SEMICONDUCTOR DEVICE INCLUDING OVERVOLTAGE PROTECTION DIODE

Abstract: A semiconductor device includes a diode element for protecting a transistor against an overvoltage. A first region (11) of p-type conductivity is formed on an upper surface of an n-type semiconductor substrate (10) in which base and emitter regions of the transistor are formed. A second region (12) of n⁺ -type conductivity whose impurity concentration is higher than that of the n-type semiconductor substrate is formed on its upper surface to be spaced apart from the first region (11). An insulating film (13) is formed to cover the upper surface of the semiconductor substrate (10). Furthermore, a conductive film (14) is formed to partially overlap the first and second regions (11, 12) through the insulating film (13). The first region (11) serves as an anode, the second region (12) serves as a cathode, and the conductive film (14) serves as a gate electrode; thus an overvoltage protection diode is obtained.

Description: The present invention relates to an overvoltage protection device, according to claim 1. In a high power transistor circuit, a combination of overvoltage protection diodes is used to protect the transistor elements. These protection diodes must be connected to the collector-base paths of the transistors on one chip. Furthermore, a breakdown voltage V Z of the protection diode must be lower than a DC collector-to-emitter voltage (base open) V CEO . Such a protection diode is connected to the collector-base path of the transistor on one chip in the following manner: an impurity is deeply diffused from part of an inactive base region to a collector region of a high concentration, thereby determining the breakdown voltage V Z . Alternatively, an impurity is ion-implanted in part of the inactive base region to form a high-impurity region of the same conductivity type as that of the collector region, thus determining the breakdown voltage V Z . However, the protection diode of this type described above must be formed in a separate process from a process for forming a general transistor. Furthermore, in the case of an ignitor output circuit for an ignition control circuit of an internal combustion engine, the breakdown voltage of the breakdown diode must fall within a range of 360 V±10%. It is very difficult to set the breakdown voltage in the above range. It is therefore a primary object of the present invention to provide an overvoltage protection device having an overvoltage protection diode which can be formed during the same process as used for manufacturing a transistor and which has stable electric characteristics, in particular a controllable breakdown voltage. Solution of this object is achieved by the features of claim 1. A first main advantageous effect of the present invention is that the breakdown voltage V Z of the protection diode may not vary even if the diode is influenced by an external electric charge. A further advantageous effect is that the overvoltage protection diode is a device of a high withstand voltage on one chip for an ignitor output circuit of an internal combustion engine wherein the protection diode is connected between collector and base of a transistor of a Darlington circuit without increasing the area of the chip. Gate controlled diodes are known from US-A-4 037 140. The gate electrode is connected to the cathode to increase the normal reverse breakdown voltage. The overvoltage protection devices shown in Figs. 5, 6 and 9 do not represent embodiments of the invention. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a sectional view of an overvoltage protection device and, especially, of an overvoltage protection diode thereof according to an embodiment of the present invention; Fig. 2 is a circuit diagram showing an equivalent circuit of the protection diode shown in Fig. 1; Figs. 3A and 3B are graphs for respectively explaining the leakage current and the breakdown voltage of the protection diode as a function of time; Fig. 4 is a graph showing voltage-current characteristics between the anode and cathode of the protection diode with respect to the gate voltage thereof; Fig. 5A is a plan view of a semiconductor device including a protection diode of the type described above; Fig. 5B is a sectional view of the semiconductor device shown in Fig. 5A; Fig. 6 is a circuit diagram showing an equivalent circuit of the semiconductor device shown in Figs. 5A and 5B; Figs. 7 and 8 are, respectively, circuit diagrams showing Darlington circuits for ignitor output circuits; Fig. 9A is a plan view of a semiconductor device including the Darlington circuit shown in Fig. 8. and Fig. 9B is a sectional view of the semiconductor device shown in Fig. 9A. Fig. 1 shows an overvoltage protection diode formed in a semiconductor overvoltage protection device in accordance with the present invention. The semiconductor device has an n-type semiconductor substrate 10. A first region 11 which serves as a p-type anode region is formed on one major surface of the semiconductor substrate 10. A second region 12 which serves as a cathode region is also formed on the one major surface of the semiconductor