore, the straight line passes through the point h _FE =5 on the straight line A. When h _FE is changed to 2.5, 0.5, and 0.15, the change is represented by a straight line that passes through each h _FE point on straight line A and is almost parallel to straight line B. As can be seen from straight line B, when the gate resistance is changed, the change in (dv/dt) _M is very large with respect to _IFT .

Conventional photothyristor has h _FE =0.1~1.0 and
Rg=about 20 to 100 kΩ (base width d is 100 μ), and straight line A in FIG. 5 shows a part of the conventional example in which the gate resistance is set to the minimum 20 kΩ.

Now the PNP transistor of the photothyristor is h _FE =
5, if the minimum trigger current I _FT of the light emitting diode is 5 mA, in the conventional device, (dv/dt) _M is 7 v/μsec, but according to the present invention, from the straight line B 140v/μsec, which is a 20 times improvement. Furthermore, if I _FT is chosen to be 10 mA, _M can be increased by 70 times (dv/dt). The effect becomes even more pronounced as the h _FE of the PNP transistor becomes larger.

In conventional photothyristors, h _FE = 0.1 to 1.0, R _G =
However, in the present invention, by making h _FE larger than 1.0 and the resistor Rg connected to the P gate smaller than 20 kΩ, the minimum trigger current can be reduced compared to the above conventional example. Critical off-voltage rise rate (dv/dt) _M in relation to I _FT
can be significantly improved.

The above-mentioned improvement in the dv/dt value becomes more remarkable by selecting the base width d of the PNP transistor of the photothyristor shown in FIG. That is, the base width d is selected so that the dv/dt value is the largest.

Figure 6 shows the results obtained from each phototransistor when the base width d was sequentially changed to 50, 100, 150, and 300μ in phototransistors manufactured under the same manufacturing conditions as h _FE = 5.0 in Figure 5 above. The relationship between I _FT and (dv/dt) _M is shown. The numbers in parentheses in the figure indicate gate resistance. As can be seen from the figure, by setting the base width d to greater than 100μ and up to 300μ,
The critical off-voltage rise rate (dv/dt) _M with respect to the minimum trigger current I _FT can be made even steeper, and the same I _FT
For (dv/dt) _M changes significantly, and as the base width d increases, (dv/dt) _M becomes larger. I _FT
The above effect becomes more pronounced as the value of becomes larger.

In Figure 6, the values of h _FE of the PNP transistor correspond to base widths of 50, 100, 150, and 300μ.
Compatible with 8.2, 5.0, 3.8, 2.3.

The above effect becomes more noticeable when the h _FE of the PNP transistor is further increased.

By improving the structure of a package in which a photothyristor and a light emitting diode are integrated as an optical coupling device, it is possible to increase (dv/dt) _M even if I _FT is reduced. For example, a structure in which light-emitting and light-receiving elements are installed on both sides of glass or film and the distance between both elements is reduced, or when molding the outside of an element optically coupled with transparent resin, instead of surrounding it with black resin. I _FT can be reduced by using a structure that uses white resin and utilizes reflected light.

By increasing the above h _FE , decreasing the gate resistance, and selecting the widest base width.
The significant improvement in dv/dt values is explained as follows.

First, consider the response of the PNP transistor part of the photothyristor. The response of the transistor is expressed by the following equation.

t _PNP ∞h _FE ×t _D (3) t _D is a value determined by the structure of the PNP transistor, etc. In general, increasing _hFE slows down the response and makes it impossible to follow steep signals.

Next, consider the effect of gate resistance. Consider the case where a gate resistor _RG is connected between the gate PG and cathode K of the photothyristor in the photothyristor equivalent circuit shown in FIG. The displacement current based on equations (1) and (2) is
First, it flows through the gate resistor R _G , and the gate potential is given by the following equation.

V _G = i _D R _G ≒ CR _G dv/dt (4) When the value of V _G above becomes equal to or higher than the activation voltage V _GB of the thyristor, the thyristor is turned on. Therefore, if the gate resistance is decreased, the rate of increase in the critical off-voltage increases. Furthermore, t _D in equation (3) is given by the following equation.

t _D ∞d ² (5) d indicates the base width of the PNP transistor. Increasing the base width in this way slows down the response.

