JP2690050B2 - Surge protection element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surge protection device for protecting a communication terminal or a maintenance worker from a high-voltage surge such as lightning induced on a telephone line or the like.

[0002]

2. Description of the Related Art A thyristor (SC) having a pnpn four-layer structure is used.
The surge protection element utilizing R) has an SCR operation that forms a low impedance current path in the element and changes from a forward blocking state to a conducting state when an abnormal voltage higher than a breakdown voltage (breakover voltage) is applied. Is a device that absorbs a large current accompanying an abnormal voltage, clamps the voltage across the device to a certain voltage value or less, and protects the electric circuit system.

[0003] The surge protection element has a structure in which two thyristors are connected in anti-parallel on the same substrate in order to fulfill the same protection function against positive and negative surges. That is, as shown in the equivalent circuit of FIG. 4, two thyristors 51 and 52 sharing the floating base layer are connected in anti-parallel.

FIG. 5 is a sectional view showing the structure of a conventional surge protection element. Note that here, the region showing the first conductivity is p-type, and the region showing the second conductivity is n-type.
In the figure, an n-type diffusion layer region 2 is formed on both surfaces of a p-type semiconductor substrate 1. In the n-type diffusion layer region 2, p
Type diffusion layer region 3 and, if necessary, an n concentration higher than the diffusion surface concentration of the diffusion layer region 2 for forming an ohmic junction.
A mold diffusion layer region 4 is formed. Further, a protective film 6 such as a silicon oxide film is formed on those surfaces by an oxidation method, a deposition method, or another method to form an n-type diffusion layer region 2, a p-type diffusion layer region 3, and an n-type diffusion layer region 4. A window is opened at the position of, and the electrode 7 is formed on the entire surface of the window.

The above-described components are formed symmetrically on both surfaces of the semiconductor substrate 1 so that two thyristors are connected in antiparallel. In FIG. 5, a and b are added to the reference numerals of the respective portions formed on the upper surface and the lower surface to distinguish them.
That is, the p-type diffusion layer region 3a and the n-type diffusion layer region 2
The a / p-type semiconductor substrate 1, the n-type diffusion layer region 2b, and the n-type diffusion layer region 4b constitute a forward thyristor 8 having a pnpn structure, and the p-type diffusion layer region 3b and the n-type diffusion layer region are formed. 2b, p-type semiconductor substrate 1, n-type diffusion layer region 2
A reverse thyristor 9 having a pnpn structure is constituted by the a and n type diffusion layer regions 4a.

Here, the function of the surge protection element will be described. When a positive voltage is applied to the electrode 7a on the upper surface, the pn junction between the n-type diffusion layer region 2a and the p-type semiconductor substrate 1 is reverse biased, and a current is applied until the applied voltage exceeds the junction breakdown voltage. Only leaks current and is usually in a high impedance state. However, when a surge exceeding the junction breakdown voltage occurs, the breakdown of this pn junction causes p
Charge is injected into the semiconductor substrate 1 of the mold. Now, in the forward thyristor 8, the p-type semiconductor substrate 1 constitutes the base layer of the thyristor, and the thyristor is turned on by charge injection there. That is, the thyristor is turned on by application of the surge voltage, and has a low impedance to pass the surge current. In a steady state after passing an abnormal current due to the surge, the thyristor cannot automatically maintain the ON state and automatically returns to the OFF state due to charge disappearance due to charge recombination in the p-type semiconductor substrate 1 constituting the base layer. (For example, JP-A-1-307264).

On the other hand, when a positive voltage surge is applied to the lower electrode 7b due to the symmetry of the structure, the reverse thyristor 9 operates similarly. FIG. 6 is a diagram showing the voltage-current characteristics of the thyristor.

In the figure, the horizontal axis is voltage and the vertical axis is current. V _b0 are breakover voltage, is I _H holding current, V _T is the on-voltage in the lightning surge current I _F.
V _b0 ′, I _H ′, V _T ′, and _IF ′ are values in the opposite directions.