substrate 10 and is spaced apart from the first region 11. The second region 12 has an impurity concentration higher than that of the first region 11 or an n⁺-type conductivity. An insulating film 13 such as a silicon oxide film is formed to cover the entire surface. A conductive film 14 is selectively formed on the insulating film 13. The conductive film 14 partially overlaps both first and second regions 11 and 12 through the insulating film 13, and is thus simultaneously surrounded by the first and second regions 11 and 12. The first region 11 functions as an anode electrode 15 and is connected as an anode to a lead wire. Similarly, the conductive film 14 functions as a gate electrode and is connected as a gate to a lead wire. A conductive film 16 is then deposited on the other major surface of the semiconductor substrate 10 and is connected as a cathode to a lead wire. In the protection diode of the type described above, a power source E is connected between the gate and the anode, as shown in Fig. 2. A reverse-bias voltage is then applied to the anode. In this case, a gate voltage V G is set at 0 V. When a reverse-bias voltage is increased in this condition, an inverted layer 17 is formed in a surface layer of the semiconductor substrate 10 beneath the conductive film 14 as the gate electrode. The inverted layer 17 extends between the first region 11 and the second region 12. A potential difference between the gate and the semiconductor substrate 10 in this state is defined as V T . When the reverse-bias voltage is further increased, the majority carriers of the inverted layer 17 are expelled from the portion thereof which is adjacent to the second region 12. A depletion region 18 is thus formed between the inverted layer 17 and the second region 12. As a result, an avalanche breakdown effect occurs in proportion to the strength of the electric field determined by the impurity concentration of the semiconductor substrate 10. A breakdown voltage V Z is always lower than an inherent withstand voltage of the semiconductor substrate 10. According to an experiment by the present inventors, the breakdown voltage V Z is about one-third of the withstand voltage described above. Therefore, a transistor protection diode having the above-mentioned breakdown voltage can be suitably formed in a semiconductor device on a single chip. In the protection diode of the type described above, controllability of the breakdown voltage V Z may seem to depend upon a distance between the first region 11 and the second region 12. However, the inverted layer 17 is formed between the first region 11 and the second region 12 and is kept at a constant voltage. Therefore, the distance between the first and second regions 11 and 12 bears no relationship to the breakdown voltage, and may be determined only so as to obtain a required dielectric resistance or withstand voltage. In the protection diode having the above arrangement, the distance between the first and second regions 11 and 12 need not be precisely determined. As a result, the variation in the breakdown voltage V Z of this diode is about one-fifth of that of the conventional protection diode. This effect is called a buffer effect of the inverted layer. The active surface portion of the diode is shielded by the conductive film 14 through the insulating film 13. For this reason, influence of the external electric charge on the diode can be properly eliminated. Therefore, the breakdown voltage V Z becomes stable, without any change over time. The semiconductor device shown in Fig. 1 will be described in more detail. The semiconductor substrate 10 has an n-type conductivity and an impurity concentration of 1.5x 10¹⁴ atoms/cm³. The first region 11 comprises a p-type diffusion layer and has a surface concentration of 1x10¹³ atoms/cm³. The depth of the diffusion layer is 30 µm. The second region 12 comprises an n-type diffusion layer and has a surface concentration of 2x10¹⁹ atoms/cm³. The depth of the second region 12 is 18 µm. The insulating film 13 comprises a silicon oxide film (SiO₂) formed by phosphorus getter sputtering to a thickness of 4.0 µm. The conductive film 14 and the anode electrode 15 respectively comprise aluminum deposition films each having a thickness of 4.0 µm. When the protection diode of the type described above is used for protecting an output transistor circuit such as an ignitor of an internal combustion engine, the first region 11 is formed simultaneously with the formation of the base of the transistor element, whereas the second region 12 is formed when the emitter of the transistor element is formed. The field oxide film may be used as the insulating film 13. The conductive films or electrodes can be formed at the same time as the base and emitter electrodes of the transistor element are formed. In other words, when the protection diode is formed together with the transistor on the single chip, special steps for forming the protection diode are not required. A withstand voltage V BLUCK inherent in the semiconductor substrate is about 1,000 V. The collector-to-emitter voltage (base open) V CEO of the transistor having the base and emitter