JPH0337745B2 - - 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 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 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.

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

The present invention was made in view of the problems with the conventional photothyristor couplers mentioned above, and relates to a photothyristor coupler that does not require external components.

Although various reports have been made regarding the improvement of malfunction caused by the steep rise voltage dV/dt of photothyristors, the following methods are mainly known for increasing the (V/dt) _M value.

(1) Reduce h _FE of PNP transistor.

(2) Reduce gate resistance R _G.

However, when (dV/dt) _M is increased by the method described above, the minimum trigger current I _FT increases, which is a practical problem. Therefore, in order to solve (dV/dt) _M and I _FT at the same time, the following two methods (3) and (4) have been proposed.

(3) Method of controlling gate resistance using transistors Figure 4 shows an example of a circuit in which gate resistance R _G is controlled using transistors Q ₁ and Q ₂ . When a steep voltage is applied in this circuit, part of the displacement current is applied to the base of transistor Q1, turning on transistor Q1 and increasing the (dV/dt) _M value.

When irradiated with light, phototransistor Q ₂ is turned on and Q1 is turned off, keeping I _FT small.

However, there is a problem in that dielectric isolation technology is required to make the circuit shown in FIG. 4 into one chip, which complicates the process.

(4) A method in which the gate resistance is controlled using a MOS FET.

An example of a circuit using this method is shown in FIG. However, two sets of photothyristors Q ₁ , Q ₃ and Q ₂ , Q ₄ are connected in antiparallel. The operating principle is as follows.

Now consider a photothyristor represented by transistors Q ₁ and Q ₃ . Photothyristor Q ₁ , Q ₃
MOSFETQ ₆ is connected in parallel to the gate resistance R _G1 , and the gate potential of MOSFETQ ₆ is
Connected to the base of _Q3 . Therefore, the anode potential of photothyristors Q ₁ and Q ₃ is
When the threshold voltage V _T of MOSFETQ ₆ is exceeded,
MOSFETQ ₆ turns on and reduces the gate resistance of the photothyristor. It has a so-called zero-crossing function, and when the anode potential exceeds V _T , it becomes difficult for the photothyristor to turn on, and dV/dt substantially increases. In this method, a high voltage of several hundred V is applied to the gate of the MOSFET, so
MOSFETs that can withstand high voltages are required, and special processes are required when creating them.

Therefore, the present invention provides a photothyristor that improves (dV/dt) _M and prevents malfunctions due to steeply applied voltages without using the above-described complicated process.

To summarize the present invention, in a vertical photothyristor, heat treatment, etc. is used to reverse the conventional method (1) described above.
Increase h _FE and photosensitivity of PNP transistor,
This can also be achieved by reducing the gate resistance and further by incorporating the resistance into the same chip as the photothyristor.

The present invention will be explained in detail below with reference to Examples.
FIG. 6 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 with a resistivity of ~50Ω·cm and a thickness of about 200μ is used. 2, from both sides of the N-type semiconductor substrate 1, boron contains approximately P-type impurities such as gallium.
The anode was created by diffusing 100μ, separating the N-type substrate 1 through it, and then diffusing P-type impurities over the entire back surface again. 3 is a gate formed by diffusing 5 to 60 μm of P-type impurity boron into the substrate 1 from the main surface of the substrate, and is generally created at the same time as the entire back surface diffusion of the anode (diffusion depth is the withstand voltage, h (may vary depending on _FE ). 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. In each of the above regions, an anode electrode 6, a gate electrode 7, and a cathode electrode 8 are formed 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 has a large h _FE of the PNP transistor included in the photothyristor. Moreover, the gate resistance is reduced. That is, such characteristics of h _FE and gate resistance 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 a typical photothyristor is protected with _SiO2 ,
Heat treatment in an oxygen atmosphere significantly degrades the h _FE of NPN transistors. Therefore, it is necessary to add another process such as once removing the SiO ₂ film, and other methods such as dislocation-free diffusion technology and optimization of impurity concentration may also be used.

In the vertical photothyristor structure described above, when the h _FE of the PNP transistor is changed by heat treatment, etc., the light emitting diode required to transition the photothyristor from OFF to ON state in the photothyristor coupler shown in Figure 1 is Figure 7 shows the relationship between the minimum value of the forward current (minimum trigger current: I _FT ) and the rate of increase in critical off-voltage (dV/dt) _M. The value of h _FE shown in the figure is calculated by setting V _CE to 5V for each element.
The peak value of h _FE is shown when the collector current is changed by setting .

Straight line A in Figure 7 shows when the gate resistance R _G is fixed at 20KΩ (h _FE of the NPN transistor is also fixed).
The relationship between I _FT and (dV/dt) _M is shown when the h _FE of the PNP transistor is sequentially changed to 0.5, 1.0, 2.5, and 5. Straight line A has a relatively gentle slope, indicating that the change in (dV/dt) _M with respect to I _FT is small.

