SEMICONDUCTOR SWITCH CIRCUIT

Abstract: A semiconductor switch circuit of field-drive type in­ cludes a bipolar components (8, 9) having a forward block­ ing and reverse blocking junctions, and unipolar compo­ nents (10, 11) of one and another conductivity types con­ nected across the bipolar component so as to short-circuit its forward blocking junction. The switch circuit operates to turn on or off in response to the voltage signal to the gate circuit which is isolated from the bipolar component, irre­ spective of the polarity of a voltage applied across the bi­ polar component.

Description: BACKGROUND OF THE INVENTION The present invention relates to a semiconductor switch circuit of field-drive type in which the main drive section is isolated electrically from its control section. With the ever progressing electronic technologies in extensive fields of industry, there is an increasing demand of devices for controlling a large electric power with a small control signal, with electrical isolation being made between the control section and the main drive section of the device. Typical semiconductor devices that meet the demand are photo-coupling devices known as "photocouplers". Among the devices photo-coupling thyristors having superior features such as: (1) forward and reverse blocking ability, (2) small power loss after switching, and (3) self-holding ability, are used extensively for switches of electronic exchange and solid state relays. The photo-coupling thyristor, however, has some crucial problems as will be described in the following. Fig. 1 shows the basic arrangement of a switch circuit including a photo-coupling thyristor. With a switch 1 being closed, a current flows in a light emitting device 2, and it radiates the light. The light emission causes a photo-thyristor 3 to produce a photocurrent, and when the photo-thyristor is forwardly biased by an a.c. power source 4, it is turned on by the photocurrent. Reference numbers 5, 6 and 7 denote resistors. This arrangement, with the photo-thyristor and light emitting device isolated electrically, has the following advantages in contrast with the usual electrical coupling systems. (a) There can be a voltage difference between terminals B and D in controlling the conducting operation. (b) The currents flowing in the light emitting component 2 and in the thyristor circuit are totally independent. On the other hand, the above arrangement has the following drawbacks. (1) The photo-thyristor 3 and transistor 1 are made of silicon, while the light emitting device 2 is made of compound semiconductor of III-V group or II-VI group, typically GaAs. Due to different materials used, the device needs to be fabricated in a hybrid integrated circuit that requires a precise assembling work, and therefore the cost is high. The processing technology of compound semiconductor wafer lagging behind that of silicon wafer also contributes to high cost. (2) Efficiencies are low in generating the light by the light emitting diode, transmitting the light to the photo-thyristor and receiving the light by the photo-thyristor. A low total photo-coupling efficiency requires a large control current of several milliamperes in the light emitting diode for activating the photo-thyristor. Japanese Patent Examined Publications JP-A- 42-24863 and JP-A- 53-46589 disclose arrangements for activating a pnpn component by a MOS gate or MOS FET. Japanese Patent Unexamined Publications JP-A- 57-196626 discloses an arrangement for turning on and off a pnpn component by a MOS FET. Although in any of the above cases the gate section is isolated from the main switch section, the main switch in a floating state cannot be turned on. Namely, the main switch is turned on only when the gate voltage is higher or lower than the cathode voltage of the main switch, and these arrangements do not function identically to the photocoupler. FR-A- 2 524 711 discloses a semiconductor device wherein the forward blocking junction of a bipolar component is short-circuited by a n-MOS FET. This structure cannot perform the floating operation, because the n-MOS FET would not operate and fails to control the bipolar component when the gate electrode of the n-MOS FET has a lower potential with respect to the emitter electrode or the collector electrode of the bipolar component. Said document discloses also a semiconductor device wherein the reverse blocking junction of a bipolar component is short-circuited by a p-MOS FET. This structure also cannot perform the floating operation for similar reasons as in the first case. Said document further discloses a semiconductor device wherein a forward blocking junction of a bipolar component is short-circuited by a n-MOS FET, while the reverse blocking junction thereof is short-circuited by a p-MOS FET. Also in this structure the floating operation cannot be performed, because the n-MOS FET would not be turned on and hence fails to control the bipolar component when the potential at the control terminal is lower than that at the main terminal of the bipolar component, and similarly, the p-MOS FET would not be turned on and hence fails to control the bipolar component when the potential at the control terminal is higher than that at the main terminal of the bipolar component. EP-A- 47 392 discloses a semiconductor device wherein the forward blocking junction of a bipolar component is short-circuited by an n-MOS FET. In this structure, the floating operation cannot be performed because the n-MOS FET would not operate when the potential at the gate electrode of the n-MOS FET is lower than that at the collector of the bipolar component. SUMMARY OF THE INVENTION An object of this invention is to provide a field-drive semiconductor