By the way, the displacement current due to dv/dt is a transient phenomenon. Therefore, the above effects can be expected to act synergistically. In this way, by increasing the base width of the PNP transistor, further increasing h _FE , and decreasing the gate resistance, the (dv/dt) _M width can be significantly improved.

Furthermore, the method of increasing h _FE of a PNP transistor generally has the effect of increasing photosensitivity. Therefore, it has the effect of reducing I _FT and further increases the effect of improving the (dv/dt) _M value. Generally 900 as mentioned above
When annealed in _N2 at °C, the photosensitivity is approximately 20~
Improved by 30%.

Furthermore, the gate resistor _RG can be easily integrated into a single chip with the photothyristor body. An example is shown in FIG. Reference numeral 9 denotes a resistor made by diffusing P-type impurity boron into the substrate 1. One end of the resistor is made to overlap the gate portion 4, and the other end is connected to the cathode electrode 8 through an electrode 10. When comparing a structure with an external resistor and a structure with a built-in resistor using the same resistance value, the (dv/dt) _M value of the built-in resistor is two to three times larger. This means that the dv/dt transient phenomenon needs to be considered as a distribution function, and the gate resistor needs to be installed close to the photothyristor. This effect allows further improvement of the present invention.

By using the process of manufacturing the resistor 9 by diffusion, the base width d can be increased. That is, since the N-type substrate 1 is penetrated for isolation and the P-type impurity is diffused, a very thick substrate cannot be used. Therefore, in order to substantially widen the base width, the above-mentioned resistance diffusion is used instead of performing the gate diffusion and the entire back surface diffusion for the anode as explained in FIG. 4. e.g. gate diffusion depth
50μ, and the depth of the resistance diffusion is 5μ, the base width becomes substantially larger by 45μ, making it possible to improve dv/dt.

When integrated into a single chip including the gate resistance as described above, electron holes are generated in the semiconductor by light irradiation from the light emitting diode, and the gate resistance value changes due to conductivity modulation. As an example, if R _G = 20kΩ, if 10mA is applied to the light emitting diode, the resistance value will be
_However , according to the present invention, the gate resistance value can be significantly reduced, and the resistance change can be virtually ignored. Furthermore, since the resistance value can be reduced, the chip area can also be reduced.

Further, in FIG. 8, if the resistive portion 9 is covered with Al as shown by 11, the change in gate resistance due to light becomes even smaller.

As described above, according to the present invention, the dv/dt value can be made very large by selecting the base width to an optimal value, increasing h _FE and decreasing the gate resistance to configure a vertical photothyristor. It is possible to make a photothyristor coupler that does not require external parts. In addition, the manufacturing process of the device does not involve any complicated steps, so it has great practical value.

Although the present invention has been described with respect to a photothyristor coupler, it is an improvement on the photothyristor itself. It is not limited to the structure shown in Figures 4 and 8,
It can also be applied to amplification gate type photothyristors and general thyristors.

[Brief explanation of drawings]

Figure 1 is a diagram showing an optically coupled photothyristor, Figure 2 is an equivalent circuit diagram of a photothyristor, and Figure 3 is a diagram showing an optically coupled photothyristor.
The figure shows a conventional improved optically coupled photothyristor, FIG. 4 is a sectional view of a vertical photothyristor according to the present invention, and FIGS. 5 and 6 are diagrams for explaining the operation of the photothyristor according to the present invention. dv／dt)−I _FT
7 is an equivalent circuit diagram for explaining the operation of the photothyristor according to the present invention,
FIG. 8 is a sectional view of another embodiment of the present invention. GL...Light emitting diode, PT...Photothyristor, R _G ...Gate resistance, d...Base width.

Claims (1)

[Claims] 1. In a vertical photothyristor having a PNPN laminated structure, the photothyristor includes:
h _FE of PNP transistor is greater than 1.0 and P
The resistance connected to the gate should be smaller than 20kΩ, and the base width of the PNP transistor should be 100μ.
300μ to 300μ to improve the critical off-voltage rise rate of the photothyristor in relation to the minimum trigger current of the photothyristor. 2. The semiconductor device according to claim 1, wherein the resistor connected to the gate is formed at the same time as the anode by diffusion over the entire back surface of the substrate, and is integrated with the photothyristor.