JPH08316456A - Surge protecting element - Google Patents

Abstract

PURPOSE: To independently design break-over voltage and to decrease junction capacitance by a method wherein the impurity concentration of the N-base layer in a through hole is made higher than the other part, and the P-N junction, having the impurity concentration grade larger than the other junction part, is provided directly under the through hole. CONSTITUTION: When surge voltage is applied to an electrode 1, junctions J2 and J2' are invertedly biased. As the junction J2' has the impurity concentration gradient larger than the junction J2 and it has low break-over voltage, avalanche currents Ia1 and Ia2 are allowed to flow through the junction J2. When the voltage drop, generated by the resistance of N-base, exceeds the built-in potential of a junction J1, the injection of carrier is started from a P-emitter, a forward direction thyristor is switched to on-state, and a large current ION is allowed to flow. To be more precise, as the break-over voltage is determined by the junction J2', the impurity concentration of the junction J2 can be made low, and the junction capacitance can be sharply decreased on the whole element.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bidirectional two-terminal thyristor, and more particularly to a surge protection element suitable for protecting a communication device from a high voltage surge such as lightning.

[0002]

2. Description of the Related Art A conventional bidirectional two-terminal thyristor, which is widely used for surge protection of communication devices, is shown in FIG.
It has a structure shown in (a). That is, a forward thyristor composed of P1N2P3N4 and a reverse thyristor composed of P5N4P3N2 are connected in anti-parallel in the same substrate. FIG. 4B shows the current-voltage characteristics of the forward thyristor, where VB0 is the breakover voltage, IS is the switching current, and IH is the holding current. The operating principle of the device is as follows. When a positive voltage is applied to the electrode 1, the junction J2
Is reverse biased. When the voltage across the junction J2 exceeds the breakover voltage VB0, an avalanche current Ia1 flows through the junction. N base (N2) resistance and current Ia1
When the voltage drop caused by the product of the two exceeds the built-in voltage of the junction J1, holes are injected from the P emitter (P1), the forward thyristor switches to the ON state, and the ON current ION flows.

When the above-described conventional bidirectional two-terminal thyristor is used for protection against lightning surge of a communication device in which a power supply current is constantly flowing, the ON state (low resistance state) is maintained only against lightning surge. However, it is necessary that the power supply current is turned off. That is, after the lightning surge current has flown away, the element must return to the off state without delay. Therefore, it is necessary to design the holding current IH to be large (greater than the feeding current). The holding current IH can be arbitrarily set by the diffusion profile of the P emitter and the N base, but the breakover voltage of the device is also defined by the diffusion profile. As a result, there is a problem in that the degree of freedom in design is significantly limited in order to satisfy both target performances at the same time. Furthermore, when applying conventional elements for surge protection of high-speed digital communication equipment,
There is a problem that communication quality is deteriorated due to the junction capacitance in the junctions J2 and J3. In particular, an element having a low breakover voltage has a problem that the junction capacitance inevitably becomes large because it is necessary to set the impurity concentrations of N2, P3 and N4 to be high.

Further, in the conventional technique of FIG.
Part of the central part of P1 is removed to expose the N2 layer on the surface,
There is also a technique in which the structure in which the electrodes 1 are short-circuited (so-called short-circuited emitter holes) is adopted and the degree of freedom in designing the holding current is increased by adjusting the number and size of the short-circuited emitter holes. However, the impurity concentration of the short-circuit hole is the same as that of the N2 layer, and the junction surface J2 has a uniform impurity gradient regardless of where the short-circuit emitter hole is provided, so that the junction capacitance is still large. The problem of becoming will remain. Furthermore, if the size and number of the short-circuit holes are increased, Is in FIG. 4 (b) is increased, which causes a problem that switching loss increases and surge withstand capability decreases.

[0005]

As described above, in the conventional device, the holding current, the breakover voltage, and the capacitance are not uniquely determined, so that the degree of freedom in design is limited. , In some cases, there was a problem that could not be realized.

It is an object of the present invention to provide a surge protection element which has a large degree of freedom in design and which can independently design the holding current and the breakover voltage and can reduce the junction capacitance.