switch circuit arranged in a monolithic integrated circuit, with the main drive section being isolated from the control section. Another object of this invention is to provide a field-drive semiconductor switch circuit operable to control the main drive section even in a floating state. Still another object of this invention is to provide a field-drive semiconductor switch circuit operable to drive the main drive section by a small control current. To achieve the foregoing objects, the present invention provides a semiconductor device comprising a pnp or npn transistor having at least one forward blocking junction and at least one reverse blocking junction, and a unipolar component of one conductivity type connected so as to short-circuit at least one of the forward blocking junction and the reverse blocking junction of said transistor, characterized in that a unipolar component of the other conductivity type is connected in parallel with said unipolar component of one conductivity type, and a control terminal of said unipolar component of one conductivity type is electrically connected to a control terminal of said unipolar component of the other conductivity type. To achieve the foregoing objects, the present invention also provides a semiconductor device comprising a thyristor having at least one forward blocking junction and at least one reverse blocking junction; and a unipolar component of one conductivity type connected so as to short-circuit at least one of the forward blocking junction and the reverse blocking junction of said thyristor, characterized in that a unipolar component of the other conductivity type is connected in parallel with said unipolar component of one conductivity type, and a control terminal of said unipolar component of one conductivity type is electrically connected to a control terminal of said unipolar component of the other conductivity type. According to a preferred form of this invention, said unipolar component of the other conductivity type is a p-channel MOS FET. According to another preferred form of this invention, said unipolar component of the other conductivity type is a n-channel MOS FET. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram showing the conventional semiconductor switch circuit employing a photocoupler; and Figs. 2 through 7 are schematic diagrams showing the first through 6th embodiments of this invention, respectively. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 2 shows the first embodiment of this invention. The arrangement includes an npn transistor 9, a p-channel MOS FET 10 having its source and drain connected to the collector and base of the npn transistor 9 so as to short-circuit the forward blocking junction, i.e., collector junction, of the transistor 9,and an n-channel MOS FET 11 having its source and drain connected to the collector and base of the transistor 9 in parallel to the p-channel MOS FET 10 so as to short-circuit the collector junction of the npn transistor 9. These components are formed independently in separate cristalline islands of a dielectric isolation substrate produced by the EPIC process, and wiring is made between them using aluminum conductors to complete a monolithic integrated circuit. Control terminals G1 and G2 are connected together to the switch 1. In fabricating the arrangement in a monolithic integrated circuit, each component formed in a monocristalline island has a high breakdown voltage above 250 volts. The npn transistor 9 has a usual known structure, the n-channel MOS FET 11 is a high voltage vertical FET having a structure as disclosed in publication "Digest of ISSCC", p. 238, Feb. 1981, by W.H.A. Mattheus, and the p-channel MOS FET 10 is a high voltage lateral FET having a structure as disclosed in a semiconductor transistor committee report "High voltage MOS FET", SSD73-14, June 1973, by Yoshida et al. In Fig. 2, when the switch 1 is open, the transistor 9 in any bias condition is not supplied with a base current, and the current path between A2 and B2 is in a non-conductive state. When the switch 1 is closed so that a voltage higher than the threshold voltage of MOS FETs 10 and 11 is applied to the control terminals G1 and G2 by the voltage source 15, the npn transistor 9 becomes conductive. In this case, a voltage source 20 operates on the npn transistor 9 to become conductive, even though the terminals A2 and B2 are floating, irrespective of the voltage relation between the control terminals G1 and G2 and the terminals A2 and B2, but in different operating manners depending on the voltage relation between the terminals, as follows. In case the control terminals G1 and G2 are lower in voltage than the terminals A2 and B2, the MOS FETs 10 and 11 have a higher source-drain voltage than the gate voltage. Accordingly, the n-channel MOS FET 11 is non-conductive, and the p-channel MOS FET 10 is conductive. Consequently, the source-drain current of the p-channel MOS FET 10 flows into the base of the npn transistor 9, causing it to become conductive. In another case, if the control terminals G1 and G2 are higher in voltage than the terminals A2 and B2, the MOS FETs 10 and 11 have a higher source-drain voltage than the gate voltage. Accordingly, the p-channel MOS FET 10 is non-conductive, and the n-channel MOS FET 11 is conductive. Consequently, the npn transistor 9 is supplied with a base current, and it becomes conductive. In case the control terminals G1 and G2 are higher in voltage than the terminal B2, but lower than the terminal A2, the operation of one of the above two cases takes place, causing the npn transistor 9 to become conductive. According to this embodiment, the npn transistor 8 of the main drive section is surely driven even if the