which are formed in the above-mentioned semiconductor substrate is about 400V. The breakdown voltage V Z of the protection diode shown in Fig. 1 is found to be given by the following equation: V Z =V T +(1/3)V BLUCK When a surface charge density Q SS of the insulating film 13 is given as 2x10¹¹ atoms/cm³, V T is given as about 50 V. Therefore, the breakdown voltage V Z turns out to be about 383 V. This breakdown voltage satisfies a rated range of 330 V<V z <V CEO for a protection Zener diode used for the output transistor circuit of the conventional ignitor. Furthermore, a change in the breakdown voltage V Z falls within ±5% due to the buffer effect of the inverted layer 17, which is not dependent upon the distance between the anode and the cathode. The stability of the breakdown voltage V Z over time is considered. The active portion is shielded by the conductive film 14 from the external electric charge. The mobile electric charge within the insulating film 13 comprises positive electric charge such as Na⁺ ions when getter sputtering is performed. However, in the protection diode shown in the above embodiment, the semiconductor substrate 10 has an n-type conductivity and the gate and the anode are commonly used with the base of the transistor. Therefore, the gate is kept at a negative potential. The positive electric charge described above is thus attracted to the gate and does not influence the active portion of the diode in the semiconductor substrate 10; thus a stable breakdown voltage V Z is constantly obtained. Fig. 3A is a graph for explaining a leakage current I as a function of time when a BT test of the semiconductor device obtained in the above embodiment as a sample is made at a reversebiase voltage of 300 V and at a high temperature of 170°C. Fig. 3B is a graph for explaining the breakdown voltage V Z as a function of time when the same test is performed under the same conditions as in Fig. 3A. As may be apparent from the above tests, the leakage current I and the breakdown voltage V Z are stably maintained without variations. Fig. 4 shows the relationship between a current I AC and a voltage V AC between the anode and the cathode when the gate voltage V G in an equivalent circuit shown in Fig. 2 is changed using the test sample as described above. The breakdown voltage V Z is expressed by the following equation: V Z =aV G·A +V T +bV BLUCK where a: a constant related to the quality of the insulating film and the surface charge density of the semiconductor substrate; in this case, a=0.9; and b: a constant related to the quality and thickness of the insulating film and the surface charge density of the semiconductor substrate; in this case, b=1/3. Since the breakdown voltage V Z varies linearly with respect to the voltage V G·A between the gate and the anode, the breakdown voltage V Z can be arbitrarily set by setting the gate voltage. The diode thus breaks down with a minimum area of the depletion region 18 due to the buffer effect of the inverted layer 17. If the same operation area is given, it is confirmed by experiment that the operation resistance, or Zener diode, can be decreased to between half and one-fifth of that of a diffusion region obtained by diffusing an impurity in the conventional base region, or of that of an impurity-doped region obtained by ion-implanting an impurity in part of the inactive base region. Figs. 5A and 5B shown transistor device including a protection diode. The overvoltage protection device included in the transistor device shown in Figs. 5A and 5B is not an embodiment of the invention. A semiconductor substrate 10 has an n-type conductivity and its impurity concentration is 1.4x 10¹⁴atoms/cm³. An n⁺-type diffusion/collector region 20 is formed in the rear surface (the other major surface) of the semconductor substrate 10. A p-type base region 21 is formed in the front surface (one major surface) of the semiconductor substrate 10. A first region 11 which corresponds to a p-type anode region of the diode is formed to surround the base region 21. The first region 11 is formed to be spaced apart from the base region 21. If parasitic capacitance need not be considered, the inactive region of the base region 21 may be also used as the first region 11. An n-type emitter region 22 is formed in the base region 21. A second region 12 which has an n-type conductivity is formed together with the emitter region 22 so as to surround the first region 11. The second region 12 serves as the cathode region of the protection diode. The second region 12 is spaced apart from the first region 11 by a predetermined distance. An insulating film 13 which comprises a silicon oxide film is formed on the front surface of the semiconductor substrate 10. A conductive layer 14 is formed on the insulating layer 13 so as to partially overlap the first and second regions 11 and 12. The conductive film 14 is formed by deposition of aluminum together with a base electrode 23 and an emitter electrode 24 and is electrically connected to the first region 11. Fig. 6 shows an equivalent circuit of the transistor circuit shown in Figs. 5A and 5B. The semiconductor device of the type described