Next, a straight line _B5 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 ₅ is a straight line when h _FE of the PNP transistor is set to 5, and therefore passes through the point h _FE =5 on straight line A. When h _FE is changed to 2.5, 1.0, and 0.5, the change is represented by a straight line that passes through each h _FE point on straight line A and is approximately parallel to straight line B ₅ . As can be seen from the straight line _B5 , when the gate resistance is changed, the change in (dV/dt) _M is very large with respect to I _FT .

Conventional photothyristor has h _FE = 0.1 to 1.0, R _G =
Assuming that the current is 20KΩ and that 5mA is required as the minimum trigger current I _FT of the light emitting diode,
From straight line A in Figure 7, when R _G = 20 KΩ and h _FE = 0.5, (dV/dt) _M is 7 V/μsec, but according to the present invention, from straight line B ₅ , it becomes 440 V/μsec, an improvement of 63 times. is obtained. Furthermore, if I _FT is selected to be 10 mA, _M can be increased by 300 times (dV/dt). The effect becomes even more pronounced as the h _FE of the PNP transistor becomes larger.

Generally, the average current amplification factor of the h _FE of an NPN transistor is larger than that of a PNP transistor because a resistor is connected to the P gate, and here, a value of about 5 is used as the average h _FE .

In conventional photothyristors, h _FE = 0.1 to 1.0, R _G =
Although it is used at about 20KΩ, in the present invention, as mentioned above, it is desirable that h _FE be large and R _G be small.

In other words, in the conventional photothyristor, h _EF =0.1
20KΩ in the range of ~1.0 and with R _G at the minimum
As is clear from Figure 7, it is approximately 2
For a minimum trigger current I _FT between mA and 10mA,
(dV/dt) _M ranges from approximately 4v/μsec to 10v/μsec.

In contrast, the present invention makes h _FE larger than 1.0 and R _G smaller than 20KΩ,
Minimum trigger current I _FT in the same 2mA to 10mA range
For h _FE = 1.0, (dV/dt) _M changes on a straight line that is almost parallel to the straight line B ₅ passing through the point h _FE = 1.0 on straight line A in Figure 7, In the case of the present invention, by being located above this straight line, (dV/dt) _M for each minimum trigger current I _FT can be dramatically increased. Furthermore, the present invention can easily reduce the minimum trigger current I _FT to 2 mA or less compared to the conventional one, and can obtain an improved (dV/dt) _M with respect to these minimum trigger currents I _FT .

The significant improvement in dV/dt value by the present invention is as follows. It is explained in detail.

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 depending on 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.

By the way, the displacement current due to dV/dt is a transient phenomenon. Therefore, the above two effects can be expected to have a synergistic effect. In this way, by increasing the h _FE of the PNP transistor and decreasing the gate resistance, the (dV/dt) _M value can be significantly improved.

Furthermore, the method of increasing h _FE of a PNP transistor generally has the effect of increasing photosensitivity. This has the effect of reducing I _FT , further increasing the effect of improving the (dV/dt) _M value. Generally 900℃ as mentioned above
When annealed in _N2 , the photosensitivity is approximately 20-30.
% improved.

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 the way, when integrated into a single chip, electron holes are generated in the semiconductor by light irradiation from a light emitting diode, and the gate resistance value changes due to conductivity modulation. As an example, when R _G = 20KΩ, when 10mA flows through the light emitting diode, the resistance decreases by 20% and I _FT increases, but according to the present invention, the gate resistance value can be significantly reduced, and the resistance change can be virtually ignored. can.
Furthermore, since the resistance value can be reduced, the chip area can also be reduced.

Further, in FIG. 9, if the resistance 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 in a vertical photothyristor, and a photothyristor coupler that does not require external components can be manufactured.

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 FIGS. 6 and 9,
It can also be applied to amplification gate type photothyristors and 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, FIGS. 4 and 5 are equivalent circuit diagrams of other conventional improved photothyristors, and FIG. 6 is a sectional view of a vertical photothyristor according to the present invention. FIG. 7 is a characteristic diagram showing the relationship between (dV/dt)-I _FT for explaining the operation of the photothyristor according to the present invention, and FIG. 8 is an equivalent circuit diagram for explaining the operation of the photothyristor according to the present invention. , FIG. 9 is a sectional view of another embodiment according to the present invention. GL: Light emitting diode, PT: Photothyristor,
R _G : Gate resistance.

Claims (1)

[Claims] 1. In a vertical photothyristor having a PNPN laminated structure,
The feature is that the h _FE of the PNP transistor is made larger than 1.0, and the resistance connected to the P gate is made smaller than 20KΩ, thereby improving the critical off-voltage rise rate of the photothyristor in relation to the minimum trigger current of the photothyristor. semiconductor device. 2. The semiconductor device according to claim 1, wherein the resistor connected to the gate is integrally formed in the same chip as the photothyristor.