main terminals A2 and B2 are in a floating state. The forward breakdown voltage between A2 and B2 is about 260 volts, and the insulation voltage between the terminals A2 and B2 and the control terminals G1 and G2 is about 600 volts. The threshold voltage for turning on the device is about 3.5 volts, and the on-resistance with a current conduction of 10 mA is about 15 Ω. Fig. 3 shows the second embodiment of this invention, which differs from the previous embodiment in that the main drive section employs a high voltage pnp transistor 8. The operation is substantially identical to the case of the first embodiment, with the npn transistor 9 of the former being replaced with the pnp transistor 8, and the further explanation will be omitted. The new arrangement provides a breakdown voltage comparable to that of the first embodiment, while the threshold voltage is about 4 volts and the on-resistance is about 10 Ω for a current conduction of 3 mA. Fig. 4 shows the third embodiment of this invention, in which an npn transistor 9 and a pnp transistor 8 are formed in a common monocristalline island of a dielectric isolation substrate, thereby implementing a thyristor operation. Otherwise, the npn transistor 9 and pnp transistor 8 may be formed in separate monocristalline islands with connection being made therebetween using aluminum conductors. MOS FETs 10 and 11 have their source and drain electrodes connected in parallel so that the collector junctions of the npn transistor 9 and pnp transistor 8 are short-circuited. Accordingly, with this embodiment placed on the block of Fig. 2 and with the d.c. voltage source 14 replaced with an a.c. voltage source, when the switch 1 is closed and a forward bias voltage is applied across A2 and B2 by the a.c. voltage source for the case of the control terminals G1 and G2 lower in voltage than the terminals A2 and B2, the p-channel MOS FET 10 becomes conductive to drive the npn transistor 9 and pnp transistor 8, and consequently a thyristor arrangement between A2 and B2 is turned on. In another case where the control terminals G1 and G2 are higher in voltage than A2 and B2, the n-channel MOS FET 11 becomes conductive, driving the npn transistor 9 and pnp transistor 8, and consequently the thyristor is turned on. This embodiment slightly differs in the operating manner from the first and second embodiments. This embodiment is adapted to switch an a.c. power source, which requires both the forward and reverse voltage withstanding, and all of the thyristor and n, p-channel MOS FETs 10 and 11 are made to have forward and reverse blocking abilities. Namely, the source and drain junctions can withstand 250 volts. The main circuit between A2 and B2 has a breakdown voltage of about 200 volts forward and reverse, and a breakdown voltage between the terminals A2 and B2 and the control terminals G1 and G2 is about 700 volts. The threshold voltage for turning on the main circuit is about 4 volts, and the on-resistance for conducting 30 mA is about 5.5 Ω. Fig. 5 shows the fourth embodiment of this invention, in which a pnp transistor 8, npn transistor 9, first p-channel MOS FET 10 and first n-channel MOS FET 11 are connected in the same manner as Fig. 4. Further provided are the second p-channel MOS FET 21 and second n-channel MOS FET 22 having their source and drain electrodes connected in parallel to the base of the npn transistor 9 (collector of the pnp transistor 8) and the emitter of the transistor 9 so as to short-circuit the reverse blocking junction, i.e., emitter junction, of the npn transistor 9. The first p-channel MOS transistor 10 and first n-channel MOS transistor 11 have their control terminals connected together to form a common control terminal G3, while the second p-channel MOS transistor 21 and second n-channel MOS transistor 22 have their control terminals connected together to form another common control terminal G4. The FET 11 and FET 10 with their gate electrodes connected to the control terminal G3 are used to turn on the thyristor arranged between terminals A2 and B2, while the FET 22 and FET 21 with their gate electrodes connected to the control terminal G4 are used to turn off the thyristor. A typical operational sequence of this embodiment is as follows. (1) A forward bias voltage is applied between A2 and B2. (2) An ON control voltage is applied to the control terminal G3 to turn on the thyristor. (3) The control voltage is removed from the control terminal G3, and the thyristor is left in a conductive state. (4) An OFF control voltage is applied to the control terminal G4 to turn off the thyristor. (5) The control voltage is removed from the control terminal G4, and the thyristor is left in a non-conductive state. The turn-on operation of this embodiment is exactly identical to the case of the third embodiment, and only the turn-off operation will be described in the following. This embodiment is capable of turn-off control for the conductive thyristor even if the terminals A2 and B2 are floating, by application of an OFF control voltage higher than the threshold voltage of FETs 21 and 22 to the control terminal G4, but in different operating manners depending on the voltage relation between the control terminal G4 and the temrinals A2 and B2. In case the control terminal G4 is lower in voltage than the terminals A2 and B2, the MOS FETs 21 and 22 have their source and drain voltages higher than the base voltage. Therefore, the second n-channel MOS FET 22 is non-conductive, while the second p-channel MOS FET 21 is conductive. Making the FET 21 to have a sufficiently small on-resistance, the above condition causes the npn transistor section 9 of the thyristor to have its emitter junction short-circuited. Consequently, the