[0007]

To achieve the above object, in the present invention, for example, when the first conductivity type is P type and the second conductivity type is N type (conversely, the first conductivity type is May be N-type and the second conductivity type may be P-type), N-base layers are formed on both sides of the P-type semiconductor substrate, and a P-emitter layer is formed at a position deviated from the center of the N-base layer. Have and
The P emitter layer and the N base layer have a structure in which they are in common contact with the electrodes on the surface, and at least one part of the P emitter layer has a through hole formed by the N base layer. In a semiconductor device having a bidirectional two-terminal thyristor structure having a structure in which electrodes are short-circuited, for example, as shown in FIG. 1, an N base layer (N2 layer in the drawing) in the through hole (the portion shown as CH) Has a higher impurity concentration than that of the N base layer (N2 layer in the figure) of the other part, and has a larger impurity concentration gradient just below the through hole than the other junction (J2 in the figure) (see the figure). J2 ') inside.

Alternatively, according to another aspect of the present invention for achieving the above object, in a semiconductor device having a bidirectional two-terminal thyristor structure having the same structure as described above, for example, as shown in FIG. The impurity concentration of a part of the N base layer (N2 layer in the figure) located outside the P emitter layer (P1 layer in the figure) when viewed from the center of the electrode 1) is made higher than that of the other part, and A PN junction (junction J2 'in the figure) having an impurity concentration gradient larger than that of another junction (junction J2 in the figure) is provided immediately below.

[0009]

According to the present invention, the impurity concentration of the N base layer portion in the through hole penetrating the P emitter layer is made higher than that of the other N base layer portions, and the impurity concentration gradient immediately below that is higher than that of the other junction J2 portion. By forming a large PN junction J2 ',
The breakover voltage will be determined by the junction J2 '.
Therefore, the impurity concentration gradient in the other junction J2 can be made small, which can significantly reduce the junction capacitance of the entire element. Further, the magnitude of the holding current can be independently determined by adjusting the size and number of the through holes in the P layer.

According to another aspect of the present invention, the impurity concentration of a part of the N base layer located outside the P emitter layer is made higher than that of the other part, and the impurity concentration is directly below that of the other junction J2 part. By forming the PN junction J2 'having a large concentration gradient, the junction J2' located outside the P emitter layer is formed.
Since the path of the avalanche current (Ia1 in FIG. 3) of the N base layer that flows through the element is long, the voltage drop due to this causes the injection of carriers from the P layer, as will be described later in detail, and It can be easily turned on. The holding current and the breakover voltage can be independently determined, and the capacitance can be reduced as in the case of the present invention.

[0011]

1 is a cross-sectional view of a first embodiment of the present invention, and FIG. 2 is a plan view of FIG. 1 with the electrodes and insulators removed. In this device, N bases (N2, N4) in which N type impurities are diffused are formed on both sides of P3 which is a P type semiconductor substrate, and then P emitters (P1,
P5) is formed. A part (CH) of the P emitter is removed to expose the N base on the surface, and the P emitter and the N base are short-circuited by the electrode. Here, the short-circuit hole CH is formed by diffusion of N-type impurities so as to have a higher impurity concentration than other N-base regions, and a junction J2 'is formed immediately below the short-circuit hole with the P-base (P3). ing. That is, the concentration gradient of the junction J2 'becomes larger than the concentration gradient of the junction J2. The relationship between the concentration gradients of the junction J3 'and the junction J3 is similar, and the concentration gradient of J3' is larger than that of J3. As shown in FIG. 2, the shape of the short circuit hole is circular, square,
It is formed in any shape such as an ellipse, and the number is one or more, and any number may be provided as necessary. P according to this embodiment
The operation of the NPN surge protection element is as follows. When a positive surge is applied to electrode 1, junction J2 and junction J2 '
Is reverse biased. Since the junction J2 'has a larger impurity concentration gradient and a lower breakover voltage than the junction J2, the avalanche current Ia passes through the junction J2'.
1 and Ia2 flow. When the voltage drop caused by the avalanche current and the resistance of the N base exceeds the built-in potential of the junction J1, carrier injection starts from the P emitter, the forward thyristor switches to the ON state, and a large current ION flows. As a result, FIG. 4 (b)
The same voltage-current characteristic as that of is obtained. The feature of this device is that the breakover voltage is determined by the junction J2 ', so that the impurity concentration of the other junction portion, that is, the junction J2 can be lowered, and the junction capacitance of the entire device can be greatly reduced. Furthermore, by adjusting the size and number of short-circuit holes, the size of the holding current can be determined independently of the breakover voltage.