above can be manufactured in the same manner as described above even if it has regions (including the semiconductor substrate) which are of opposite conductivity types to those of the regions shown in Figs. 5A and 5B. In this case, if a p-type semiconductor substrate is used, the diode tends to receive an effect of the positive mobile electric charge in the insulating film 13, as compared with the n-type semiconductor substrate. The anode and cathode regions may comprise buried epitaxial layers, respectively. Furthermore, the anode and cathode regions need not be formed simultaneously with the base and emitter of the transistor. As described above, the transistor circuit of high withstand voltage which includes an overvoltage protection diode can be effectively used for an ignitor output circuit of an ignition control circuit of an internal combustion engine. Fig. 7 shows a Darlington circuit 100 which is arranged as an ignitor output circuit of the type described above. The Darlington circuit 100 has a drive transistor Tr1 and an output transistor Tr2 which are Darlington-connected. A protection diode D Z is connected between the collector and base of the transistor Tr1. A control signal to the transistor Tr1 is produced from the collector of a control transistor Tr3 and is supplied to the base of the transistor Tr1 through a resistor R1. In the circuit shown in Fig. 7, when the transistor Tr3 is ON, the transistor Tr1 is OFF. A voltage V CB between the collector and base of the transistor Tr1 is increased. When a voltage applied across the protection diode D Z reaches the breakdown voltage V Z , a current corresponding to the breakdown voltage flows in the base of the transistor Tr1. The voltage V CB stops increasing when the voltage applied to the protection diode D Z reaches the breakdown voltage. At this time, since the control transistor Tr3 is ON, the breakdown current of the protection diode D Z flows through the transistor Tr3 if the resistor R1 is not present. In this condition, the transistor Tr1 may not be turned on. However, as shown in Fig. 8, when a protection diode D Z is connected between the collector and base of the output transistor Tr2 of a Darlington circuit 100, an internal resistor R2 of the circuit operates in the same manner as the resistor R1 of the circuit shown in Fig. 7. Thus, the resistor of the transistor Tr3 may be omitted. This results in a decrease in the number of component parts externally connected to the circuit. Furthermore, since an input series-connected resistor is not present, gain of the circuit is increased, thus resulting in an advantage in circuit design. Figs. 9A and 9B show a semiconductor device having the Darlington circuit 100 shown in Fig. 8. The overvoltage protection device included in the semiconductor device shown in Figs. 9A and 9B is not an embodiment of the invention. An n-type semiconductor substrate 10 has an impurity concentration of 1 x 10¹⁴ to 2 x 10¹⁴ atoms/cm³. An n⁺-type diffusion/collector region 20 is formed to cover the entire rear surface (the other major surface) of the semiconductor substrate 10. P-type base regions 21 a and 21 b of drive and output transistors are formed in the front surface (one major surface) of the semiconductor substrate 10. A first region 11 which has the same conductivity type as that of the base regions 21a and 21b is formed at the same time as the base regions 21a and 21b to surround them. The first region 11 functions as the anode region of the protection diode. The base regions 21a and 21b and the first region 11 which are simultaneously formed are formed by diffusion so as to have a diffusion depth Xj (=30 µm). An n⁺-type second region 12 which is spaced apart from the first region 11 is formed to surround the first region 11. The second region 12 serves as the cathode region of the protection diode. The second region 12 is formed simultaneously with n-type emitter regions 22a and 22b. Phosphorus getter sputtering is performed to form an insulating film 13 to a thickness of 4 µm, using mainly silicon oxide (SiO₂), on the front surface of the semiconductor substrate 10 including the base regions 21 a and 21 b and the first and second regions 11 and 12. A conductive film 14 is formed on the insulating film 13 so as to partially overlap both first and second regions 11 and 12. The conductive film 14 is electrically connected to the first region 11 serving as the anode region, and is used as the anode electrode of the protection diode. At the same time, the conductive film 14 serves as a gate electrode of the protection diode by partially overlapping the second region 12 which serves as the cathode region of the protection diode. An aluminum base electrode 25 of the drive transistor, and an emitter electrode 26 and a collector electrode 27 of the output transistor are formed. The conductive film 14 which serves as the anode electrode of the protection diode is commonly connected to an emitter electrode 28 of the drive transistor and a base electrode 29 of the output transistor. The semiconductor device of the type described above has a Darlington circuit (Fig. 8) which is integrally formed on