JPH03215979A - Bidirectional thyristor - Google Patents

Abstract

PURPOSE:To surely protect an integrated circuit against a surge, where the integrated circuit concerned is low in breakdown strength, small in capacitance and variation of surge current withstanding strength, and possessed of a digital data processing function, by a method wherein a region whose breakdown strength is determined on the basis of punch-through is provided. CONSTITUTION:Punch-through sections C and C' protruding into a P layer are provided to a part of an N1 and an N2 layer on the end opposite to the setting section of a P1 and a P2 layer, the thickness of the part of the P layer sandwiched between the sections C and C' is represented by W'P smaller than the thickness WP of the main body of the P layer, and the sections C and C' are subjected to punch-through at a voltage lower than the avalanche breakdown voltage of junctions J2 and J3. As mentioned above, a breakdown strength is determined basing on the impurity concentration of the P layer and the thickness W'P of the punch-through sections C and C', and when the P layer is constant in impurity concentration, the breakdown strength is determined only by W'P, so that a required breakdown strength can be obtained by selecting the thickness W'P. By this setup, an integrated circuit can be surely protected against a surge, where the integrated circuit concerned is low in breakdown strength, small in capacitance and variation of withstand surge current, and possessed of a digital data processing function.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a PNPNP (NPNPN) bidirectional thyristor, particularly to lowering the withstand voltage (■■0) and reducing the capacitance.

(Prior Art) A bidirectional silicon risk having the basic structure as shown in FIG. 1(a), that is, N. layer and N2 layer, and further N,,N. P exposed on the surface in a part of the layer,
A bidirectional thyristor is well known, which has a five-layer structure in which a P2 layer is formed and the P1 layer and N layer and the P2 layer and N2 layer are short-circuited by a metal electrode 1'T2, respectively, and operates as follows. ing.

It is assumed that a voltage is applied in a direction that causes a current to flow in the direction of the arrow shown in the diffusion cross-sectional view of FIG. 1(a). When this voltage exceeds the withstand voltage (vno) of junction J3, no current flows through junction J3. Then, the current component (2) of this current flowing laterally through the N2 layer and the voltage drop based on the lateral resistance R forwardly bias the junction J4, causing holes to be injected from the P2 layer. For this purpose, the electrodes T, T2 are turned on as shown in the voltage-current characteristic diagram shown in FIG. 1(b).

When a voltage in the opposite direction is applied, the junction J is forward biased by the current component flowing laterally through the N layer, turning on the electrode TT2 as shown in Figure 1(b). Since it is a bidirectional one-way resistor and two terminals that perform switching operations, it is easy to use, and is small and lightweight, so it is widely used as a surge protection element for various electronic circuits connected to low-power circuits, such as communication lines. It's starting to look like this.

However, as electronic circuits have recently become more integrated and their withstand voltages have become lower, there has been a strong demand for bidirectional thyristors with low withstand voltages (VBO), and the recent progress in digitalization has increased. Demand for elements with low capacitance is increasing.

(Prior art and its problems) However, in the structure of a conventional bidirectional thyristor using such ahalanche breakdown, the breakdown voltage difference between T'F2 in FIG. As a general rule, the smaller the impurity concentration, the higher the L ball becomes.

On the other hand, the junction determines the capacitance of the element. 12,. 1. The capacitance is essentially determined by the impurity concentration of the P layer, and as is well known, as the impurity concentration decreases, the capacitance also decreases in proportion to this.

Therefore, if the impurity concentration of the I) layer is increased in order to lower the 101 voltage, the capacitance will also increase, resulting in the so-called 1·Re 1ζ off relationship, so a bidirectional thyristor with low stiffness pressure and low capacitance can be realized. is difficult.

Therefore, as a means to solve this problem, the impurity concentration is increased by reducing the thickness WP of the P layer shown in FIG. A possible method is to punch through two layers, N and PN.

However, when trying to obtain a device with the required low withstand voltage and low capacitance using this method, the thickness W of the layer 1) in Fig. 1(a)
, becomes too thin, making it difficult to manufacture and difficult to realize.

For example, if the impurity concentration of the P layer is 10"/cc, N.
The surface concentration of the N2 layer is 10'8/cc, and its diffusion depth is 3.
If we assume that the impurity distribution is of an error function type and the punch-through voltage is 150μ, then the depletion layer spreads to the layer (■), and therefore the N. ,N. The thickness W of the P layer in the layer is 35
It will be about μ. As a result, N. , the thickness of the entire device including the N2 layers is approximately 100 μm, which poses significant difficulties in manufacturing using 4114 wafers, which are commonly used at present.

(Object of the Invention) The present invention provides a structure in which an element with a required low breakdown voltage and low capacitance can be easily manufactured by a known means such as ordinary selective diffusion using the punch-through method. The aim is to realize a two-way security risk that can reliably protect this type of circuit, such as the integrated circuit with the digitalization processing function.

