JPH0312458B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor device including an overvoltage protection element that can be built into a transistor or the like and has a controlled breakdown voltage. Note that the Zener diode herein refers to a diode that uses a breakdown phenomenon such as avalanche breakdown as its element function.

Conventionally, a protection diode, which is one of the overvoltage protection elements built into a device and used, has been devised in various ways to make the breakdown voltage V _Z of the diode lower than the V _CEO voltage of the transistor to be protected. As shown in the circle A in Figure 1, this diode can be formed by deeply diffusing a part of the inactive base region and determining V _Z by reach-through with the collector high concentration layer. Also, as shown in circle B in Figure 2, a highly concentrated region of the same conductivity type as the collector is formed in a part of the inactive base region by ion implantation, etc., as shown in circle B in Figure 2. There is a method to determine _{V Z} using As shown in the circle D in Fig. 4, there is a method of determining V _Z by punch-through in the horizontal direction. FIG. 5 shows an equivalent circuit of the semiconductor structure shown in FIGS. 1 to 4. In addition, 2 in Figures 1 to 4
1 is an N-type collector substrate, 22 is an N-type collector high concentration region, 23 is a P-type base region, and 23A is a P-type collector substrate.
24 is an N-type emitter region, 25 is a base electrode, 26 is an emitter electrode, 27 is a silicon oxide film, 28 is an N-type high-precision diffusion region, and 29 is a P-type diffusion region. .

Of these, only the structures shown in FIGS. 1 and 2 have been confirmed as products, and the structures shown in FIGS. 3 and 4 are only proposed. This is because the devices shown in FIGS. 3 and 4 are formed on the surface of the substrate, and V _Z changes significantly due to the influence of external charges, making them completely useless. Now, regarding the structure shown in Figures 1 and 2, since the operating part is inside the semiconductor substrate, fluctuations in V _Z are not a problem.
Naturally, a process for forming the diode portion is required, which increases costs. In addition, the 360V± required for controlling the ignition coil of the ignition system for internal combustion engines
Controlling _VZ of 10% is extremely difficult and increases cost in terms of yield. For the structures shown in Figures 1 and 2, it has been actually measured that the variation is approximately ±25%.

The present invention has been made in consideration of the above points, and is a novel overvoltage that can be obtained by a relatively easy manufacturing method compared to the above, has stable characteristics, and has a breakdown voltage with good controllability. An object of the present invention is to provide a semiconductor device including a protection element.

The basic element structure of the present invention includes a semiconductor substrate of a predetermined conductivity type, a first region formed on one main surface of the semiconductor substrate and of a conductivity type opposite to that of the substrate;
a second region formed on one main surface of the semiconductor substrate apart from the first region and having the same conductivity type as the substrate and having a higher impurity concentration than the substrate; and a second region between the first region and the second region. A gate electrode covers a part or all of the surface of the substrate with an insulating film interposed therebetween, and at least a part thereof overlaps a part of the first region and the second region with the insulating film interposed therebetween. A conductive film, an anode electrode electrically connected to the first region, and a cathode electrode electrically connected to the back surface of the semiconductor substrate, and a predetermined potential difference is applied between the gate electrode and the anode electrode. This is a characteristic feature.

Therefore, the basic structure of the present invention will be explained using an example of a substrate shown in FIG. On one main surface of the N-type semiconductor substrate 1, a P-type region 2 (first region) and a region 3 (second region) having a higher impurity concentration than the N ⁺ type, that is, the N-type substrate 1 are formed. Then, these surfaces are covered with an insulating film 4 such as a silicon oxide film, and the region 2, the region 3, and the substrate surface surrounded by the regions 2 and 3 are covered with at least a partial overlap of the regions 2 and 3 from above the insulating film 4. This electrode 5 is used as a gate G. Electrodes 6 are appropriately provided on the substrate 1 and the region 2, the electrode electrically connected to the substrate 1 is designated as a cathode K, and the electrode electrically connected to region 2 is designated as an anode A. Then, apply a reverse bias to this anode A, connect the gate G and the anode A via the power supply S as shown in Fig. 7 (in this case, the gate voltage V _G =0), and then increase the reverse bias voltage. First, an inversion layer 7 is formed on the surface of the substrate under the gate G and reaches from region 2 to region 3. Let the voltage between the gate and the substrate at this point be V _T . When the reverse bias voltage is further increased, this inversion layer 7 is driven away from the region 3, and a depletion region 71 is formed on the surface between the inversion layer 7 and the region 3 (of course, it is formed in all the PN junctions in the substrate). (omitted for the sake of explanation), causes an avalanche breakdown phenomenon with the electric field strength determined by the substrate concentration. The breakdown voltage V _Z at this time is always lower than the withstand voltage (V _BLUK ) originally possessed by the substrate, and according to the inventors' experiments, it is 1/
It has been confirmed that the diode is about 3, which is convenient as a built-in diode for protecting the device.

