JPH0547989B2 - - Google Patents

Description

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

As shown in Figure 1, a light emitting diode is installed on the input side.
A photothyristor coupler that uses a photothyristor PT on the GL and output side to form a one-piece cage has been put into practical use. Compared to electromagnetic relays, these photothyristor couplers have extremely good insulation between input and output, operate faster, have a longer lifespan, generate less noise, and are not affected by external magnetic fields.
They have advantages such as being compact, and as the use of electronic circuits in various devices progresses, they are used in a wide range of fields such as signal transmission system isolation and AC control.

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

i _D = d _Q / dt = d (C ₀ V) / dt = C ₀ dv / dt + V
dc ₀ /dt (1) Here, assuming that C ₀ 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 thyristor 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, in actual use,
Installing resistors and capacitors externally requires
This is extremely inconvenient in terms of increased costs.

The following method has been reported as an improvement method for preventing malfunction caused by the steep rising voltage 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, with either method, even if it is possible to increase the (dv/dt) _M value, there are drawbacks such as the minimum trigger current I _F becoming large and the circuit manufacturing requiring 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.

The above purpose is to use PNP in horizontal thyristors.
Considering that by increasing the base width of the transistor and increasing hFE, the characteristic of the critical off-voltage rise rate with respect to the minimum trigger current becomes steeper, we decided to increase the base width of the PNP transistor to 150μ.
No, 300μ, and h _FE of PNP transistor
This can be achieved by setting the value to 0.48 to 0.75.

The present invention will be explained below using examples.

FIG. 4 shows the structure of a lateral type photothyristor according to the present invention. 1 is an N-type semiconductor substrate, usually 20~
Silicon with a resistivity of 50Ωcm and a thickness of 200 to 400μ is used. 2 and 3 are those in which 5 to 60 μm of boron, which is a P type impurity, is diffused into a predetermined location of the N type substrate 1, and the diffusion depth is changed depending on the breakdown voltage, _hFE , etc. 2 is formed as an anode, 3 is formed as a gate, and a base width d is set between both regions. Next, N-type impurity phosphorus is diffused into the gate 3 to form a cathode. The diffusion depth varies depending on the diffusion depth of the gate 3, and is approximately 2 to 20 microns. 5 on the semiconductor substrate 1 is an insulating film, generally SiO ₂
is used. 6, 7, and 8 provided in each of the regions 2, 3, and 4 are anode electrodes, gate electrodes,
It is a cathode electrode, and Al is generally used.

As mentioned above, the cross-sectional structure is the same as the conventional horizontal 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. At the same time, h _FE is increased and gate resistance is decreased.

Among the above conditions, desired characteristics of _hFE and gate resistance can be obtained by, for example, performing the following heat treatment, providing a diffusion region, or using both together.

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 1.5-2 times. In this case, the surface of a typical photothyristor is protected with SiO ₂ , and when it is heat-treated in an oxygen atmosphere, the h _FE of the other NPN transistor deteriorates significantly. Therefore, it is necessary to add another process such as once removing the SiO ₂ film.
h Any method such as dislocation-free diffusion technology or optimization of impurity concentration may be used to increase _FE .

Furthermore, if phosphorus treatment is applied to the back surface of the wafer, as is applied to light-receiving elements, the h _FE of the PNP transistor increases by 1.5 to 2.5 times. Additionally, this treatment can increase photosensitivity by 20-30%.

Furthermore, a phosphorus diffusion region S is provided around the chip, surrounding the device region where the anode 2 and cathode 3 are provided.
By providing , it is possible to increase h _FE and decrease gate resistance. That is, the phosphorus diffusion region S
As a result, the holes injected from the anode 3 are reflected, and _hFE further increases by about 1.5 to 2.0 times. If the phosphorus diffusion region S is deep and penetrates the wafer, the effect will be even greater.