FIG. 3 shows a second embodiment of the present invention in which a short-circuit hole CH is provided in a part of the P emitter as in the first embodiment. However, the impurity concentration distribution of the short-circuit hole of this embodiment is the same as that of the N base (N2), and P immediately below the N base (N2).
N-junction does not have a particularly large concentration gradient. In this embodiment, a high concentration gradient junction J2 'that determines the breakover voltage of the device is provided at a position other than directly under the P emitter.
That is, the junction J2 'has a part of the impurity concentration of the N base layer (N2 layer in the figure) located outside the P emitter layer (P1 layer in the figure) when viewed from the center of the electrode (for example, the electrode 1). It is formed higher than that of the other portion and is formed as a PN junction immediately below that having a larger impurity concentration gradient than the other junction (junction J2 in the figure). The joint J3 'is similarly formed. With the above configuration, the avalanche current Ia1 flows for a long distance in the N base layer immediately below the P emitter, and the built-in voltage for causing carrier injection from the P emitter can be easily secured. . As a result, the element can be switched to the ON state without increasing the switching current Is shown in FIG. 4B, and the energy consumption in the element can be reduced. Of course, as in the case of the first embodiment, the magnitude of the holding current and the breakover voltage can be designed independently, and the capacitance can be reduced.

Although the PNPN surge protection device has been described in the above embodiments, it goes without saying that the NPNP surge protection device can be formed in the same manner as this embodiment by only reversing the conductivity type.

[0014]

As described above, in the surge protection element according to the present invention, for example, a short-circuit hole in which the N base appears on the surface is provided in a part of the P emitter, and a reverse bias is applied to the P element.
PN with a larger concentration gradient in part of the N junction than in other parts
Since the junction is provided, the magnitude of the holding current and the breakover voltage can be designed independently, and an element having a small junction capacitance can be realized.

[Brief description of drawings]

FIG. 1 is a sectional view of a first embodiment of the present invention.

FIG. 2 is a plan view of the first embodiment of the present invention.

FIG. 3 is a sectional view of a second embodiment of the present invention.

FIG. 4 is a sectional view of a conventional PNPN surge protection device and a voltage-current characteristic of the device.

[Explanation of symbols]

P1, P5 ... P Emitter N2, N4 ... N
Base P3 ... P Base J1, J2, J3, J2 ', J3' ... Join CH ...
Short-circuit holes (through holes) Ia1, Ia2 ... Avalanche current VB0 ... Breakover voltage IS ...
Switching current IH ... holding current

Claims (2)

[Claims] 1. A base layer of the second conductivity type is formed on both sides of a semiconductor substrate of the first conductivity type, and an emitter layer of the first conductivity type is formed at a position deviated from the center of the base layer of the second conductivity type. And a structure in which the first-conductivity-type emitter layer and the second-conductivity-type base layer are in common contact with a surface electrode, and a part of the first-conductivity-type emitter layer In a semiconductor element having a bidirectional two-terminal thyristor structure having a through-hole formed by the second conductivity type base layer at one or more positions of the second conductive type base layer, the surface of which is short-circuited by the electrode. The impurity concentration of the conductivity type base layer is made higher than that of the second conductivity type base layer of the other portion, and a junction having an impurity concentration gradient larger than that of the other junction portion is provided immediately below the through hole. Surge protection element. 2. A second conductivity type base layer is formed on both sides of the first conductivity type semiconductor substrate, and a first conductivity type emitter layer is formed at a position deviated from the center of the second conductivity type base layer. And a structure in which the first-conductivity-type emitter layer and the second-conductivity-type base layer are in common contact with a surface electrode, and a part of the first-conductivity-type emitter layer In a semiconductor device having a bidirectional two-terminal thyristor structure having a through hole formed by the second conductivity type base layer at one or more positions thereof and having a surface short-circuited by the electrode, viewed from the center of the electrode. The impurity concentration of a part of the second conductivity type base layer located outside the first conductivity type emitter layer is set higher than that of the other part, and the impurity concentration gradient is larger immediately below it than the other junction part. Surge characterized by having a junction Mamoru element.