Here, the energy loss E in the surge protection element when a lightning surge is applied is:

[0010]

(Equation 1)

It can be expressed as Note that this is equal to the calorific value Q of the surge protection element.

[0012]

By the way, the breakdown of the element depends on the heat generation temperature inside the element. Since the lightning surge is a short-time pulse, the internal heat generation of the element when the lightning surge is applied is instantaneous, whereas the heat transmitted to the outside has a large thermal time constant. Therefore, when, for example, a positive lightning surge is applied to the electrode 7a shown in FIG.
The forward thyristor 8 generates heat and the reverse thyristor 9 functions as a heat sink. However, it takes a time corresponding to the thermal time constant for the entire element to reach a uniform temperature, and the temperature gradient inside the element temporarily increases.

FIG. 7 shows the time variation of the temperature distribution inside the conventional surge protection element (when the forward thyristor 8 generates heat).
FIG. In the figure, the horizontal axis represents the position of the surge protection element shown in FIG. 5 in the x-axis direction, and the vertical axis represents the temperature at each position. It can be seen that the temperature gradient becomes gentle as time elapses from time t ₁ to time t ₄ , but it can be seen that there is a considerable temperature difference between the forward thyristor 8 and its surroundings temporarily.

Therefore, it is inevitable to generate mechanical distortion due to the difference in thermal expansion of the surge protection element, and the element may be destroyed depending on the magnitude of the surge current. That is,
The destruction of the element means the heat generation temperature (peak temperature) inside it
However, in the conventional structure, the surge current resistance is small because the structure itself cannot be reduced.

To solve this problem, Japanese Patent Application No. 3-
In the specification of 189038, a structure in which a plurality of forward thyristors and reverse thyristors are alternately formed is shown. A cross-sectional structure of an example thereof is shown in FIG.

In FIG. 8, components having the same functions as those of the conventional surge protection element (FIG. 5) are designated by the same reference numerals, but in addition, a forward thyristor connected to the same electrodes 7a and 7b and a reverse direction thyristor are connected. A plurality of thyristors are arranged alternately. However, forward thyristors 11 and 1
Reference numerals 2 and 13 denote p-type diffusion layer regions 3a, n-type diffusion layer regions 2a, p-type semiconductor substrate 1, n-type diffusion layer regions 2b and n.
The reverse thyristors 14, 15 and 16 are composed of a p-type diffusion layer region 3b, an n-type diffusion layer region 2b, a p-type semiconductor substrate 1 and an n-type diffusion layer region 2a. , N-type diffusion layer region 4a.

Here, n-type diffusion layer regions 4a and 4b are formed in order to form ohmic junctions in the thyristor regions in each direction.
Is arranged, the contact potential difference between the semiconductor portion and the electrode can be reduced.

As described above, since the subdivided forward thyristor and reverse thyristor are arranged adjacent to each other, for example, when a positive lightning surge is applied to the electrode 7a, plural thyristors are provided. Forward thyristors 11, 12,
13 generates heat and the heat source is dispersed.

FIG. 9 is a diagram showing the time variation of the temperature distribution inside the surge protection element of the prior application. In the figure, the horizontal axis represents the position of the surge protection element shown in FIG. 8 in the x-axis direction, and the vertical axis represents the temperature at each position. Since the plurality of subdivided forward thyristors 11, 12, and 13 generate heat in a distributed manner, the amount of heat (peak temperature) per one forward thyristor can be reduced. Adjacent reverse thyristors 14, 15, 16 absorb heat as heat sinks, but since they are also subdivided, the heat conduction distance is short, and the temperature of the entire device can be quickly made uniform.

Therefore, when the surge is applied, the temperature gradient generated inside the element becomes gentle, and the mechanical strain caused by the difference in thermal expansion can be reduced. Further, since the peak temperature is lowered, the reliability of the surge protection element and the surge current withstand capability can be improved.