a single chip. Electrical characteristics of the Darlington transistor device including the protection diode are shown as follows. The withstand voltage of the semiconductor substrate 10 is about 1,000 V, and the withstand voltage between the collector and the emitter of the transistor is 450 V. The breakdown voltage at the base end portion which is surrounded by a guard ring can be more than 600 V by a cancelling effect of electric fields when the depletion region extending from the anode region of the protection diode overlaps the depletion region extending from the transistor. Therefore, this breakdown voltage is sufficiently large to satisfy the conventional requirement of the breakdown operation voltage (340 to 380 V) of the protection diode of the ignitor output circuit. In the semiconductor device having the configuration described above, the first region 11 which serves as the anode region of the protection diode can be formed simultaneously with the formation of the base regions 21 a and 21 b of the transistor. Furthermore, the second region 12 which serves as the cathode region can be formed simultaneously with the formation of the emitter regions 22a and 22b of the transistor. A portion of the insulating film 13 which is under the conductive film 14 as the gate electrode of the protection diode may be substituted by a field oxide film used for forming the transistor. The Darlington transistor circuit including the monolithic protection diode can be manufactured during the same process as is used for forming the conventional transistor. Furthermore, even if the protection diode is formed between the collector and the base of the transistor circuit (Fig. 7), the conductive film 14 which is electrically connected to the first region 11 can be connected to the aluminum base electrode 25 of the drive transistor. In this case, the base electrode 29 of the output transistor is isolated from the conductive film 14. In summary, the Darlington circuit having a protection diode can be manufactured with the same process as is used for forming the transistor. Furthermore, regardless of the presence or absence of the pad portion, the diode can be formed together with the transistor on the same, single chip. Therefore, the number of externally connected components can be eliminated. Even if the protection diode is connected between the collector and the base of the output transistor (the base region of the output transistor does not have a pad portion), the chip size need not be increased.

Claims: 1. An overvoltage protection device, comprising: a semiconductor substrate (10) of one conductivity type; a first region (11) of the opposite conductivity type formed on one major surface of said semiconductor substrate (10) and functioning as an anode (15) and not containing any regions of different conductivity type; a second region (12) serving as a cathode region of the one conductivity type which is formed on said one major surface of said semiconductor substrate (10) and is spaced apart from said first region (11), and has an impurity concentration higher than said semiconductor substrate (10), an insulation film (13) being formed to cover the entire surface including said first and second regions (11, 12), and a single conductive film (14) for use as a gate electrode being formed on a portion of said insulation film (13) in such a manner as to overlap the substrate layer between the first and second regions (11, 12) as well as partially each of the first and second regions (11, 12), the characteristics of the breakdonw voltage (V Z ) being determined by a control voltage (VGA) applied between the gate electrode (14) and the anode (15), said control voltage (V GA ) not resulting from a short circuit connection between the gate electrode and the anode. 2. A device according to claim 1 characterized in that a base region having the same conductivity type as the conductivity type of said first region is formed on said one major surface of said semiconductor substrate at a location different from a location of said first region, said base being connected to said first region, and an emitter region having the same conductivity type as the conductivity type of said second region is formed in said base region, thereby aligning said transistor circuit and said overvoltage protection element on said one major surface of said semiconductor substrate. 3. A device according to claim 2, characterized in that said base region of said transistor is simultaneously formed with said first region. 4. A device according to claim 2, characterized in that said first region is formed to surround said base region formed on said one major surface of said semiconductor substrate, and said second region is spaced apart from said first region and surrounds said first region. 5. A device according to claim 2 characterized in that said conductive film partially formed on said first and second regions through said insulating film is electrically connected to said base region.

Inventors: TOYOSHIMA, SHOJI, YAMAOKA, MASAMI
Applicants: NIPPONDENSO CO., LTD.
Application year: 1982
Main classification: H01L27
IPC codes: H01L29, H01L27