(Means of the present invention for solving the problems) The present invention is characterized by the fact that the P
, NP2 (N, PN2 ) layer by providing a region whose breakdown voltage is determined by punch-through.
By making it possible to realize a device with low breakdown voltage while keeping the thickness of the main part of the layer, and therefore the thickness of the device, similar to that of conventional devices due to ahalanche breakdown, it is possible to easily manufacture devices with the required low breakdown voltage and low capacitance. It can be done. Next, the present invention will be explained by way of an example.

(Example) Figures 2 (a), (b), and (c) are a plan view showing an example of the present invention (the electrodes are not shown), a diffusion cross-sectional view taken along the line A-A' in the plan view, and a diffusion cross-sectional view of the plan view. It is an equivalent circuit diagram. In the present invention, P
,, Punch through parts C and C' that protrude into the P layer are provided in a part of the N,, N2 layer at the opposite end of the setting part of the P2 layer, and the thickness of the part of the P layer sandwiched by these parts is The thickness of the main portion of the P layer is W, and the thickness is W'P, which is thinner than that of the main portion of the P layer, so that this portion punches through at a voltage lower than the ahalanche breakdown voltage of the junctions J2 and J3. Next, its operation will be explained.

Now assume that a voltage is applied with a polarity that causes a current to flow in the direction of the arrow in FIG. 2(b). Then, the applied voltage becomes the thickness W'P
When the punch-through voltage corresponding to , is reached, the current T2,I, no longer flows through the protruding portion C.

When the current (2) increases, the voltage drop due to the effective lateral resistance R of the N2 layer directly below the P2 layer causes the junction J4 to be forward-biased, so holes are injected from the P2 layer into this part.
Portion C in FIG. 2(C) is first turned on as a punch-through cyrisk. In this case, the current ■1 is the junction J1
It only reverses high-assistance and does not contribute to turn-on. C
When a part is turned on, part C becomes an auxiliary/reel resistor and the turn-on state spreads to other parts, as shown in the equivalent circuit diagram in Fig. 2 (C). Since the above operation is symmetrical in structure, the same operation is performed by the C° portion even when the direction of voltage application is reversed.

As shown in Figure 2 below, the breakdown voltage (Ilo) of the device of the present invention is determined by the impurity concentration of the P layer and the thickness W'P of the bunches c and c', and at the same impurity concentration it is determined only by W', By selecting , it is possible to achieve a desired low breakdown voltage.

Also, the capacitance of the element and hence the junctions J2, J. The capacitance of is determined only by the impurity concentration, but the thickness of the P layer parts other than the C and C'' parts, which are related to the capacitance value, and therefore the main part of the P layer is small and the impurity concentration is small. , the capacitance can be reduced, the problem of 1-rade-off is solved at once, and a bidirectional thyristor with low breakdown voltage and small capacitance can be provided.

Further, according to the present invention, the thickness of the P layer portions other than the punch-through portions c and c', that is, the main portion of the P layer, can be made as thick as in the conventional device using avalanche breakdown, and the thickness of the device itself can be increased. Therefore, compared to the case where the punch-through structure is obtained by reducing the overall thickness of the P layer described above, difficulties in manufacturing processing can be significantly reduced.

In addition, according to the present invention, it is possible to use an element with less variation in surge current resistance, which is a performance required for surge protection. In other words, the surge current withstand capacity is determined by the power loss in the turn-on transition region shown in FIG. 1(b),
It is greatly influenced by the speed at which the turn-on area spreads over the entire surface from the initial turn-on position, that is, the initial ignition position.

However, in conventional elements, the ignition position is not constant, which tends to cause variations in surge current withstand capacity.

However, in the present invention, the initial firing position is the punch-through portion C,
Since the surge current is always localized to C", it is possible to provide bidirectional surge risk with almost no variation in surge current resistance.

Hereinafter, an embodiment of the present invention has been described. In forming the punch through portions c and c', there is a means for forming the starting wafer parts c and c' thin in advance as shown in the sectional view of FIG. 3. can be adopted.

Also, the punch-through operation part is N. It can be a PN2 bunch-through diode PD. Figure 4(a)(b)
is its cross-sectional view and equivalent circuit.

Further, the shape and position of the bunch-through region can be variously modified, and known methods other than diffusion can be employed in manufacturing. Furthermore, the present invention provides PNPNP (NPNPN)
It can be effectively applied to a composite thyristor whose basic structure is a bidirectional thyristor.

(Effects of the Invention) As is clear from the above description, according to the present invention, 9. Suitable for surge protection of integrated circuits that handle digital signals that have low withstand voltage, small capacitance, and less variation in surge current withstand capacity. Bi-directional thyristors can be provided.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of a conventional element, FIG. 2 is an explanatory diagram of one embodiment of the present invention, and FIGS. 3 and 4 are explanatory diagrams of other embodiments of the present invention.

Claims (1)

[Claims] P_1N_1PN_2P_2(N_1P_1NP_2N
It consists of five layers of N_1 (P_1) exposed on the surface.
) and P_1 (N_1) layers are short-circuited with the P_1 and P_2 layers to form one electrode, respectively, and the bidirectional thyristor is characterized in that a region is provided whose breakdown voltage is determined by punch-through.