However, this is the case where all the thyristors 11 to 16 have the same characteristics, and in reality, due to the non-uniformity of the impurity concentration of the semiconductor substrate 1, the diffusion layer regions 2 and 3, crystal defects and other causes, It is difficult to manufacture them so that they all have the same characteristics. Therefore, generally, in the thyristor, the first switching occurs in a minute portion at one point, the adjacent regions are sequentially switched, the operation region is expanded, and the entire region is switched. Similarly to a general thyristor, the surge protection element has the same polarity thyristors 11 to 13 or 14 to 1 when a lightning surge is applied.
The first switching in 6 is a minute portion at a certain point, and the adjacent regions are sequentially switched to expand the operation region.

When the emitter width W is longer than the lateral diffusion length L of the majority carriers in the base layer, the plurality of same-polarity thyristors are independent of each other, which is the same as when the individual thyristors are connected in parallel. become. Under such circumstances, only one of the thyristors may operate, and in such a case, a lightning surge is concentrated on the thyristor, and the heat generation is remarkably increased, resulting in a decrease in surge current resistance. There was a point.

It is an object of the present invention to provide a surge protection element which has improved surge current withstanding capability and high reliability.

[0024]

According to a first aspect of the present invention, a diffusion layer region having a second conductivity is formed on both surfaces of a semiconductor substrate having a first conductivity, and a second conductivity on both surfaces is formed. A diffusion layer region having the first conductivity is formed in each diffusion layer region having the conductivity in a comb shape with a width of 500 μm or less, and the diffusion layer regions having the first conductivity on both surfaces are alternately formed. A diffusion layer region having a second conductivity on both sides and a first
The electrode is connected to the diffusion layer region having the conductivity of 1.

According to a second aspect of the present invention, the diffusion layer regions having the second conductivity are formed on both surfaces of the semiconductor substrate having the first conductivity, and the diffusion layers having the second conductivity on both surfaces are formed. A diffusion layer region having a first conductivity is formed in each region with a width of 5
It is formed in a comb shape with a thickness of 00 μm or less, and the diffusion layer regions having the first conductivity are formed alternately on both surfaces, and the surface concentration is higher than the surface concentration in each diffusion layer region having the second conductivity on both surfaces. Each of the second conductive diffusion layer regions having a surface concentration is formed in a comb shape with a width of 500 μm or less so as to fill the space between the first conductive diffusion layer regions, and the second conductive layers on both sides are formed. An electrode is connected to the diffusion layer region and the diffusion layer region having the first conductivity.

[0026]

According to the present invention, the diffusion layer region having the first conductivity is formed into a comb shape to integrate the plurality of diffusion layer regions,
All of the same-polarity thyristors can be operated reliably. As a result, it is possible to effectively operate the structure in which a plurality of forward and backward thyristors are formed in parallel, and it is possible to surely disperse the heat generating portions inside the element. Therefore, the peak temperature inside the element when a lightning surge is applied can be reduced, and the temperature distribution can be quickly made uniform.

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram showing the structure of an embodiment of the surge protection element according to the first aspect of the invention. Note that (1) is a top view, (2) is a cross-sectional view taken along one-dot chain line of the top view, and (3) is a bottom view.

In the figure, components having the same functions as those of the parts of the surge protection device of the prior application (FIG. 8) are designated by the same reference numerals and will not be described. The protective films 6a and 6b and the electrodes 7a and 7b are omitted in the top view and the bottom view.

A feature of this embodiment is that when a plurality of forward thyristors and reverse thyristors connected to the same electrode 7a, 7b are alternately arranged, a plurality of p-type diffusion layer regions (emitters) are provided. Each of 3a and 3b is formed in a comb shape. Further, at this time, the p-type diffusion layer region 3a and the p-type diffusion layer region 3b formed in a comb shape are arranged so as not to overlap on both sides.