In the above lateral type photothyristor structure,
First, set the base width of the PNP transistor to 100μ, for example.
For those manufactured to a constant value of , the above heat treatment,
Figure 5 shows measured values of h _FE of a PNP transistor that was changed by back surface treatment. The figure shows the minimum forward current value (minimum trigger current: I _FT ) of the light emitting diode required to transition the photothyristor from off to on state and the critical Off-voltage rise rate dv/dt
shows the relationship between The value of h _FE shown in parentheses in the figure indicates the peak value of h _FE when the collector current is changed for each element.

Straight line A in Fig. 5 shows that when the gate resistance R _G is fixed at 51KΩ (h _FE of the NPN transistor is also fixed),
The relationship between I _FE and (dv/dt) _M is shown when h _FE of a PNP transistor is sequentially changed to 0.2, 0.5, 1.0, and 2. Straight line A has a relatively gentle slope, and I _FT
(dv/dt) indicates that the change in _M is small.

Next, set the h _FE of the PNP transistor constant (the h _FE of the NPN transistor is also constant), and set the gate resistance R _G ([ ]
I _FT and (dv/dt) _M when changing )
The relationship with is shown in straight line B. Straight line B is a straight line when h _FE of the PNP transistor is set to 1, and therefore passes through the point on straight line A where h _FE =1. h _FE 2.0,
When the value is changed to 0.5, 0.2, the change is represented by a straight line that passes through each _hFE 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 .

Now the PNP transistor of the photothyristor is h _FE =
1, if the minimum trigger current I _FT of the light emitting diode is 5 mA, (dv/dt) _M is 12 V/μsec in the conventional device, but according to the present invention, it is 130 V/μsec from the straight line B. μsec, resulting in an improvement of 10.8 times. For I _FT = 10 mA, _M can be increased by a factor of 34 (dv/dt). The effect becomes even more pronounced as the h _FE of the PNP transistor becomes larger.

In the conventional lateral type photothyristor, h _FE = 0.03 to 0.3, R _G = 50, assuming the straight line A shown in Figure 5, that is, the base width of the PNP transistor to be 100μ.
~100KΩ, but the photothyristor of the present invention is PNP as described below.
This was achieved by setting the base width of the transistor as a significant parameter for improving (dv/dt) _M .

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 for each photothyristor when the base width d was sequentially changed to 50, 100, 150, and 300μ in a photothyristor fabricated under the same manufacturing conditions as when h _FE = 1.0 in Figure 5 above. The relationship between I _FT obtained from a thyristor and (dv/dt) _M is shown. The numbers in parentheses in the figure indicate gate resistance. As can be seen from the figure, by increasing the base width d, (dv/dt) _M changes significantly for the same I _FT , and as the base width d increases, (dv/dt) _M becomes larger.
The above effect becomes more pronounced as the value of I _FT increases.

In addition, in Figure 6 above, the PNP transistor
The values of h _FE correspond to 1.5, 1.0, 0.75, and 0.48 for base widths of 50, 100, 150, and 300μ. The above effect becomes more noticeable when the h _FE of the PNP transistor is further increased.

In this way, it is possible to improve (dv/dt) _M in relation to the minimum trigger current of the thyristor, especially when the base width is 150μ to 300μ and h _FE is 0.48 to 300μ.
For those in the 0.75 range, the conventional base width
100μ, h _FE = 0.03 to 0.3, Rg = 50 to 100KΩ Not only can the critical off-voltage rise rate be improved compared to that of 100μ, h FE = 0.03 to 0.3, but also the h _FE can be
Compared to the case where h FE = 0.3 or more (see straight line B in FIG. 5, h _FE = 1.0), the characteristic line can be made even steeper, and a dramatic improvement effect can be expected. Although it is more effective to reduce the gate resistance Rg, as is clear from the brackets in Figure 6, by increasing the base width, the parameter of the gate resistance Rg at the critical off-voltage rise rate can be reduced. According to this example, even if the resistance value of Rg is increased, the critical off-voltage rise rate can be improved without reducing the minimum trigger current, which is advantageous.