Furthermore, regarding the controllability of V _Z , if there is a certain distance necessary to obtain withstand voltage with respect to the distance between region 3 and region 2, which is estimated to be most relevant,
An inversion layer 7 is formed in the remaining portion, and since the voltage within the inversion layer is constant, it is not related to V _Z . That is,
According to the structure of the present invention, distance accuracy between regions 2 and 3 is not required, and therefore the variation in V _Z is about 1/5 compared to the conventional structure. This effect will be referred to as the buffer effect of the inversion layer.

Furthermore, since the surface of the operating part is always shielded by the electrode 5 via the insulating film 4, it is not affected by external charges and a stable breakdown voltage _VZ that does not change over time can be obtained. . Also,
In the semiconductor structures shown in FIGS. 3 and 4, there is a variation in V _Z that is twice the initial value, whereas the structure of the present invention has the advantage that the variation in V _Z can be almost completely eliminated. Note that the semiconductor substrate 1 and each region 2,
The basic structure of the present invention can also be achieved when the conductivity type 3 is the opposite conductivity type.

Hereinafter, the present invention will be explained in detail with reference to embodiments shown in the drawings. In FIG. 6, semiconductor substrate 1 is an N-type diffusion layer with a concentration of 1.5×10 ¹⁴ atms/cm ³ , and region 2 is a P-type diffusion layer with a surface concentration of 1×10 ¹⁸ atms/cm ³ and a layer depth of 30 μm.
It is. An electrode 6 is provided on this region 2 to constitute an anode A in a diode structure. Region 3 is an N-type diffusion layer with a surface concentration of 2×10 ¹⁹ atms/cm ³ ,
The depth of the layer is 18 μm. Further, the insulating film 4 is a SiO ₂ film, subjected to ring etching, and has a thickness of 4.0 μm. Electrodes 5 and 6 are made of aluminum with a thickness of 4.0 μm.
For example, when a transistor is built in as a protection diode, such as an output transistor for an ignition device for an internal combustion engine, the semiconductor substrate 1 is formed in common with the collector, the region 2 is formed as the base, and the region 3 is formed simultaneously with the emitter, and is insulated. Membrane 4 is the field
The SiO ₂ film can be used as is, and the electrodes can be formed at the same time as the base and emitter electrodes, and no special steps are required to incorporate the device of the present invention into a transistor. Here, normally gate G is directly connected to region 2.
It is common to connect with Al wiring, and region 2 and the inactive part of the base region of the transistor may be shared.

Therefore, the operation based on the structure of the present invention will be explained.
The original withstand voltage V _BLUK of the semiconductor substrate is about 1000v,
A transistor with a base and an emitter inside the substrate.
V _CEO is about 400v. As a result of experiments, it was found that the breakdown voltage V _Z based on the structure of the present invention is given by V _Z =V _T +(1/3)V _BLUK . Here, V _T is SiO ₂ film 4
If the surface charge density Q _SS is 2×10 ¹¹ atms/cm ² , it will be about 50 V. Yotsute V _Z ≒50×1000/3≒388
(v) This value is, for example, the standard for a protective Zener diode normally used in an output transistor for an ignition system.
According to the experimental results, the above-mentioned inversion layer 7 satisfies _330t < _V
3. In other words, due to the buffer effect of the inversion layer where the distance between the anode and cathode no longer contributes to V _Z , ±
This is about 5%, which is much smaller than the actually measured element variation of ±20% in the conventional structure as shown in FIGS.

Also, regarding the stability of V _Z due to changes over time,
In particular, the operating parts are shielded from external charges,
Furthermore, regarding the influence of mobile charges inside the film, when ordinary gettering is performed, most of the positive charges are Na ions, etc., and as in this example, the basic type is N type, and the gate G is connected to the anode A and the base of the transistor. In the structure common to both, since the gate G has a negative potential, positive charges are attracted to the gate side, and there is almost no influence on the surface of the base body, which is the operating part, and a very stable V _Z can be obtained.

Figures 9a and 9b show the results of a BT test (in this case, a reverse voltage of 300 V and a high temperature atmosphere of 170° C.) of a prototype according to this example. It shows changes in leakage current I and breakdown voltage V _Z over time measured at a high temperature of 170° C., and they do not change at all.
Furthermore, the operating characteristics shown in Fig. 8 are obtained by changing the gate voltage and looking at the I-V characteristics of this device using the equivalent circuit shown in Fig. 7, where V _Z = aV _G

Claims (1)

[Scope of Claims] 1. A semiconductor substrate having a predetermined conductivity type, a first region formed on one main surface of the semiconductor substrate and having a conductivity type opposite to that of the substrate, and a first region formed on one main surface of the semiconductor substrate. a second region formed apart from the first region, having the same conductivity type as the substrate and having a higher impurity concentration than the substrate; and a part or all of the surface of the substrate between the first region and the second region. a conductive film that serves as a gate electrode and is disposed such that at least a portion of the conductive film overlaps a portion of the first region and a portion of the second region with the insulating film interposed therebetween; A semiconductor device comprising: an anode electrode electrically connected to the semiconductor substrate; and a cathode electrode electrically connected to the back surface of the semiconductor substrate, and a predetermined potential difference is applied between the gate electrode and the anode electrode.