That is, a plurality of forward thyristors having a pnpn structure, which are composed of the p-type diffusion layer region 3a, the n-type diffusion layer region 2a, the p-type semiconductor substrate 1, and the n-type diffusion layer region 2b, are integrally formed. Formed and p-type diffusion layer region 3
A plurality of reverse thyristors having a pnpn structure, which are composed of the b and n type diffusion layer regions 2b, the p type semiconductor substrate 1, and the n type diffusion layer region 2a, are integrally formed.

With the comb-shaped emitter structure as described above, the plurality of forward thyristors and the plurality of reverse thyristors each become one thyristor, and the thyristors are alternately arranged. Can obtain the same effect (dispersion of heat source) as the previous application.

Furthermore, since the emitter is comb-shaped and not individualized as in the previous application, the on-region spreads over the entire comb-shaped region with the passage of time centering on the initial on-region. The thyristor can be operated reliably.

Therefore, when the surge is applied, the temperature gradient generated inside the element becomes gentle, and the mechanical strain caused by the difference in thermal expansion can be reduced. Further, since the peak temperature is lowered, the reliability of the surge protection element and the surge current withstand capability can be improved.

FIG. 2 is a diagram showing the relationship between the width (emitter width) of the p-type diffusion layer regions 3a and 3b and the peak temperature.
In the figure, the horizontal axis shows the emitter width (μm), and the vertical axis shows the peak temperature. The example shown here is a lightning surge waveform with a rise time of 15 μs and a fall time of 100 μs.
It is the result of having performed unsteady thermal analysis at the time of applying 600A. As shown in the figure, it can be seen that the peak temperature can be significantly reduced by setting the emitter width to 500 μm or less.

By the way, some high-frequency and high-power transistors also have a comb-type emitter structure, which has a reduced ratio Ae / Le between the emitter area Ae and its peripheral length Le in order to effectively work the emitter. Is. In this case, the emitter width We is determined by the lateral diffusion distance of majority carriers, which is L _PB ≈ (D _PB τ _PB ) ^1/2 in the case of the npn structure, and the emitter width of the diffusion base type silicon transistor is It will be about 10 μm. On the other hand, if the emitter width used in the surge protection device of the present invention is 500 μm or less, the peak temperature can be sufficiently reduced.

FIG. 3 is a diagram showing the structure of an embodiment of the surge protection element according to the second aspect of the invention. Note that (1) is a top view, (2) is a cross-sectional view taken along one-dot chain line of the top view, and (3) is a bottom view.

In the figure, components having the same functions as those of the surge protection element of the prior application (FIG. 8) are designated by the same reference numerals and will not be described. The protective films 6a and 6b and the electrodes 7a and 7b are omitted in the top view and the bottom view.

The feature of this embodiment is that when a plurality of forward thyristors and reverse thyristors connected to the same electrode 7a, 7b are alternately arranged, a plurality of p-type diffusion layer regions (emitters) are provided. 3a and 3b are formed in a comb shape, and n-type diffusion layer regions 4a and 4b are also formed in a comb shape in the thyristor region in each direction to form an ohmic junction. It is arranged to fill the space.

At this time, the p-type diffusion layer region 3a and the p-type diffusion layer region 3b formed in a comb shape are arranged so as not to overlap each other on both sides, and the n-type diffusion formed in a comb shape is formed. The layer region 4a and the n-type diffusion layer region 4b are also arranged so as not to overlap each other.

That is, a pnp formed by the p-type diffusion layer region 3a, the n-type diffusion layer region 2a, the p-type semiconductor substrate 1, the n-type diffusion layer region 2b, and the n-type diffusion layer region 4b.
A plurality of forward thyristors of n structure are integrally formed, and a p-type diffusion layer region 3b, an n-type diffusion layer region 2b, a p-type semiconductor substrate 1, an n-type diffusion layer region 2a, and an n-type diffusion are formed. A plurality of reverse thyristors having a pnpn structure formed by the layer regions 4a are integrally formed.