current flowing in the thyristor is pulled out, and it is turned off. In another case where the control terminal G4 is higher in voltage than the terminals A2 and B2, the second p-channel MOS FET 21 is non-conductive, while the second n-channel MOS FET 22 is conductive, short-circuiting the emitter junction of the npn transistor section 9 of the thyristor, and the thyristor is turned off. When the control terminal G4 is higher than B2 and lower than A2, the operation of one of the above two cases takes place, and the thyristor is turned off. As described, this embodiment ensures the turn-on and turn-off operations of the device even if the main terminals A2 and B2 are in a floating state. In this case, the on-controlling MOS FETs 10 and 11 are capable of control even with their relatively large on-resistance, whereas the off-controlling MOS FETs 21 and 22 need to have a relatively small on-resistance, e.g., 500 Ω or less, in order to facilitate and ensure the control. This embodiment has a breakdown voltage and turn-on threshold voltage substantially identical to those of the third embodiment. The turn-off control voltage to the control terminal G4 is about 6 volts, and the on-resistance is about 8 Ω for a current conduction of 30 mA. The fifth and sixth embodiments are shown in Figs. 6 and 7, respectively. In these arrangements, the transistor collector junction is connected in series with another junction, i.e., the emitter junction of a pnp transistor 8 or npn transistor 9, and thereafter connected in parallel to an n-channel MOS FET 11 and p-channel MOS FET 10. The operation of these arrangements is virtually identical to the case of parallel connection for short-circuiting only the collector junction. Namely, the case of Fig. 6 is the same as the first embodiment and the case of Fig. 7 is the same as the second embodiment, with the exception that once the transistors are activated a positive feedback action arises between the npn and pnp transistors, resulting in a turn-on thyristor. Various forms of the present invention have been described, however, the invention is not limited to these embodiments, but many modifications are possible. For example, in Figs. 4, 5, 6 and 7, integration may be made only for the base n1 of the pnp transistor 8 and the collector n2 of the npn transistor 9, or the collector 02 of the pnp transistor 8 and the base p3 of the npn transistor 9, to form a thyristor. Although the arrangements shown in Figs. 4, 5, 6 and 7 are reverse blocking switches solely operable to turn on when biased forwardly in the case of a.c. driving, bidirectional switches can be arranged through the anti-parallel connection of each device in pair. The semiconductor devices shown in Figs. 4, 5, 6 and 7 are not sufficiently immune to voltage noises in the forward bias direction. It is possible to enhance the noise immunity significantly without impairing the features of this invention through the attachment of a voltage noise protection circuit disclosed in Japanese Patent Examined Publication No. 53-46588. As an alternative arrangement for turning off the device, the off-controlling second p-channel MOS FET 21 and second n-channel MOS FET 22 shown in Fig. 5 may be connected to the bipolar component of Figs. 6 and 7 so as to short-circuit at least one of the reverse blocking junctions. As described above, the present invention enables a current-control bipolar component to be controlled by a voltage-control unipolar component. The circuit is fablicated in a monolithic structure, with a d.c. isolation being made between the control section and the main drive section. Moreover, the switching device can surely be controlled even in a floating state of the unipolar components, and a control current needed can be made smaller.

Claims: A semiconductor device comprising: a pnp or npn transistor (8, 9) having at least one forward blocking junction and at least one reverse blocking junction; and a unipolar component (10, 11) of one conductivity type connected so as to short-circuit at least one of the forward blocking junction and the reverse blocking junction of said transistor (8, 9); characterized in that a unipolar component (11, 10) of the other conductivity type is connected in parallel with said unipolar component of one conductivity type, and a control terminal (G1) of said unipolar component of one conductivity type is electrically connected to a control terminal (G2) of said unipolar component of the other conductivity type. A semiconductor device comprising: a thyristor (8, 9) having at least one forward blocking junction and at least one reverse blocking junction; and a unipolar component (10, 11) of one conductivity type connected so as to short-circuit at least one of the forward blocking junction and the reverse blocking junction of said thyristor (8, 9); characterized in that a unipolar component (11, 10) of the other conductivity type is connected in parallel with said unipolar component of one conductivity type, and a control terminal (G1) of said unipolar component of one conductivity type is electrically connected to a control terminal (G2) of said unipolar component of the other conductivity type. A semiconductor device according to claim 1 or 2, characterized in that said unipolar component of the other conductivity type is a p-channel MOS FET (10). A semiconductor device according to claim 1 or 2, characterized in that said unipolar component of the other conductivity type is a n-channel MOS FET (11).

Inventors: INABE, YASUNOBU, KIMURA, TADAKATSU, KITANO, JUNJIRO, SUGAWARA, YOSHITAKA, TANABE, MASA-AKI
Applicants: HITACHI, LTD., NIPPON TELEGRAPH AND TELEPHONE CORPORATION
Application year: 1985
Main classification: H03K17
IPC codes: H03K17