By improving the structure of a package that integrates a photothyristor and a light emitting diode 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 a glass film and the distance between the two elements is reduced, or when molding the outside of an element optically coupled with transparent resin, instead of surrounding it with black resin, white I _FT can be reduced by a structure that uses resin to utilize 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. Generally, when h _FE is increased, the response becomes slower and it becomes 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 equivalent circuit shown in FIG. The displacement current based on the above equations (1) and (2) first flows through the gate resistor _RG , and the potential of the gate 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 R _G is reduced, the rate of increase in the critical off-voltage increases.

Further, 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. When the base width d is increased in this way, the reaction becomes slower.

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 _hFE , and decreasing the gate resistance, dv/dt
value 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 enhances the effect of improving the dv/dt value. Generally 900 as mentioned above
When annealed in ℃ _N2 , the photosensitivity is about 20-30%.
Improved.

Furthermore, the gate resistor PG can be easily integrated into a single chip with a photothyristor. An example is shown in FIG. 9
is a resistor created by diffusing P-type impurity boron into the semiconductor substrate 1, and one end of the resistor is connected to the gate portion 3.
and the other is connected to the cathode electrode 8 through the electrode 10. When comparing the case of using an external resistor and a built-in resistor using the same resistance value, the dv/dt value is 2 to 3 times larger with the built-in resistor. This means that it is necessary to consider the dv/dt transient phenomenon as a distribution function, and it is necessary to install the gate resistor close to the photothyristor. This effect allows the present invention to be further improved.

However, when integrated into a single chip, electrons and holes are generated in the semiconductor by light irradiation from a light emitting diode, and the gate resistance value changes due to conductance modulation.

As an example, when the gate resistance R _G =50KΩ, when 10mA is applied to the light emitting diode, the resistance value changes by about 1/2. Therefore, I _FT becomes large. However, according to the present invention, the gate resistance can be significantly reduced and changes in resistance can be virtually ignored. Furthermore, since the resistance value can be reduced, the chip area can also be reduced.

Furthermore, if the resistance portion 9 is covered with Al as shown in 11 in FIG. 8, the change in gate resistance due to light will be further reduced.

As described above, by configuring a lateral photothyristor with the base width of the PNP transistor being 150μ to 300μ and the h _FE of the PNP transistor being 0.48 to 0.75 as in the present invention, the dv/dt value can be greatly increased, and the external part You can make unnecessary photothyristor couplers. Furthermore, the manufacturing process of the device does not involve any complicated steps, so it has great practical value.

Although the embodiment has been described with respect to one photothyristor, it can also be applied to one chip connected in antiparallel. Moreover, it is applicable not only to photothyristors but also to general thyristors.

[Brief explanation of the drawing]

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 horizontal photothyristor according to the present invention, and FIGS. /dt) _M −
_FIG . 7 is an equivalent circuit diagram for explaining the operation of the photothyristor according to the present invention, and FIG. 8 is a sectional view of another embodiment according to the present invention. GL...Light emitting diode, PT...Photothyristor, R _G ...Gate resistance, d...Base width.

Claims (1)

[Claims] 1. A horizontal thyristor made of PNPN,
By setting the base width of the PNP transistor included in the photothyristor to 150 μ to 300 μ and the h _FE of the PNP transistor to 0.48 to 0.75, the critical off-voltage rise rate of the photothyristor was improved in relation to the minimum trigger current of the photothyristor. Characteristic semiconductor devices. 2. The semiconductor device according to claim 1, wherein the photothyristor is provided with a phosphorus diffusion layer on the back surface of the semiconductor substrate to increase the h _FE of the PNP transistor. 3. The photothyristor has a phosphorus diffusion layer formed around the thyristor element region, so that
2. The semiconductor device according to claim 1, wherein h _FE of a PNP transistor is increased.