Thus, the n-type diffusion layer region (base)
By forming the n-type diffusion layer regions 4a and 4b for making ohmic contact with a surface concentration higher than the surface concentrations of 2a and 2b, the contact potential difference (loss due to Schottky barrier) between the semiconductor portion and the electrode is reduced. The surge current withstand capability can be improved. The p-type diffusion layer region 3
Since a and 3b and the n-type diffusion layer regions 4a and 4b are short-circuited via the electrodes 7a and 7b, respectively, the same function is obtained whether or not the diffusion layer regions are in contact with each other.

The thermal operation and effect of this embodiment are similar to those of the embodiment shown in FIG.
When it is 00 μm or less, the peak temperature can be sufficiently reduced.

[0043]

As described above, according to the present invention, the peak temperature inside the element can be reduced and the temperature inside the element can be made uniform in a short time. Therefore, the mechanical strain caused by the difference in thermal expansion is minimized. It is possible to prevent the element from being destroyed. Therefore, the reliability as a protection element against lightning surge or static electricity to which a high-voltage pulse having a short time width is applied, and as a switching noise and other absorbing elements can be improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing a structure of an embodiment of a surge protective element according to the first aspect of the invention.

FIG. 2 is a diagram showing a relationship between a width (emitter width) of p-type diffusion layer regions 3a and 3b and a peak temperature.

FIG. 3 is a diagram showing an embodiment structure of a surge protection element of the invention according to claim 2;

FIG. 4 is a diagram showing an equivalent circuit of a surge protection device.

FIG. 5 is a sectional view showing a conventional structure of a surge protection element.

FIG. 6 is a diagram showing voltage-current characteristics of a thyristor.

FIG. 7 is a view showing a time change of temperature distribution inside a conventional surge protection element.

FIG. 8 is a cross-sectional view showing the structure of the surge protection element described in the specification of the prior application (Japanese Patent Application No. 3-189038).

FIG. 9 is a view showing a time change of temperature distribution inside the surge protection element of the prior application.

[Explanation of symbols]

 1 p-type semiconductor substrate 2 n-type diffusion layer region (base region) 3 p-type diffusion layer region (emitter region) 4 n-type diffusion layer region (ohmic region) 6 protective film 7 electrode 8 forward thyristor 9 reverse Directional thyristor 11, 12, 13 Forward direction thyristor 14, 15, 16 Reverse direction thyristor 51, 52 Thyristor

Continuation of the front page (56) References JP-A-55-62768 (JP, A) JP-A-63-233565 (JP, A) Jikken-44-23247 (JP, Y1)

Claims (2)

(57) [Claims] 1. A diffusion layer region having a second conductivity is formed on both surfaces of a semiconductor substrate having a first conductivity, and first diffusion layer regions having the second conductivity are formed on both surfaces of the diffusion layer region. The diffusion layer regions having conductivity are formed in a comb shape with a width of 500 μm or less, and the diffusion layer regions having the first conductivity on both surfaces are alternately arranged, and the diffusion layer regions having the second conductivity on both surfaces. And a surge protection element comprising an electrode connected to the diffusion layer region having the first conductivity. 2. A diffusion layer region having a second conductivity is formed on both surfaces of a semiconductor substrate having a first conductivity, and first diffusion layer regions having the second conductivity are formed on both surfaces of the first diffusion layer region. The diffusion layer regions having conductivity are formed in a comb shape with a width of 500 μm or less, and the diffusion layer regions having the first conductivity on both surfaces are alternately formed, and the diffusions having the second conductivity on both surfaces are formed. Each of the second conductive diffusion layer regions having a surface concentration higher than the surface concentration is formed in a comb shape with a width of 500 μm or less so as to fill the space between the first conductive diffusion layer regions. A surge protection element, characterized in that electrodes are connected to the diffusion layer region having the second conductivity and the diffusion layer region having the first conductivity on both surfaces.